Toxoplasma risk factors for cats

There's been a lot of publicity (aka hype bordering on paranoia, including a recent article in the Toronto Star) about the cat-associated parasite Toxoplasma gondii lately. Cats are the definitive host of this parasite and it can cause serious disease in certain people: in pregnant women who have not been previously exposed to the parasite it can infect the unborn fetus, and it can cause severe illness (including neurological disease) in people with severely compromised immune systems. It's also been very loosely implicated in various other conditions, but much of the information gets overblown, as there is lack of solid evidence of a role of Toxoplasma in most of these cases.  Unfortunately, cats end up getting a bum rap in the process, even though most Toxoplasma infections don't come directly from cats.

Nonetheless, toxoplasmosis is a potentially devastating disease in some circumstances. and taking measures to reduce exposure to the parasite makes sense. To do this you need to know what makes cats more likely to be infected, so that these factors can be modified. A recent paper in Preventive Veterinary Medicine (Opsteegh et al. 2012) investigated risk factors for cats having antibodies against Toxoplasma. It's important to note that the presence of antibodies means the cat was exposed at some point and mounted an immune response, not that it's currently shedding the parasite in its feces. Most cats only shed Toxoplasma in their feces for a very short window of time (a week or two) after initial exposure, and that usually occurs early in life. Therefore, it's rare for older cats in households to be shedding the parasite.

The research group found 18% of cats they tested had antibodies against Toxoplasma, and they identified a few factors associated with previous Toxoplasma infection:

  • Age: Younger cats were less likely to have antibodies. The likelihood of having Toxoplasma antibodies increased steadily from 1-4 years of age.
  • Hunting
  • Presence of a dog in the house
  • Being a former stray
  • Feeding raw meat

Most of these make perfect sense and are consistent with other studies. Cats typically get infected by ingesting Toxoplasma cysts found in the muscle of other animals. So, cats that are outside (indoor/outdoor cats, former strays) and hunt, or cats that are fed raw meat are more likely to be exposed. Analysis of the data indicated that hunting contributed the most.

So, while the risk of Toxoplasma infection for the average person is pretty low, some basic management practices can further reduce any risk:

  • Keep cats indoors: This greatly reduces the chance they will be exposed to the parasite. It is also good idea for several other reasons.
  • Don't feed cats raw meat: Cooking meat to the recommended temperature and time will kill any encysted parasites - this also helps prevent exposure of people eating the meat (to Toxoplasma and lots of other bacteria).
  • Control rodents in the house (not by getting a cat!): Indoor cats can still be exposed to various infectious agents through catching mice. I know it's not always easy or even possible (my cat still catches the odd indoor critter) but taking measures to reduce the likelihood of this is wise.

Other important preventive measures include:

  • Changing the litterbox regularly, especially if a high-risk person has to do it. Toxoplasma oocysts need at least 24-48h in the environment to become infective. If feces are removed daily, they don't get that chance.
  • Clean up any fecal accidents and remove any fecal staining of the haircoat (e.g. poop stuck around the rear end of long-haired cats) promptly, before that 24-48h window expires.
  • Wash you hands regularly, especially after contact with the litterbox or any potentially contaminated areas.
  • Wash vegetables and cook meat properly. You're more likely to get Toxoplasma from food than from your cat.

More information about Toxoplasma can be found on the Worms & Germs Resources page.

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Comments (1) Read through and enter the discussion with the form at the end
Peter J. Wolf - February 26, 2012 2:35 PM

Thank you, Scott, for a much-needed dose of sanity on this subject! I’d like to offer a few additional comments regarding the alleged risks free-roaming cats, as potential hosts of T. gondii, pose to wildlife (due largely to environmental contamination)…

A recent paper published in Pacific Science reports, bluntly: “Cats are the definitive host: the disease only occurs when cats are present” (Duffy & Capece, 2011). In fact, this claim is contradicted by a number of studies:

High levels (75 percent) of congenital transmission of T. gondii, for example, in a “wild population of mice,” led UK researchers to conclude “that this phenomenon might be more widespread than previously thought” (Marshall et al., 2004). Infections in sheep also point to congenital transmission, which “may be more important than previously considered” (Hide et al., 2009).

The “high incidence of T. gondii found, among others, in free-living ruminants suggests a possibility of other, so far unknown, paths of transmission of this protozoan” (Sroka, Szymańska, & Wójcik-Fatla, 2009). “Due to the fact that they are widespread, and tick-bites occur frequently both in humans and in animals, ticks might play an important role in toxoplasmosis transmission” (Sroka et al., 2009).

Of particular interest are studies in the Arctic, where the prevalence of T. gondii infection in arctic foxes, Svalbard reindeer, sibling voles, walruses, kittiwakes, barnacle geese, and glaucous gulls “indicates that infection by oocysts is not an important mode of transmission on Svalbard” (Prestrud et al., 2007). Researchers studying infection rates in polar bears concluded: “It would… be inconceivable to assume that the few cats would play a major role in the epidemiology of T. gondii in the vast high Arctic. This is apparently the case in East Greenland as well” (Oksanen et al., 2009).

Domestic cats are often implicated in the infection of Southern sea otters, the idea being that freshwater runoff is contaminated with their feces (and T. gondii). In fact, the most common type of T. gondii found to be infecting otters off the California coast is the Type X strain (Conrad et al., 2005), which has yet to be traced to domestic cats (Miller et al., 2008).

And, according to a recent study, it seems clear that T. gondii isn’t acting alone: “dual infections of T. gondii and S. neurona were more frequently associated with mortality and protozoal encephalitis than single infections, indicating a role for polyparasitism in disease severity” (Gibson et al., 2011).

As you and your readers are no doubt aware, so much of the research on this subject is used to further vilify free-roaming cats. The more we learn about T. gondii, though, the more it seems clear that (1) there is, in fact, a great deal we still don’t know about the parasite and its transmission, and (2) even if we were able to rid the environment of free-roaming cats (as many of suggested), any reduction in T. gondii is likely to be minimal, given what’s already “out there” and the various transmission routes.

Peter J. Wolf
http://www.voxfelina.com


Literature Cited
• Conrad, P. A., Miller, M. A., Kreuder, C., James, E. R., Mazet, J., Dabritz, H., et al. (2005). Transmission of Toxoplasma: Clues from the study of sea otters as sentinels of Toxoplasma gondii flow into the marine environment. International Journal for Parasitology, 35(11-12), 1155-1168.
• Duffy, D. C., & Capece, P. (2011). Biology and Impacts of Pacific Island Invasive Species 7. The Domestic Cat (Felis catus). Pacific Science, 66(2 (Early View)), 000–000.
• Gibson, A. K., Raverty, S., Lambourn, D. M., Huggins, J., Magargal, S. L., & Grigg, M. E. (2011). Polyparasitism Is Associated with Increased Disease Severity in Toxoplasma gondii-Infected Marine Sentinel Species. PLoS Neglected Tropical Diseases, 5(5), e1142.
• Hide, G., Morley, E. K., Hughes, J. M., Gerwash, O., Elmahaishi, M. S., Elmahaishi, K. H., et al. (2009). Evidence for high levels of vertical transmission in Toxoplasma gondii. Parasitology, 136(Special Issue 14), 1877-1885.
• Marshall, P. A., Hughes, J. M., Williams, R. H., Smith, J. E., Murphy, R. G., & Hide, G. (2004). Detection of high levels of congenital transmission of Toxoplasma gondii in natural urban populations of Mus domesticus. Parasitology, 128(01), 39–42.
• Miller, M. A., Miller, W. A., Conrad, P. A., James, E. R., Melli, A. C., Leutenegger, C. M., et al. (2008). 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, 38(11), 1319-1328.
• Oksanen, A., Åsbakk, K., Prestrud, K. W., Aars, J., Derocher, A. E., Tryland, M., et al. (2009). Prevalence of Antibodies Against Toxoplasma gondii in Polar Bears (Ursus maritimus) From Svalbard and East Greenland. Journal of Parasitology, 95(1), 89–94.
• Prestrud, K. W., Åsbakk, K., Fuglei, E., Mørk, T., Stien, A., Ropstad, E., et al. (2007). Serosurvey for Toxoplasma gondii in arctic foxes and possible sources of infection in the high Arctic of Svalbard. Veterinary Parasitology, 150(1–2), 6–12.
• Sroka, J., Szymańska, J., & Wójcik-Fatla, A. (2009). The occurrence of Toxoplasma gondii and Borrelia burgdorferi sensu lato in Ixodes ricinus ticks from eastern Poland with the use of PCR. Annals of Agricultural and Environmental Medicine, 16(2), 313–319.

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