Toxoplasma Re-shedding Rates in Domestic Cats

Toxoplasmosis is a common parasitic disease found in a wide range of species, including humans. As the only known definitive hosts for Toxoplasma gondii, felids typically infect other hosts by shedding T gondii oocysts in feces.

Existing literature suggests that cats experience temporary immunity against re-shedding of oocysts upon reinfection with T gondii and that oocyst shedding is quantitatively lower after subsequent infections. Researchers in Brazil, a location known for high genetic diversity of Toxoplasma, recently explored the effect of multiple strains of T gondii on oocyst re-shedding up to 36 months after initial infection. Their findings were presented in Veterinary Parasitology.

Analysis

Thirteen domestic short-haired cats aged 3 to 6 months were chosen for the study based on negative T gondii serum titer on indirect immunofluorescence assay and negative fecal examination results. While anesthetized, the cats were inoculated orally via stomach tube with approximately 800 T gondii cysts isolated from the brain tissue of challenged mice. Inoculations were performed at 3 time points with strains belonging to the 2 most common genotypes implicated in human and animal toxoplasmosis cases.

Five cats were inoculated initially with a genotype II strain and re-infected at 12 months with the genotype III strain. Another group of 5 cats was infected with genotype II strains at 0 and 12 months, followed by re-infection with a genotype III strain at 36 months. The final group consisted of 3 cats initially inoculated with a genotype II strain and re-infected at 36 months with a genotype III strain.

Fecal samples, which were collected daily for the first 20 days after each inoculation, were used to calculate the total oocysts per gram of feces (OOPG) count for each cat. IgG, IgM, and IgA levels were measured in serum samples via enzyme-linked immunosorbent assay (ELISA) to determine antibody response.

Results

Among the 13 inoculated cats, the mean prepatent period after initial T gondii infection was 3.4 days and oocyst shedding lasted an average of 7.0 days. Peak shedding occurred, on average, 6.9 days after inoculation. All cats shed similar amounts of oocysts after primary inoculation regardless of T gondii strain and genotype, while only 1 of 10 cats shed oocysts after the 12-month re-infection and 5 of 7 cats shed oocysts after the 36-month re-infection.

Average OOPG for cats that shed oocysts was lower after the 12-month (118,500) and 36-month (179,000) re-infections compared with after initial infection (749,141). The average prepatent period during re-infection was slightly increased and shedding duration decreased by more than 50% compared with initial infection.

Serum IgM, IgA, and IgG were all measurable from just after the first infection shedding period until 3 months after the 12- and 36-month re-infections, indicating long-term antibody response. Interestingly, re-infection did not significantly change antibody concentrations.

Conclusion

T gondii infection provided temporary protection against re-shedding of multiple strains of oocysts, although protection appeared to decline between 1 and 3 years after infection.

Dr. Stilwell received her DVM from Auburn University, followed by a MS in Fisheries and Aquatic Sciences and a PhD in Veterinary Medical Sciences from the University of Florida. She provides freelance medical writing and aquatic veterinary consulting services through her business, Seastar Communications and Consulting.