toxoplasmosis-mouse-cat

A mouse infected with the Toxoplasma parasite loses its fear of cats, increasing the chance that the parasite will get into the cat's intestinal system, the only place it can sexually reproduce. (Wendy Ingram/UC Berkeley)

A parasite that, for its own reproduction, makes mice lose their fear of cats can continue to affect mouse behaviour long after its departure, a new study has found.

The surprising discovery by University of California, Berkeley, molecular biology researcher Wendy Ingram, published this week in the journal PLoS  ONE, suggests that symptoms of a disease can sometimes persist long after the agent that caused it is gone.

Toxoplasmosis is single-celled parasite that can infect many birds and mammals, but can only reproduce inside the gut of a cat. That means that, after living inside other hosts, it must eventually find its way back into a cat.

In order to achieve that, with its own limited mobility, it has evolved an interesting mind-control technique — it causes mice to lose their natural fear of cats.

"When rodents get infected, the best way back to the primary host, the cat, is by making these mice a little foolhardy and wandering into cat territory," Ingram told CBC’s Quirks & Quarks in an interview that airs Saturday.

"As soon as a mouse stumbles into cat territory, it’s pretty much at the end of the road."

For the toxoplasmosis, however, that means a good chance of reaching its desired destination — the inside of the cat’s digestive system.

Ingram and her colleagues were interested in figuring out just how the parasite manages to make mice lose their fear of cats.

In order to do that, they used genetic manipulation to produce a weakened strain of toxoplasmosis that the mouse immune system could easily clear.

To their surprise, the mice remained unafraid of cats even after the toxoplasmosis infection was long gone.

"This suggests that the parasite is actually going into the brain and causing a permanent change in the behavioural circuits in the mouse brain," Ingram said.

Mechanism unknown

She and her colleagues are still not quite sure what the change is or how it is made. However,  there is some evidence that the toxoplasmosis parasite injects proteins or other molecules into large numbers of brain cells that it doesn’t infect.

Ingram said the research could present a new way to look at symptoms and infections, which aren’t always explained by modern medicine.

"Not only we look at what you currently have, but what you have had in the past."