Impact of parasites on hosts

ALL THE knowledge and skills of the neuroscience community seem to fade into insignificance when compared with the ability of some parasites to alter the behaviour of their hosts so as to gain an evolutionary advantage. For example, a parasitic hairworm that infects a grasshopper induces the insect to seek a water source and to jump in and drown. Once the grasshopper is in the water the parasite emerges from its anal opening and swims off to seek a mate and to continue its lifecycle.

The parasitic fungus Cordyceps causes its host – an ant, for instance – to climb to a higher location, such as a twig or blade of grass and to clamp itself there. The fungus then grows out from the host’s body and distributes its spores, which are thus spread more effectively. Each species of Cordyceps is specialised to invade a particular host insect.

A species of King Crab found in British Columbia is parasitised by a species of barnacle that enters its body by boring through the crab’s shell. Once inside it grows extensively. After some time, a part of the barnacle, called the externa, protrudes between the crab’s hind legs and hangs there. This allows mating with a male barnacle.

The crab, once infected, cannot produce eggs of her own, but is induced to prepare a nest and treats the external protrusion as her own egg mass, grooming and protecting it. When the time is right, the crab emerges from her nest and disperses the barnacle larvae.

Even more amazingly, the barnacle can infect male crabs in the same manner and can somehow feminise them to perform the same functions as the female.

There are many examples of parasites inducing changes in host behaviour that facilitate replication, transmission and survival, killing the host in the process. The hosts are generally insects or crustaceans. A fascinating exception is the single-celled parasite Toxoplasma gondii, which lives in the intestines of cats. Its eggs are shed in cat faeces and can be transmitted to rodents and other animals, including humans.

As well as transmission via cat faeces, which can occur while gardening, for example, the parasite can be transmitted in the undercooked meat of other animals that have ingested it. It is estimated that 30-50 per cent of the world’s population has been infected with T gondii.

Infection in humans rarely causes disease symptoms unless the immune system is compromised, so it can be particularly dangerous in organ-transplant patients or those with Aids.

Pregnant women are advised to avoid cats, as infection with T gondii can result in serious damage to the foetus. The impact of this parasite on host behaviour is most obvious in infected rats. Rats will normally avoid cat urine, but once infected with T gondii rats will approach cat urine in a state of sexual arousal. This behaviour increases the likelihood of being eaten by a cat and the parasite continues to thrive. Infected rats appear normal in every other way and respond appropriately to the presence of signs of natural predators other than cats.

While the precise mechanism of behavioural manipulation of host organisms is poorly understood, some models have been proposed. In the case of T gondii, Patrick House and his colleagues in a paper published in 2011 noted that T gondii cysts in the rat brain accumulated in greater numbers in the limbic system.

There are parallel but distinct pathways in this area of the brain that mediate defensive and reproductive behaviours. They suggest that in infected rats the processing of the presence of cat urine is biased toward the reproductive pathway, resulting in approach rather than avoidance behaviour.

Infection with T gondii has been implicated in an increased risk of schizophrenia and obsessive compulsive disorder in which dopamine levels are elevated. It is important to note that this data is correlational and tentative. There is also data to suggest that men infected with T gondii are several times more likely to be involved in traffic accidents.

While much research remains to be done, the potential role of parasites in the modification of human behaviour constitutes an intriguing puzzle.

Paul O’Donoghue is a clinical psychologist and founder member of the Irish Skeptics Society. contact@irishskeptics.org.