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To stem disease, keep cats
indoors, stop feeding strays, scientist urges
Life Sciences Editor
photo to enlarge
by Jim Barlow
M. McAllister, a professor of pathobiology in the
College of Veterinary Medicine, recommends keeping
cats indoors to decrease the transmission of Toxoplasma
gondii. Domestic cats and some wild cats are the only
animals that can transmit the parasite by shedding
the organism in feces.
— Keep pet cats inside, stop feeding strays, cook meat sufficiently
and reconsider the way the veterinary profession and public health agencies
think – and teach – about the zoonotic pathogen Toxoplasma
Such are the recommendations of Milton M. McAllister, a professor of
pathobiology in the College of Veterinary
Medicine at the University of Illinois at Urbana-Champaign. He delivered
that message at 8:30 a.m., Oct. 19 (2:30 p.m. CDT Tuesday, Oct. 18)
in Christchurch, New Zealand, at the 20th International Conference of
the World Association for the Advancement of Veterinary Parasitology.
McAllister, also a clinical professor of pathology in the U. of I. College
of Medicine at Urbana-Champaign, made his case based on his review
of numerous studies on the animal-carried pathogen during the past decade.
His review, prepared for the conference, appeared in the Sept. 30 issue
of the journal Veterinary Parasitology.
“Our profession needs to come to grip with the accumulating body
of evidence about the tremendous burden wrought on society by toxoplasmosis,”
McAllister wrote. “Further research is needed to clarify the association
between toxoplasmosis and mental health, but until such time that this
association may be refuted, it is my opinion that the current evidence
is strong enough to warrant an assumption of validity.”
Toxoplasma can infect most warm-blooded animals, as well as humans and
birds. Domestic cats and some wild cats are the only animals that can
transmit the parasite by shedding the organism in feces. Other animals
become infected when they consume the organisms shed by cats. This method
of parasite transmission is called fecal-oral, but it doesn’t
actually mean that feces are directly ingested. The organisms survive
in soil long after feces have decomposed.
Dust contaminates paws, fingers, feedstuffs and water, ultimately leading
to ingestion by animals and people.
McAllister and colleagues are beginning to work on a new vaccine, which,
if successful, would be administered to cats orally, possibly incorporated
into a treat. The vaccine would be used to prevent cats from shedding
“Cats usually become infected with toxoplasma by ingesting an
infected animal, or raw meat from an infected animal,” he said.
“So a cat gets infected by catching and eating mice or birds,
or by eating meat scraps from such things as poultry, pork, lamb or
In his review, McAllister noted a long list of maladies made worse by
toxoplasma infection in people with suppressed immunity, and he cited
a growing list of studies that link problems in people whose immune
systems are not impaired. Among the latter problems are fever, enlarged
lymph nodes, weakness and debilitation, damaged vision, or multi-systemic
infections with serious complications such as pneumonia and hepatitis.
Toxoplasma also is a causative agent of encephalitis in AIDS patients.
People can get infections either by fecal-oral transmission –
even through inhaling oocysts in dusty conditions – or by eating
undercooked infected meat. Oocysts, the egg-like forms of a parasite,
can survive for more than a year in soil, dust or water, McAllister
“Cats that remain indoors have a low potential to become infected
if they don’t have access to mice and if they are not fed raw
meat or meat products,” McAllister said. Owners can safely keep
an indoor cat simply by practicing good hygiene with the litter box
and washing hands after daily cleanings, he added.
Infected mice, he noted, show altered behavior, including being less
aware of cats in an area, leaving the mice open to predation that renews
the parasite’s life cycle. Mice may not be the only creatures
susceptible to behavioral changes from infections, he said.
“Evidence is mounting to link toxoplasmosis with schizophrenia
or similar psychiatric disorders (in people),” McAllister wrote.
“Recent studies from three countries found that schizophrenic
patients had higher antibody levels to T. gondii than did matched control
He also cited older studies that used a toxoplasma skin test that “showed
highly significant associations between toxoplasmosis and psychiatric
disorders.” Recent studies also have linked infections with reduced
Toxoplasmosis is the third leading cause of food-related deaths in the
United States, behind salmonella and listeria infections. Exposure in
the womb is considered “one of the most common infectious causes
of birth defects, mental retardation and visual problems worldwide,
including industrialized nations,” McAllister wrote. Studies in
the last three years have estimated that toxoplasma has infected 25
percent of adult Americans, 40 percent of adults in the Netherlands
and 70 percent in Jakarta, Indonesia.
Marine mammals also are at risk, possibly from cat-feces-contaminated
surface water going into the oceans. He cited infections of seals, dolphins
and sea otters.
Simply put, McAllister said, domestic cats should not be allowed to
roam outdoors. The feeding of stray cats, he added, by cat protectionist
groups including some veterinary organizations that wish to spare homeless
cats from the threat of euthanasia, unfortunately increases the spread
of toxoplasmosis to wildlife, domestic animals and people.
“Public lawmakers should consider developing effective solutions
that protect the best interests of society,” he wrote.
Health educators teaching new students, McAllister writes, “should
be careful to distinguish sub-clinical infections from the possibilities
of undiagnosed infections and latent disease.” He noted that proper
diagnosis likely is frequently missed.
If a practical vaccine to prevent cats from shedding toxoplasma organisms
can be developed, he said, then its use could be made mandatory, similar
to rabies vaccine laws in many states.