Subcooled flow boiling CHF correlations
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Flow boiling has an extremely high heat transfer coefficient, and is applied in variety of practices. However, once the heat flux exceeds a certain high level the heated surface can no longer support continuous liquid contact, associated with substantial reduction in the heat transfer efficiency. It may result in a sudden rise of surface temperature in a heat flux controlled system, or a drastic decrease in power transferred in a temperature controlled system. This phenomenon is called the boiling crisis, and the maximum heat flux just before the boiling crisis is usually referred to as critical heat flux (CHF).
Depending on the flow regimes, two types of CHF are classified: (i) in subcooled or low quality region the CHF is characterized by the transition from nucleate boiling to film boiling, and it is termed as the departure from nucleate boiling (DNB); (ii) in higher quality region the CHF is characterized by the dryout of liquid film of annular flow. The DNB and dryout have substantially different mechanisms, and are generally cataloged as the first and the second kind of critical heat flux, respectively (Tong and Tang, 1997).
The CHF is an important subject to many engineering applications. Especially, in a nuclear reactor the occurrence of critical heat flux could lead to a failure of fuel element, and thus the CHF is a major limit for the reactor safety. During past five decades the CHF has been investigated extensively over the world theoretically and experimentally (IAEA-TECDOC-1203, 2001). A great number of empirical correlations and physical models have been proposed. In recent years, a Look-Up Table method (LUT) is widely accepted due to its advantages of higher accuracy, wider range of application, correct asymptotic trend and convenience for updating (Groeneveld, et al., 1996). This table is applied in the system code RELAP5 for reactor safety analysis. The LUT contains a tabulation of normalized CHF data of a uniformly heated tube of 8 mm in diameter at discrete local parameters of pressure, mass flux and quality. Several correction factors are incorporated for utilization of the LUT in other conditions to account for the effects of diameter, bundle, spacer, flux distribution, flow orientation, etc.. Unfortunately, there exists a scarcity of CHF data in low pressure/low flow/ subcooled region, as shown in Fig.1. Because of the extreme complexity of the phenomenon and the lack of adequate knowledge of the mechanisms, all these predictive methods are heavily relied on experimental data, and can not be extrapolated out of their ranges with confidence.
In China Institute of Atomic Energy (CIAE), in the past four decades a great number of CHF experimental data of subcooled boiling of flowing water were obtained in tubes and annuli at lower pressure with different diameter or gap to support the designs of research reactors, HFR and CARR, which were first put into operation in 1980 and 2011, respectively. In recent years the experiments were extended to the region of near-critical pressure with lower flow for the R&D of Supercritical Water-Cooled Reactor (SCWR). In these experiments the CHF (DNB) phenomena are studied with emphases on lower pressure and higher pressure with lower flow. The characteristics and parametric trends of the CHF are clarified, and the physical models are derived.
A comprehensive review on the subcooled flow boiling CHF was given by Berglest (1977). As concluded, the effect of heating length disappears when exceeding 200 mm and the subcooled boiling CHF depends only on local parameter (“local condition hypothesis”). The CHF increases with the local mass flux and subcooling increasing. The effect of pressure is more complicated: the CHF increases with pressure in low pressure region, is relatively constant over an intermediate range of pressure, and decreases in high pressure region. The effect of geometry on CHF is observed in many experiments, and is accounted by a factor f = (De/8)n, in which De is the hydraulic diameter in mm. Different value of the exponent n is obtained from different experiments, ranging from -0.1 to -0.5. For applying the LUT in reactor safety analysis code RELAP5 the value of n = -0.5 is used. While for annulus or rectangular channels the effect of gap width is not observed in many experiments.
In Savannah River lab and Columbia University the subcooled CHF experiments were conducted with D2O and H2O coolant and aluminum and stainless-steel heaters (Knoebel et al., 1973). It was concluded that the CHF for D2O is 16% greater than for H2O at constant subcooling and velocity, and the CHF of aluminum heaters is a minimum of 20% greater than for stainless steel.
Some typical subcooled boiling CHF correlations are listed in Tab.1. Similar trends of the CHF with the flow rate and subcooling are represented in various correlations, but the degrees of these effects are different significantly. The present investigation has revealed that these effects are inter-dependent, associated with complicated parametrical trends for different regions of conditions.
The onset of critical heat flux is characterized by a drastic increase in the wall temperature. For subcooled flow boiling of water with lower pressure the critical heat flux is higher and it could lead to a failure of the heated wall rapidly. This kind of CHF is also called as “fast burnout”. In experiment the onset of CHF is usually detected by thermocouples for protection of the test section. However, the occurrence of CHF generally initiates from a small area (or a spot), and for a test section of larger size the CHF spot can not be expected exactly. This presents a challenge for prevention of the test section from burnout. For this condition the photocell has advantage for the detection of CHF.
In the present experiments the pressure and flow rate are kept at constant, and the CHF is approached by increasing slowly the water temperature or the power to test section. When the CHF occurs and the wall temperature exceeds about 500°C, the photocell produces an output, which switches off the power supply to test section. This technique is used by author for all the experiments in tubes and annuli.
Experiment was performed in an uniformly heated inconel tube of 7.95 mm in diameter and 0.8 m in heating length, covering the ranges of pressure of = 1.96 – 20.4 MPa, mass flux of = 476 – 1653 kg/m2s, inlet subcooling of = 49 – 343 K, outlet subcooling of= 1 – 145 K and critical heat flux of = 0.26 – 4.95 MW/m2.
For the present low flow condition the CHF is related to the inlet condition, characterizing the mechanism of total power dominant. Totally, 193 data are obtained, and are formulated as the following empiric correlation,
where is the heat flux for the exit to reach the saturation temperature, evaluated by
where is the pressure, and the inlet enthalpy and saturation enthalpy, respectively, the mass flux, the diameter and the heating length. Fig.2 shows the comparison of the prediction of Eq.(1) with the experimental data by plotting the ratio of qCHF,c/qCHF,M versus P or G. The average deviation, AVG, is 0.75% and the standard deviation, RMS, is 5.34%.
The effects of mass flux, inlet subcooling and pressure are exemplified in Fig.3 and 4. The CHF decreases substantially with mass flux decreasing. As can be seen, at G > 1200 kg/m2s the data are close to the prediction of 96-CHF Look-Up Table (LUT), but at low mass flux the data are overpredicted significantly (Fig.3). For lower pressure the effects of inlet subcooling and mass flux are stronger than higher pressure, associated with complicated trend of the CHF with pressure. For G > 1200 kg/m2s higher CHF corresponds to lower pressure, especially in high subcooling region (Fig.4(a)). For G= 700 – 1200 kg/m2s the results of < 16 MPa are not different appreciably for different pressures. For G< 600 kg/m2s, in high subcooling region lower CHF corresponds to higher pressure, while in low subcooling region lower CHF is attained at lower pressure (Fig.4 (b))
184.108.40.206. CHF in tubes\n\t\t\t\t\t
Medium and high subcooling (Chen et al., 2000)
Experimental data were obtained in uniformly heated stainless-steel tubes of 5.17, 8.05, 10.0 and 16.0 mm in diameter, covering the ranges of pressure p = 0.13 – 1.92 MPa, velocity V = 1.47 – 22.32 m/s, as listed in Tab.2.
Fig.5 and 6 exemplify the variations of critical heat flux with local velocity and subcooling for different pressures. The CHF increases strongly with the velocity increasing (Fig.5). For subcooling higher than about 30 K, the CHF exhibits an approximately linear increase with increase of subcooling. In medium subcooling region the CHF is not different greatly between different pressures. For p < 0.3 MPa the trend of CHF with subcooling is steeper than that of higher pressure, so that in low subcooling region lower CHF is obtained at lower pressure (Fig.6). The CHF also exhibits a general increase trend with the decrease of diameter, and at lower velocity this effect appears stronger than higher velocity.
|Number of data|
|2||8.05||383, 396||0.14-1.92||1.85-16.05||8.3 – 88.2||65|
224 CHF data of subcooling higher than 35 K are formulated by the following empiric correlation with local parameters of p, V, △Ts and D:
Low subcooling (Chen et al., 2005).
The experiment was performed in a stainless-steel tube of D=15.9 mm with emphasis on the CHF characteristic in low subcooling region. The conditions cover the ranges of pressure of 0.2 - 1.7 MPa and velocity of 2.2 - 13.2 m/s. Fig.8 exemplifies the variations of CHF with subcooling. For p > 1.0 MPa, the CHF decreases with △Ts decreasing monotonously. For p < 0.3 MPa, however, this trend breaks at a certain low value of subcooling, and it turns to increase at further low subcooling. The subcooling at the minimum CHF varies from 13 to 30 K with lower value corresponding to lower velocity.
This behavior was also observed in the author’s experiment in annuli and other similar experiments (Zeigarnik, 1994, Knoeble et al., 1973). It can be attributed to the onset of net vapor generation (NVG) or the onset of significant voiding (OSV). For p < 0.3 MPa, the vapor density is very small, so that in NVG regime the volumetric flow rate increases essentially, associated with a considerable increase in the liquid velocity. Therefore, the bubbles generated on the heated surface are more likely to enter into the liquid core, resulting in higher CHF. At high pressure the vapor density is much higher, and hence the CHF behavior could not vary distinctly in low subcooling region.
The NVG or OSV was generally identified by a sharper increase in the pressure drop. The following type of empiric correlation was derived
with the values of n ranging from 0.5 to 1.0. (Siman-Tov, et al. 1995). For the present experimental condition most correlations predict the OSV at subcooling of below 40 K.
Fig.9 shows the variations of pressure drop with the increase of exit temperature. As seen, before the OSV the pressure drop increases slowly with the temperature increasing. After the OSV this trend varies distinctly. Especially at low pressure it is much steeper than that at higher pressure.
In a reactor core the fuel elements are located in parallel channels. Therefore, at low pressure with low subcooling the OSV could result in flow instability or flow excursion (FE), characterized by a continuous decrease of flow rate in a channel. This could eventually lead to a failure of fuel element. Therefore, for reactor safety the limit of OSV is taken as a cr1terion in combination with the limit of minimum ratio of DNB
220.127.116.11. CHF in annuli
Single-side heating (Chen et al., 2004)
Experiments were performed in 8 annuli made of stainless-steel tubes with single-side heating. The diameter of outer wall is 16, 32 or 70mm and gap width is 2 to 4mm. Great majority of CHF data were obtained at outer wall and less at inner wall, covering the range of pressure of 0.17 - 1.8MPa, mass flux of 1300 to 18200 kg/m2s, outlet subcooling of 27-105 K and critical heat flux of 2.0 - 18.1 MW/m2. The experimental conditions are listed in Tab.3.
|Length L (mm)||Pressure|
|Number of data|
The velocity and subcooling have predominant effects on the CHF. Under most conditions of interest the effect of pressure is not appreciable. The variation of gap width from 2.0 to 4.0mm does not make an appreciable effect on the CHF. This can probably be explained as follows: larger gap size associates with lower heat transfer coefficient to the liquid core, as suggested by the turbulent convection heat transfer correlation, on the other hand larger gap size corresponds to higher Reynolds number, which is benefit for the departure of bubbles from heated surface. For the present conditions these two opposite factors would be comparative, making the CHF not sensitive to the gap size. These results are consistent with many similar experiments (see Tab.1).
In the present experiment the effect of curvature of heated surface is not observed. This is understandable, because the curvature of heated surface is small, compared to the bubble, and it could not have a noticeable effect on the bubble behavior.
For the sake of simplicity in engineering applications, the effects of pressure, diameter and gap width are ignored, and the experimental data are formulated as the following empiric correlation
Bilateral-side heating (Chen, et al., 1996)
Experiment was performed in a stainless-steel annulus of D1=28 mm and D2=35.7 mm and heating length of 160 mm with bilateral-side heating. The critical heat flux data were obtained at the outer wall. The conditions cover the range of pressure of 0.31 – 0.39 MPa, velocity of 2.55 – 8.12 mm, subcooling of 49.1 – 76.6 K and the ratio of inner to outer-wall heat fluxes of q1/q2 of 0 – 0.94.
For convenience in comparison, the result of outer-side heating is formulated by
as shown in Fig.11.
The experimental results of bilateral-side heating are displayed in Fig.12 by plotting the ratio of qc/qc,0 against q1/q2 (1 and 2 denote the inner-wall and outer-wall, respectively). As seen, the critical heat flux exhibits an increase trend with q1/q2 increasing. When q1/q2 closes to 1.0 the CHF is increased by 15 – 20%. It can be attributed to the variation of temperature profile in liquid core, which results in an increase in condensation efficiency of the bubbly layer by subcooled liquid core, that is similar with that in single-phase convection heat transfer. This effect can be clarified further by the model analysis latter in paragraph 4.3.
Critical heat flux is more likely to occur under off-normal or accident conditions, in which a transient would experiences of flow rate, pressure and/or power. Many studies have focused on the transient CHF (Leung, 1978, Chang, et al., 1989, Iwamura, et al., 1987, 1994, Celata, et al., 1991, Weisman, 1993, Moon, et al., 2002). In higher quality region the experimental results revealed the inadequacy of the steady-state CHF correlation for transient conditions. While in subcooled and low quality region, the effect of transient on the CHF was found not appreciable. In general, the transient CHF has not been studied adequately for wider range of condition, and in evaluation of nuclear reactor safety the CHF for transient conditions is predicted with the correlations derived at steady-state conditions (IAEA-TECDOC-1203, 2001).
In the present study an experiment of flow-reduction transient CHF was performed in a stainless-steel tube of 15.9 mm in diameter, covering the range of pressure of 0.2 - 1.4 MPa, initial velocity of 4.5 - 13.5 m/s, and the initial inlet subcooling of 80 – 160 K. The flow rate was reduced linearly as
where and are the instantaneous and initial flow rate, respectively, t is the time, and k is the flow reduction rate, ranging from 0.0075 to 0.24 1/s.
The experimental results are shown in Fig.13, in which the P, V and △Ts are the instantaneous values. For P > 1.0 MPa the effect of flow transient on CHF appears not prominent. For P < 0.3 MPa, in high subcooling region the effect of transient is not appreciable, while for subcooling lower than about 50 K the result departs from the trend of steady-state distinctly, and higher CHF is attained at higher flow reduction rate.
Different effect on the CHF observed at different conditions can probably be explained by different mechanisms of the CHF. For low subcooling the CHF is induced by a limit of enthalpy of bubbly layer, while for high subcooling the CHF is induced by a limit of bubbly-layer condensation by the subcooled liquid core (Thorogerson et al., 1974). Compared to the steady-state, at a flow-reduction transient with constant heat flux and constant subcooling the enthalpy in the bubbly layer is higher and the temperature in the liquid core is lower. At high subcooling the thickness of bubbly layer is small, thus for a constant CHF the subcooling would not be different greatly from the steady-state. While at low subcooling the thickness of bubbly layer is larger, and a constant CHF would occur at a higher subcooling than that of steady-state. It would lead to the premature of the OSV, associated with prominent effect on the CHF, as observed at low pressure.
Fig 14 shows the variation of pressure drop before the onset of CHF in flow-reduction transients of p =0.23 MPa and k = 0.24 with heat flux of q = 890, 775 and 750 W/cm2 for run iii, iv and v respectively. In run iii, the pressure drop exhibits a monotonous decrease with flow rate decrease, until the onset of CHF. While in run iv and v, before the onset of CHF the decrease of pressure drop is followed by a sharp increase. In these three runs the CHF are higher than those of steady-state by about 5%, 18% and 36%. The critical subcoolings are 54.7, 46.5 and 37 K respectively, all of which are higher than the values of steady-state, evidencing the premature of OSV.
In subcooled flow boiling the enthalpy of bubbly layer is determined by the heat transfer from the interface of bubbly layer to the liquid core, and the excessive bubble crowding serves as a thermal shield, leading to the onset of CHF. For the DNB with low subcooling or saturated condition, the critical enthalpy models were proposed by Weisman and Pei (1983) and Tong (1968), in which the heat transfer coefficient from the interface of bubbly layer to liquid core was estimated by correlations derived from their experiments. For the DNB with high subcooling, the liquid sublayer dryout models were proposed by Katto (1992), Lee and Mudawar (1983) and Celata et al. (1994), in which the bubble diameter, the thickness of liquid sublayer and the length of vapor blanket were determinant for the sublayer dryout.
For higher subcooling the major part of heat from wall is transferred to the liquid core and a minor part for increase of the enthalpy of bubbly layer. Therefore, the characteristic of bubbly layer is primarily controlled by the heat transfer from the bubbly layer to liquid core. This heat transfer is closely relative to the turbulence near the edge of bubbly layer, and is sensitive to the distance from the wall. The increase in thickness of bubbly layer has positive effect on the CHF due to increase in the heat transfer efficiency to liquid core, but negative effect due to increase in the thermal resistance of the bubbly layer. The balance of these two factors gives a critical value of the thickness. Therefore, in the liquid sublayer dryout model the thickness of bubbly layer is a determinant factor.
The model is based on the mechanism of liquid sublayer dryout by modifying the Celata’s model for the thickness of bubbly layer to cover both high and low subcooling region.
At high flow and high subcooling the minimum thickness of bubbly layer is determined by the size of a bubble, while at low subcooling it could be larger due to bubble crowding. For the present experimental condition the following expression on the thickness of bubbly layer is attempted,
where is the surface tension, the liquid density, the mass flux, is a function with parameter of contact angle and recommended as = 0.02-0.03. In the present model it is taken as
where p is the pressure in MPa.
The friction factor, , is calculated by Colebrook-White equation combined with Levy’s rough surface model (1967), as
where D is the tube diameter, Re the Reynolds number, is the surface roughness, accounted by.
The velocity distribution in the liquid core is represented by Karman’s universal law, as in Celata’s model,
where is the liquid velocity, the distance from the wall, the liquid viscosity and the liquid density, the friction velocity, and is the wall shear stress, evaluated by
The temperature distribution in the liquid core is as follows (Martinelli, 1947),
Equations (5) to (7) are assumed valid in the region of, and the is a referent value, which is determined by at.
Calculation of critical heat flux
The local enthalpy, , is calculated by
where is the total flow rate, and are the vapor and liquid flow rate in the bubbly layer, respectively, and are the vapor and liquid enthalpy, and is the enthalpy of liquid core.The, and are evaluated by
where is the void fraction in the bubbly layer, and it is taken as = 0.9, is the average velocity of bubbly layer, estimated by
where r is the radius of tube, and is the distance from wall at which the temperature is equal to the saturation value.
The exit enthalpy, , is evaluated from the heat balance equation, as
where is the inlet enthalpy, and the heating length.
Calculation is started with a test heat flux (), and the, , and are calculated by Eq. (4), (10), (11) and (12). Then, theis calculated by Eq.(9) and compared to that calculated by Eq.(13). The result of CHF is obtained through an iterative process.
To get the calculations better fit to the experimental data, the constants in Eq. (4) are as: = 0.75, = 1000, and = 1.0. At low subcooling is close to, and not sensitive to the, thus the maximum value of is simply set as 0.1D.
Fig.17 shows schematically the profiles of velocity and temperature in the liquid core of an annulus. Some assumptions are made as follows:
Each wall is heated uniformly, and the flow and heat transfer conditions are fully developed;
At the edge of liquid bubbly layer the heat flux is equal to that of outer-wall
In liquid core the properties are evaluated at the bulk temperature.
In liquid core the energy balance equation is written as
For fully developed flow with uniform heating, the is a constant. Integrating Eq.(14) gives
The average bulk temperature is approximated by
Integrating Eq.(16) by parts gives
where is the temperature at the outer edge of bubbly layer. For low pressure with higher subcooling the enthalpy of bubbly layer is negligible small.
The velocity distribution is assumed in power law, as
where r1 and r2 are the radius of inner and outer wall, respectively, and rm is the radius of maximum velocity, calculated from
where andare the shear stress at the inner and outer wall, respectively.
The friction factor is estimated by (Xu et al., 1979)
The momentum eddy diffusivity is evaluated by (Levy, 1967)
The turbulent Prandtl number, , is taken as =1/1.2.
Both the experimental data of 8 annuli with single-side heating and the annulus with bilateral-side heating are predicted by this model. The comparison is shown in Fig.18 to 20.
Critical heat flux is an important subject to many applications. Especially for nuclear reactors, it is a major limit for the safety. The CHF has been investigated extensively over the world, and various prediction methods have been available. Unfortunately, there exists a scarcity of experimental data in certain regions. Because of extreme complexity of the phenomenon and the lack of adequate knowledge of the mechanisms, all these predictive methods are heavily relied on the experimental data, and can not be extrapolated out of the range with confidence.
In the present lab a great number of critical heat flux data of subcooled water have been obtained in tubes and annuli with different diameter and gap size over wide range of parameters with emphasis on lower pressure and higher pressure with low flow. The results fill the gap of database and the knowledge of the phenomenon.
The velocity and subcooling are the predominant parameters for the CHF. At lower pressure these effects are stronger. At the pressure below 0.3 MPa, when the subcooling decreases below a certain low value the CHF behavior varies substantially as a result of significant voiding. The effect of geometry is related to the pressure, subcooling and velocity. All these effects are inter-dependent, and are hardly to be represented in a single correlation for wide range of conditions. In the present study two models have been developed for the subcooled flow boiling CHF in circular tube and annulus, based on the mechanisms of CHF and the present experimental data. They will be validated and improved for extended range of conditions.
G mass flux
heat transfer coefficient
U velocity in axial direction
V average liquid velocity at CHF
Prt turbulent Prandtl numbe
Re Reynolds number
W channel width
distance from the wall
momentum eddy diffusivity
wall shear stress
c critical, calculatiom
Traditional medicine is still recognized as the preferred primary health care system in many communities, with over 60% of the world’s population and about 80% in developing countries depending directly on medicinal plants for their medical purposes . This is due to a number of reasons including affordability, accessibility and low cost .
The use of plants to cure several kinds of human diseases has a long history. Various parts of plants such as leaf, stem, bark, root, etc. are being used to prevent, allay symptoms or revert abnormalities back to normal. Since the practice of “herbal remedies” does not adhere strictly to facts accrued using scientific approaches, orthodox medicine sees “herbal medicines” as an alternative medicine. However, most of the pharmaceutical products currently dispensed by physicians have a long history of use as herbal remedies, including opium, aspirin, digitalis and quinine. Modern medicine today utilizes active compounds isolated from higher plants, and about 80% of these active ingredients indicate a positive correlation between their modern therapeutic use and the traditional uses .
The search for, and use of drugs and dietary supplements obtained from plants have increased in recent years. Scientist such as pharmacologists, microbiologists, botanists, and phytochemists are combing the Earth for phytochemicals and clues that could be developed into medicines for various diseases treatment. This study therefore reviewed electronic database (Google Scholar, SciFinder, PubMed, etc.) for medicinal plants that have potent activity in treating some prevalent and common ailments like malaria, diarrhea, tuberculosis, pneumonia and asthma.
Malaria is one of the world’s most important parasitic disease and a leading cause of death especially in developing countries . It is endemic in about 100 developing countries, leading to about 1.2 million estimated deaths each year in Africa , with pregnant women and children below 5 years being mostly affected . A wide range of medicinal plants is employed for the treatment of malaria, since majority of the people who get infected cannot afford the existing expensive orthodox medicines . The problem of resistance to existing antimalarial agents by parasite has necessitated the search for new and potent agents, and the focus of researchers is on natural products especially medicinal plants since active compounds like quinine and artemisinin were isolated from plants and have been lead compounds for antimalarial drug development [8, 9]. Various medicinal plants have been investigated for their anti-malarial activity and some with demonstrated potent in vitro activity have been reviewed below.
C. sanguinolenta (Lindl.) Schlechter (Apocynaceae) is known by Ghanaians as ‘Ghana quinine’ and specifically by the Asantes and Ewes as ‘Nibima’ and ‘Kadze,’ respectively . It is a twining and scrambling thin-stemmed shrub, indigenous to Africa, with much ethno-medicinal importance and interest in the West African sub-region . It is used traditionally for the treatment of malaria, upper respiratory and urinary tract infections, diarrhea, hypertension and as cicatrizant of wounds [12, 13]. The ethanolic and aqueous extracts of C. sanguinolenta exhibited an in vitro antiplasmodial activity against multi-drug resistance Plasmodium falciparum (K1) strain, with all the extracts inhibiting 90% of parasite growth at concentrations below 23 μg/mL. The ethanolic roots and leaves extracts showed potent activity with IC50 of 0.895 ± 0.02 and 3.01 ± 0.02 μg/mL, respectively. While the aqueous roots and leaves extracts had IC50 of 2.32 ± 0.3 and 13.5 ± 0.7 μg/mL, respectively . Evaluating the clinical efficacy of a tea bag formulation of the root of C. sanguinolenta in patients with uncomplicated malaria showed that within 72 h, Fifty percent (50%) of the patients had their P. falciparum parasitaemia cleared, and all patients, by Day 7. By Day 3, all presenting symptoms such as fever, chills, nausea and vomiting were completely no more. The overall cure rate when one tea bag of C. sanguinolenta was taken three times a day for 5 days was 93.5%, due to two cases of recrudescence on Days 21 and 28 .
T. ivorensis A. Chev. belongs to the family Combretaceae and is commonly known as ‘black afara’ and by the Asantes as ‘amire.’ It is a large deciduous forest tree of 15–46 m high, normally grown as timber plantation in many tropical countries . In traditional medicine, various parts of the plant is used to treat malaria, yellow fever, pile, stomach ulcer, wounds and other infections [17, 18]. A study by Komlaga et al.  revealed an active in vitro antiplasmodial activity of T. ivorensis aqueous leaf extract, against P. falciparum chloroquine-sensitive (3D7) and chloroquine resistant (W2) strains with IC50 of 0.64 ± 0.14 and 10.52 ± 3.55 μg/mL, respectively. The ethanolic stem bark extract also showed an in vitro antimalarial activity against chloroquine-resistant strains of P. falciparum with an IC50 of 6.949 μg/mL .
E. guineensis Jacq (Arecaceae), popularly known as oil palm is a monocotyledonous plant which belongs to the coccoid group of palms. It grows up to 15 m high with a lifetime of over 100 years and occurs throughout the tropical rainforest belt of West Africa . E. guineensis is commonly used for treating gonorrhea, rheumatism, headache, wounds . An in vitro anti-plasmodial assay revealed that, the ethanolic extract of E. guineensis leaves has potent antimalarial activity with IC50 of 1.195 μg/mL, against chloroquine-resistant P. falciparum .
P. emblica L. of the family Euphorbiaceae is a deciduous medium-sized plant (10–18 m high), native to tropical south eastern Asia and widely distributed in most subtropical and tropical countries. It is commonly known as Indian gooseberry, rich in vitamin C, minerals and amino acids which helps to build up lost vitality and vigor [23, 24]. Various parts of the plant is used traditionally for the treatment of diarrhea, inflammation, diabetes, jaundice, cough, asthma, peptic ulcer, skin diseases, leprosy, intermittent fevers, headache, anemia, dizziness, snakebite and scorpion-sting . In an SYBR green I-based fluorescence assay to assess the anti-plasmodial potential of P. emblica, the methanol leaf extract exhibited potent activity against CQ-sensitive (3D7) and CQ-resistant (Dd2 and INDO) strains of P. falciparum with IC50 of 3.125, 4.8 and 5 μg/mL, respectively. Also the ethyl acetate leaf extract showed activity with IC50 of 7.25, 15 and 9 μg/mL against 3D7, Dd2 and INDO P. falciparum strains, respectively .
S. aromaticum (L.) Merril. & Perry, syn. Eugenia caryophyllata, an ancient and valuable spice is a member of the family Myrtaceae and is commonly known as clove. It is mostly used as a spice to flavor all kinds of foods and has other medicinal values including anthelmintic, anti-asthma and other allergic disorders, anti-inflammatory, antioxidant, antiviral and anti-parasitic properties . A study by Bagavan et al. , revealed the antimalarial activity of methanol extract of S. aromaticum flower buds with IC50 of 6.25, 9.5 and 10 μg/mL against P. falciparum CQ-sensitive (3D7) and CQ-resistant (Dd2 and INDO) strains, respectively.
G. tamirensis Pierre ex Finet & Gagnep is an accepted synonym for the species and is from the family Annonaceae. It occurs naturally in tropical and subtropical parts of Southeast Asia. 80%-EtOH extracts showed an in vitro antimalarial activity (IC50 = 6.3 μg/mL) against the drug resistant K1 strain of P. falciparum .
C. sylvestris var. lingua (Cambess.) Eichler, (Salicaceae) is an evergreen shrub or small tree with long, slender branches and a very dense globose crown. Usually 4–6 m tall, but can grow up to 20 m high, with wide distribution throughout South America. It has been employed in traditional medicine for treating snake bites, wounds, inflammation, fevers, gastric ulcers and diarrhea . The hexane extracts of C. sylvestris stem wood, stem bark, root bark, leaf and root wood as well as ethanol extract of the root bark, exhibited potent in vitro antiplasmodial activity against chloroquine-resistance FcB1/Colombia P. falciparum strain with IC50 values of 0.9 ± 0.2, 1.0 ± 0.4, 1.2 ± 0.4, 1.3 ± 0.1, 2.3 ± 0.5 and 7.7 ± 1.1 μg/mL, respectively .
C. vernalis Cambess. (Sapindaceae) is a semi-deciduous tree with elongated and dense crown, which can grow up to 10–22 m tall. It can be found in almost all forest formations in Brazil, South America, Argentina, Uruguay, Paraguay and Bolivia. The tree serves as source of tannins and wood locally, and in traditional medicine as diuretic, stimulant, expectorant, natural surfactant, sedative and for treating stomach-ache and dermatitis . The hexane and ethanol leaf extracts showed active antimalarial activity against chloroquine-resistance (FcB1/Colombia) P. falciparum with IC50 of 0.9 ± 0.3 and 6.6 ± 0.2 μg/mL, respectively .
X. emarginata Mart. is a species of plant in the Annonaceae family. It is native to Cerrado vegetation in Brazil. It is an evergreen tree with a very narrow, almost columnar crown which can grow up to 10–20 m tall and 30–40 cm in diameter. It usually grows in large clusters, forming a homogeneous mass. It is a species characteristic of swamp forest, and does not grow in the driest places. It is used as a condiment in food, a carminative and aphrodisiac in traditional medicine . X. emarginata hexane root bark and stem bark extracts were able to inhibit P. falciparum (chloroquine-resistance FcB1/Colombia strains) with IC50 of 4.9 ± 0.2 and 5.2 ± 0.4 μg/mL, respectively .
X. aromatica (Lam.) Mart. belongs to the family Annonaceae and the accepted name is X. xylopioides. It is a tree native to Cerrado grassland vegetation, particularly in the states of Goiás and Minas Gerais, in eastern Brazil. It is a medium-sized tree with long, hanging branches that can make the crown look like a Christmas tree. Leaves are alternate, narrow, pointed, in a flat plane and arranged regularly along the branches. It is a common roadside and farmland species of the Pacific slope, not in the forest . The root wood and root bark hexane extracts demonstrated an in vitro antimalarial activity against chloroquine-resistance (FcB1/Colombia) strains of P. falciparum with IC50 of 4.7 ± 0.9 and 6.8 ± 0.6 μg/mL, respectively .
A. macrocarpon Mart. (Apocynaceae) is a deciduous tree with an open crown growing up to 3–25 m tall and 25-35 cm in diameter. It is a timber tree, native to Brazil, Venezuela, Bolivia, Paraguay and Peru. Traditionally, it is employed in the treatment of fever . The in vitro antiplasmodial study of the ethanol extract revealed an effective activity against P. falciparum (chloroquine-resistance FcB1/Colombia) with an IC50 of 4.9 ± 1.1 μg/mL .
A. indica A. Juss is commonly known as neem tree or Indian lilac and belongs to the mahogany family Meliaceae. It is an evergreen, fast-growing tree that can reach a height of 15–20 m with few of them growing up to 35–40 m, but in severe drought it may shed most of its leaves or nearly all leaves. It is typically grown in tropical and semi-tropical regions. Neem is effective against certain fungi that infect humans and hence used to treat skin diseases like eczema, psoriasis . The 80% methanol leaf extract showed in vitro anti-plasmodial activity against chloroquine and pyrimethamine sensitive, 3D7 strain, and chloroquine resistant and pyrimethamine sensitive, Dd2 strain, with IC50 of 5.8 and 1.7 μg/mL, respectively .
H. abyssinica Oliv. of the family Rutaceae, is a spiny, evergreen shrub that branches from the base and can become a spreading or much-branched tree. It usually grows up to 6–13 m tall and commonly found in Tropical Africa, in the areas of Sierra Leone, Cameroon, Sudan, Ethiopia, Uganda, Kenya, Angola, Zambia and Mozambique . The methanolic stem bark extract inhibited chloroquine resistant P. falciparum strain Dd2, with IC50 value of 4.7 ± 0.113 while in chloroquine sensitive P. falciparum strain 3D7, the IC50 value was 10 ± 0.114 μg/mL .
M. senegalensis Lam. Exell which belongs to the family Celastraceae is an African shrubs or trees widely distributed throughout Central and South America, Southeast Asia, Micronesia and Australasia, the Indian Ocean and Africa, growing up to 15 m high with spines up to 7 cm long. Traditionally, it is an anti-inflammatory herbal drug and is useful in treating toothaches . The stem bark methanol extract showed anti-plasmodial activity with IC50 of 3.9 and 10 μg/mL when treated in vitro on chloroquine sensitive, 3D7 and chloroquine resistant, Dd2 strains, respectively .
Cholera is an acute intestinal disease caused by a facultative anaerobic, Gram-negative, comma-shaped rod bacterium, known as V. cholerae. Cholera is a life threatening disease transmitted by the fecal-oral route. The organisms adhere to and colonize the small bowel within a short incubation period, where they secrete cholera enterotoxin leading to severe and watery diarrhea accompanied with vomiting, dehydration and eventually death if not treated promptly . Various antibiotics have been effective for the treatment of cholera; however, the worldwide problem of microbial resistance to existing antimicrobial medicines has led to most antibiotic failure. Researchers are therefore shifting their focus to natural products, especially medicinal plant, with effective antimicrobial properties. Some medicinal plants with potent anti-cholera activity are reviewed below.
T. chebula Retz. (Combretaceae) commonly known as black or chebulic myrobalan is a medium to large deciduous tree growing up to 30 m tall, with a trunk of 1 m in diameter. It leaves are oval, alternate to subopposite in arrangement and is a native to South Asia, from India and Nepal east to southwest China, Sri Lanka, Malaysia and Vietnam. Traditionally, it has been used for treatment of indigestion, diarrhea and diabetes . The plant extract used to treat Cholera worked effectively against the strains of V. cholera the causative agent. The methanol fruit extract of T. chebula had strong bactericidal activity with MIC ranging from 0.125 to 1.5 mg/mL and MBC ranging from 0.25 to 2 mg/mL, against multi-drug resistance strains of V. cholerae (serotypes O1, O139, and non-O1, non-O139) .
S. cumini (L.) Skeels (Myrtaceae), known as Jam is an evergreen tropical tree, native to the Indian Subcontinent, adjoining regions of Southeast Asia, China and Queensland. It Grows up to 30 m and can live more than 100 years, with a dense foliage which provides shade and is grown just for its ornamental value. The leaves are pinkish when young, and changes to dark green with a yellow midrib as they mature . The seeds have traditionally been used to treat diarrhea, dysentery, piles, indigestion and diabetes. S. cumini methanol seed extract exhibited a bactericidal anti-cholera activity against multi-drug resistance strains of V. cholerae (serotypes O1, O139, and non-O1, non-O139), with MICs and MBCs ranging from 1.25–3 mg/mL . Also Sharma et al.  reported the in vitro anti-vibrio activity of the ethanolic stem bark extract against different strains of V. cholera with MICS ranging from 2.5 to 20 mg/mL.
S. indica auct. L. commonly known as Asoka-tree or Ashok is a plant belonging to the Detarioideae subfamily of the Fabaceae family. Asoka tree is an evergreen tree with a spreading crown which can grow up to 24 m tall and 34 cm in diameter. The original plant specimen came from Java. Some traditional uses of the plant include treatment of dyspepsia, fever, burning sensation, colic, ulcers, menorrhagia, leucorrhoea, pimples . S. indica evoked strong bactericidal activity against different strains of multi-drug resistance V. cholera, with MBCs ranging from 1 to 4 mg/mL . A study by Sharma et al.  also showed the anti-vibrio potential of the ethanolic stem bark extract, with MICs range of 2.5–10 mg/mL against 13 strains of V. cholera.
B. monosperma (Lam.) Taub. (Papilionaceae) is a native to tropical and sub-tropical parts of the Indian Subcontinent and Southeast Asia, ranging across India, Bangladesh, Nepal, Sri Lanka, Myanmar, Thailand, Laos, Cambodia, Vietnam, Malaysia and western Indonesia. Common names include flame-of-the-forest and bastard teak. It is a medium-sized dry season-deciduous tree, growing to 15 m tall. Leaves are pinnate, with (8–16 cm) petiole and three leaflets of 10–20 cm long. Its flowers are used in traditional medicine for the treatment of ulcer, inflammation, hepatic disorder and eye diseases . The methanol flower extract showed anti-cholera activity with MIC and MBC ranging from 1.75 to 5 mg/mL against different strains of multi-drug resistance V. cholera .
E. serpens Kunth is a member of the Euphorbiaceae family. It is native to South America but it can be found on most continents as an introduced species and often a weed. This is an annual herb forming a mat of prostrate stems . Purified bioactive fraction of aqueous extract of E. serpens exhibited an anti-Vibrio activity at a Minimum Inhibitory Concentration of 3.92 mg/mL .
Vachellia farnesiana, also known as A. farnesiana (L.) Willd, commonly known as sweet acacia or needle bush, is a species of shrub or small tree in the legume family, Fabaceae. The species grows to a height of 4.6–9.1 m and grows multiple trunks. V. farnesiana has been used in Colombia to treat malaria, in the Philippines the leaves are traditionally rubbed on the skin to treat skin diseases in livestock. In Malaysia, an infusion of the plant’s flowers and leaves is mixed with turmeric for post-partum treatment . The bark methanolic extract revealed a potent bactericidal activity against two strains of V. cholera, O139 (AI-1837) and O1 (569-B) with MBCs of 0.5 ± 0.1 and 0.9 ± 0.1, respectively .
A. ludoviciana (Nutt.) White sagebrush of the family Asteraceae is native to North America where it is widespread across most of the United States, Canada and Mexico. It is a rhizomatous perennial plant growing to height of 0.33–1 m. Medicinally, it is used for dermatological purposes and for treating cold . The anti-cholera activity of the methanol whole plant extract was effective and bactericidal against O139 (AI-1837) and O1 (569-B) V. cholera strains. The minimum bactericidal concentrations against the two strains were 0.7 ± 0.2 and 1 ± 0.3, respectively .
O. basilicum (L.) Basil (Lamiaceae) can be found in Tropical Asia. It is a perennial growing up to 0.5 m tall and by 0.3 m in diameter. Medicinally it is used for the treatment of fever, colds, influenza, poor digestion, nausea, abdominal cramps, gastro-enteritis, migraine, insomnia, depression and exhaustion . The methanol whole plant extract exhibited a bactericidal activity against V. cholera O139 (AI-1837) and O1 (569-B) strains with MBCs of 2 ± 0.6 and 3 ± 0.5, respectively .
O. ficus-indica (L.) of the family Cactaceae is species of cactus that has long been domesticated. It is commonly known as prickly pear or Nopal cactus. It originated from Mexico and cultivated in other parts of the world including Mediterranean Basin, Middle East and northern Africa . A study by Sánchez et al. , revealed the anti-cholera activity of the methanol cladode extract of O. ficus, with minimum bactericidal concentrations against O139 (AI-1837) and O1 (569-B) V. cholera strains to be 3 ± 0.05 and 3 ± 0.1, respectively.
L. inermis Linn. (Apocynaceae) commonly known in India as Henna is a flowering plant and the sole species of the genus Lawsonia. It is a tall shrub or small tree, standing 1.8–7.6 m tall, glabrous and multi-branched, with spine-tipped branchlets. The henna plant is native to northern Africa, western and southern Asia, northern Australia, and thrives well in semi-arid zones and tropical areas. It is useful medicinally for burning sensation, leprosy, skin diseases, amenorrhoea, and dysmenorrhea and as abortifacient . The ethanolic leaf extract exhibited an in vitro anti-vibrio activity with MICs ranging from 2.5 to 10 mg/mL against 13 strains of V. cholera .
Tuberculosis (TB) is an airborne infectious disease which does not only affect the lungs but also other parts of the body such as the brain and spine . The main cause of TB is Mycobacterium tuberculosis. Other M. tuberculosis complex that causes TB include M. bovis, M. africanum, M. canetti and M. microti . The predominant symptoms of active TB are fever, night sweat, weight loss and chronic cough with blood containing sputum. However, most TB infections are latent which may progress into active disease if left untreated . Treatment of TB is very tedious and requires a long course with multiple antibiotics involved. However, this fastidious bacteria have become resistant to most antibiotics, and hence researchers are working tirelessly to come up with new and effective products especially from natural products such as medicinal plant. Some medicinal plants that have been investigated to possess active anti-tuberculosis activity are reviewed below.
A. leiocarpa (Combretaceae) commonly called African birch is a tall deciduous tree which is indigenous to the savannas of tropical Africa. Traditionally, its stem and root barks are used to treat gonorrhea, worm infestation, cough, asthma and tuberculosis . The susceptibility of clinical isolates of M. tuberculosis to the methanolic extract of A. leiocarpa was investigated using the broth dilution method. The results demonstrated anti-mycobacterial property (MIC 78 μg/mL). A. leiocarpa fraction showed an increased anti-mycobacterial activity (MIC 7.8 μg/mL) .
T. avicennioides (Combretaceae) is a tree commonly found in West Africa. Its root bark, fruit and mistletoes are used traditionally to treat diarrhea, hemoptysis, sore throat, TB, asthma and cough . The in vitro antibacterial studies using broth dilution method of methanolic extract of T. avicennioides showed a significant anti-mycobacterial activity (MIC 78 μg/mL) against clinical isolates of M. tuberculosis. The n-hexane and ethyl acetate fractions obtained from the crude methanol extract of T. avicennioides showed inhibitory activity (MIC 200 and 625 μg/mL, respectively) against attenuated strains of M. bovis. A further study of T. avicennioides fraction obtained demonstrated anti-mycobacterial activity (MIC 4.7 μg/mL) .
C. brassii (Capparidaceae), the narrow-leaf caper bush is distributed in the coastal forest and mixed woodland from tropical West Africa to South-East Africa. The root bark is used to treat TB in folk medicine . The methanol extract of C. brassii has demonstrated some level of anti-mycobacterial activity (MIC 1.25 mg/mL) against clinically isolated strains of M. tuberculosis .
Combretum (Combretaceae) commonly called the bush willows has about 370 species of shrubs and trees, predominant in southern and tropical Africa, Madagascar, Asia and tropical America. Traditionally, its root and stem barks are used to treat cough, bronchitis and TB . The methanol extract exhibited anti-mycobacterial activity (MIC 1.25 mg/mL) against M. tuberculosis clinical isolates when evaluated in vitro using the broth microdilution method .
S. torvum (Solanaceae) also called turkey berry is an upright bushy and spiny perennial plant which is native to the Caribbean, southern Mexico, tropical and central America. However, it is also widely naturalized in the warmer and coastal regions of New South Wales, northern and eastern Australia, tropical Africa, Asia, Papua New Guinea, South-Eastern USA and on several pacific islands. The juice from this plant is used for the treatment of fever, sore throat, dropsy, rheumatism, gonorrhea, stomach ache, chest ailment, and asthma, while leaves and fruits can also be used to control a wide range of microbial activity . The crude leave extract of S. torvum has demonstrated a significant inhibitory activity against two stains of M. tuberculosis (H37Ra and H37Rv) with MIC of 156.3 and 1250 μg/mL, respectively .
G. africana (Aizoaceae) is an upright green to yellow-green aromatic woody perennial shrublet commonly found on the western and southern edges of Karoo . The ethanolic extract of G. africana demonstrated anti-mycobacterial activity (MIC 1.2 mg/mL) against M. tuberculosis. A further study of flavone, 5,7,2′-trihydroxyflavone which was isolated from G. africana showed an increased activity (MIC 0.1 mg/mL) against M. tuberculosis .
A. sativum (Amaryllidaceae) popularly called garlic is a bulbous plant, native to northern and eastern Iran and Central Asia , however, garlic can grow in the wild and in places where it has become naturalized. During World War I and II, garlic was used as an antiseptic to prevent gangrene . Aside its reported nutritional value, garlic can demonstrate antimicrobial effect at temperature as high as 120°C. The aqueous and ethanolic extracts of A. sativum has shown anti-tuberculosis activity (MIC 0.05 and 0.1 mg/mL, respectively) against M. tuberculosis, H37Ra via the use of Microplate Alamar Blue Assay (MABA) . A study by Gupta et al.  also showed the inhibitory activity of A. sativum against multidrug resistant isolates DKU-156 and JAL-1236, as well as sensitive M. tuberculosis H37Ra with percentage inhibition of 72, 72 and 63%, respectively.
A. cepa commonly called onions is from the family Liliaceae. Onions have several pharmacological activity such as antidiabetic, antioxidant, anticancer, cardiovascular, antimicrobial and others . The minimum inhibitory concentration by which the ethanolic and aqueous extracts of the tissue of A. cepa inhibited the growth of M. tuberculosis H37Ra was recorded to be 0.1 mg/mL for both extracts . Another in vitro study showed a 79% proportion of inhibition of aqueous extract of the bulb of A. cepa against MDR isolate JAL-1236 .
C. verum, (formerly C. zeylanicum) of the family Lauraceae, commonly known as cinnamon tree is an evergreen small tropical plant native to Sri Lanka, it is also cultivated in Madagascar and Seychelles on commercial scale . Its anti-tuberculosis activity reported by Sivakumar and Jayaraman,  revealed that, the aqueous and ethanolic extracts of the bark of C. verum exhibited anti-mycobacterial activity (MIC 0.1 and 0.2 mg/mL, respectively) against M. tuberculosis H37Ra.
A. indica popularly known as Indian nettle is from the family Euphorbiaceae. In Africa, it is distributed in Nigeria, from eastern part of Sudan to Somalia and south through DR Congo and East Africa to Southern Africa. It also occurs in South-East Asia, India, Oceania and widely in the Indian Ocean islands. Traditionally, it is used as an antifungal and antibacterial agent for both human and plant pathogens. It is also used as an expectorant to treat pneumonia and asthma . The in-vitro study of the aqueous leave extract of A. indica against MDR isolate DKU-156, JAL-1236 and sensitive M. tuberculosis H37Rv, demonstrated 95, 68 and 68% inhibition, respectively .
Pneumonia is a respiratory tract infection characterized by the inflammation of one or both lungs as a results of the accumulation of pus in the alveoli. Pneumonia which can be caused by bacteria, viruses or fungi can be mild, severe or life threatening. Bacterial pneumonia can be caused by Streptococcus pneumoniae which is the commonest cause, Staphylococcus aureus, Moraxella catarrhalis, Klebsiella pneumoniae, Haemophilus influenza, Chlamydophila pneumonia and Legionella pneumophila. Pneumocystis jirovecii pneumonia (PCP) is a fungal pneumonia commonly found in immunocompromised patients. Viral pneumonia can also be caused by adenovirus, Varicella zoster, Influenza virus and respiratory syncytial virus [64, 65]. Traditionally, medicinal plants have been employed for treating pneumonia and hence the need to prove, scientifically, their folkloric uses. Researchers have investigated such plant, and below is a review on some of the reported plants with demonstrated activity.
In Ethiopian herbal medicine, members of the genus Echinops from family Asteraceae are used for the treatment of diarrhea, intestinal worm infestation, hemorrhoids, migraine and different forms of infections . Zamzam water extract of E. adenocaulos demonstrated an antibacterial activity against multidrug resistance S. pneumoniae with a minimum inhibitory concentration (MIC) of 0.781 mg/mL .
Various species of Verbascum, of the family Scrophulariaceae, have been used to treat pulmonary diseases in traditional medicine as a results of its antibacterial activity against Klebsiella pneumonia and Staphylococcus aureus . The in vitro antimicrobial activity of aqueous extract of V. fruticulosum against multidrug resistant clinical isolate of S. pneumoniae showed a high antibacterial activity with MIC value of 0.195 mg/mL .
P. judaica commonly known as pellitory of wall from family Urticaceae has been valued for its use as a diuretic, balm for wounds and burns and also as a soother for chronic cough in herbal medicine . The micro-broth dilution method was used to study the inhibitory activity of aqueous extract of P. Judaica. The extract was able to inhibit multidrug resistant S. pneumonia at an MIC value of 3.125 mg/mL .
U. urens commonly known as dwarf nettle or annual nettle from family Urticaceae is used medicinally for the treatment of pulmonary diseases . A study by Saleh Fares et al.  on the inhibitory activity of the aqueous extract of this plant against multidrug resistant clinical isolates of S. pneumoniae, using micro-broth dilution method, gave an MIC of 6.25 mg/mL. This illustrates its potential to be used as medicine in the treatment pneumonia caused by multidrug resistant S. pneumoniae.
B. vulgaris popularly known as sugar beet from family Amaranthaceae is a sugar producing plant. Sugar-producing plants contain bioactive compounds, which are active against microbes and hence are able to protect the sugar from fermenting or from undergoing any alteration . The study of the antimicrobial activity of the crude ethanolic leaf (lamina and midrib) extracts as well as fractions (n-hexane and chloroform) against K. pneumonia, showed zones of growth inhibition at different concentrations tested. At 1 mg/12 μL, the lamina and midrib crude extracts recorded 19 and 9 mm inhibition zone. The chloroform lamina and midrib fraction recorded 12 and 14 mm at concentration 1 mg/6 μL, while at concentration 1 mg/12 μL, their inhibition zones were 15 and 20 mm, respectively. Also the n-hexane lamina and midrib fractions had 20 and 16 mm inhibition zones (1 mg/6 μL),while 36 and 32 mm zones of inhibition (1 mg/12 μL) were recorded, respectively .
Asthma is a complex inflammatory disease and congestive respiratory disorder brought about by airway narrowing. It symptoms may include episodic wheezing, cough and chest tightness resulting in airflow block. It leads to changes in the levels of eosinophils, mast cells, lymphocytes, cytokines and other inflammatory cell products. There is increased prevalence worldwide especially in industrialized countries and among children with increased morbidity and mortality rate [73, 74]. Medicinal plants have been screened for properties that enhance their activity as anti-asthmatic agents, since current medications have adverse side effects. Few of such plants with demonstrated activity are reviewed below.
C. longa L. is a rhizomatous herbaceous perennial flowering plant of the ginger family, Zingiberaceae. It is native to the Indian subcontinent and Southeast Asia, and requires temperatures between 20 and 30°C and a considerable amount of annual rainfall to thrive. Methanolic extracts (curcumin-II at 200 mg/kg and curcumin-I at 100 mg/kg) of the finger rhizomes of C. longa reduced significantly (P < 0.01) estimated white blood cells count in ovalbumin (OVA) sensitized Wistar rat models for both long and short term. At a higher dosage, curcumin-II (200 mg/kg) tends to protect intact mast cells from degranulation . This suggests that curcumin can be used as complementary medicine in the treatment of Asthma.
A. lanata (L.) A. L. Juss. ex Schult (Amaranthaceae) is a perennial herb, frequently becoming more or less woody at the base. The stems can be erect to prostrate, sometimes scrambling or climbing into other plants for support. It is widespread in the tropics and subtropics of Africa through Asia to the Philippines and New Guinea. It is used traditionally for treating cough, sore throat, indigestion, wounds, and diabetics and as a vermifuge for children . The ethanol extract of aerial parts of A. lanata at 100 μg/mL significantly (***p < 0.01) exhibited percentage decreased contraction in the isolated goat tracheal chain preparation model. Also in clonidine induced mast cell degranulation, the extract at 30 and 60 mg/kg administered orally, showed percentage protection of 64.2 and 68.9%, respectively .
C. dactylon (L.) Pers, of the family Poaceae is a short-lived, prostrate, perennial grass. It is widely naturalized in the temperate to tropical zones of Europe, Africa, Asia, the Pacific and the Americas. Its habitat is along roadsides and in exposed rocky or sandy sites. It use in traditional medicine to stop bleeding in minor injuries, for weak vision and eye disorders, piles, asthma, tumors among others . The findings of Savali et al. , indicated that isolated C. dactylon compound was potent and has significant (p < 0.01 and p < 0.001) inhibitory effect on compound 48/80 induced anaphylactic reaction and mast cell activation. Also, compound 48/80 induced increased level of nitric oxide in rat serum and rat peritoneal mast cells were significantly inhibited.
P. betle L. (Piperaceae) commonly referred to as Betel pepper, is an evergreen climbing shrub producing woody stems, 5–20 m long, and distributed in Southeast Asia—probably originally from Malaysia. It is traditionally used to cure cough, cold, pruritis, asthma and rheumatism . Ethanol and aqueous extract of leaves at doses 100 and 200 mg/kg possesses anti-asthmatic activity on histamine induced bronchoconstriction in guinea pig and histamine induced dose dependent contraction of guinea pig tracheal chain .
L. sativum L. (Brassicaceae) also referred to as Garden cress is a profusely-branched, erect, annual plant growing up to 80 cm tall . It commonly grown in many regions of Saudi Arabia and the Eastern Province. The seeds are used to cure bronchitis, asthma, cough, and useful as abortifacient, antibacterial, aphrodisiac, diuretic, expectorant, gastrointestinal stimulant, gastroprotective, laxative and stomachic . The bronchodilatory effect of ethanolic seed extract and ethyl acetate, n-butanol and methanol fractions, against histamine and acetylcholine induced acute bronchospasm in guinea pigs, exhibited significant inhibition of bronchospasm, with n-butanol fraction showing a significant (p < 0.001) protection comparable to the reference standards used in the study . Rehman et al.  also confirmed the bronchodilatory effect of L. sativum crude extract by investigating the various pathways for its activity in airway disorders. It was revealed that, the extract’s activity was mediated through a combination of anticholinergic, Ca++ antagonist and phosphodiesterase inhibitory pathways.
C. orchioides Gaertn. (Hypoxidaceae) is a stemless evergreen perennial herb producing a cluster of leaves from the roots and spreading to form a clump. It grows up to 50 cm tall. It ranges from East Asia—South China, Japan, Indian subcontinent, Myanmar, Thailand, Cambodia, Laos, Vietnam, Malaysia, Indonesia, Philippines, New Guinea, W. Pacific. Alcoholic extract of C. orchioides rhizomes at doses (100–400 mg/kg) shows mast cell stabilizing and antihistaminic activity on Compound 48/80-induced mast cell degranulation and systemic anaphylaxis . Also Pandit et al.  established the usefulness of the ethanol extract in treating asthma, as it was reported to exhibit significant relaxant effect (p < 0.01) at concentrations 100 and 25 μg/mL in isolated goat tracheal chain and isolated guinea pig ileum preparations respectively. In an in vivo study using histamine induced bronchoconstriction in guinea pigs, egg albumin induced passive paw anaphylaxis in rats and haloperidol-induced catalepsy in mice, there was significant (p < 0.01) protection at lower doses. Again, maximum increase in leucocytes and lymphocytes (99%) and maximum decrease in eosinophils up to 0% at dose 375 mg/kg p.o. was reported in milk-induced total leukocytes and differential leukocyte counts.
C. equisetifolia L. (Casuarinaceae) also commonly known as Common Ru, is an evergreen tree with a finely branched, feathery crown usually growing from 6 to 35 m and 20–100 cm in diameter. The tree is widely planted throughout the tropics, and ranges from East Asia to Bangladesh, Myanmar, Thailand, Vietnam, Malaysia, Indonesia, Philippines, Australia and the Pacific . The methanol extract of wood and bark (10–80 mcg/mL) exhibited a significant dose dependent (p < 0.05) antihistaminic activity by inhibiting the histamine induced contraction of trachea. The wood extract (100 mg/kg, i.p.) significantly reduced clonidine induced catalepsy (p < 0.05) and mast cell degranulation (p < 0.001) .
All the plants reviewed exhibited potent activity confirming their various traditional uses and their ability to treat prevalent diseases. There is therefore the need to subject these plants to further studies, by isolating active compounds which can be processed into new and potent medicines and the need to study their mechanisms of action.