Mind-altering parasites

Recent coverage in the national media about a man found to have a 1cm-long worm burrowing through his brain highlights the adaptability and tenacity of parasites.

The man, who lives in East Anglia but is of Chinese descent, is thought to have picked up the Spirometra erinaceieuropaei infection on a visit to China, possibly via infected meat or water. This is one of only three reported cases of human infections with this tapeworm in Europe since 1953.

Rather than living on its victim’s brain tissue, the parasite is thought to have simply absorbed nutrients from his brain and remained as a larva. The worm is normally found in amphibians and crustaceans, infecting the guts of cats and dogs later in its lifecycle when it can grow to 1.5m long. The patient experienced headaches, seizures, memory flashbacks and strange smells, and still suffers from the after-effects of infection even though surgeons have removed the worm.

A far more common parasitic infection, which affects 30-60 per cent of people worldwide, has no effect on human health in its latent form but has been shown to alter personality and reduce psychomotor performance. The single-celled Toxoplasma gondii usually infects humans via consumption of undercooked meat or food contaminated by cat faeces containing its oocysts. These cysts can remain in the human brain and muscles for life.

Psychological testing has shown that men infected with toxoplasma have lower novelty-seeking scores, are less impulsive, extravagant and disorderly, as well as having lower IQ scores. But the effects on women, however are quite different. Infected females are more outgoing, happy-go-lucky and confident. Although the mechanism behind these changes is unknown, researchers have suggested they are due to increased dopamine levels.

The significance of these behavioural changes is unclear, but in other animals and with other parasitic infections these types of changes often serve to increase the spread of the parasite. Rats infected with Toxoplasma gondii (which can only reproduce in cats’ intestines), for example, are attracted to the smell of cat urine, while normally they are repelled by it. And humans infected with the flu virus have been shown to initially become more sociable, thereby enabling spread of the disease.

Another fine example of neuroparasitology is the Costa Rican parasitoid wasp, which lays its single egg on the abdomen of the orb spider. The larva injects a chemical into the spider, causing it to stop spinning its normal web and instead create a structure to support the wasp’s cocoon. The spider waits in the centre of the new structure for the larva to kill and eat it before building the cocoon.