It is well documented that infections with internal parasites can reduce weight gains, feed efficiency and reproductive performance in cattle, but just why different parasites have evolved to affect or manipulate their hosts in different ways is unclear.

Researchers Eric J. McElroy and I. de Buron at the College of Charleston in South Carolina recently completed a meta analysis of 101 studies of parasite affects in different animal species. The authors eventually narrowed the study to 49 published papers that had sufficient statistical data, and their report, titled “Host performance as a target of manipulation by parasites: A meta-analysis,”  appears in the current edition of the Journal of Parasitology.

The researchers maintain that performance capacity is an important target of parasitic manipulation, and set out to integrate the study of performance with that of parasitic manipulations of host behavior and fitness.

In their review of research, they sought answers to three questions:

  • Do parasites exert an important effect on host performance capacity?
  • Is that effect routinely to decrease or enhance performance capacity?
  • What factors explain variation in the effect sizes that have been quantified?

The studies included in the review covered a wide range of host species, including fish, reptiles and birds, but many of the effects parallel those we see in cattle.

The parasites’ effects on hosts varied depending on factors such as host age, the tissue infected, and whether a study was based on natural infections or experimental. The type of performance affected was also a factor, with parasites more often affecting endurance than speed.

In all but a few cases, parasitic infection negatively affected host performance capacity, often significantly. Several studies found infected hosts' fitness reduced by poor body condition, increased metabolism or depressed immune function. “This result agrees with the general idea that parasites harm or live at the expense of their hosts,” the authors wrote. For example, the mortality rate of animals with parasites is more than twice that of animals without parasites, and infected animals may not be able to reach spawning grounds. Consequently, changes resulting from parasitic infections can affect not just the individual host but host populations, ecosystems, and even the evolution of host species.

Interestingly, some parasite-induced behavioral alterations may benefit the host. The authors found three examples in which specific parasite infections appeared to enhance the performance of the host, such as the swimming speed of a species of marine crustacean. The researchers speculate this adaptation could make the host less vulnerable to most predators, thus making it more available to specific predators that serve as hosts to the parasite during the next stage in its lifecycle.

Read the full article from the Journal of Parasitology (free registration required).