Study breaks species barrier on widespread parasite

Researchers harvest Toxoplasma oocysts from mice
Published on September 25, 2019

About one-quarter of the world's population has been infected with Toxoplasma gondii, including 40-50 million people in the U.S., according to the U.S. Department of Agriculture.

A small portion of those infected become ill, yet the parasite is so ubiquitous that toxoplasmosis kills more people in the U.S. than any other foodborne disease. One of the protozoan parasite's life phases occurs only in cats, so research on the organism's reproduction has required infecting cats to harvest T gondii oocysts.

In a scientific article published this summer, a research team from the University of Wisconsin and USDA Agricultural Research Service report that they identified why cats are the hosts of T gondii and used that knowledge to induce T gondii oocyte production in mice. The findings could make T gondii studies easier for more laboratories, aid work on vaccines that could protect people and animals from infection, and help investigators understand how other parasites reproduce.

The article, "Intestinal delta-6-desaturase activity determines host range for Toxoplasma sexual reproduction," was published Aug. 20 by PLOS Biology..

How cats become carriers

Cats become infected with T gondii by eating infected animals, mostly birds and rodents, or through exposure to oocysts shed by other cats. People and other animals are infected through oocysts in contaminated soil, food, water, and litter boxes or, often, through tissue cysts in undercooked meat.

Laura Knoll, PhD, who led the research at Wisconsin's School of Medicine and Public Health, said T gondii needs an environment high in linoleic acid as a signal to start sexual reproduction. Felids are the only mammals without an enzyme in their intestines called delta-6-desaturase, which other animals use in the first step of converting linoleic acid to arachidonic acid. Arachidonic acid, in turn, is important in production of immune system mediators known as eicosanoids.

Cats evolved in desert environments with scarce calories, Dr. Knoll said. They still express delta-6-desaturase in their livers and brains, but their immune systems adapted to work without it in their intestines, Dr. Knoll said.

"What we think is that they evolved that way so that they wouldn't waste the lipid calories on signaling molecules so all of their lipid calories would go into energy production," she said.

Dr. Knoll's research team infected mice with T gondii, fed them diets with added linoleic acid, and tube fed an anti-inflammatory agent called SC26196, which binds to the delta-6-desaturase enzyme, halting metabolism of linoleic acid. The mice started shedding oocytes in their feces five days later, with shedding peaking eight to nine days after infection.

Dr. Knoll said it's unclear why T gondii needs linoleic acid to start sexual reproduction, but she thinks the parasite may be using linoleic acid to create its own signaling molecules.

White laboratory mice
Cats have been the only known hosts for the sexual reproduction phase of Toxoplasma gondii. A research team recently identified why and used this knowledge to induce T gondii oocyte production in mice.

'Costly and burdensome'

This spring, ARS officials halted federal T gondii research that used cats, announcing in April that other researchers would need to continue the work. The announcement stated that toxoplasmosis remains the most common fatal foodborne illness in the U.S. and USDA researchers had helped reduce T gondii prevalence by as much as half.

But the announcement said the ARS had achieved its objectives, and further research regarding human health was outside the USDA's mission.

Of the small portion of people infected with T gondii who develop toxoplasmosis, most of them develop mild flu-like symptoms that last weeks to months, according to the World Health Organization. But infection is particularly dangerous for pregnant women because the organism can be transmitted to the fetus, with potentially lethal consequences. In addition, in people who have been infected, the organism can reactivate and cause severe neurologic damage if the person becomes immunocompromised.

Researchers from the Universite Montpellier in France found in 2012 that toxoplasmosis was linked epidemiologically with schizophrenia, and the disease has been associated with an increased risk of suicide.

Dr. Knoll said the methods she used are costly and burdensome, and the mice fared poorly. But, she is working on creating a line of mice lacking delta-6-desaturase production in their intestines that could potentially be used to replace cats in future studies. She hopes such mice will be available by early 2020.

"The inhibitor is incredibly expensive," she said. "It's about $400 per mouse per experiment." In addition, the protocol requires tube feeding every 12 hours, which would be difficult during long-term studies.

The mice that received the inhibitor and were infected with T gondii also became so ill that the researchers typically euthanized them within a week after starting the protocol. They reduced illness in some mice by stopping administration of the inhibitor at day five.

Dr. Knoll thinks understanding the conditions needed for T gondii reproduction could also help in creating a vaccine strain that is unable to go through the sexual reproduction cycle. Cats tend to build good immune responses after their first exposure to Toxoplasma organisms, which usually is the only time they would shed many oocysts, she said.

"I think that means you could dramatically reduce shedding by having a strain of toxo that was genetically modified such that it couldn't undergo the sexual development, and then you could give that to cats before women get pregnant, and then they wouldn't have to worry about their cats giving them toxo," Dr. Knoll said.

Implications of the research

Michael S. Behnke, PhD, who studies developmental regulation and host-parasite interactions of T gondii at Louisiana State University, said Dr. Knoll's findings not only advanced research on Toxoplasma but also may aid studies on how other parasites—such as Cryptosporidium, Eimeria, and Cystoisospora—sense that they are in the right environments for growth or reproduction.

Research on these parasites that uses cats is impossible for most laboratories because of the cost and housing needs, as well as the ethical concerns the researchers would need to address, Dr. Behnke said. Being able to study the full life cycle of T gondii in mice would make that work more accessible, especially if Dr. Knoll's team is able to develop mice modified to lack the delta-6-desaturase enzyme in their intestines.

Dr. Knoll was one of three authors of the paper, along with Jitender P. Dubey, PhD, who is a microbiologist for the ARS, and Bruno Martorelli Di Genova, PhD, who is a postdoctoral researcher at UW. Dr. Dubey is one of the authors of a 1970 article in Science that initially described T gondii's sexual reproduction in cats.

Related JAVMA content:

USDA ends toxoplasmosis research that used cats (June 15, 2019)

Study to assess veterinarians' exposure to toxoplasmosis (May 1, 2015)