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

Cite this dataset

Zhu, Sophie et al. (2023). Toxoplasma gondii microscopy and PCR detection results between July, 2020 and August, 2022 in greater Monterey Bay, CA [Dataset]. Dryad.


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.


Four colonies in Gilroy (Santa Clara County), Moss Landing (Monterey County), Aptos (Santa Cruz County), and Santa Cruz (Santa Cruz County) were sampled monthly between July 2020 and August 2022 (24 months). Sampling did not occur in February or March 2021 due to COVID-19 restrictions, and sampling occurred twice in April 2022 due to the low sample size (n = 5) during the first collection trip earlier that month. Three sites were located directly on the coastline, and one (Gilroy) was located inland (25.4 km from coast) in the Uvas-Llagas watershed, which drains into Monterey Bay. Samples with confirmed T. gondii are indicated with a 1 under the "Toxoplasma confirmed" column, while samples without T. gondii DNA are listed as 0. Samples that were not tested with PCR are indicated by "NA".

Usage notes

CSV file can be opened by R or other statistical software.


University of California, Davis

University of California, Davis, Award: Graduate Student Support Program