Starving the Enemy? Feeding Behavior Shapes Host-Parasite Interactions.

The loss of appetite that typically accompanies infection or mere exposure to parasites is traditionally considered a negative byproduct of infection, benefitting neither the host nor the parasite. Numerous medical and veterinary practices directly or indirectly subvert this 'illness-mediated anorexia'. However, the ecological factors that influence it, its effects on disease outcomes, and why it evolved remain poorly resolved. We explore how hosts use anorexia to defend against infection and how parasites manipulate anorexia to enhance transmission. Then, we use a coevolutionary model to illustrate how shifts in the magnitude of anorexia (e.g., via drugs) affect disease dynamics and virulence evolution. Anorexia could be exploited to improve disease management; we propose an interdisciplinary approach to minimize unintended consequences.

Egg fibrils and transmission in the acanthocephalan Acanthocephalus dirus.

Acanthocephalans have multi-host life cycles that include arthropods as intermediate hosts and vertebrates as definitive hosts. Eggs are dispersed into the habitat from definitive hosts and in some species eggs possess fibrils, which have been proposed to facilitate transmission to intermediate hosts. We examined the potential role of fibrils in transmission of the acanthocephalan Acanthocephalus dirus to its intermediate host Caecidotea intermedius, a stream-dwelling isopod. We identify three properties of fibrils that could favor transmission. First, there was a slow rate of fibril release, which was dependent on the actions of stream microorganisms. Second, eggs with fibrils were more likely to adhere to the substrate than those without fibrils. Third, in feeding trials, isopods exposed to eggs with fibrils had a higher infection prevalence than isopods exposed to eggs without fibrils. These properties could favor transmission by increasing the likelihood that eggs sink to the sediment occupied by their target hosts before adhering to items on the substrate (e.g., leaves) and by increasing recruitment after the eggs have been consumed.

Competitive growth, energy allocation, and host modification in the acanthocephalan Acanthocephalus dirus: field data.

The acanthocephalan Acanthocephalus dirus is a trophically transmitted parasite that modifies both the physiology and behavior of its intermediate host (isopod) prior to transmission to its definitive host (fish). Infected isopods often contain multiple A. dirus individuals and we examined the relationships between host sharing, body size, energy content, and host modification to determine if host sharing was costly and if these costs could influence the modification of host behavior (mating behavior). Using field-based measures of parasite energy content (glycogen, lipid) and parasite body size (volume), we showed that host sharing was costly in terms of energy content but not in terms of body size. Analysis of the predictors of host behavior revealed that energy content, and body size, were not predictors of host behavior. Of the variables examined, parasite intensity was the only predictor of host behavior. Hosts that contained more parasites were less likely to be modified (i.e., less likely to undergo mating suppression). We suggest that intraspecific competition influenced parasite energy content and that the costs associated with competition are likely to shape the strategy of growth and energy allocation adopted by the parasites. These costs did not appear to have a direct effect on the modification of host mating behavior.