Toxoplasmosis In Cats: A Review

Toxoplasmosis In Cats: A Review.

 

Maigan Espinili Maruquin

 

Structure and Replication

Fig. 01 Structure of Toxoplasma gondii
(https://www.123rf.com/photo_81668845_stock-vector-toxoplasma-gondii-structure-.html)

 

The family Felidae is the only animal species that hosts infective oocysts of Toxoplasma gondii and passes through their feces, however, this parasite infects most species of birds and mammals (Elmore, Jones et al. 2010). This pathogen is an obligate intracellular coccidian parasite an can infect warm-blooded animals, including people (Hartmann, Addie et al. 2013) (Dubey JP, 2005; Dubey JP and Lappin MR, 2006).

 

The complex life cycle of T. gondii undergoes three distinguished stages. The tachyzoites, formerly called the trophozoite or endozoite, is the active multiplying stage and most likely to cause clinical disease and spread to almost all organs. The next stage is the bradyzoite stage where multiplication is slow and usually within a tissue cyst, leading to a life-long chronic infection. This stage penetrates the small intestine epithelial cells. Finally, the oocysts, which are excreted and shed in feces of infected felid, are the result of sexual reproduction within the intestine and constitute the environmentally resistant stage. (Dubey JP, 2005; Dubey JP and Lappin MR, 2006) (Dabritz, Gardner et al. 2007, Dabritz, Miller et al. 2007, Hartmann, Addie et al. 2013, Wyrosdick and Schaefer 2015, Calero-Bernal and Gennari 2019).

 

Oocysts are non- infectious when excreted in feces but begin to become sporulate after 1-5 days of exposure to air and moisture. These are round to oval in shape and around 10 x 12 μm in size. Most naïve cats who get infected take 3–10 days of ingestion of tissue cysts to complete the cycle (Hartmann, Addie et al. 2013) (Dubey JP, 2005; Dubey JP and Lappin MR, 2006).

 

Infection/ Pathogenesis

The very first case of toxoplasmosis in cats was diagnosed from a domestic cat in Middletown, NY, in 1942 (Olafson and Monlux, 1942; Dubey, 2008)(Wyrosdick and Schaefer 2015). Generally, most cats at 6 to 10 weeks were detected to have antibodies to T. gondii while maternally transferred antibodies (MTAs) disappear by 12 weeks of age. Seropositivity increases with the age and varies according to the cat’s lifestyle (like hunting food) (Dubey, J.P., 2010)(Dubey, Cerqueira-Cézar et al. 2020). The oocysts were noted to remain infectious in the environment for at least 12 months (Hutchison 1965, Wyrosdick and Schaefer 2015).

 

Generally, transmissions of the parasite are congenital infection, ingestion of the infected tissue, and ingestion of oocyst-contaminated food or water (Dubey JP and Lappin MR., 2006) (Hartmann, Addie et al. 2013). Most likely, congenitally infected kittens show clinical signs while post-natal infections are usually through ingestion of infected tissue cysts and in some cases, oocysts (Dubey and Jones 2008, Elmore, Jones et al. 2010). Queens giving birthe to infected kittens during gestation can become infected transplacentally or via suckling (Dubey JP, et al., 1996) (Calero-Bernal and Gennari 2019)

 

The most common method of feline infection due to predation of intermediate hosts is tissue cyst ingestion where shedding occurs in 3 to 5 days, while ingestion of tachyzoites takes 8- 10 days, and 21-24 days after ingestion of oocysts (Dubey, J.P., 2010) (Schares, Vrhovec et al. 2008, Wyrosdick and Schaefer 2015).

 

Clinical Signs

Feline toxoplasmosis develops clinical signs rarely but causes inflammation and tissue necrosis from intracellular growth of tachyzoites (Dubey JP and Lappin MR, 2006)(Hartmann, Addie et al. 2013). It frequently results to hepatitis, pneumonia, and encephalitis with signs of ascites, lethargy, and dyspnea while infected adults do not show specific clinical signs (Brennan A, et al, 2016) (Dubey and Carpenter 1993, Calero-Bernal and Gennari 2019). Moreover, observations also showed extra- intestinal enteritis (Cohen, Blois et al. 2016) and inflammatory intestinal disease (Peterson, Willard et al. 1991).

 

Tissues that are most commonly affected are the central nervous system, the muscles, the lungs, and the eyes. Infected cats show neurological signs, muscle hyperesthesia, jaundice, diarrhea, fever, depression, anorexia, vomiting, paresis, dermatitis and weight loss (Dubey JP and Lappin MR, 2006) (Hartmann, Addie et al. 2013, Dubey, Cerqueira-Cézar et al. 2020). When severe respiratory and neurological signs were observed, it’s usually fatal (Dubey and Carpenter 1993).

 

Diagnosis

Diagnosis in cats for toxoplasmosis include ante-mortem fecal examination for oocysts and serologic testing (Johnson, Tinker et al. 2009, Elmore, Jones et al. 2010). However, geographical location may influence differential diagnosis (Calero-Bernal and Gennari 2019).

 

The shedding of the oocysts of an infected cats may only be once in their lifetime (Elmore, Jones et al. 2010) which can be diagnosed in their fecal samples via microscopy (Hartmann, Addie et al. 2013). However, there is low probability of finding oocysts in the fecal samples of infected cats and there is a confusing morphological resemblance of the T. godii oocysts to other coccidian like Hammondia hammondi and Besnoitia spp  (Elmore, Jones et al. 2010). Therefore, molecular and bioassay techniques can be used to distinguish them while only mouse bioassay is the definitive confirmation method (Dubey 2009, Elmore, Jones et al. 2010). In diagnosing T. gondii, confirmation is when the organism is found in body fluids or tissue (Hartmann, Addie et al. 2013).

 

For tachyzoites detection, ante- mortem diagnosis in tissues and body fluids during acute illness may use cytology or polymerase chain reaction (PCR). A definitive diagnosis is when tachyzoites were detected rarely in blood but aqueous humour, lymph nodes, and transtracheal or bronchoalveolar lavage fluid can be used (Hartmann, Addie et al. 2013).

 

On the other hand, antibodies of the IgM, IgG and IgA isotypes can be detected by immunofluorescence assay (IFA). For antibody- negative, cats are likely to shed oocysts while antibody- positive cats don’t shed oocysts wherein antibodies need 2–3 weeks to develop (Dubey JP, 2005)(Hartmann, Addie et al. 2013).

 

Despite the development of immunofluorescence and ELISA tests, requirement for species-specific protein conjugate makes it limitedly used in veterinary diagnostics (Wyrosdick and Schaefer 2015). After comparing indirect hemagglutination test, latex agglutination test, Feldman dye test and modified agglutination tests, the aqueous humour resulted to be the most sensitive of the agglutination methods (Dubey and Thulliez 1989, Elmore, Jones et al. 2010). Further, agglutination testing is still the most commonly used for diagnosis of toxoplasmosis in animals (Elmore, Jones et al. 2010).

 

Further, currently, VETlabs Rapid diagnostic kits in Toxoplasma is already available for fast and accurate results. This rapid test kits are manufactured by Bioguard Corporation and being distributed globally.

 

Disease Management and Vaccination

Since effective vaccine is still absent for humans, limiting exposure by practicing good hygiene cooking meat, and not drinking untreated water are still best approaches (Jones, Kruszon-Moran et al. 2007, Dubey and Jones 2008, Elmore, Jones et al. 2010). For cats, keeping them indoors, feeding commercial diets, and cleaning their litters daily are highly recommended (Vollaire, Radecki et al. 2005, Elmore, Jones et al. 2010, Hartmann, Addie et al. 2013). Reduction of the incidence of infections and the shedding of oocysts into the environment is an important measure (Hartmann, Addie et al. 2013). Also, we should make efforts for our pets not to hunt and eat intermediate hosts like rodents or mechanical vectors, such as cockroaches and earthworms. Preventing them from entering buildings where food-producing animals are housed or where feed storage areas are located will also reduce the risk (Dubey JP, 2005) (Hartmann, Addie et al. 2013)

 

Clindamycin in combination with topical, oral or parenteral glucocorticoids are used to treat cats with systemic disease (Lappin, Greene et al. 1989) while prednisolone acetate (1% solution) is applied 3 to 4 times daily in the eyes (Hartmann, Addie et al. 2013). On the other hand, radiographic abnormalities for pulmonary toxoplasmosis cases might not be solved for several weeks (Hartmann, Addie et al. 2013).

 

Most vaccines under development are involving live vaccines to prevent feline oocyst shedding (Elmore, Jones et al. 2010). Several attempted vaccination of cats using T. gondii strains modified by irradiation, chemical treatments, selected recombinant antigens, and new delivery systems, including a feline herpes-virus type 1 vehicle for delivery have been reported (Elmore, Jones et al. 2010).

 

References

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