Parasitic aphrodisiacs: manipulation of the hosts' behavioral defenses by sexually transmitted parasites.

Animals have a number of behavioral defenses against infection. For example, they typically avoid sick conspecifics, especially during mating. Most animals also alter their behavior after infection and thereby promote recovery (i.e., sickness behavior). For example, sick animals typically reduce the performance of energetically demanding behaviors, such as sexual behavior. Finally, some animals can increase their reproductive output when they face a life-threatening immune challenge (i.e., terminal reproductive investment). All of these behavioral responses probably rely on immune/neural communication signals for their initiation. Unfortunately, this communication channel is prone to manipulation by parasites. In the case of sexually transmitted infections (STIs), these parasites/pathogens must subvert some of these behavioral defenses for successful transmission. There is evidence that STIs suppress systemic signals of immune activation (e.g., pro-inflammatory cytokines). This manipulation is probably important for the suppression of sickness behavior and other behavioral defenses, as well as for the prevention of attack by the host's immune system. For example, the cricket, Gryllus texensis, is infected with an STI, the iridovirus IIV-6/CrIV. The virus attacks the immune system, which suffers a dramatic decline in its ability to make proteins important for immune function. This attack also hampers the ability of the immune system to activate sickness behavior. Infected crickets cannot express sickness behavior, even when challenged with heat-killed bacteria. Understanding how STIs suppress sickness behavior in humans and other animals will significantly advance the field of psychoneuroimmunology and could also provide practical benefits.