“Toxoplasma gondii, Source to Sea: Higher Contribution of Domestic Felids to Terrestrial Parasite Loading Despite Lower Infection Prevalence,” published in EcoHealth, 2013. Abstract available online here.
In this study , researchers presented the results of their work investigating the rates of infection for the common parasite Toxoplasma gondii (T. gondii) among both domestic cats and wild felids (bobcats and mountain lions) along the coast of central California. Infection rates among “managed feral cats,” who lived in residential areas, were approximately 79% lower than infection rates of feral cats living in undeveloped areas (81%).
Authors of this study reported the lowest rate of T. gondii infection among “managed feral cats” (121 of 720 cats, or 17%) most of which were “collected from small to large colonies in close proximity to people, where they had access to provided food sources (e.g., commercial cat food or discarded human foods)” .
Higher infection rates among unmanaged feral cats
“Unmanaged feral cats,” by contrast, were “solitary, feral domestic cats living in undeveloped landscapes” who “likely subsisted primarily on wild prey and had minimal association with humans” . Infection rates among these cats (13 of 16, of 81%) were similar to those observed for bobcats (16 of 22, or 73%) and mountain lions (58 of 72, or 81%). In addition, the proportion of managed feral cats shedding infectious spore-like oocysts in their feces was 0.5% (2 of 435), compared to 5.9% (1 of 17) for unmanaged feral cats, 6.3% (1 of 16) for bobcats, and 2.0% (1 of 51) for mountain lions.
Results indicate that cats living in residential areas are at much lower risk of becoming infected with, and transmitting, the T. gondii parasite.
These results indicate that cats living in residential areas are at much lower risk of becoming infected with, and transmitting, the T. gondii parasite. This is likely related to their consumption of anthropogenic food sources (i.e., provided by humans). Other studies, too, have suggested that free-roaming cats are less likely to hunt wildlife when food is provided by nearby residents [2,3].
Where feral cats are found
The sampling of domestic cats for this study sheds light on the relative number of free-roaming cats living in residential areas compared to those living in undeveloped areas. Researchers obtained 720 serum samples from managed feral cats over three years compared to just 16 from unmanaged feral cats over the same period. These findings correspond reasonably well with the observations of other researchers that cats are more likely to be found near human habitations , and with population estimates based on much higher cat densities in urban areas compared to natural areas .
Implications for endangered California sea otters
The study’s authors warn that, “due to their large populations, pet and feral domestic cats likely contribute more oocysts to lands bordering the sea otter range than native wild felids” . More recent research conducted by the U.S. Geological Survey (USGS), however, suggests that “spillover from wildlife, not pets, dominates spatial patterns of disease transmission” .
USGS: “spillover from wildlife, not pets, dominates spatial patterns of disease transmission”
California’s sea otter population has also been increasing steadily in recent years , raising questions about the extent to which free-roaming domestic cats impact sea otter populations.
See related Issue Brief: Wildlife impacts of free-roaming cats: Estimates vs. evidence
VanWormer, E.; Conrad, P.A.; Miller, M.A.; Melli, A.C.; Carpenter, T.E.; Mazet, J.A.K. Toxoplasma gondii, Source to Sea: Higher Contribution of Domestic Felids to Terrestrial Parasite Loading Despite Lower Infection Prevalence. EcoHealth 2013, 1–13.
Silva-Rodríguez, E.A.; Sieving, K.E. Influence of Care of Domestic Carnivores on Their Predation on Vertebrates. Conservation Biology 2012, 25, 808–815.
Cove, M.V.; Gardner, B.; Simons, T.R.; Kays, R.; O’Connell, A.F. Free-ranging domestic cats (Felis catus) on public lands: estimating density, activity, and diet in the Florida Keys. Biological Invasions 2018, 20, 333–344.
Morin, D.J.; Lesmeister, D.B.; Nielsen, C.K.; Schauber, E.M. The truth about cats and dogs: Landscape composition and human occupation mediate the distribution and potential impact of non-native carnivores. Global Ecology and Conservation 2018, 15, e00413.
Rowan, A.N.; Kartal, T.; Hadidian, J. Cat Demographics & Impact on Wildlife in the USA, the UK, Australia and New Zealand: Facts and Values. Journal of Applied Animal Ethics Research 2019.
Lafferty, K.D. Sea otter health: Challenging a pet hypothesis. International Journal for Parasitology: Parasites and Wildlife 2015, 4, 291–294.
Tinker, M.T.; Hatfield, B.B. California Sea Otter (Enhydra lutris nereis) Census Results, Spring 2016; U.S. Geological Survey Data Series 1018, 2016; p. 10.