Ten years of field studies and a complex computer simulation suggest that producers should keep cats out of hog units to guard against Toxoplasma gondii (T. gondii) infection in pigs and to reduce the risk to human health.

Cats are the only known full-life-cycle host of the protozoan parasite which causes toxoplasmosis. Pigs become infected from feed or soil contaminated with cat feces. If uninfected, pregnant women eat raw or improperly cooked pork, the disease can damage the fetus, triggering mental and physical retardation, blindness and hearing loss.

Based on U.S. serological studies, prevalence rate of T. gondii is estimated at 42% of breeding stock and 23% of market hogs.

University of Illinois field studies found that limiting cat access to hog facilities is the best solution. A vaccine against the parasite is not commercially available, reports Nohra E. Mateus-Pinilla, who conducted graduate studies on the parasite.

"The simple recommendation is that producers should deny cats access to pigs to prevent infection with T. gondii," says study co-author Ronald Weigel, veterinary epidemiologist at the university.

"The easiest way to accomplish this is to build total confinement facilities, with screened windows and no portals for cats to enter, and monitor cats when doors are open," he adds. "If feed is stored in buildings of this type, there should be minimal risk of cat fecal contamination of feed."

In 1992, Illinois scientists began research on 47 hog farms in the state, studying cats on hog farms and testing for T. gondii infection. They found infected cats shed the oocysts – egg-like forms of the parasite – that often survive for a year or more in the soil. They also found that vaccinating cats with an experimental vaccine reduced the prevalence of infection.

Aided by a computer model, they found that the prevalence of T. gondii in cats had little to do with the rate of transmission in pigs.

The key is how infection works in cats. Newly infected cats shed the oocycts for two weeks after infection. Cats then enter a chronic but non-infectious state. "They will be positive to T. gondii, but they are done shedding and no longer contributing to infection on a swine farm," observes Mateus-Pinilla.

The problem occurs with new, uninfected cats entering the farm. "It is hard to control cat populations, but all of our simulations said that decreasing the number of cats will decrease infection in pigs," she observes. Eliminating food sources, not feeding strays and controlling rodent populations can also reduce cat numbers.