Until now, my posts have focused almost exclusively on wildlife impacts (real and otherwise) related to predation by cats, a topic I’ll be returning to soon enough. Over the past week or so, however, I’ve been researching the Toxoplasma gondii parasite (another subject that will keep me busy well into the future). As it turns out, there’s big news on the T. gondii front—though in this case, the “news” is actually two years old.
Toxoplasma gondii is found in many mammals and birds, but its definitive host—the animal in which the parasite reproduces—is the cat, both domestic and wild species. Cats pass the mature, infective form of the parasite in their feces—a process called “shedding oocysts.” T. gondii infection, or toxoplasmosis, in humans can be traced to “ingestion of oocyst-contaminated soil and water, from tissue cysts in undercooked meat, by transplantation, blood transfusion, laboratory accidents, or congenitally.” 
How often cats shed oocysts, and to what extent, is a complex issue—one I’ll save for later. For now, I will simply note that, in general, it is thought that most cats build up immunity to re-shedding oocysts (though exceptions have been documented in laboratory testing).  (For a concise overview of T. gondii’s prevalence in, and risks to, humans, download “Toxoplasma gondii: Epidemiology, feline clinical aspects, and prevention.”)
T. Gondii, Cats, and Sea Otters
In recent years, T. gondii has been linked to the illness and death of marine life, primarily sea otters , thereby prompting investigation into the possible role of free-roaming (both owned and feral) cats. [3, 4] It’s generally thought that oocysts are transferred from soil contaminated with infected feces to coastal waterways by way of freshwater run-off.  And it’s also generally thought that domestic cats are the culprits—or at least it was.
As I was sifting through my growing pile of T. gondii studies, I was rather shocked to find this:
“Three of the Type X-infected carnivores were wild felids (two mountain lions and a bobcat), but no domestic cats were Type X-positive. Examination of larger samples of wild and domestic felids will help clarify these initial findings. If Type X strains are detected more commonly from wild felids in subsequent studies, this could suggest that these animals are more important land-based sources of T. gondii for marine wildlife than are domestic cats.”  (italics mine)
Let me explain. There are multiple strains of T. Gondii. Studies of southern sea otters from coastal California found that 36 of 50 otters were infected with the Type X strain.  In other words, 72% of the otters were infected with a strain of T. gondii that has yet to be traced to domestic cats.
Now, I’ll be the first to admit that these results are to be treated with caution—as Miller et al. note, “subsequent studies” are in order. For one thing, their sample size was quite small: three bobcats, 26 mountain lions, and seven domestic cats (although the authors suggest at one point that only five domestic cats were included). In addition, this area of research is quite active—and, as this study illustrates, the results can be surprising. Future research intended to confirm or refute this work could just as easily take us off in another direction altogether.
That said, this is still big news. Nearly two years old now, however, it’s not exactly breaking news. So why is this the first I’ve heard about these important findings?
What’s the Story?
For some reason, Miller et al. downplay their findings. Worse, they confuse matters by going into detail about the estimated mass of “feline fecal deposition” created by domestic cats in the communities adjacent to their study site. Suddenly, the focus is back on domestic cats. Given the authors’ findings, I’m not sure how this is relevant, other than as background—previous assumptions being called into question by their results. Perhaps it’s merely the inevitable result of 14 co-authors (one of whom, it should be noted, is David Jessup, of whose work I have been critical in the past) collaborating on a single paper.
But I’m unwilling to give Longcore et al. the same benefit of the doubt. In their essay, Longcore et al.  dissemble to such an extent that readers are likely to come away missing the point entirely:
“The large quantity of waste from feral and free-roaming cats containing Toxoplasma oocysts [3, 7] and the correlation between freshwater runoff and toxoplasmosis in marine mammals  has led researchers to suspect domestic cats as the source of the infections, although further research is needed to determine the relative importance of native versus exotic felids as sources of this parasite .”
While technically correct, Longcore et al. gloss over the fact that, based on the very study they cite, “the relative importance of native versus exotic felids as sources of this parasite” might be something like three-to-one.
And it’s not as if these authors are unwilling to consider speculative findings—such as those by Baker et al.  and Hawkins . Longcore et al. even take seriously the Wisconsin Study  and its findings that “aren’t actual data.”  And they leave out plenty, too—which in the case of the Miller et al. work, might have been a more honorable approach.
Something else they should have omitted:
“Felids, including feral and free-roaming cats, shed Toxoplasma oocysts that infect southern sea otters [8, 5], Pacific harbor seals, and California sea lions.” 
In fact, Conrad et al. examined just one harbor seal and one sea lion—and in both cases found the Type X strain of T. gondii.  Which, when combined with the results from Miller et al., suggests wild felids as the more likely source, rather than domestic cats.
These two studies not only contradict the specific claims made by Longcore et al., they also challenge the native-good/non-native-bad dichotomy that seems to be at the root of so many feral cat/TNR complaints.
* * *
I sent an e-mail to Melissa Miller, lead author of “Type X Toxoplasma gondii in a wild mussel and terrestrial carnivores from coastal California: New linkages between terrestrial mammals, runoff and toxoplasmosis of sea otters,” asking her to comment on my reading of the study. I have not yet received a response.
1. Elmore, S.A., et al., “Toxoplasma gondii: epidemiology, feline clinical aspects, and prevention.” Trends in Parasitology. 26(4): p. 190-196.
2. Jones, J.L. and Dubey, J.P., “Waterborne toxoplasmosis—Recent developments.” Experimental Parasitology. 124(1): p. 10-25.
3. Dabritz, H.A., et al., “Outdoor fecal deposition by free-roaming cats and attitudes of cat owners and nonowners toward stray pets, wildlife, and water pollution.” Journal of the American Veterinary Medical Association. 2006. 229(1): p. 74-81.
4. Miller, M.A., et al., “Type X Toxoplasma gondii in a wild mussel and terrestrial carnivores from coastal California: New linkages between terrestrial mammals, runoff and toxoplasmosis of sea otters.” International Journal for Parasitology. 2008. 38(11): p. 1319-1328.
5. Conrad, P.A., et al., “Transmission of Toxoplasma: Clues from the study of sea otters as sentinels of Toxoplasma gondii flow into the marine environment.” International Journal for Parasitology. 2005. 35(11-12): p. 1155-1168.
6. Longcore, T., Rich, C., and Sullivan, L.M., “Critical Assessment of Claims Regarding Management of Feral Cats by Trap–Neuter–Return.” Conservation Biology. 2009. 23(4): p. 887–894.
7. Dabritz, H.A., et al., “Detection of Toxoplasma gondii-like oocysts in cat feces and estimates of the environmental oocyst burden.” Journal of the American Veterinary Medical Association. 2007. 231(11): p. 1676-1684.
8. Miller, M.A., et al., “Coastal freshwater runoff is a risk factor for Toxoplasma gondii infection of southern sea otters (Enhydra lutris nereis).” International Journal for Parasitology. 2002. 32(8): p. 997-1006.
9. Baker, P.J., et al., “Impact of predation by domestic cats Felis catus in an urban area.” Mammal Review. 2005. 35(3/4): p. 302-312.
10. Hawkins, C.C., Impact of a subsidized exotic predator on native biota: Effect of house cats (Felis catus) on California birds and rodents. 1998, Texas A&M University
11. Coleman, J.S. and Temple, S.A., On the Prowl, in Wisconsin Natural Resources. 1996, Wisconsin Department of Natural Resources: Madison, WI. p. 4–8. http://dnr.wi.gov/wnrmag/html/stories/1996/dec96/cats.htm
12. Elliott, J., The Accused, in The Sonoma County Independent. 1994. p. 1, 10.