Evidence is mounting to link toxoplasmosis with schizophrenia

Oct 18 2005

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 gondii.

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 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 toxoplasma.

"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 game."

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 said.

"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 subjects."

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 average intelligence.

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.