Clockwork intruders: Do parasites manipulate their hosts' circadian rhythms?

Most organisms have developed circadian clocks to adapt to 24-hour cycles in the environment. These clocks have become crucial for modulating and synchronizing complex behavioral and biological processes. A number of parasites seem to have evolved to take advantage of their hosts' circadian rhythms to favor their own infection and survival. Some species, such as Microphallus sp. and Trypanosoma cruzi, can alter the patterns of locomotor behavior of infected intermediate hosts, which can promote transmission to a subsequent primary host. Some fungi of the genera Ophiocordyceps and Entomophthora, as well as hairworms (Nematomorpha), elicit complex behaviors that promote their host's death at a time and place that optimizes continuation of the parasite's life-cycle. At least in some cases, a proposed mechanism might involve a change in the expression of clock-controlled genes. Lastly, some disease-causing protozoan parasites of the genera Trypanosoma, Plasmodium, and Leishmania induce changes in the circadian rhythms of their primary hosts upon infection. Some of these changes may be attributed to circadian alterations resulting from the host's inflammatory response to the infection or other unexplored responses or adaptations to the illness. Thus, a distinction must be made between manipulation of the parasite and response of the host when studying these alterations in the future. Parasitic manipulation of circadian rhythms, which vastly modulates behavior and physiology, is an essential issue that has been relatively understudied. A deeper understanding of this phenomenon could lead to the development of novel therapeutic approaches for the diseases that these parasites convey.