Community ecology of infectious disease Flashcards
Parasitism and other ecological interactions
o Host organism might be a prey item for a predator
o Parasite might make infected host more susceptible to predation
o Host organism might be in competition with another host species
o Parasite might change outcome of competition between two host species
Snowshoe hares and predation
- Hares infected with nematode parasites
- 3 main predators: red-tailed hawk, coyotes, great-horned owl
Most abundant parasite of hares
- Obeliscoides cuniculi
- Lives in stomach
Parasites of hares
- 5 different nematode parasites
- 3/5 have direct life cycles
- 2/5 have indirect life cycles with hares as final host
What type of life cycle is likely to have a parasite that manipulates host behaviour?
- A parasite with an indirect life cycle.
- Wouldn’t want to manipulate behaviour of hares that are the final host because then the host would be more likely be eaten, BUT would want to manipulate behaviour of a host in indirect life cycle if it helps them get into the final host
Ivermectin treatment to kill parasitic nematodes and tracking
Had treated and untreated groups of hares
Tracked with radio collars
95% were killed by predators. Looked at predation risk between parasite reduced and parasite normal hares
- Higher survival rate (2.4x) was seen in parasite-reduced hares compared with parasite-normal hares
Parasites and predation
- Parasites increase risk of predation
- However, predation is not adaptive for parasitic nematodes! The parasites don’t want to be eaten by predators. Not good for their lifecycle
- NOTE: both parasites and predators interact to control the density of the host/prey species
Lifecycle of Euhaplorchis californiensis
- Marine birds are final host for adult trematode parasites
- Trematode eggs are released in bird droppings
- Marine snails are the intermediate host 1. They ingest eggs that develop into infective cercariae
- Snail releases infective cercariae and they infect killifish (intermediate host #2)
- Trematode parasites encyst as metacercariae in brain of killifish
- Avian predators eat infected killifish to complete the life cycle
Trematode parasites in killifish
Cause weird behaviours such as surfacing, flashing, contorting, shimmying, jerking in killifish. These flash-dancing weird behaviours resulted in the light-coloured lateral and ventral surfaces being made visible to avian predators.
Infected fish were more visible and at the surface more making them much easier targets than uninfected fish
- Parasites increases behaviours by factor of 4
Dose-dependent effect of trematode parasites in killifish
Parasites increased strange behaviours of killifish by a factor of 4. Showed a dose-dependent effects
Trematode effect on killifish brain
Metacercaria stage of bacteria targeted the brain and influenced the neurotransmitters in intermediate host to manipulate behaviour
Trematode infection intensity and its effect on predation
Killifish of unknown parasitism were put into two pens. One pen covered with a net to prevent predation. Birds were able to predate the fish in the open pens and natural selection took over. All fish euthanized and assayed for parasite cyst number
Small changes in behaviour can cause large changes in predation
- Heavily parasitized fish (>1400 cysts) were 30x more susceptible to predation than unparasitized fish
Changing in behaviour as an adaptive strategy
- In complex cycles, when changes in behaviour increase the chances of the parasite being passed on in the lifecycle
- Compared with the parasitized snowshoe hares which was not an adaptive strategy because it did not benefit the parasite to have the hare be eaten (lifecycle would end)
Rinderpest
- Introduced to Africa in 1880 by Italians. Virus spread throughout Africa and killed 90% of domestic cattle and wildlife. Hugh effect on food security.
- Devastated domestic and wild ungulate populations in Africa
- Vaccine created in 1957; eliminated from wild in 2011
Elimination of rinderpest and its effect on wildebeest
Vaccinations and elimination of rinderpest from the domestic cattle allowed for protection of wild animals and the wildebeest population to recover and grow
- Wildebeest are very important grazers. They keep grasses and shrubs under control, without them there is an increase in wildfires and destruction of young trees.
- They are keystone species and their increase caused an increase in other species such as lions and hyenas, as well as recovery of woodlands.
