Parasite manipulations can be either direct or indirect. Indirect manipulation is the most frequent method used by behaviour-altering parasites while the direct approach is far less common. Direct manipulation is when the parasite itself affects the host and induces a behavioural response, for example by creating neuroactive compounds that stimulate a response in the host’s central nervous system (CNS), a method mostly practised by parasites that reside within the CNS.
The more common approach for parasites is to indirectly induce behavioural responses by interacting with the host’s immune system to create the necessary neuroactive compounds to induce a desired behavioural response. Parasites can also indirectly affect the behaviour of their hosts by disturbing their metabolism, development or immunity. Parasitic castrators drastically modify their hosts’ metabolism and reproduction, sometimes by secreting castrating hormones, changing their behaviour and physiology to benefit the parasite.
Parasites may alter hosts’ behaviours in ways that increase their likelihood of transmission (eg by the host being in- gested by a predator), result in the parasite’s release at appropriate sites (eg by changes in the host’s preferences for habitats) or increase parasite survival or increase the host’s likelihood of being infected with more parasites.
The way in which parasites induce behavioural changes in hosts has been compared to the way a neurobiologist would affect a similar change in a lab. A scientist may stimulate a certain pathway in order to produce a specific behaviour, such as increased appetite or lowered anxiety; parasites also produce specific behavioural changes in their hosts, but rather than stimulate specific neurological pathways, they appear to target broader areas of the central nervous system. While the proximate mechanisms underlying this broad targeting have not been fully characterised, two mechanisms used by parasites to alter behaviour in vertebrate hosts have been identified: infection of the central nervous system and altered neurochemical communication.