Curiosity Kills the Rat

Rat hunting, 1850


One of the many diseases harboured by rats is toxoplasmosis.

Toxoplasmosis is a major livestock disease and is a serious problem for humans. In the U.S., it is said that toxoplasmosis causes more congenital abnormalities than rubella, syphilis and herpes combined.

The Toxoplasma gondii parasite (a protozoan) passes from rats to cats to humans. It has long been known that toxoplasmosis changes rat behavior and makes them more susceptible to predation by domestic cats, which are the parasite's definitive host.

Dr. David Macdonald's team from the Wildlife Conservation Research Unit at Oxford looked into how toxoplasmosis effects rat behaviour. They found that compared with healthy rats, infected rats were more active (and hence more prone to cat predation), more curious (even approaching humans), and more likely to overcome their innate fear of cat odour. Some infected rats even preferred areas scented with predator odors!

Obviously, a disease that makes rats lose fear of predators must have some adaptive benefit for the microrganism. And it does. Since infected rats are more likely to be killed by cats, the Toxoplasma cunningly helps move itself on to its final cat host.

Ironically, curiosity kills the rat, not the cat.

In most other respects, rats infected by Toxoplasma behave normally. They are not totally deranged and their social status within their warrens is unchanged. However, infected rats are less phobic of novel foods and are also more easily trapped. So, in addition to tempting fate with respect to predation, these unfortunate rats are also more susceptible to control measures.

Toxoplasmosis is very rarely carried by dogs. It is theoretically possible for a dog to get toxoplasmosis, but this is so rare (only occuring in dogs with seriously compromised immune systems that are already fighting off some other type of disease) that dogs are not considered a vector.
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