Toxoplasma gondii › Page 2

“Have you seen this already? This is awful.”

That’s what somebody posted on the Vox Felina Facebook page late last night—along with a link to an MSNBC news story. The headline was an attention-getter, no doubt about it: “Report: Kill feral cats to control their colonies.”

But beyond that, MSNBC had practically no details. A little digging around, however, led me to New England Cable News (NECN), which has the complete story.

“The report began in an undergraduate wildlife management class, with students writing reports on feral cats based on existing research. The students’ professor and other [University of Nebraska] researchers then compiled the report from the students’ work.” [1]

“Feral Cats and Their Management” claims, straightforwardly enough, to provide “research-based information on the management of feral cats.” [2] Management, in this case, meaning—as is so often the case in such contexts—killing, extermination, eradication, and so forth. Detailed advice is provided (e.g., “Body-gripping traps and snares can be used to quickly kill feral cats”).

And research? In this case, nothing more than a cursory review of all of the usual suspects: Coleman and Temple, Pamela Jo Hatley, Cole Hawkins, The Wildlife Society, Linda Winter. In other words, lots of Kool-Aid drinking.

It’s Like Science, Only Different
Among the research misinterpreted and/or misrepresented (none of which is cited in the text):

“As instinctive hunters, feral cats pose a serious threat to native wildlife, particularly birds.”

It’s no surprise that the authors of the report offer no evidence to support such a sweeping claim. “There are few if any studies,” write Mike Fitzgerald and Dennis Turner in their contribution to The Domestic Cat: The biology of its behaviour, “apart from island ones that actually demonstrate that cats have reduced bird populations.” [3]

Biologist C.J. Mead, reviewing the deaths of “ringed” (banded) birds reported by the British public, suggests that cats may be responsible for 6.2–31.3% of bird deaths. “Overall,” writes Mead, “it is clear that cat predation is a significant cause of death for most of the species examined.” Nevertheless, Mead concludes, “there is no clear evidence of cats threatening to harm the overall population level of any particular species… Indeed, cats have been kept as pets for many years and hundreds of generations of birds breeding in suburban and rural areas have had to contend with their predatory intentions.” [4]

The German zoologist Paul Leyhausen (1916–1998), who spent the bulk of his career studying the behavior of cats, found that cats, frustrated by the difficulties of catching them, “may soon give up hunting birds.” [5]

“During years in the field,” wrote Leyhausen, “I have observed countless times how cats have caught a mouse or a rat and just as often how they have stalked a bird. But I never saw them catch a healthy songbird that was capable of flying. Certainly it does happen, but, as I have said, seldom. I should feel sorry for the average domestic cat that had to live solely on catching birds.” [5]

“Cats kill an estimated 480 million birds per year (assuming eight birds killed per feral cat per year).”

Fitzgerald and Turner (whose work is not referenced in the report) argue that “we do not have enough information yet to attempt to estimate on average how many birds a cat kills each year.” [3] Though, of course, many studies have tried to do exactly that—few, it should be said, involve feral cats.

Unfortunately—and as I have pointed out time and time again—such work typically suffers from a range of methodological and analytical problems (e.g., statistical errors, small sample sizes, and inappropriate/baseless assumptions).

And—as with the UNL report—obvious bias.

“Estimates from Wisconsin indicate that between 500,000 and 8 million birds are killed by rural cats each year in that state…”

How anybody could misquote the numbers from the Wisconsin Study—easily the most widely circulated work on the subject—is a mystery. (On the other hand, the figures were, as Stanley Temple has said, “not actual data” [6] in the first place, so I suppose that does allow for some rather liberal interpretation.)

“The diets of well-fed house-based cats in Sweden consisted of 15 percent to 90 percent native prey, depending on availability.”

How important is it that the prey of feral cats is native, versus non-native? That’s a point of some debate—but not in this case. See, what Liberg actually wrote was this: “Most cats (80-85%) were house-based and obtained from 15 to 90% of their food from natural prey, depending on abundance and availability of the latter.” [7, emphasis mine] He was merely drawing the distinction between food provided by humans and any prey that cats might eat as food.

Liberg goes on to point out that the predation he documented did not, “justify a conclusive assessment of the effects of cats on their prey populations, but… indicate[s] that cats by themselves were not limiting any of their prey.” [7] Even high rates of predation do not equate to population declines.

“In California, 67 percent of rodents, 95 percent of birds, and 100 percent of lizards brought home by cats were native species, and native birds were twice as likely to be seen in areas without cats.”

What looks to be truly damning evidence loses much of its impact when it’s seen in context. The reference to Crooks and Soulé’s 1999 paper, for example, omitted the sample size involved: “Identification of 68 prey items returned by cats bordering the fragments indicated that 67% of 26 rodents, 95% of 21 birds and 100% of 11 lizards were native species.” [8] It’s important to note, too, that these researchers asked residents to recall what kind of prey their cats returned—no prey items were collected—thereby raising questions about the accuracy of species attribution.

Furthermore, the cats involved with Crooks and Soulé’s study were all pet cats. How their habits compare with those of feral cats is an open question. Merritt Clifton of Animal People, an independent newspaper dedicated to animal protection issues, suggests, “feral cats appear to hunt no more, and perhaps less, than free-roaming pet cats. This is because, like other wild predators, they hunt not for sport but for food, and hunting more prey than they can eat is a pointless waste of energy.”

The second portion of the quote refers to Cole Hawkins’ PhD dissertation. Hawkins’ research methods and analysis are so problematic that the suggestion of a causal relationship between the presence of cats and the absence of birds (native or otherwise) is highly inappropriate (indeed, Hawkins scarcely investigates predation at all).

Among the key issues: Hawkins had no idea what the “cat” area of his study site was like before the cats were there; he merely assumes it was identical to the “no cat” area in terms of its fauna (though the two landscapes are actually quite different). It’s also interesting to note Hawkins’ emphasis on “the preference of ground feeding birds for the no-cat treatment” while downplaying the fact that five of the nine ground-feeding species included in the study showed no preference for either area. (For a more comprehensive analysis, please see my previous post on the subject.)

“…cats are the most important species in the life cycle of the parasite responsible for toxoplasmosis, and in 3 separate studies, most feral cats (62 percent to 80 percent) tested positive for toxoplasmosis.”

While cats are the “definitive host,” it’s important to note that “wild game can be a source of T. gondii infection in humans, cats, and other carnivores. Serologic data show that a significant number of feral pigs, bears, and cervids are exposed to T. gondii.” [9]

“Humans,” write Elmore et al., “usually become infected through ingestion of oocyst-contaminated soil and water, tissue cysts in undercooked meat, or congenitally. Because of their fastidious nature, the passing of non-infective oocysts, and the short duration of oocyst shedding, direct contact with cats is not thought to be a primary risk for human infection.” [10]

Toxoplasma gondii has been linked to the illness and death of marine life, primarily sea otters [11], prompting investigation into the possible role of free-roaming (both owned and feral) cats. [12, 13] It’s generally thought that oocysts (the mature, infective form of the parasite) are transferred from soil contaminated with infected feces to coastal waterways by way of freshwater run-off. [13]

However, a 2005 study found that 36 of 50 sea otters from coastal California were infected with the Type X strain of T. gondii [14], a type linked to wild felids (mountain lions and a bobcat, in this case), but not to domestic cats. [13] A recently published study from Germany seems to corroborate these findings. Herrmann et al. analyzed 18,259 fecal samples (all from pet cats) for T. gondii and found no Type X strain. (It’s interesting to note, too, that only 0.25% of the samples tested positive for T. gondii). [15]

[NOTE: Please see follow-up post for additional information about cats and T. gondii.]

“Predation by cats on birds has an economic impact of more than $17 billion dollars [sic] per year in the U.S. The estimated cost per bird is $30, based on literature citing that bird watchers spend $0.40 per bird observed, hunters spend $216 per bird shot, and bird rearers spend $800 per bird released.”

According to this bizarre form of accounting, hunters value an individual bird more than 500 times as much as a birdwatcher does—suggesting, it seems, that dead birds are far more valuable than live birds. This is the kind of estimate that can be developed only through university (or perhaps government) research efforts.

Public Indecency
“Stephen Vantassel, a wildlife damage project coordinator who worked on the study, said researchers were aware that some people would be ‘very offended that we offered any type of lethal control method.’ But he said the report was written for public consumption and wasn’t submitted to any science journals for publication.” [1]

For the record, Dr. Vantassel, I’m more offended by the way you’ve allowed such sloppy, grossly irresponsible work to pass for “research.” And the idea that such an undertaking is somehow acceptable because it’s meant for a mass audience is simply absurd!

Naturally, the American Bird Conservancy (ABC) embraced the report immediately, “with one official calling it ‘a must read for any community or government official thinking about what to do about feral cats.’” [1]

“‘Not surprisingly, the report validates everything American Bird Conservancy has been saying about the feral cat issue for many years—namely, TNR doesn’t work in controlling feral cat populations,’ Darin Schroeder, vice president of the Conservation Advocacy for American Bird Conservancy, said Tuesday.”

But validation requires far more than this report provides—beginning with a real interest in scientific inquiry and some basic critical thinking skills. And while we’re at it, a refresher in ethics wouldn’t hurt, either.

* * *

In my previous post, I’d indicated that my next post—this post—was going to focus on The American Bird Conservancy Guide to Bird Conservation. Obviously, something came up. Anyhow, the book will keep for a few more days…

Literature Cited
1. n.a. (2010) Report: Kill feral cats to control their colonies. http://www.necn.com/11/30/10/Report-Kill-feral-cats-to-control-their-/landing_scitech.html?&blockID=3&apID=95afccc4d9564caf8e264f9d087f5732 Accessed December 1, 2010.

2. Hildreth, A.M., Vantassel, S.M., and Hygnstrom, S.E., Feral Cats and Their Managment. 2010, University of Nebraska-Lincoln Extension: Lincoln, NE. http://elkhorn.unl.edu/epublic/live/ec1781/build/ec1781.pdf

3. Fitzgerald, B.M. and Turner, D.C., Hunting Behaviour of domestic cats and their impact on prey populations, in The Domestic Cat: The biology of its behaviour, D.C. Turner and P.P.G. Bateson, Editors. 2000, Cambridge University Press: Cambridge, U.K.; New York. p. 151–175.

4. Mead, C.J., “Ringed birds killed by cats.” Mammal Review. 1982. 12(4): p. 183-186. http://dx.doi.org/10.1111/j.1365-2907.1982.tb00014.x

5. Leyhausen, P., Cat behavior: The predatory and social behavior of domestic and wild cats. Garland series in ethology. 1979, New York: Garland STPM Press.

6. Elliott, J. (1994, March 3–16). The Accused. The Sonoma County Independent, pp. 1, 10

7. Liberg, O., “Food Habits and Prey Impact by Feral and House-Based Domestic Cats in a Rural Area in Southern Sweden.” Journal of Mammalogy. 1984. 65(3): p. 424-432. http://www.jstor.org/stable/1381089

8. Crooks, K.R. and Soule, M.E., “Mesopredator release and avifaunal extinctions in a fragmented system.” Nature. 1999. 400(6744): p. 563.

9. Hill, D.E., Chirukandoth, S., and Dubey, J.P., “Biology and epidemiology of Toxoplasma gondii in man and animals.” Animal Health Research Reviews. 2005. 6(01): p. 41-61. http://journals.cambridge.org/action/displayAbstract?fromPage=online&aid=775956&fulltextType=RA&fileId=S1466252305000034

10. Elmore, S.A., et al., “Toxoplasma gondii: epidemiology, feline clinical aspects, and prevention.” Trends in Parasitology. 26(4): p. 190-196. http://www.sciencedirect.com/science/article/B6W7G-4YHFWNM-1/2/2a468a936eb06649fde0463deae4e92f

11. Jones, J.L. and Dubey, J.P., “Waterborne toxoplasmosis – Recent developments.” Experimental Parasitology. 124(1): p. 10-25. http://www.sciencedirect.com/science/article/B6WFH-4VXB8YT-2/2/8f9562f64497fe1a30513ba3f000c8dc

12. 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. http://avmajournals.avma.org/doi/abs/10.2460/javma.229.1.74

13. 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. http://www.sciencedirect.com/science/article/B6T7F-4RXJYTT-2/2/32d387fa3048882d7bd91083e7566117

14. 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. http://www.sciencedirect.com/science/article/B6T7F-4GWC8KV-2/2/2845abdbb0fd82c37b952f18ce9d0a5f

15. Herrmann, D.C., et al., “Atypical Toxoplasma gondii genotypes identified in oocysts shed by cats in Germany.” International Journal for Parasitology. 2010. 40(3): p. 285–292. http://www.sciencedirect.com/science/article/B6T7F-4X1J771-2/2/dc32f5bba34a6cce28041d144acf1e7c

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
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.” [1]

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). [2] (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 [2], 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. [4] 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.” [4] (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. [5] 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. [6] 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 [8] 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 [4].”

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. [9] and Hawkins [10]. Longcore et al. even take seriously the Wisconsin Study [11] and its findings that “aren’t actual data.” [12] 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.” [6]

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. [5] 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.

Literature Cited
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.

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