Toxoplasmosis risks target of research, vaccine development

URBANA, ILL. — Toxoplasmosis might be a greater risk to humans than what is perceived, according to a paper written by a University of Illinois (UI) DVM on subclinical effects of the parasite.

Three UI veterinarians are working on a toxoplasmosis vaccine, labeling the infection a serious public health risk. If the vaccine is proven effective, it will prevent cats from shedding the parasite, says Dr. Milton McAllister, a UI professor of pathobiology.

"Toxoplasmosis is a public health policy issue, and should be treated as such," McAllister contends. "Cats that run throughout neighborhoods are potential carriers of the parasite and are a danger to anyone coming into contact with the cat's environment. With estimates of 25 percent of the U.S. population being infected with the parasite, it is a problem."

Although only preliminary data is currently being examined and vaccine development is in its early stages, researchers say a vaccine would likely need to be made mandatory to make a significant impact on preventing the parasite's spread.

McAllister stresses that many risks are involved with a toxoplasmosis infection.

Toxoplasmosis is the third leading cause of food-related deaths in the United States, behind Salmonella and Listeria infections.

Taking the Good With the Bad

After gathering information on the parasite, McAllister presented his findings at the 20th International Conference of the World Association for the Advancement of Veterinary Parasitology. The research also was published in the September 30 issue of Veterinary Parisitology.

"I found a lot of contrary information about toxoplasmosis than what we were taught in veterinary school," he adds. "It's a matter of emphasis. We are taught toxoplasmosis can affect immuno suppressed people but isn't a great threat to the general population. But it has greater consequences than realized."

"Schizophrenia is one possible effect of the parasite. I believe the evidence is strong, but there is not enough proof to say it positively is a cause," McAllister says. "Recent studies from three countries found that schizophrenic patients had higher antibody levels to T. gondii than did matched control subjects."

Other effects under investigation include:

• Retinal choroiditis and other eye infections that ultimately can cause blindness.

• Chronic fatigue syndrome, weakness

• Headaches

• Enlarged lymph nodes

• Multi-systemic infections

• A lower IQ.

• Personality: As a group toxoplasmosis-infected people score differently.

"It's these things the profession has gotten wrong," McAllister says. " It's not a small risk if you aren't careful with hygiene with a cat. I think a step veterinarians can take to minimize disease spread is to encourage owners to keep their pets indoors and feed them dry or moist cat food only."

Trap, neuter and release programs perpetuate the problem, exposing humans to the parasite when handling feral cats, McAllister adds.

Exposure to toxoplasmosis 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 says.

Research

In studies conducted on the disease, mice infected with the pathogen show altered behavior, including being less aware of predators, allowing the toxoplasmosis life cycle to be renewed in a new host when the mouse is consumed.

"Our profession needs to come to grips with the accumulating body of evidence about the tremendous burden wrought on society by toxoplasmosis," McAllister says. "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."

Although McAllister is an advocate of limiting human exposure to toxoplasmosis, he says indoor cats can be kept safely by maintaining good hygiene.

"I don't want to appear anti-cat," McAllister says. "I simply want people to be aware of the severity of the disease."