Daredevil Zombie Mice!

Ramisa Maliha

Freshman

School of Life Sciences

Independent University, Bangladesh

August 25th, 2017

Cats and mice are born enemies. This is widely known through numerous pop culture references and watching shows like “Tom and Jerry”. It shouldn’t surprise us, then, that mice have a natural instinct to run away at the very smell of their furry predators. But what if mice brains suddenly lost this inbuilt fear of cats? It can happen, and the consequences are as dire as you would think. How, then, do mice brains suddenly undergo reprogramming that makes them forget to run from predators? The answer to that question is directly tied to a small parasite called Toxoplasma gondii.

Toxoplasma gondii is a parasite that infects up to fifty percent of people around the world, and can cause Toxoplasmosis. In adults, infection is generally asymptomatic, but there can sometimes be minor symptoms such as muscle pain and swollen lymph nodes near the neck and chin. Neurological symptoms such as seizures and loss of motor coordination can occur in immunocompromised individuals. If pregnant women are infected by the parasite, it may result in fetal death or abortion, or the infant may be born with neurological deficits. The parasite spreads through ingestion of improperly cooked food that contains cysts (a stage in the lifecycle of the parasite), contact with cat faeces, and from mother to child during pregnancy. Toxoplasmosis is generally diagnosed by detection of antibodies (specific defensive molecules produced by our bodies in response to infection) against the parasite in blood, or parasite DNA in the amniotic fluid (in pregnant women). The Toxoplasma lifecycle is composed of a sexual stage, which is only possible inside feline (of the cat family) hosts, and an asexual stage which can take place inside virtually all warm-blooded mammals.

A literal daredevil mouse. Unrelated to Toxoplasma other than its fearlessness in the face of death. Daily Mail

Since the parasite absolutely needs cats (or other feline species) to complete its lifecycle, what could be a better strategy than “mind-controlling” mice (and possibly other rodent hosts) to get into the tummies of cats? So the parasite infects the mice's brains, and alters behavior, possibly through making changes that increase the production of a neurotransmitter called dopamine. Infection with the parasite basically makes a mouse lose its hard-wired terror of cat odors, turning it into a kind of partial zombie owing to this loss of agency.  Some mice even become mildly attracted to cat odors! The parasite seems to have evolved behavior modification as an adaptation to complete its life cycle. Toxoplasma has also been linked to mental illness in humans, suggesting that its ability to affect behavior is not restricted to rodents (although we don't it to get us eaten by cats).

What is even more amazing is that studies have shown that the loss of fear in mice may be permanent. It can remain long after the parasite has been completely cleared by the mice, suggesting that the parasite is able to make permanent changes in the rodents' brains.

Amidst of all this, I wonder how puzzled cats are when they see their little preys walking up to them casually. It must be like pizza delivery for cats! The cats get a meal and the Toxoplasma get to complete their lifecycle, only for the little rodents to pay the penalty.

Ramisa is a first-year Microbiology student. She writes:

Biology has been the most intriguing subject to me for as long as I can remember. After being introduced to Microbiology, I found my passion in learning all about microscopic life that we interact with every day. I hope to indulge my passion by getting into the field of research and contribute to knowledge about microbes.