VARIABLE EPIZOOTICS AND HOST MORTALITY

Rodrigo Ortega

Background

Effects of parasite infection in host populations are more evident in periods when:

host reproductive success is reduced directly by infection (increased mortality); indirectly by debilitation, diminished performance or

behavioural changes (from manipulation)

times when infections result in the death of larger host fish

Background Reduced survivorship and reproduction can lead to large

fluctuations in host population size, and thus, in host population parameters

Research QuestionsHow do these fluctuations affect parasite population dynamics in 2 different epizootic events?

What is the role of host life history traits in re-establishing equilibrium after epizootics?

The parasite-host system

Life cycle of Schistocephalus solidus• Hermaphrodite cestode that reproduces in the intestine of

fish-eating waterfowl. Eggs pass into the water with faeces.

• Coracidium is ingested by the 1st intermediate host (copepod), where it develops at its 2nd larval stage in the hemocoel.

• The 2nd intermediate host (juvenile stickleback) becomes infected upon ingestion of an infected copepod within one to two weeks. Here, the plerocercoid grows in the body cavity of the fish and reaches infectivity to the final host after one to three months

Life History traits of Gasterosteus aculeatus

• Infection occurs during 1st summer following spring hatching

• Small, young sticklebacks feed on copepods.

• Sexual maturity achieved at 2 years of age (if uninfected)

Epizootics• An epizootic ended between 1996 and 1998 and another occurred between 1998 and 2003

• First epizootic ended with a dramatic reduction in host population size not seen in the second epizootic.

• Suggests that parasitic infection strongly influences host population dynamics over time.

The site of study

Walby Lake, Alaska

• Usually covered with ice from October to May

• Average air temperature in July: 14–15 °C • Average warmest summer temperatures : 18–20 °C.

Design

Annual collections during host reproductive season

Presence of plerocercoids?

Removal + weight

Hs population size

Ps prevalence, intensity

Parasite index

PI=P/H

Catch-per-unit-effort (CPUE) (# adult fish /

trap/ hour)

Relative population size of threespine stickleback

1997:• 2 replicate catches• Mortality of larger

fish• Juveniles survived

and reproduced in 1998

Prevalence and Mean Intensity of S.solidus Infection

Parasite:Host biomass ratio

Interpretation• Unstable epizootics

Possible causes of the first epizootic: ‘perfect storm’

• Mortality of larger host fish from the stress of infection• Parasitic impact on host reproduction• Environmental stresses• Metabolic drain that lead to death by starvation during the

winter. • Variation of winter survival between years

Mechanisms

• Behavioural modifications that lead to increased buoyancy, and decreased swimming speed

• Decreased responsiveness to predator attack

• De-melanization of the fish host increases conspicuousness increases predation (also from predators that are not definitive hosts)

Conclusion• This study provides evidence about the dependence of parasites in host demographics

• Sometimes epizootics depend on a set of unique circumstances

• Possible support for compensatory mortality model

References• Heins, D.C., Birden, E.L., Baker, J.A. 2010. Host mortality

and variability in epizootics of Schistocephalus solidus infecting the threespine stickleback, Gasterosteus aculeatus. Parasitology. 137: 1681–1686

• http://evolution.unibas.ch/scharer/research/former_research/schistocephalus/schisto_overview.html

• http://www.fishbase.org/photos/PicturesSummary.php?need2save=&tosave=&TRPP=999999&id=2420&what=species&personnel=&user_session=&lme=&StartRow=1&TotRec=15&show_all=on&SortBy=iucn

• http://www3.imperial.ac.uk/people/s.schjorring