Toxoplasma gondii microscopy and PCR detection results between July, 2020 and August, 2022 in greater Monterey Bay, CA

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 to water bodies through
rain-driven surface runoff. Certain genotypes of T. gondii were found in
otters that died from toxoplasmosis, however, the source of these
parasites has not been determined. Despite their large population sizes
and ability to contribute to environmental oocyst contamination, there is
limited knowledge on the frequency of oocyst shedding and genotypes of
oocysts shed by free-ranging cats. We aimed to determine the presence of
T. gondii oocyst shedding in free-ranging domestic cats in central coastal
California and identify genotypes shed in feces. We utilized a
longitudinal field study of four free-ranging cat colonies to assess
oocyst fecal shedding prevalence using microscopy and molecular
confirmation with polymerase chain reaction (PCR). Initial DNA screening
targeting the ITS1 locus confirmed presence of T. gondii and positive
samples were further 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 samples at one or more loci using multi locus sequence
typing (MLST). Parasite DNA detection was higher during the wet season
(16.3%, 59/362) compared to the dry season (9.7%; 35/362), suggesting
seasonal variation in T. gondii oocyst shedding. High diversity of T.
gondii strains was characterized at the B1 locus in cat feces, including
atypical strains previously associated with sea otter mortalities.
Free-ranging cats may thus play an important role in the transmission of
virulent parasite genotypes that cause morbidity and mortality in marine
wildlife. Proper management of free-ranging cat colonies could reduce
environmental contamination with oocysts and subsequent T. gondii
infection in endangered marine mammals and people.