Worms make rabbits peaky

Isabella Cattadori, Ottar Bjornstad, Peter Hudson and collaborators from the U.K. have found strong empirical evidence for the "peak shift", a pattern predicted by epidemiological theory but not described in a free-living animal population until now.

In a June 2005 paper in Proceedings of the Royal Society B, they analyzed  parasite burdens in a rabbit population sampled monthly for 26 years.

Intensity of infection with the gastrointestinal nematode Trichostrongylus retortaeformis increased, peaked, then decreased with rabbit age.  

Moreover, intensity of peak infection and rabbit age at this peak depended on when the rabbit was born:

• The highest peaks were seen in summer-born rabbits at a relatively young age
• Spring-born cohorts not only had lower peak infections, but were older when infections peaked.

This peak shift suggests that rabbits acquire immunity  to the parasites as they age, and that this happens faster in summer-born cohorts than in spring-born ones.

These results illustrate how seasonality in host reproduction and parasite transmission can play an important role in host-parasite interactions.