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Fatal feline attraction

Joanne Webster of Imperial College, London, has been studying Toxoplasma for years. Like Plasmodium, which cycles between mosquitoes and man, Toxoplasma cycles between its rodent and feline hosts, living out different phases of its existence in each. In cats, it resides in the wall of the small intestine and passes out of the host in its faeces. These are then picked up by rats and mice (and also by other mammal species, including humans), where they form cysts in brain, liver and muscle tissue. Eventually, if the parasites are lucky, their rodent host is eaten by a cat and the whole cycle starts again.

Unlike Plasmodium, however, which can rely on the natural behaviour of mosquitoes to spread it around, Toxoplasma's rodent hosts have a strong aversion to helping it into its next home. Which is where, in Dr Webster's elegant phrase, fatal feline attraction comes in. Rats and mice infected with Toxoplasma start wandering around and drawing attention to themselves—in other words, behaving in ways that will bring them to the attention of cats. They are even, Dr Webster's work suggests, attracted to the smell of cats.

How these behavioural changes come about was, until recently, obscure. But in 2009 Glenn McConkey of the University of Leeds, in England, analysed Toxoplasma's DNA. When he compared the results with those of other species, he discovered that two of the bug's genes encode enzymes involved in the production of a molecule called dopamine. This molecule acts, in animals that have nervous systems, as a chemical messenger between nerve cells. It does not, however, have any known function in single-celled critters. Moreover, dopamine is particularly implicated in schizophrenia. Haloperidol, an antipsychotic drug, works by blocking dopamine receptors.

Intriguingly, Dr Webster has found that haloperidol serves to reverse fatal feline attraction in rats. This suggests the parasite is indeed interfering with the brain's dopamine system—and thus that it might be doing the same thing in people. Dr McConkey is now making a version of Toxoplasma with the dopamine genes excised, to see if rats infected with this modified bug are protected from the fatal attraction.