While attending the 6th annual conference on Ecology and Evolution of Infectious Disease (EEID) last week, I had a chance to listen to the latest research on host-parasite interactions from some of the leading scientists both within and outside USA.

It is, therefore, quite timely to read this new finding from World Science, about a parasite changing the behavior of its host, making it a suicidal bodyguard.

But, first thing first. What is a parasite, and a host?

Host-parasite (and pathogen) interaction

A host-parasite interaction, also called parasitism, is an ecological interaction between two biological species - a host species and a parasite species - where the parasite lives (grows, feeds, shelters and reproduces) on or within the body of its host.

Depending on the species, this is either a win-nothing or a win-lose interaction, where the parasite always benefits from the host, but the host gets nothing in return, and may be even harmed, resulting in a disease.

A pathogen is a parasitic micro-organism (virus or bacteria) that usually lives within its much larger host body (such as us), and a host-pathogen interaction almost always leads to a disease in the host, which can be relatively harmless (for example, common cold) or even fatal.

Every biological species, including parasites, always tries to promote its own persistence in a generally hostile environment (Darwinian struggle for existence). For a parasite that is not free-living (cannot live without a host), this means that it must try to maximize its own transmission from one host to another.

When the transmission is between individuals of the same host species, maximizing transmission will facilitate parasite survival when, for example, the first host individual dies (of disease). And when transmission is from one host species to a new host species, maximizing transmission helps the parasite to survive by completing its life cycle, as we will see in some examples below.

Manipulating host behavior

One of the several strategies that a parasite uses to achieve high host-to-host transmission is by manipulating host behavior. Some of the fascinating examples of such manipulation, including the present case, are as follows:

• Ants parasitized by the trematode Dicrocoelium dendriticum climb up the blades of grass, to be accidentally eaten along with the grass by grazing sheep. The trematode is then transmitted from the ants (intermediate host species) to sheep (definitive host species).
• Killifish (intermediate host) infected by the trematode Euhaplorchis californiensis start swimming near the water surface, and are captured by birds (definitive host) more frequently than uninfected fish.
• Rats (intermediate host) infected with the protozoan Toxoplasma gondii tend to lose their anti-predatory instinct, and are therefore more heavily predated by cats (definitive host).
• Crickets parasitized by the nematomorph Paragordius tricuspidatus exhibit erratic behavior and jump into water (in what appears like suicide) more often than the unparasitized insects, which allow the parasite to access the aquatic environment needed for its reproduction.
• Spider infected with the parasitic wasp Hymenoepimecis sp. is induced to build a special “cocoon web” to protect the wasp pupae from heavy rain.
• The latest example: After the larva of the parasitic wasp Glyptapanteles sp. crawls out of the body of its host caterpillar (Thyrinteina leucocerae) to build a cocoon, the caterpillar becomes a devoted security guard for the cocoon. It stops eating and stays close by the cocoon, wraps it into a protective silk web, and defends it against predators with violent head swings. The caterpillar continues to do this till the wasp emerges from the cocoon, and then it dies.

In the first three cases, the intermediate host species is usually host to the parasitic egg or larva, which then must be transmitted to its definitive host species to develop into an adult, thereby completing its life cycle.

Parasites achieve such manipulative capability often by affecting the host’s central nervous system, for example by forming cysts in the brain of killifish and rat, which then behave abnormally compared to their unparasitized cousins.

Survival of the fittest

Of course, parasites cannot learn to manipulate host behavior by trial and error like we do (you can amuse yourself by trying to guess a worm’s IQ).

Instead, it is a result of evolutionary selection acting on many different interaction patterns between parasites and their hosts over millions of years, which selects the strategy that provides the best chance of the parasite’s survival in its struggle for existence.

Thus, in the Darwinian sense, a smart host manipulation strategy for maximizing transmission is a tool for survival of the fittest parasite. Others that adopted less efficient mechanisms went extinct along the way.