While analyzing interactions between parasites and hosts, a substantial amount of research has been devoted to studying the methods parasitic organisms use to control host behavior. In "Invisible Designers: Brain Evolution Through the Lens of Parasite Manipulation," published in the September 2019 issue of The Quarterly Review of Biology, Marco Del Giudice explores an overlooked aspect of this relationship by systematically discussing the ways in which parasitic behavior manipulation may encourage the evolution of mechanisms in the host's nervous and endocrine systems. Examining this evolutionary history, Del Giudice investigates the hypothetical methods hosts may have adopted to counteract attempts at behavioral hijacking.
Parasites, such as viruses, insects, helminths, and bacteria, seek to manipulate host behavior for numerous reasons. Parasitic organisms may induce behavioral changes in order to increase their chances of transmission from one host to another. In a similar vein, parasites may disrupt a host's normal neural functioning to prompt the organism to travel to an environment that is more hospitable for the parasite or more conducive for reproduction. Host bodies are sometimes co-opted and utilized as safe environments for the development of the parasite's offspring.
The means by which parasites attempt to alter host behavior also vary. Parasites may use an immunological approach by disrupting responses in an organism's immune system. A more direct option may be to employ neuropharmacological manipulation by secreting substances that interfere with the host's neurotransmitters. Parasites may also take the genomic/proteomic route by changing gene expression.
Expanding beyond the motivations and tactics of parasites, Del Giudice posits that attacks from these biochemical mechanisms place significant pressure on the nervous system to adapt and develop countermeasures. Drawing upon previous literature and real-world examples, the article proposes four categories of potential host countermeasures. Analyzing this taxonomy, Del Giudice argues that when encountering manipulation, hosts may prevent parasites from bypassing the brain's protective barrier, force parasites to work harder and release greater amounts of neuroactive substances, make signals more complex, or strengthen the brain's ability to endure disturbances.
Elaborating further on these countermeasures, the author considers the potential evolutionary constraints and the associated "robustness-fragility tradeoffs." Although countermeasures may increase complexity and deter parasites, new adaptations may simultaneously weaken another section of the system and provide alternative targets for parasites.
Taking into account the proposed effectiveness of the mechanisms discussed, the article suggests studying host-parasite interactions could aid in furthering neuroscience and psychopharmacology research. By studying adaptations to parasitic attacks, neuroscientists could gain additional perspective into how continuous evolutionary battles produce inefficiency or how once-critical mechanisms over time serve unrelated functions. In psychopharmacology, the biochemical methods parasites use mimic those of pharmacological drugs. Researchers could analyze the ways brain structures reject pharmacological interventions by parasites and, in turn, use that information to make psychoactive drugs more effective for patients.
"The unrelenting pressure exerted by parasites must have shaped the evolution of nervous and endocrine systems at all levels, with important consequences even for animals that are not (or no longer) manipulation targets. If this is true, many aspects of neurobiology are destined to remain mysterious or poorly understood until parasites--the brain's invisible designers--are finally included in the picture," Del Giudice writes.