Examples of parasites altering host behavior
Strategy: Alter behavior of intermediate host to make it more susceptible to predation.
· Adults reside in ducks
Anterior region of adult Polymorphus sp., from
· Invertebrate stages reside in Gammarus lacustris, about 5-10 mm in length, a small crustacean that feeds in ponds and rivers.
· Uninfected gammarids are negatively phototactic and positively geotactic, but infected ones are positively phototactic and negatively geotactic, and engage in a “skimming” behavior at the surface of the water. Furthermore, they will find a rock or plant on the surface, and latch on. Hence, they become easy prey to ducks, which are the definitive host.
The evidence for selective predation of infected individuals?:
Comparative vulnerability of uninfected Gammarus lacustris and ones infected with Polymorphus paradoxus. (Adapted from Table 1 of Bethel and Holmes , Increased vulnerability of amphipods to predation owing to altered behavior induced by larval acanthocephalans. Canadian Journal of Zoology 55(1):110-115.
*P = probability, by chi-square contingency tests
· What is the mechanism? It is thought that when Gammarus lacustris is infected with a thorny-headed worm, the parasite causes serotonin to be massively expressed. Serotonin is a neurotransmitter involved in emotions and mood. Researchers have found that during mating Gammarus lacustris expresses high levels of serotonin. Also during mating, the male Gammarus lacustris clamps down on the female and holds on for days. Researchers have additionally found that blocking serotonin releases clamping. Another experiment found that serotonin also reduces the photophobic behavior in Gammarus lacustris. Thus, it is thought that the thorny-headed worm physiologically changes the behavior of the Gammarus lacustris in order to enter its final host, the bird.
Strategy: Parasitic castration, leading to behaviors of infected hosts to care for brood pouches as if they were their own.
Definitive hosts are crabs
cypris form, which looks equivalent to other barnacle larvae, attaches onto
host. The following 3 images are from:
· Female injects kentrogen larva into body of suitable host first.
· Two males inject their kentrogen larvae into suitable pore of female
· Female and accompanying females grow tumorlike, especially in gonads of host crab, castrating it.
Suitable hosts are found in waters near Hong Kong and Japan.
Males acquire physical characteristics of females, and both infected males and females “fan” and care for the egg pouch as if it were an authentic brood pouch.
Sacculina brood pouch
1. Alter “fragrance” of food source for intermediate hosts, making it more likely for intermediate hosts to feed on food sources infected with ova.
2. Parasitic castration of the intermediate host, to provide more nutrition for the parasite, rather than allocating it to gamete production.
Adults normally reside in rodents, especially rats (about 18-30 cm in length)
Ova are passed in the feces
Cysticercoid larvae are found in beetles, which have ingested ova in feces from infected rat.
· “Beetles are lured to egg-laden droppings by an aroma that’s apparently irresistible to the insects.”
o Source of “fragrance” could be the eggs themselves, something produced by the adult worms, or perhaps something secreted by the infected hosts.
o Difference in droppings increases likelihood that beetles will feed on sources of tapeworm larvae.