Chlamydias as a Zooonosis and Antibiotic Resistance in Chlamydiae

Abstract

Chlamydiosis is a disease that can be seen in different forms in the animals. In the genus Chlamydia, two species have been reported in the studies. The first is C. trachomatis, which is responsible for infections in humans and C. psittaci, which has a wide host distribution, including many animals and humans. C. psittaci is usually transmitted from poultry to humans. Along with causing flu-like conditions in humans, it has also caused abortions in pregnant women by contact with sheep and goats that have been infected and have offspring. The likelihood of pregnant women contracting the Chlamydia pathogen through contact with sheep and goats increases the zoonotic importance of the disease. There are few reports documenting antibiotic resistance in Chlamydiae. Furthermore, there are no examples of natural or permanent antibiotic resistance in strains that cause disease in humans. In some strains, the detected antibiotic resistance cannot be identified in vitro, which hinders the recognition and interpretation of antibiotic resistance.

1. Introduction

The most important characteristic that distinguishes Chlamydia genus bacteria from other bacteria is their biphasic growth cycle and their status as obligate intracellular pathogens. The Chlamydia genus belongs to the Chlamdiaceae family within the Chylamdiades order. In classification, their antigenic structures, intracellular inclusion bodies, sulfonamide sensitivity, and disease presentations are taken into account. Their three-layered outer membrane makes them resemble Gram-negative bacteria because they have a non-peptidoglycan envelope. The proteins they encode are referred to as major outer membrane proteins, which all Chlamydia species produce. Chlamydia’s reproduction is unique to itself, and in its growth cycle, it has infectious and reproductive forms known as EC: Elementary Body and RC: Reticulate Body. The Chlamydia genus includes C. pneumonia, C. Trachomatis, C. pisittaci, and C. pecorum [1, 2].

In previous years, the Chlamydiaceae family was divided into several classes, but recently, it has been assessed that it may be separated into two possible genera. Genus 1 includes Chlamydia trachomatis, Chlamydia suis, and Chlamydia muridarum. Genus 2 includes Chlamydophila abortus (Chlamydia psittaci serotype 1), Chlamydophila caviae, Chlamydophila felis, Chlamydophila pecorum, Chlamydophila pneumoniae, and Chlamydophila psittaci [3].

C. pneumoniae is involved in respiratory tract infections such as sinusitis, pharyngitis, bronchitis, asthma, pneumonia, and atherosclerotic diseases in the heart, brain, and peripheral arterial system. It also plays a role in most cases of acute ischemia [4]. The bacteria C. trachomatis causes psittacosis/ornithosis in humans, urogenital infections, and lymphogranuloma venereum (LGV) [5].

Serological tests are often used for the diagnosis of the disease because the causative agent can only be produced in vitro environments. This requires living environments such as cell culture and embryonated eggs, which are difficult and time-consuming. This is the main reason why serological tests are used in the diagnosis of the disease [3, 6].

The tests used in the diagnosis of the disease are immunofluorescence assay (IFA), enzyme-linked immunosorbent assay (ELISA), and complement fixation test (CFT). The IFA is used to diagnose and differentiate Chlamydia species, as its sensitivity and specificity are high. However, ELISA and CFT have limitations in diagnosis and species identification [3, 6].

2. Potential Danger Chlamydia family

The Chlamydiaceae family is made up of bacteria that can only survive inside other cells and are unable to survive outside of them. These Gram-negative bacteria can cause various illnesses in humans, animals, and birds infections in animals can lead to a variety of negative effects on reproductive health, such as causing abortions or infertility. It can also cause issues with the digestive system, such as enteritis, and can affect the brain and nervous system, resulting in encephalomyelitis. Additionally, it can cause eye inflammation, known as conjunctivitis, inflammation in the joints, arthritis, and respiratory diseases [7, 8, 9].

Chlamydiae are a type of bacteria that can cause a variety of health problems in humans, including preventable blindness, sexually transmitted diseases, respiratory infections, and potentially cardiovascular disease. They are a common cause of preventable blindness and sexually transmitted diseases. They also can cause respiratory infections and have been linked to cardiovascular disease [9, 10].

3. Discovery of chlamydial organisms from past to present

Chlamydial organisms, first described by Halberstaedter and von Prowazek in 1907, were isolated from human trachoma. During isolation, conjunctival cells were injected into orangutan cells, and intracellular inclusion bodies were identified in this way [11, 12]. These organisms were classified as protozoa, not bacteria, and given the name Chlamydozoa. The Greek word “chlamys” means a cloak, referring to the red elementary bodies (EBs) embedded in a blue matrix in bacteria [13].

Psittacosis, a zoonotic disease, was first described by Ritter in 1879. In 1985, Harris and Williams identified respiratory disease in people who had contact with tropical pet birds, which was found to be psittacosis [13]. The disease was named Psittokos after an outbreak in parrots in 1892. The etiologic agent was isolated in 1930, and research was carried out on lovebirds. In the same year, Chlamydia, the etiologic agent of lymphogranuloma venereum (LGV) in humans, was isolated, and the two agents, which had similar characteristics, were known as psittacosis-LGV group viruses [5, 13].

Until the 1930s, Chlamydia psittaci was known to cause disease only in exotic and psittacine birds. With the reporting of new cases, it was discovered that the agent could also cause disease in other birds, including fulmar petrels, domestic pigeons, and ducks. The term ornithosis was used to describe the disease that developed in non-psittacine birds. By the 1950s, the zoonotic importance of Chlamydia was further understood, as outbreaks of human psittacosis-related ornithosis were seen in people in contact with ducks and turkeys [14].

The first notable case in farm animals occurred in 1936 when researcher Greig observed reproductive disorders and abortions in a flock of sheep, which he called enzootic abortion of ewes. He argued that the disease was caused by nutritional disorders, as he could not identify the etiologic agent. However, in the 1950s, it was proven by Stamp et al. that the etiologic agent of this disease was the psittacosis-LGV group organism. In addition, a respiratory disease seen in cats was also associated with this group of organisms in later years [13].

After all these developments, it was proven that Chlamydia was not a virus. They had a cell wall containing RNA and DNA, their cell wall structure was similar to Gram-negative bacteria, their reproductive cycle was different from viruses, and they had ribosomes sensitive to antibiotics. These characteristics classify them as part of the prokaryotic family [13].

4. Taxonomy of Clamydiae

The taxonomy used for classifying living organisms was discovered by Carl von Linne (Linnaeus) in 1735. This system, which is used by all biologists, still remains the basis for classifying living organisms today. In this classification, Chlamydiae, which have a prokaryotic cell structure, were initially considered as bacteria-like organisms. In the taxonomic classification, Chlamydiae belong to the domain bacteria and the order Chlamydiales [2].

Until the molecular studies conducted by Everet, Bush, and Anderson, Chlamydiae, considered a single family, was divided into four different families and included in the classification (Table 1). These families are Chlamydiaceae, Simkaniaceae, Parachlamydiaceae, and Waddliaceae within the Chlamydiales order. Within the Chlamydiaceae family, based on the sequence analysis of the 16S and 23S rRNA genes, two genera (Chlamydia and Chlamydophila) and nine species were reclassified [15].

Classification	Old	Last
Order	Chlamydiales	Chlamydiales
Family	Chlamydiaceae	Chlamydiaceae-Simkaniaceae	Parachlamydiaceae; Waddliaceae
Genus	Chlamydia	Chlamydia	Chlamydophila
Species	C. trachomatis	C. trachomatis	C. pneumoniae
	C. pneumoniae	C. muridanum	C. psittaci
	C. psittaci	C. suis	C. abortus
			C. felis
	C. pecorum		C. caviae
			C. pecorum

Table 1.

Chlamydial taxonomy [2, 13].

The truth is that the changes in the taxonomic classification of Chlamydia proposed by Everett and Everett et al. (1999) are not accepted by many. The fact that this new classification is not used in studies conducted in the field of human medicine is an indication of this. The use of more molecular markers in classification can eliminate these uncertainties. However, there is no confusion in the classification of Chlamydia agents in animals [13].

5. Animal diseases

Chlamydia, as a host, selectively infects humans, other mammals and birds by colonizing certain systems and causing various health issues. Some of these species are zoonotic and can cause human and animal diseases. In this regard, C. abortus and C. psittaci are essential. C. abortus causes abortion, while C. psittaci causes a respiratory disease known as psittacosis [13].

The Chlamydia diseases in animals that have been confirmed to be transmitted to humans to date are Ovine Enzootic Abortion, and Avian Chlamyiosis. All disease factors are listed below.

1. C. abortus: Causes abortion in animals and pregnancy-related diseases such as endometritis and placentitis in humans.

2. C. psittaci: Causes psittacosis, a respiratory infection.

3. C. trachomatis: Causes a sexually transmitted infection in humans and can cause conjunctivitis in animals.

4. C. pneumoniae: Can cause respiratory infection in humans and animals.

5. C. suis: Causes swine enzootic pneumonia, a respiratory infection in pigs.

6. C. caviae: Causes guinea pig enzootic abortion, triggers abortions in cavy.

7. C. muridarum: Causes murine pneumonitis, a respiratory infection in mice.

5.1 Clamidya abortus at Ovine

C. abortus is responsible for causing Ovine enzootic abortion, a disease that is encountered in almost every region of the world. It is a major factor in lamb losses in sheep herds. Countries that engage in sheep breeding, particularly in Europe, have recognized C. abortus as one of the responsible factors for these losses [7, 8].

After 1 or 2 weeks of sheep contracting the disease, discharge related to birth disappears. Pregnant animals that have had a miscarriage due to this disease will not be affected by the disease again. They will not encounter another abortion due to Chlamydia. Animals that are infected but have not had an abortion do not fully develop immunity to this agent, so they spread the agent to their surroundings during other breeding seasons and lambing periods. Sheep flocks can always be infected with Chlamydia, but the most vulnerable period for them to the disease is during lambing [16]. Clamydia can survive for weeks or months at low temperatures but has a shorter lifespan at higher temperatures and in harsher environments. There is no evidence that sheep can get infected through sexual transmission [9, 13]. However, if a sheep encounters infection during mating season, the resulting low-weight live offspring or abortion during lambing season suggests sexual transmission. Experimental studies have shown that infection with C. abortus did not result in abortion when sheep were artificially inseminated with infected semen or mated with infected rams. However, the infection was still detected in the flock even though it did not result in abortion [15].

Male animals are thought to contribute more to the spread of the disease, as they are not always housed with female animals. Experiments have shown that an infected ram can infect a healthy ewe and her offspring [16]. The presence of C. abortus has been detected in the semen of infected 12-month-old rams. When C. abortus enters a flock, the first symptoms are not necessarily abortions; general discomfort and discharge from the genital organs within 48 hours are more common. Later on, abortions 2–3 weeks before normal birth indicate that the flock is infected [17]. The waste material is either normal in appearance or has swelling in the umbilical area. The placental membranes have thickened and become reddened. Seven to 10 days after the waste is produced, the infected ewe may have a dirty pink vaginal fluid, which spreads the disease. If the agent remains in the birth canal, it can lead to general discomfort and metritis. Secondary infections result in death. In goats and cattle, vaginitis, endometritis, and placental retention are most commonly seen [15].

The fleece of a lamb that has been aborted may be discolored with a pink-brown substance from placental secretions. In addition to abortion, premature or full-term delivery of stillborn, sickly, or weak lambs can also occur, and most weak lambs do not survive more than 48 hours, even with nursing. Some infected ewes may give birth to healthy lambs, and it is not uncommon for an infected ewe to deliver both a dead and a weak or healthy lamb [16]. The normal pattern of infection in a previously unaffected flock is that a small number of abortions occur in the first year, usually due to the addition of infected replacement female sheep, followed by a sudden increase in abortions in the second year, affecting 30% or more of the female sheep population [7, 8].

The pattern of infection in a previously unaffected flock usually begins with a small number of abortions in the first year, caused by the addition of infected replacement female sheep. In the second year, there is a sudden increase in abortions affecting 30% or more of the female sheep population. Then, in the third year, there is a final enzootic phase in which younger ewes are mainly affected and are generally in their second year of lambing after becoming infected in their first [16]. If adequate control measures are not introduced, the annual incidence of abortion is likely to be around 5–10% [7]. In flocks with an extended lambing season, this pattern may be different, as infected ewes can infect other pregnant ewes later in the same season. In general, the current pregnancy is not at risk unless more than 3–4 weeks remain until lambing [15].

It is believed that the primary infection in sheep is initially established in the tonsils and then spreads to other organs via blood or lymph. In non-pregnant animals, a latent infection is formed, likely in lymphoid tissue, through a process that is influenced by cytokines, especially the pro-inflammatory cytokine interferon-gamma (IFN-g). IFN-g produced in response to infection with C. abortus restricts the organism’s growth in living organisms and in the laboratory by inducing the enzyme indolamine 2,3-dioxygenase, which breaks down tryptophan. The Chlamydia static effect can be reversed by adding exogenous l-tryptophan [16].

Latently infected sheep do not show any signs of having chlamydial organisms [16]. During pregnancy, the immune system may become weakened, allowing the organisms to multiply and cause a low-grade chlamydaemia, leading to placental infection. In sheep, the placenta is structured with cotyledons, is non-deciduous, and has a syndesmo-epitheliochorial design [16]. Around day 60 of gestation, the mother develops haematomas at the interface between the mother and fetus in the hilus of each placentome these haematomas are believed to allow the Chlamydiae to come into contact with the chorionic epithelium and infect the fetus. No significant changes occur until after day 90 of gestation [13]. The Chlamydiae invade the fetal trophoblastic cells in the hilus of the cotyledon and replicate, causing visible cytoplasmic chlamydial inclusions [8, 13]. Once the infection has taken hold in the trophoblastic cells in the hilus of several placentomes, it spreads to the peri-placentome and intercotyledonary regions of the chorion, causing inflammation, edema, and damage to the epithelial cells and resulting in red, thickened placental membranes. Not all placentomes will become infected, and the degree of inflammation and necrotic damage to the cotyledons and intercotyledonary membrane can vary. In the fetus, the primary pathological changes occur in the liver and can also occur in the lung, spleen, brain, and lymph nodes, although less frequently [16].

The mechanisms behind abortions in sheep infected with Chlamydia abortus are not well understood. However, it is believed that the destruction of the chorionic epithelium and related damage may be the cause. A partial impairment of the placentomes can affect the transfer of oxygen and nutrients between the mother and fetus, leading to fetal death [13]. Examination of the infected placental tissues shows a mixture of inflammatory cells, with inflammation and blood clots in the intercotyledonary membranes. The infected tissues also have high levels of TNF-alpha, which is not present in normal ovine placentas. This TNF-alpha is believed to cause damage to the placenta and contribute to abortion or premature birth. In vitro studies have shown that the infected trophoblast cells produce PGE2, which may be induced by chlamydial lipopolysaccharide (LPS) through TNF-alpha. The levels of hormones like progesterone, 17b-estradiol, and PGE2 change during chlamydial infection of the placenta and may also play a role in triggering premature labor [13].

5.1.1 Effects on people

Compared to the human infections caused by C. psittaci, which is more commonly found in birds, infection with C. abortus is relatively uncommon. A study of 1157 cases of human chlamydiosis in Scotland from 1967 to 1987 showed that only 11 cases were linked to sheep and cattle, while 94 cases were linked to birds [1]. However, a survey of antibody levels in an area in northwest England found no significant difference between the frequency of chlamydial infections in adults working in sheep farming and those in other types of farming or non-farming [8]. Nevertheless, there have been reports of respiratory illness in laboratory staff and workers in vaccine plants and abattoirs, indicating a potential risk for those working in such environments [7, 13].

Pregnancy poses the greatest risk of human infection due to the ability of C. abortus to settle in the human placenta. Cases of transmission have been reported in various countries, including the United Kingdom, France, the Netherlands and the United States. Although the number of cases each year is low, the potential danger to the pregnant woman and her unborn baby is significant. In the first trimester of pregnancy, human infection will likely lead to spontaneous abortion, while later infections can result in stillbirths or premature labor. These conditions usually follow several days of flu-like symptoms. Pregnant women with the infection may also experience kidney failure, liver problems, and a spreading blood clotting disorder, which can be fatal. Diagnosing C. abortus infection can be done through tests such as cell culture or PCR of swabs and fetal samples [7, 8].

Humans can experience abortion caused by C. abortus due to exposure to infected sheep or goats, usually transmitted through oral contact, such as handling infected animals or contaminated clothing. Other causes include contaminated food, smoking with unwashed hands, or mouth-to-mouth resuscitation of weak lambs, as well as inhaling contaminated air, for instance from infected lambs placed in front of fan heaters [13].

5.1.2 Diagnosis

The early and correct identification of the reason for an abortion during pregnancy is essential to implement appropriate measures to limit or stop the spread of the infection. If an abortion occurs in the final two to three weeks of pregnancy and is accompanied by inflamed and necrotic placental membranes, it suggests a potential chlamydial infection, although other microorganisms such as Coxiella burnetii, Campylobacter fetus ssp. fetus, and Toxoplasma gondii may also lead to placental damage [7, 8].

Placentae and dead lambs should be immediately sent to a national veterinary laboratory for examination [13]. The process involves placing the samples into strong plastic bags with the use of disposable gloves. The placentae are examined for necrotic placentitis, and stained smears of infected cotyledons are analyzed under a microscope for chlamydial organisms. In case of a possible delay, a piece of affected placental tissue containing an infected cotyledon should be removed and placed in a Chlamydia transport medium called SPG. This medium contains 10% fetal calf serum and antibiotics such as streptomycin and gentamicin, but not penicillin. If the placenta is not available, swabs should be taken from the vagina and the moist coats of the lambs to detect any organisms. Suitable staining procedures include Macchiavello, Giemsa, or MZN, with MZN being the preferred choice for routine use. Positive MZN staining, under high-power microscopy, should reveal many small, round, coccoid EBs, either individually or in clusters, stained red against a blue background of cellular debris. Under dark field illumination, the organisms will appear as bright, pale green, round objects [7, 8].

C. abortus can be grown from samples of infected tissues, such as eggs from infected hens, endocervical swabs, intercervical membranes, fetal liver, vaginal tampon samples, embryonic eggs of laying hens, and cell cultures. It is important to avoid gut contamination, as there is evidence of enteric C. abortus non-pathogenic or commensal strains in ovine infections, and another chlamydial species, C. pecorum, is frequently found in feces. Tissue samples or tampons should not be preserved or dried in SPG. C. abortus can be isolated from many cell types, but the most commonly used are McCoy, L929, and baby hamster kidney cells [6, 7]. The growth of C. abortus in cell culture can be improved by the presence of cycloheximide in the infection inoculum, pre-treatment of cells with emetine, or exposure of cells to 5-iodo-2-deoxyuridine. After growth, Chlamydiae can be detected by staining smears made from infected yolk sac membranes with Giemsa or staining growing cells in culture with MZN or Giemsa [8, 10].

5.1.3 Control and prevention

It is necessary to isolate sheep immediately after abortion. As the lambing process continues, attention should be paid to promptly identify and isolate all affected sheep, particularly those giving birth to live lambs instead of stillborns. All dead lambs, placentas, and bedding must be properly disposed of, and lambing pens must be cleaned and disinfected in order to reduce the risk of contamination. It is crucial to thoroughly clean one’s hands after handling infectious material before caring for other animals [7, 8].

Long-acting oxytetracycline can be given to reduce the severity of OEA infection in pregnant ewes, but it should only be used in exceptional circumstances. The treatment should start as soon as possible after day 95 of gestation, and multiple doses should be given every 2 weeks until lambing. However, it is important to note that this treatment will not eliminate the infection and cannot undo any damage that has already been done to the placenta. Controlling OEA through proper flock management and vaccination is more effective than relying solely on antibiotics [13].

Pregnant women are recommended to avoid working with sheep, particularly during the lambing period, and to keep away from any potential sources of infection, such as contaminated work clothing. People with compromised immune systems should also exercise caution to avoid contact with sources of infection during the lambing season [7, 8]. Basic hygiene practices like washing hands before eating, drinking, or smoking, and using disinfectants are general steps to be taken but should be strictly followed if an infection is suspected. Disposable gloves should always be used when handling placentas, and under no circumstances should mouth-to-mouth resuscitation be performed on a lamb. Early diagnosis is crucial in treating the infection, which is responsive to antibiotics like tetracyclines and erythromycin when given early [13].

5.2 Avian Chlamydiosis (Psittacosis, Ornithosis)

Chlamydophila psittaci infections can be found all over the world and the bacteria has been found in a wide range of birds, both wild and domesticated, including psittacine birds such as parrots and macaws, game birds, seabirds, garden birds, pigeons, and poultry [14]. These infections are most commonly found in psittacine birds, and the disease is referred to as psittacosis. In other bird species, the same disease is referred to as ornithosis. However, since the disease is similar in all bird species, the term avian chlamydiosis can be used to describe all bird infections caused by Chlamydophila psittaci [8, 9].

Studies by Schwartz and Fraser (1982), Bracewell and Bevan (1986), Grimes and Clark (1986), Dorrestein and Wiegman (1989), and Vanrompay et al. (1992) show that the highest incidence of C. psittaci infection occurs in psittacine birds. Meanwhile, according to research by Panigrahy et al. (1982), Chiba (1984), Bracewell and Bevan (1986), and Alexander et al. (1989), pigeons have the highest rates of C. psittaci infection [13]. Additionally, several economically significant outbreaks of chlamydiosis have been reported in turkeys, and wild birds. Human infections have also been linked to some of these outbreaks [9, 14].

The spread of C. psittaci, a type of microorganism, occurs mostly among birds through breathing in dried-up excrement and secretions, including both eye and nose, from infected birds or by ingesting contaminated feces [13, 14]. The parent birds that are shedding the organism can also pass the infection to their young in the nest, and there is evidence that it can also be transmitted through eggs [14]. C. psittaci can also be spread from bird to bird through blood-sucking ectoparasites like lice, mites, and flies, or less frequently through bites or wounds. To avoid the spread of infection, contact between wild birds and poultry should be prevented, as wild birds can act as a potential source of infection [7, 8].

Exposure to C. psittaci can lead to a variety of infections with varying symptoms, depending on factors like the bird species, virulence, and bird health and stress. The onset of symptoms can range from 3 days to several weeks. In turkeys, the organism can be detected within 48 hours, but symptoms may not appear until 5–10 days later. While subclinical infections may not display symptoms, infected birds can act as carriers and intermittently shed the organism. Acute avian chlamydiosis is a serious infection that affects all major organs, causing symptoms like respiratory distress, lethargy, reduced appetite, ruffled feathers, diarrhea, and discharges from the eyes and nose. The mortality rate from this infection can vary greatly [13].

In summary, birds belonging to the psittacine species can display several clinical signs when infected with Chlamydia, such as anorexia, diarrhea, respiratory issues, sinusitis, conjunctivitis, yellow droppings, and central nervous system disturbances [7, 9]. Pigeons can exhibit anorexia, diarrhea, conjunctivitis, swollen eyelids, and rhinitis in acute cases and lameness, torticollis, opisthotonus, tremor, and convulsions in chronic cases [13, 14]. Turkeys infected with highly virulent serovar D strains can experience severe symptoms such as anorexia, cachexia, yellow-green diarrhea, low egg production, conjunctivitis, sinusitis, and sneezing, with mortality rates ranging from 10 to 30% [15]. If turkeys are infected with a virulent B serovar instead of a serovar D, the infection will run a milder course. Symptoms such as mild anorexia and diarrhea may be noticeable [7, 15]. Ducks can show symptoms such as trembling, unsteady gait, conjunctivitis, nasal discharge, and depression. Chickens tend to be relatively resistant to Chlamydia, but in some cases, they may develop blindness, weight loss, and an increase in mortality rate [13].

5.2.1 Effects on people

Psittacosis or ornithosis is a disease that can be transmitted from birds to humans. The largest group affected by this disease are bird fanciers and pet bird owners, as well as people whose jobs put them at risk of exposure such as pet shop employees, aviary workers, veterinarians, laboratory workers, poultry processing plant employees, farmers, and zoo workers [13]. From 1996 to 2001, there were reports of 1620 cases of human psittacosis in the United Kingdom, 661 in Germany, and 165 in the United States. These figures are believed to underestimate the actual number of cases, as psittacosis can be difficult to diagnose. The disease is considered to be an occupational hazard and is recognized as a prescribed disease for Industrial Injuries Disablement Benefit in the United Kingdom [15]. The OIE Central Bureau maintains a world database of animal diseases and zoonoses, which can be accessed through the Handistatus web interface [13].

Infection with C. psittaci can occur through inhaling dried feces or respiratory secretions from infected birds, or by having direct contact with their feathers, tissues, or secretions, including through mouth-to-beak contact or open skin wounds. Some virus strains are very contagious to humans, and even a short exposure could lead to infection. Although there have been suggestions for person-to-person transmission, it is not considered a common occurrence [7, 8].

The human disease known as psittacosis has an incubation period that is generally estimated to be between 5 to 14 days, but in some instances, it may last up to a month. Symptoms of psittacosis can be either mild, with flu-like symptoms such as fever, headache, joint and muscle pain, photophobia and sore throat, or severe, with symptoms of atypical pneumonia including a non-productive cough and difficulty breathing [14]. During the acute phase of the illness, the white blood cell count is often normal, although a decrease in white blood cells, known as leucopenia, can occur in approximately 25% of cases. The pulse rate is typically slow relative to the elevated body temperature, and a rash may also be present. Chest X-rays often reveal signs of pneumonia [14, 15].

In addition to affecting the respiratory system, psittacosis can also result in complications in other organs, such as myocarditis, endocarditis, hepatitis, encephalitis, meningitis, and kidney and neurological issues [7, 8]. Those most at risk of developing these complications are the very young, the elderly, and individuals with weakened immune systems [13].

In order to diagnose chlamydiosis in birds, it is important to detect the presence of the Chlamydia organism or antibodies to the infection. Symptoms such as an increased white blood cell count, changes in liver enzyme activity, radiographic signs of liver and spleen enlargement, and air sac inflammation may indicate the presence of chlamydiosis. However, a definitive diagnosis requires the detection of the organism or the presence of antibodies. The tests used to diagnose avian chlamydiosis are similar to those used to diagnose other chlamydiosis [15].

The most effective methods for detecting antigens involve the use of PCR technology. There are several PCR tests mentioned in the scientific literature that target different genes such as ompA (MOMP), pmp (or pomp) genes, and 16S and 23S rRNA [15]. These tests have been successfully used on bird samples, but more research is needed to fully validate their use for diagnosing psittacosis. These PCR tests and the MIF test can be used to differentiate infections caused by the bacterium C. psittaci [13, 15].

The methods for diagnosing human psittacosis are similar to those for birds. The organism may be cultured from bodily fluids such as sputum, blood, pleural fluid, or biopsy material, but this is not typically done. Instead, infection is usually diagnosed based on a serological analysis of paired sera taken at least 2 weeks apart. Chest X-rays can also help with the diagnosis and may show signs of lobar, patchy, or interstitial infiltrates. MIF assays and several PCR tests can be used to differentiate C. psittaci infections from other chlamydial infections like C. pneumoniae and C. trachomatis in human patients [15].

5.2.2 Control and prevention

In short, controlling human psittacosis requires controlling the disease in birds. This can be done by maintaining high standards of aviary husbandry, including daily cleaning and disinfection of cages with effective disinfectants such as 70% isopropyl alcohol, 1% lysol, or a 1 in 100 dilutions of household bleach. All waste feed and litter must be disposed of as they can remain infectious for several months. Minimizing aerosols can be done by using litter that does not produce dust, spraying floors with disinfectant or water, and providing sufficient exhaust ventilation [13].

These measures are aimed at reducing the risk of infection with avian influenza or bird flu, which is a highly contagious virus that can cause severe disease in birds and in some cases, can be transmitted to humans [7, 8]. The use of protective clothing and respirators helps to prevent airborne transmission of the virus, while the use of biological safety cabinets and local exhaust ventilation helps to minimize the risk of exposure to the virus through contaminated surfaces and respiratory droplets. The use of detergent and water when performing necropsies helps to reduce the spread of the virus through aerosols, while heat-treating birds and providing respiratory protection in poultry processing plants helps to reduce the risk of exposure to the virus for workers handling infected birds [8, 14].

Treatment for avian chlamydiosis typically involves the use of tetracycline drugs, as recommended by Vanrompay et al. and the Centers for Disease Control [13]. This treatment involves the administration of CTC through medicated feed, which is commercially available as pellets for larger birds and millet for smaller ones. However, it is important to monitor the bird’s food consumption, as acceptance of the medicated feed may be variable. The diet should not have a calcium content higher than 0.07% as it interferes with the uptake of CTC. Most companion birds should receive the medicated diet for a minimum of 45 days, while poultry should receive it for a minimum of two weeks, but it should be discontinued two days before slaughter. In cases where the bird is severely ill, oral or parenteral treatment may be necessary, with doxycycline being the preferred drug for oral treatment with a recommended dosage of 25 to 50 mg/kg once a day on an empty crop to aid absorption [14]. Injectable formulations of doxycycline or oxytetracycline may be given intramuscularly, but the latter should only be given to stabilize severely ill birds as it can cause tissue necrosis. Enrofloxacin, a quinolone approved for use in domestic animals, is also being evaluated for the treatment of avian chlamydiosis. During treatment, supportive care such as intravenous fluid therapy and a heated, uncrowded environment may be necessary. It is important to eliminate the possibility of reinfection by thoroughly cleaning and disinfecting the environment after recovery [13].

Currently, there are no commercially available vaccines for avian chlamydiosis. However, research has been done in the past, and some promising results have been seen. In the 1970s, a vaccine was created using an inactivated form of the bacteria that causes chlamydiosis, Chlamydia psittaci, which successfully induced a cell-mediated immune response and protected 90% of turkeys against the disease after being challenged [15]. More recently, another vaccine was developed using DNA from Chlamydia psittaci, which has resulted in a significant level of protection in turkeys, generating both humoral and cell-mediated responses [13].

5.3 Other Animal Chlamydioses

In simpler terms, Chlamydia psittaci and Chlamydia abortus are both known to cause human illnesses, but Chlamydia felis is the only other type of Chlamydia that has been linked to human infections. This type of Chlamydia is commonly found in cats and can cause eye infections, especially in young kittens [7, 8].

In simpler terms, there have been several reports of people getting sick from being in close contact with cats, including eye infections, abnormal liver function, heart and kidney problems, and atypical pneumonia [4]. A recent study showed that a patient with a chronic eye infection and one of the patient’s cats had the same type of Chlamydia (C. felis), which suggests that the infection was transmitted from the cat to the person. However, such infections are not commonly reported, which could be because they are rare or because they are not properly diagnosed [13].

Although we have some knowledge about the potential of C. abortus, C. psittaci, and C. felis to cause disease in humans, we do not know much about the disease-causing potential of other chlamydial agents in humans [13]. C. pecorum, which causes significant diseases in animals, affects many domestic farm animals such as sheep, goats, cows, horses, and pigs, causing pneumonia, conjunctivitis, polyarthritis (“stiff lamb disease”), inapparent intestinal infections, mastitis, metritis, and encephalomyelitis [8]. However, no zoonotic cases have been reported so far. C. suis in pigs causes conjunctivitis, enteritis, and pneumonia. C. pneumoniae, which causes disease in humans’ respiratory tract, has also been isolated from frogs, koalas, and horses [7, 8]. It has been associated with chronic heart disease, Alzheimer’s disease, reactive arthritis (Reiter’s syndrome), and asthma [7]. Waddlia chondrophila, recently isolated from aborted bovine fetuses, belongs to the Chlamydiales order, and more data is needed to determine its zoonotic potential [13].

6. Antibiotic resistance in Chlamydiae

There are few reports documenting antibiotic resistance in Chlamydiae. Furthermore, there are no examples of natural or permanent antibiotic resistance in strains that cause disease in humans. In some strains, the detected antibiotic resistance cannot be identified in vitro, which hinders the recognition and interpretation of antibiotic resistance. This is due to differences in laboratory procedures for chlamydial culture, low recovery rates of clinical isolates, and the unknown significance of heterotypic resistance observed in culture [4]. Although antibiotic resistance in Chlamydiae has not been reported throughout history, a few reports indicate that they have the ability to develop resistant phenotypes to a significant extent. In vitro antibiotic resistance in Chlamydiae is demonstrated by contemporary examples of mutagenesis, recombination, and genetic transformation. In addition, tetra-cyclone-resistant Chlamydia strains can be isolated from pigs, producing tetra-cycline-resistant genes under extreme pressure [17].

In Chlamydia infections without any complex infections present, tetracyclines (TETs) and azithromycin (AZM) are commonly used as they are highly effective in treating these diseases [8]. However, data obtained so far indicate that Chlamydia may develop long-term infections that are resistant to antibiotic treatment and involve persistence in the reproductive cycle of the microorganism, leading to a deterioration of the infection. Further data is required to confirm these descriptions [10].

Antibiotic treatment may fail due to the development of chlamydial persistence in laboratory studies, where the infection can become unresponsive to antibiotics. The use of penicillin, in particular, can trigger this persistence, which hinders the differentiation of reticulate bodies into elementary bodies and interrupts cell division [15]. Distinguishing between persistence or phenotypic resistance and antibiotic resistance is a difficult task. However, in vivo evidence suggests that persistence is a common occurrence, with the presence of chlamydial RNA, DNA, and abnormal reticulate bodies often found in culture-negative cases [1].

In Chlamydia, there is no one-size-fits-all method to test for antibiotic resistance. The accuracy of antibiotic susceptibility analysis can be affected by several factors, including the type of cell lines utilized, the passage number of both the host cells and Chlamydia, the size of the inoculum, and the time at which antibiotics are added. In addition, due to the bacteria’s fastidious nature, the success rate of isolating clinical samples for culture-based diagnostic methods can vary, making them less favorable when compared to nucleic acid amplification tests with high sensitivity [17].

Stable strains of C. suis that are resistant to tetracycline have been discovered in the United States and Italy. Researchers have identified seven isolates that contain genomic islands of varying lengths and compositions. These genomic islands encode a tetracycline efflux pump called tet(C) and a novel insertion sequence element that likely helps integrate the genomic islands into the chlamydial genome at specific sites [17].

Growing Chlamydia bacteria in conditions with sub-inhibitory or excessively high concentrations of six different types of antibiotics can encourage the development of mutations and genetic resistance in various Chlamydia strains. Some spontaneously occurring mutations that cause resistance have minimal impact on the chlamydial growth characteristics, while others may cause competitive disadvantages in resistant strains [6].

A technique was developed to facilitate the transfer of genes in the laboratory by using chlamydial strains with mutations that make them resistant to antibiotics and infecting them simultaneously with tetracycline-resistant C. suis strains. This process, which involves introducing dissimilar antibiotic-resistant markers into co-infecting strains, can cause genetic exchange between different species of Chlamydia and result in genomic rearrangement and mosaicism. Researchers demonstrated the first successful transformation of Chlamydia, both naturally and using electroporation, by introducing spontaneous mutations that conferred aminoglycoside resistance. However, significant limitations are still associated with the techniques used for Chlamydia recombination and transformation [17].

7. Conclusion

The Chlamydiales order is a diverse group of organisms that are important for human and animal health and have a significant economic impact worldwide. The zoonotic potential of these organisms requires further investigation by studying animal chlamydiosis and evaluating the risks to human health from contact with domestic, wild, and synanthropic animals. There is still much to learn about these organisms in terms of biology, immunology, and disease pathogenesis in order to improve diagnosis and vaccines to control and prevent infections in humans and animals. Antibiotics are also important in treating these infections, as overuse can lead to antibiotic resistance.

Conflict of interest

“The author declare no conflict of interest.”