Recently, I’ve been reading about sickness behaviours – the behavioural changes we often experience when we’re ill (e.g., see my previous snippet on sickness-induced sleep). These are generally thought to be host-adaptive, meaning that they help us get better. However, there are some examples of infection-induced behavioural changes that clearly benefit the invading pathogen rather than the host. The most famous example is rabies: infected animals become more aggressive and therefore more likely to spread the virus through bites or scratches. Another example is Toxoplasma gondii, a protozoan parasite that turns rodents’ innate fear of cats into a “fatal feline attraction” – the rodents are more likely to be caught by their feline predators, which suits the parasite just fine, given that cats are the only known hosts in which T. gondii can sexually reproduce.
How does a parasite turn fear into attraction? There have been numerous hints of a role (at least in part) for dopamine, the brain’s “reward chemical”. T. gondii itself may be a source of this dopamine, because it encodes tyrosine hydroxylases similar to those required to make dopamine in mammals. However, the link has remained controversial.
In a recent Nature Communications paper, scientists addressed the controversy by comparing rats infected with wild-type T. gondii to those infected with T. gondii engineered to express varying levels of tyrosine hydroxylase. Using high-welfare, specific behavioural assays, the authors found that, indeed, rat behaviour correlates with tyrosine hydroxylase expression.
The effects in rats are interesting enough, but the implications may even extend to humans. T. gondii is one of the most common parasites, with around a quarter of the global population thought to be chronically infected. Infection in humans, as in rats, generally doesn’t cause illness, but it has been linked to mental disorders associated with dopamine imbalances, particularly schizophrenia. Might parasite-produced dopamine be affecting human behaviour too?
The role of parasite-produced dopamine in Toxoplasma gondii-altered host behaviour
In Nature Communications, December 2025
From the group of Joanne Webster, Royal Veterinary College, University of London, UK
Snippet by Katrina Woolcock
Image credit: Figure 1 from Calvo-Urbano et al cited above (CC BY 4.0).