High prevalence and diversity of Toxoplasma gondii DNA in feral cat feces from coastal California

Abstract

Toxoplasma gondii is a zoonotic parasite that can cause severe morbidity and mortality in warm-blooded animals, including marine mammals such as sea otters. Free-ranging cats can shed environmentally resistant T. gondii oocysts in their feces, which are transported through rain-driven runoff from land to sea. Despite their large population sizes and ability to contribute to environmental oocyst contamination, there are limited studies on T. gondii oocyst shedding by free-ranging cats. We aimed to determine the frequency and genotypes of T. gondii oocysts shed by free-ranging domestic cats in central coastal California and evaluate whether genotypes present in feces are similar to those identified in sea otters that died from fatal toxoplasmosis. We utilized a longitudinal field study of four free-ranging cat colonies to assess oocyst shedding prevalence using microscopy and molecular testing with polymerase chain reaction (PCR). T. gondii DNA was confirmed with primers targeting the ITS1 locus and positive samples were genotyped at the B1 locus. While oocysts were not visualized using microscopy (0/404), we detected T. gondii DNA in 25.9% (94/362) of fecal samples. We genotyped 27 samples at the B1 locus and characterized 13 of these samples at one to three additional loci using multi locus sequence typing (MLST). Parasite DNA detection was significantly higher during the wet season (16.3%, 59/362) compared to the dry season (9.7%; 35/362), suggesting seasonal variation in T. gondii DNA presence in feces. High diversity of T. gondii strains was characterized at the B1 locus, including non-archetypal strains previously associated with sea otter mortalities. Free-ranging cats may thus play an important role in the transmission of virulent T. gondii genotypes that cause morbidity and mortality in marine wildlife. Management of free-ranging cat colonies could reduce environmental contamination with oocysts and subsequent T. gondii infection in endangered marine mammals and people.

Fig 1. Seasonal variation of T. gondii DNA detection from feral cat feces (n = 362).

Parasites were not detected in June 2022 (0% prevalence). Error bars correspond to 95% confidence intervals.

Fig 2. Prevalence of fecal samples that tested positive for T. gondii DNA in feral cats based in four colonies, and the relative distribution of parasite strains as characterized via RFLP at the B1 locus.

Relative pie chart size is proportional to the overall T. gondii DNA prevalence. The California base map was extracted from Natural Earth using the rnaturalearth package [37].

Fig 3

A. Distribution of T. gondii strains detected in feral cat feces (n = 27) as determined via virtual RFLP at the B1 locus. 3B. Variant-specific sequence typing at the B1 locus of T. gondii strains detected in feral cat feces (n = 27). Variants were determined by the presence of one or more unique snps not located at restriction enzyme sites.

Fig 4. Interpretation of T. gondii detection in cat feces using microscopy and PCR methods.

Fig 5. Mechanistic diagram showing how cats can be exposed to multiple T. gondii strains with subsequent shedding of mixed oocyst strains in feces.

Red and blue indicate different T. gondii strain types in bradyzoite cysts and oocysts, and plus signs indicate consumption of T. gondii infected prey by felids. Mixed infection and shedding of more than one T. gondii strain can occur after animals are exposed to different parasite strains sequentially (Scenario A1 + A2), or simultaneously (Scenario B).