Ten years of University of Illinois research and a complex computer simulation varifies that there's no place for cats in swine facilities. The point is to prevent Toxoplasma gondii infection of pigs– and in turn reduce a human health risk.
Evidence of T. gondii infection in U.S. swine is based on serological studies, showing prevalence rates of 42 percent in breeding pigs and 23 percent in market hogs. Of course rates on any given farm can vary greatly.
Cats are the only known full-life-cycle host of the protozoan parasite, which causes toxoplasmosis. Feed or soil contaminated with cat feces can infect pigs. At risk are uninfected pregnant women and indiviuals with compromised immune systems. In pregnant women, consuming improperly cooked infected pork can affect their fetuses, leading to mental and physical retardation, blindness and hearing loss. Immune-compromised people face encephalitis, a swelling of the brain that can be fatal.
The University of Illinois researchers concluded, by way of computer simulation, that keeping cats away from swine herds reduces the exposure to T. gondii better than vaccinating cats against the parasite. The computer simulation included data from a previous field study that found vaccination to be effective in reducing infections in cats and pigs.
Nohra E. Mateus-Pinilla, a veterinary epidemiologist with the Illinois Natural History Survey's Center for Wildlife Ecology, conducted the doctoral research.
"The simple recommendation is that producers should deny cats access to pigs to prevent infection with Toxoplasma gondii," says study co-author Ronald Weigel, professor of epidemiology in the veterinary pathobiology department. The easiest way to accomplish this is to build total confinement facilities. But producers also need to sreen the windows and secure any portals where cats could enter, he points out. Feed storage facilities also need to be secure from cats and potential fecal contamination.
Researchers studied Illinois swine farms, looking to identify the prevalence, sources and reservoirs of T. gondii. Infected cats shed the oocysts – egg-like forms of the parasite – that have high survival rates, often for a year or more in the soil.
With the help of co-author Bruce Hannon, a professor of geography and the National Center for Supercomputing Applications, researchers developed a model in which they used parameters from their field research and tested them in all possible scenarios of vaccination, environmental variation in oocyst survival and cat population sizes.
They found that only newly infected cats shed the oocysts, for about two weeks beginning a week after infection. After shedding oocysts, cats enter a chronic but not infectious state. "They will be positive to T. gondii, but they are done shedding and no longer contributing to infection," Mateus-Pinilla says.
"The problem, we learned, is with new cats that come in uninfected," she notes. "It is hard to control cat populations, but all of our simulations showed that decreasing the number of cats will decrease infection in pigs."
Capturing farm cats, many of them feral, would be difficult and unrealistic. There is no T gondii vaccine available and destroying cats would upset existing ecosystems, Mateus-Pinilla says.
An ideal solution, she adds, would be to develop a non-live vaccine that could be administered in feed destined for both cats and swine.
"We're very comfortable with this model, because it uses data based on known information obtained from swine farms in Illinois," Mateus-Pinilla says. "While cat-management practices may be things that farmers should be doing, our study did not address their potential impact. The idea for those farm practices is to reduce pig exposure to areas where oocysts are likely to be found."
She also suggests that farmers reduce the availability of food to cats, avoid feeding strays and control rodent populations in an effort to reduce cat populations.
University of Illinois