Taking on Toxoplasma

It invades your body through food and water and has been found nearly everywhere warm-blooded animal populations exist. A mother can unknowingly pass it to her unborn child but may not get sick herself. One of the most common and beloved house pets in many cultures around the world is the parasite’s most prolific spreader, passing it on to adoring owners.

The life cycle of Toxoplasma, © Marcia Hartsock.

It may sound like something out of a horror movie, but Toxoplasma is very real and infects nearly one-third of the world’s human population. The parasite has been found almost everywhere, but Dr. Chunlei Su, associate professor in the Department of Microbiology, and his lab team are trying to map out and study Toxoplasma (“toxo” for short) to learn how to make it more manageable and less problematic on a global scale.

“[Toxo is] a single-cell pathogen that must infect human or animal cells,” explains Su. “Then they reside inside the cell and replicate, and in that process they cause disease. This parasite can infect mammals and birds in general. They hide in the cells, and then you cannot remove them.”

It is believed that infected people carry the parasite for life, but in most cases, people’s healthy immune systems can suppress this parasite so it won’t replicate as much and thus remains in the body only as a chronic infection.

Sometimes, though, it can cause eye lesions in healthy people. Infection acquired during pregnancy in a healthy mother may spread to the fetus and cause severe damage. In immunocompromised persons, such as those with AIDS or recipients of organ transplant, infections of toxo can cause life-threatening inflammation of the brain.

One of the most interesting aspects of this parasite is that it has no host specificity. Toxo can infect almost any mammal, including humans. According to Su, in other parasites, like malaria, a particular species of the parasite can infect only certain hosts.

Toxo’s ability to infect a wide range of hosts presents a particularly difficult challenge for those trying to study and treat it. Because the parasite can live in just about every mammal, it can be extremely prolific on nearly every continent, and because felines, including the common house cat, are the main propagators of toxo, human populations are especially affected by the disease.

Su explains that toxo probably began its spread around the world when humans moved from being hunter-gatherers constantly on the move to becoming more sedentary farmers with agriculture as their main source of food.

When early modern humans started settling in permanent housing, mice moved in as well. This led the always-resourceful humans to adopt cats to keep the mice out of their food. Unfortunately, this resourcefulness also facilitated a cycle of toxo transmission between cat and mouse. In just a few days, a single infected cat can shed hundreds of millions of parasites in their feces, which may contaminate food and water and cause infection in animals—mice, pigs, goats, sheep, cattle, chickens, and humans—if the parasite is ingested. So as agriculture-based human society grew, cat and mouse—and therefore toxo—populations expanded with it.

Su and his lab are trying to track the spread of toxo throughout the world by genotyping the different strains found on different continents and inside various mammals. He hopes eventually to put together a world map showing the different strains of toxo around the globe. This new research has already yielded some very compelling results.

“So now we see a very interesting pattern at the global level,” says Su. “For example, when you look at Europe, Africa, Asia, and North America, there is one particular genotype, type II, which is dominant. You see it eighty percent of the time. The question is why do you see this particular strain? Why is it dominant at the global level?”

One reason Su thinks this strain might be so prolific is its age. He hypothesizes that this type of toxo was around for a longer time than other strains, so it had more time to spread to several continents of the world via cats.  Another reason Su thinks type II might be dominant is that it may have a biological advantage over other strains of the organism, so that it can spread more efficiently. Part of Su’s work is to understand whether age or biological advantage is the major factor contributing to the success of type II toxo.

“On the other hand,” says Su, “when you look at toxo strains from South America, they’re very different. You don’t see type II strains there very often.”

One possible explanation for this variation is the effect of the many species of felines in the jungles of South America. The only time toxo can go through sexual recombination is when it is inside the gut of a feline host. So if more feline hosts are eating a wide range of prey animals infected with various types of toxo, then toxo naturally becomes more diverse through sexual recombination.

“A different ecosystem may facilitate a different way of transmission and generate a different population structure,” says Su. “By studying this, we try to understand different transmission patterns and eventually have some idea of how we should control the spread of this parasite.”

The ultimate goal of Su’s research is to find a way to manage, and even manipulate, toxo to keep it from infecting such a large proportion of the world’s creatures. Through understanding the basic biology of the parasite, they might find a way to turn off its particularly virulent aspects and perhaps develop a way to cure the disease entirely.

“If you reduce the possibility of toxo’s infecting livestock, then you can reduce the possibility of its infecting humans. Understanding the mechanism of virulence will help us identify a target to treat the disease,” explains Su. “It’s a long way to get there, but for now we need to understand the basic biology so that toxo can be better controlled.”

—Miriam Kramer