The Cat/Pig Toxoplasmosis Connection
Biological lab technician Diane Hawkins-Cooper and microbiologist Sam Shen
collect blood samples from pigs to examine for Toxoplasma antibodies.
A sparkling, crisp fall day, a tidy enclosure where young hogs happily munch
from brimming feed troughs, a plump farm cat sitting just outside the
enclosure, calmly licking her paws and rubbing her whiskers: What's wrong with
J.P. Dubey's reply would probably be "plenty." Dubey is an
Agricultural Research Service parasitologist and expert on Toxoplasma
gondii, a parasite that infects animals and humans worldwide.
Of all the creatures infected, cats are the only ones known to excrete T.
gondii oocysts, a form of the parasite that easily withstands nature's
harshness. And an uncovered feed bin would be a tempting spot for a wandering
farm cat to deposit oocyst-laden spores, opening the door to possible
swallowing of the oocysts by hungry hogs, Dubey warns.
"Infection of swine with Toxoplasma gondii creates a public
health concern because the parasite can be transmitted to humans through the
handling and consumption of raw or undercooked pork containing tissue cysts of
the parasite." explains Dubey. He is with the ARS Parasite Biology and
Epidemiology Laboratory at Beltsville, Maryland.
"However, it's not known how much T. gondii transmission to
humans is through pork and how much is through direct contact with farm or
Toxoplasma gondii can exact a terrible toll in vulnerable humans.
Healthy people other than pregnant women can weather the infection with few ill
effects. But if a pregnant woman becomes infected, there is a 20 to 50 percent
probability that her baby will be infected, possibly resulting in blindness,
mental retardation, or other medical problems for the child. In about 1 in
every 1,000 pregnancies in this country, a child is born infected with T.
gondii. The national cost of raising children infected in this way was
estimated in 1993 to be $5.3 billion.
Infection with T. gondii can also be devastating for people whose
immune systems are impaired, such as through chemotherapy treatment, in
conjunction with an organ transplant, or through acquired immunodeficiency
syndrome (AIDS). It has been estimated that as many as 1 in 10 AIDS patients in
the United States dies of toxoplasmosis.
In 1990, ARS was asked by the National Pork Producers Council to investigate
how pigs become infected with T. gondii under farm conditions.
"Working with R.M. Weigel of the University of Illinois, we surveyed 47
Illinois swine farms in 1992 and 1993," Dubey recalls. "We trapped
wildlife around the farm and checked their blood for T. gondii
infection, took blood samples from the pigs, and checked samples of feed,
water, and soil six different times."
The results: Farm cats were found to be the most likely source of T.
gondii infection, with about two-thirds of the 300-plus cats checked
showing signs of previous exposure to the parasite.
"The significant point is that if a cat has antibodies to T.
gondii in its blood, it's already shed the oocysts and they're sitting
somewhere in the environment." warns Dubey. "A cat will shed oocysts
within a week of getting infected, but it won't have antibodies in its blood
until 3 or 4 weeks later."
While an infected cat sheds for only about a week, the oocysts released
number in the millions. To make matters worse, Dubey says, the oocysts can
survive freezing and thawing, allowing them to linger in the environment for
"Of all the factors we examined in swine exposure to T. gondii,
the access of T. gondii-infected cats to the feed was the most
importantmore than mice in the feed," Dubey notes. "Outdoor
housing of the pigs was not a contributing factor.
"This means producers need to make sure their feed supplies are covered
and cats can't get into themespecially young cats, because they're more
likely to shed the oocysts. Also, producers should keep cats away from the
water and soil around their hog operations."
On the positive side, the incidence of T. gondii infestation in swine
appears to be declining, Dubey reports. A national survey in 1983-84 calculated
23 percent of market-aged swine had been infected with T. gondii at some
point. The more recent Illinois check of 7,000 market pigs and about the same
number of sows found indication of infection in only 3 percent of the market
pigs and 15 percent of the sows.
In his lab, microbiologist J.P. Dubey examines swine tissues for
More good news involves testing of a commercially developed vaccine that
could help break the T. gondii-cat connection.
"The vaccine contains a living form of the parasite. But it's a genetic
mutant, so its life cycle is not completed, and it can't produce oocysts."
"Cats vaccinated with this become immune to infection by T.
gondii and don't shed the oocysts. But you'd have to give this vaccine
every yearand especially to newborn weaned kittens, because they're a
main source of infection."
The vaccine is not yet on the market, however, it may come in 1 to 2 years,
when review of test data has been completed.
This protection is more important than ever, in light of a startling
discovery by Dubey and his research team. It was previously believed that once
a cat had been infected with T. gondii and shed oocysts, that cat would
not become re-infected and shed more oocysts.
Dubey's team has found that's probably wishful thinking. They have shown
that cats infected years earlier and that shed millions of oocysts then could
be reinfected 6 years later and begin shedding oocysts again.
That wasn't the only surprise. The same study showed that cats could have
very high levels of antibodies against T. gondii in their blood years
after a previous infection and still become reinfected. "So you can't use
high levels of antibodies as an indicator of immunity,'" says Dubey.
As part of their efforts to protect the public against T. gondii
infection, the Beltsville researchers are working on development of a vaccine
"If the pig has had this vaccine and gets infected, the vaccine would
reduce the number of parasites present in the pig's tissue," Dubey says.
"We know the effectiveness of the vaccine lasts at least 9 months. We hope
to have this vaccine ready within the next 5 years."
In the meantime, consumers already have excellent weapons at hand to protect
themselves against T. gondii ingestion through meat.
"Freezing the meal for a single day in a domestic freezer is very
effective for killing the tissue cysts, which are the form of the parasite
that's in meat," Dubey points out. "Also, cooking the meat to an
internal temperature of 153° F will kill them. Tissue cysts are much more
vulnerable than the oocysts."
Collaborative studies with microbiologist Donald W. Thayer at ARS
laboratories in Philadelphia, Pennsylvania, have shown irradiation is also
"Irradiating the meal at 0.5 kilogram will kill the T.
gondii,'" Dubey says. "And the FDA has already approved
irradiation of fresh pork carcasses to 1 kiloGray for control of Trichinella
spiralis." By Sandy Miller Hays, ARS.
Dubey is at the USDA-ARS Animal Parasitic Diseases Laboratory, Beltsville,
MD 20705; phone: (301) 504-8128.
"The Cat/Pig Toxoplasmosis Connection" was published in the
issue of Agricultural Research magazine.