Brain washing parasites

Some parasites have developed very complicated life cycles, moving between several different organisms in order to complete various stages of development. However, a quintessential problem faces these parasites: How do you ensure movement from one host to the next in order to carry on your life cycle? Various mechanisms have been established for this but some of the most ingenious are from those parasites which affect the host’s behavior. There are a number of parasites which use this technique however, one of the ones I find most interesting is Toxoplasma gondii, the protozoan which is responsible for the disease Toxoplasmosis. This is a very common infection in humans and is usually very mild. In individuals with a compromised immune system or in fetuses and newborn babies, the infection can cause blindness and mental retardation. Yet, no behavioral differences have been observed in humans (Stibbs, 1985). However, this is far from true in infected rodents.

Toxoplasma gondii is capable of infecting all mammals. However, its definitive host is the cat. This is the host in which reproduction occurs. In other mammals infected, the Toxoplasma gondii merely produces cysts which can infect other mammals which come into contact with the infected individual. In order for the Toxoplasma gondii to successfully complete its life cycle, it must make its way into a cat. This can occur if a cat comes in contact with infected medium or eats an infected organism. This is where the brainwashing comes into play. Mice have an innate aversion to odors of cats, suggesting high predation risk in the area. This is a problem for the Toxoplasma gondii if it has infected a mouse and the mouse is avoiding coming into contact with the definitive host. So how does the parasite ensure its arrival in the definitive host? It alters the behavior of the mouse of course! Mice infected with Toxoplasma gondii have shown an increase in activity and a decrease in novelty seeking behavior. With these two behavioral changes acting in conjunction with one another, the mice do not avoid cats as they are normally inclined to, but rather, they place themselves in the way of their predators (Berdoy, Webster & Macdonald, 2000). This is exactly what the parasite wants because it is then transmitted into the environment where it can reproduce and lay eggs.

The question is, how does Toxoplasma gondii accomplish this incredible feat? Stibbs (1985) conducted a study on the chemical composition in infected mice. In his study, he found that mice with acute infections had an increase of homovanillic acid levels and a decrease in norepinephrine levels but showed no changes in dopamine levels. In contrast, chronically infected mice had an increased dopamine level. Berdoy, Webster & Maconald (2000) suggest that the suppression of the predator-induced response may be triggered by serotonin antagonists. However, Stibbs found no alteration in serotonin concentration in his study (1985). What type of mechanism may be responsible for the alterations in behavior caused by Toxoplama gondii in mice?

References

Berdoy, M. , Webster, J.P. & Maconald, D.W. (2000). Fatal attraction in rats infected with Toxoplasma gondii. The Royal Society: 267, 1591-1594. Doi: 101098/rspb.2000.1182.

Stibbs, H.H. (1985). Changes in brain concentration of catecholamines and indoleamines in Toxoplasma gondii infected mice. Annals of Tropical Medicine and Parasitology. 79(2): 153-157