Total Snail catch from the study sites surveyed.
1.1. Incidence of schistosomiasis
Schistosomiasis is one of the fifteen neglected tropical diseases (NTDS) namely: schistosomiasis, ascariasis, buruli ulcer, chagas disease, cysticercosis, food borne trematodiases, hookworm disease, leprosy,lymphatic filariasis, trachoma, trichuriasis, leishmaniasis, guinea worm, trypanosomiasis and oncocerciasis. It is a resurgent disease and
Another concern with respect to the future of praziquantel treatment is the ever-present worry over the emergence of drug resistance. Praziquantel has been in use for 4 decades, during which time it has been the drug of choice for many human and veterinary parasitic infections worldwide. The European Commission has established an International Initiative of Praziquantel Use to review reports of low efficacy in clinical trials in Senegal and Egypt, and reports of resistant
The Life Cycle and Pathophysiology of
2. Fresh water habitats harbouring snail hosts of schistosomes
Fresh water habitats can be divided into six (6) main categories having the following characteristics.
3. Danger of schistosomiasis
Schistosomiasis cannot be eradicated worldwide at this time; neither will many of these diseases be cured by sophisticated health care services, epitomized by medical technology and large hospitals due to the insidious and complex nature of the disease. Instead the development process itself will perpetuate them [2, 3]. Irrigation projects-designed to increase the standard of living, agricultural production to improve quality of life are often planned in or near breeding ground for vectors of water-related diseases. For instance, the prevalence of schistosomiasis in many areas around the Aswan Dam in Egypt tripled after a constant bed of slow-moving water in open irrigation channels was provided for the snail – an intermediate host for the disease. However, a safe adequate water supply is generally associated with a healthier population. When Japan installed water supplies in 30 rural areas, for example, the number of cases of intestinal diseases there was reduced by 72% - (a rather substantial decrease) showing that schistosomiasis may be curtailed in endemic areas when major improvements in the standard of living have been introduced together with control of snails and reduction in both water contamination and contact.
Focus and attention on the new challenges in control programmes for schistosomiasis as well as priority research areas in the new century include: (1) status of schistosomiasis control programme; (2) progress in applied field research; (3) biology and control approaches of snail host; (4) novel approaches for schistosomiasis control; (5) pathogenesis and mobility of the disease; (6) immunology and vaccine development; (7) screening of population for chemotherapy in low transmission areas; (8) sustainable intervention methods in different endemic settings; (9) impact of animal schistosomiasis on agricultural development and importance of its control; (10) GIS/RS application and environmental changes that are very essential (3).
4. Continental distribution of schistosomiasis
Epidemiology of schistosomiasis refers to the prevalence, incidence and intensity of infection by a particular schistosome in man or animal. The prevalence rate gives the proportion of subjects who are infected at a given point in time, while intensity of infection” is a measure of vector burden of a subject or a group of subjects. On the other hand, the incidence rate indicates the proportion of initially uninfected subjects who become infected during a given period. Within an endemic area, focal transmission of schistosomiasis, whether human or animal, is the most common while prevalence and intensity of infection vary widely from one locality to another (4). Studies of the distribution of
A previous cross-sectional epidemiological survey on intestinal schistosomiasis due to
Information on the prevalence of morbidity is needed for re-calculation of the Global Burden of disease (WHO) due to
4.1. Advancement in methodologies for schistosomiasis survey
Recent advances in schistosomiasis epidemiological surveys have resulted in the development of models such as Geographic information system (GIS) risk models for the snail-borne diseases caused by
Similarly, In a 4 year study a geographic information system (GIS) risk model for predicting the relative risk of schistosomiasis in Kafr Elsheikh governorate, Egypt was constructed. The model, using data collected on snail population bionomics-infection rates, water quality, underground water table and cercariometry at 13 hydrologically representative sites enabling the study of role of soil type, water table and water quality at 79 of 154 rural health unit sites, validated previous models. The model permitted retrieval of relevant data by RHU 10 (rural health unit) point location. The model for the first time in Egypt supported Ministry of Health efforts to make more accurate control programme decisions based on environmental predilection sites of endemic
Presently, there is a global network for the control of snail borne diseases using satellite surveillances and GIS. It came from a team residency sponsored by the Rockefeller Foundation in Bellagio, Italy, 10-14 April 2000, where an organizational plan was conceived to create a global network of collaborating health workers and earth scientists dedicated to the development of computer–based models that can be used for improved control programs for schistosomiasis and other snail-borne diseases of medical and veterinary importance. The models consist of GIS methods; global climate model data, sensor data from earth observing satellites, disease prevalence data, distribution and abundance of snail hosts, and digital maps of key environmental factors that affect development and propagation of snail-borne disease agents. The collaboration plan calls for linking a ‘central resource group’ at the World Health Organization, the Food and Agricultural organization, Louisiana State University and the Danish Bilharzias Laboratory with regional GIS networks to be initiated in East Africa, South Africa, West Africa, Latin America and South Asia. An Internet site, www.gnosis GIS. Org (GIS Network on Snail borne infections with special reference to schistosomiasis) has been initiated to allow interaction of team members as a “virtual research group”. The sites point users to a tool box of common resources resident on computer at member organizations, provide assistance on routine use of GIS health maps in selected national disease control programme and provide a forum for development projects and climate variation and the advancement through computerized models such as Remote Sensing (RS) by earth observing satellite, a technology particularly well suited to pinpointing constraining endemic factors. Imaging techniques such as ultrasonography echo Doppler cardiography, computerized tomography (CT scan) and Magnetic Resonance Imaging (MRI) introduced a new perspective, and expanded our knowledge on morbidity [ 10,11, 12,13,14].
4.2. Morbidities associated with schistosomiasis
Three well-defined syndromes caused by
The resurgence and emergence of old and new infectious diseases in twenty first century is a major source of morbidity and mortality. Recent among these are HIV/AIDS, hanta virus pulmonary syndrome, lyme disease, haemolytic uremic syndrome, rift valley fever, dengue haemorrhagic fever, malaria, cryptosporidiosis and schistosomiasis. Related to this are the treatment modalities which include permissive use of antibiotics, the industrial use of antibiotics, demographic changes, social behavior patterns, changes in ecology, global warming and the inability to deliver minimal healthcare and the neglect of well established public health priorities that is a source of major concern globally. Since the distribution of schistosomiasis is focal, then if the resources available for control are to be used most effectively, they need to be directed towards the individuals and/or communities at highest risk of morbidity from schistosomiasis [15, 16].
5. Approaches in schistosomiasis control
The use of chemotherapy plays a leading role in the control of schistosomiasis. Snail control by the use of molluscicides is being explored in integrated control programmes. The level of effectiveness of this method however, is still subjective.It has been shown that male and female schistosomes’ exhibit regional and sexual differences in susceptibility to chemotherapy that represent a form of evasive strategy of the parasite. The control of schistosomiasis therefore by molluscicides is linked to the interdependence of immunotherapy and chemotherapy. Mass chemotherapy alone in regions of high prevalence, may provide good results initially, but it might be unable to give a lasting effect due to great environmental variations., hence, the need for back up approaches such as snail control.
For snail control, molluscicides such as frescon, sodium penta chlorophenate, organotin, dinitrophenol, carbamates, niclosamide and copper sulphate are effective against aquatic and amphibious snails and their eggs.
Combining low doses of praziquantel and oxamniquine (⅓ the curative dose of praziquantel plus ⅓ the curative dose of oxamniquine) for instance can result in potentiating effect in animals receiving combination therapy. Also low concentration of aridanin (0.25 mg/ml) reduced the production of cercariae by snails already shedding cercariae. Aridanin and Aridan both produced profound reduction in the worm recovery of mice infected with pretreated cercariae of
It has been reported that snails infected with
6. Survey, identification and distribution of species of snail hosts, in Nigeria
Certain species of African fresh water snails are exceedingly important both from a medical and veterinary viewpoint. However, there is dearth of literature through which these snails can be identified in a reliable manner according to species. This is so as more species are still being discovered. Snails’ species are however identifiable using criteria spelt out in the bulletin published by WHO snail identification center, Danish Bilharziasis Laboratory. The bulletin is a field guides to African fresh water snails [19, 20]. Snails of interest could be identified by the following criteria:
It is usually 11 x 7.7mm when fully matured but frequently smaller. The shell is usually very light, almost white and with a low spire. The distinctly arrowhead shaped mesocone of the lateral teeth only partially separated from the endocone.
It is usually in the range of 16 x 12mm and 22.5 x 14mm but sometimes larger. The distinctive character is the copulatory organ with a vergic sheath, which is not wider than preputium.
With a few exceptions all mollusks of medical or veterinary importance live in fresh water. According to their great age, several of the families have an almost worldwide distribution. Some are represented on all continents while others are missing only in South America or Australia. Others are only known from the tropics but then, as a rule, are found in the New as well as the Old World. Almost all members of one and the same family of fresh water mollusks are often to great extent alike, no matter from where in the world they descend [21,22].
The determination of species is often a very difficult problem, and without sufficient experience with regard to the family and country in question, correct identification is almost impossible except by the use of good field guide. This is due to two factors, i.e. partly the great variability found within most freshwater mollusks and partly the lack of good distinctive characters.
In these areas, temporary water bodies (being tropical zone) are the principal foci of transmission of schistosomiasis in humans. Perennial habitats were important transmission sites that represent only a small portion of the overall problem .
7. Ecological factors affecting intermediate snail hosts
A combination of both abiotic and biotic factors exert their influences on the fecundity and hence population density of snails in a given habitat. The effects of the two are interrelated and are discussed in the following sections.
7.1. Fresh water malacology
With a few exceptions, all mollusks of medical or veterinary importance live in fresh water. Through familiarity with the snails and bivalves within a certain area, mere looking at the shell can help determine from which lake or river an individual snail originates. This is due to the fresh water mollusks being influenced to a high degree by the environment under which they live. Most fresh water mollusks prefer stagnant or slowly running water. On the exposed shores of big lakes and in fast-flowing rivers there are few if any pulmonates whereas prosobranchs and bivalves may be present. They are usually lacking in very acid or alkaline water, but apart from these exceptions they can be found in all types of freshwater bodies from the greatest lakes to small rainwater pools. In great lakes they are most plentiful in sheltered bays with shallow water, but sometimes they live at greater depths, down to 10 meters or more for pulmonates and 150 – 200 meters for prosobranchs and bivalves. When these snails are absent at greater depths, the reason is lack of oxygen [26,27].
According to this report, smaller lakes, ponds, and sluggish streams are the preferred habitat for most of the species. Presence of water lilies, as a rule is indicative of good conditions for snail life while Nile lettuce seems to indicate poor conditions. Papyrus swamps are also regarded as bad habitats. Certain species of freshwater pulmonates (snails) are known to live preferably or entirely in temporary pools, even in pools that hold water during only a few months of the year. It is known that many populations of fresh water pulmonates are subjected to great fluctuations, which means that species abundant at one visit might seem to be very scarce a few months later. Repeated visits are therefore advised to be sure that all species have been found, even in small ponds. It has been noted that temporary water bodies in the tropical zone are the principal foci of transmission of schistosomiasis in humans. Perennial habitats are regarded as important transmission sites that represent only a small portion of the overall problem.
8. Habitat water chemistry parameters, snail’s availability and spread
Although the influence and effect of ecological physico-chemical properties of fresh water (water chemistry) on snails availability, distribution and survival is in no doubt, yet no clear picture has emerged on the overall influence they exert. Usually a very few distinct relationships exist in snail ecology and there is a general lack of precise data. It is very difficult to define and evaluate the significance of an individual environmental factor- whether physical, chemical or biological when all may be mutually affecting one another and their combined effect influencing a particular species or population. Fresh water snails are capable of adapting to wide range of environmental conditions such as water bodies with moderate organic content, little turbidity and substratum rich in organic matter and moderate light . It is known that major changes in patterns of transmission (i.e. availability of infected snail hosts) occur after man-made disturbances of the ecosystem, whether by physical or chemical changes to the environment. Works on the chemistry of water in different snail habitats (29, 30, 31) in Nigeria and other countries indicate that most snails are tolerant of water differing greatly in chemical content.
An outbreak of intestinal schistosomiasis reported at Richard – Toll, 130km from the dam of Diama, Senegal, was the first sign that the ecological changes caused by the dam had an impact on the prevalence of schistosomiasis. Since the main functions of the Diama dam were prevention of seawater intrusion, provision of a reservoir of fresh water for irrigation and domestic supply of water for municipal use in Dakar, it was suspected that it was likely that the dam induced both physical and chemical environmental changes. In addition to increasing the number of water bodies around the area, the Diama dam prevented salt-water intrusion into the river and the marigots of the Delta, thereby creating new habitats in areas previously unsuitable for fresh water snails .
In July 1983, salinities as high as 19.9% were recorded in the Senegal River at Richard – Toll [32,33] showed that the dam caused a gradual softening of the Lac de Guiers water: water mineralization has decreased 20% in the Northern part and nearly 50% in the Southern region. At the same time the formerly important yearly variations in salinity have distinctly diminished. However, because of the dearth or non availability of physical or chemical data from selected transmission sites at that time, it was concluded that it will be difficult predicting how the expected ecological changes influence snail habitats, especially since it is usually not possible to predict colonization of a particular habitat through chemical analysis of its water content. Interesting results have been obtained showing that the hatchability of eggs, the fecundity and survival of adult
8.1. pH of water bodies
Low pH could be harmful to snails. This is due to the possibility of denaturation of the mucus on the exposed skin surface; extremes of pH i.e. low as well very high pH will be harmful to these snails as both could denature the glyco proteins (mucus) on their exposed skin surface.
This refers to the (carbonate) CO32- and (bicarbonate) HCO3- content in the water bodies. Concentration of 50mg/L is regarded as weak, 100 mg/L as medium while 200mg/L is regarded as strong (34,35).
8.3. Total Dissolved Solids (TDS)
The presence of sewage or industrial wastes in waters bodies is regarded as an important factor in determining vector snail habitats, as they constitute pollutions and snails could only adapt to water bodies with moderate organic content. Water bodies may contain suspended solids, inorganic, organic chemicals, biodegradable organic matter, pathogenic microorganisms, and metals. Pathogenic microorganisms and indicator pathogens are found in high concentrations per gram of raw sewage, and if allowed to accumulate and sit, untreated water produces malodorous emissions as a result of decomposition of organic material.
Solids in water bodies are usually measured as solid, total and dissolved total solids. A typical water body contains solids, total as 350mg/L (Weak), 720 mg/L (Medium) and 1,200mg/L (Strong) while TDS in the range of 250mg/L (Weak); 500mg/L (Medium) and 850 mg/L (Strong) [35,36].
Pure water is colorless but the presence of pollutions in water bodies give rise to varied colors. It has been observed that vector snails appeared to prefer clean, clear water bodies to colored water .
Snail vectors are generally intolerant of high turbidities. Intermediate host snails found in most cases in naturally turbid water tend to decrease as the turbidity of the habitat pool increase. It has also been shown that while a turbidity of 360mg/L (due to suspended mineral from granite erosion) did not affect snails themselves, it prevented development and hatching of
8.6. Total hardness
This is indicated by the total concentration of Ca2+ and Mg2+ ions present in water bodies, different concentrations of calcium and magnesium ions affect the mortality rates of snails while egg production by
8.7. Chloride ion (Cl–)
The concentration of this ion is a measure of salinity of a particular water body. Salinity has been reported to exert great influence on the availability of vector snails in a given habitat. The hatchability of eggs and the fecundity and survival of adult
9. Study sites
Fresh water bodies namely: Kanye and Rimin Gado dams (Kano), Lagoon front (Unilag, Akoka, Lagos) and Oyan dam (Ogun-Osun river basin, Abeokuta) were surveyed for 3 years.
Two snail species (
9.1. Kanye and Rimin Gado
Kanye is a small village, located about 45km west of Kano city. The Kanye dam is about 600 x 800 m in size, with a shallow flat and steep margin and 100% exposure to sunlight. The vegetation consists of the aquatic plant
9.2. Lagoon front, University of Lagos
The Akoka Campus of the University of Lagos, an area of about 320 hectares located in the North East of Yaba Lagos, was a part of study area. Acquired in 1962, the acquisition notice gave the extent as 807 acres. The 320 hectares of the University of Lagos Land comprises of 174 hectares of good building ground and 146 hectares of swamp or swampy area. The swamps are divided into the northern swamp which covers an area of 61 hectares, eastern swamp 24 hectares, southern swamp 40 hectares and the center – finger swamps 16.5 hectares.
The Akoka Campus of the University of Lagos is located on latitude of 6o22’N – 6o 44’N and longitude of 3o 29’E. It is bounded in the west by the Ogbe River (traversed by a bridge between Abule Oja and the main gate of the University), in the North by Akoka Ilaje, in the South by Iwaya and East by the Lagos Lagoon.
The campus being located in the center of Lagos has the same climate as Lagos. The climate of Lagos is of the equatorial type. All the year round, temperature is usually above 65oF and average 85oF. There is minimal seasonal variation of about 10oF between the hottest month (March) and the coolest month (August). Relative humidity is usually between 80% and 100% dropping to about 70% in the afternoon during the dry season. April to October marks the extent of the rainy season with an average annual rainfall of 1830mm. Despite the relative flatness of the area, spatial variation is great.
9.3. Oyan Dam (Ogun-Osun river basin authority, Abeokuta)
The Oyan Reservoir is located on Latitude 7o 14” N and Longtitude 3o 13” E, at an elevation of 43.3 m above sea level. It has a catchment area of approximately 9000km2 within the southern climatic belt of Nigeria. The belt is characterized by a rainy season of about eight months ( March – October) and a dry season of about four months (November – February); a mean annual precipitation of 1000 – 1250 mm a mean relative humidity of 75% - 100% and a mean annual temperature of about 30oC.
The soil is generally composed of crystalline acid rocks of the ferruginous tropical type that have been moderately to strongly leached with low humus content. It is characterized by weak acidic to neutral surface layers and moderately to strongly acid sub-layers. The area falls within the ‘Ibadan Group’ which overlies the metamorphic rocks of the basement complex .
The vegetation is mainly savanna of the low forest type. This is characterized by sparsely distributed short trees in a predominantly grass land, as a result of timber lumbering, bush burning and cultivation of an original rain forest.
10.1. Collection and selection of experimental snail samples
Snail sampling for species of
|Rimin Gado & Kanye||293||450|
Survival of snails collected during the dry season subjected to aestivation for 30 days at 25oC was 20
During rainy season however, 16
Surviving snails were found buried at different depths (0.2-12.0mm) in the soil mixture while their aperture were filled with mud and were deeply retracted into the shell).
Survival was better during rainy season. The ecological conditions could be more suitable during this period survival for
There was no definite pattern in the survival but it appears that survival may be indirectly related to the infection of these snails with schistosomes.
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