Brain parasite makes rodents unafraid of cats, other predators

By Angela Betsaida B. Laguipo, BSNJan 15 2020

Cats have the power to control the minds of rodents, through a brain parasite called Toxoplasma gondii. The pathogen exerts a strange mind control on rodents that make them lose their fear of cats and become easier to catch.

This attraction theory is flawed since instead of manifesting loss of fear for cats, rodents become less scared and anxious around a broad range of threats, suggests a new study published in the journal Cell Reports. The parasite then becomes a courage booster for small rodents, because they somehow don’t exhibit fear overall.

Toxoplasma gondii. Protozoan which is transmitted from cats and other animals and causes toxoplasmosis especially dangerous for pregnant women. 3D illustration Credit: Kateryna Kon / Shutterstock

When the parasite infects a rodent’s brain, instead of being repelled by car odors or hiding, the mice are more likely to face the cat or feline. The parasite decreases anxiety, making the rodent more courageous and hence, increases the chances of being eaten.

“For 20 years, T. gondii has served as a textbook example for a parasitic adaptive manipulation, mainly because of the specificity of this manipulation. We now show that the behavioral alteration does not only affect fear of feline predators but that major changes occur in the brain of infected mice, affecting various behaviors and neural function in general,” Ivan Rodriguez of the University of Geneva, and co-author of the study, said.

Rodent courage booster

In the study, the team found that T. gondii infection does not selectively reduce the fear of cats alone, but according to a series of behavioral tests, the rodents infected with the parasite for five to 10 weeks, spent more time in the open arms of a maze compared to uninfected mice. This type of scenario is threatening for regular rodents since they’re exposed to various environmental hazards.

Further, the infected mice did not manifest the normal preference for interacting with a nonaggressive male mouse, than with objects like an apple or a metallic cube. Uninfected mice, on the other hand, exhibited defensive, avoidant, and anxiety-related behavioral responses to a potential threat, such as the hand of the scientist. Infected mice, however, even touched the hand of the experimenter, which is not normal for their nature.

“Collectively, these findings demonstrate that animals chronically infected by T. gondii display reduced anxiety levels and risk aversion, as well as increased curiosity and exploration when put in challenging situations. Furthermore, infected mice do not discriminate between live and inert stimuli. Taken together, these observations suggest an altered ability to adequately process signals that may represent potential threats,” the researchers wrote on the paper.

Brain scan of mice

To study the theory further, the scientists performed brain scans on mice about 10 to 12 weeks after being infected with the parasite. They used light-sheet microscopy to perform accurate mapping of the brain-wide distribution, size, and number of cysts in the brain of mice.

They found that the cyst density was markedly high in the cerebral cortex of the brain, which is involved in visual information processing. But the researchers also found that the cysts were widespread across the brain, hinting a random infection process.

What is toxoplasma gondii?

Toxoplasma gondii is a parasite commonly found in cat feces. One the cat is infected; the parasite begins to reproduce in their intestines before going out in the stool. It is widespread across the globe, with an estimated 40 million people in the United States being infected.

In humans, there are very few symptoms since the immune system keeps the parasite at bay. However, in pregnant women and those with compromised immune systems, the parasite can cause serious health problems. Toxoplasma gondii can pose a risk to an unborn baby, increasing the chance of the baby having hydrocephalus, intracranial calcifications, blindness, epilepsy, microcephaly, and other mental disabilities.