Examples
of parasites altering host behavior
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Phylum: Acanthocephala Strategy: Alter behavior of intermediate host to make
it more susceptible to predation. Life cycle: ·
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 [1977], 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. |
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Sacculina sp. Phylum
Arthropoda Strategy: Parasitic castration, leading to behaviors
of infected hosts to care for brood pouches as if they were their own. Definitive
hosts are crabs Larval
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 |
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Hymenolepis
diminuta Phylum
Platyhelminthes Strategies: 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. |
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