The role of transmission in the evolution of virulence of parasitic helminths of domestic animals

Parasitic worms (macroparasites) cause serious disease in humans and other animals worldwide, and worm infections cause large losses in the production of food animals. Nevertheless, the ultimate causation of virulence (harm to the host) in these organisms has not been studied. The evolution of virulence in microparasites like bacteria and viruses has been extensively studied, and it is thought that transmission is intimately related to virulence in these organisms. Increased transmission in microparasites generally results in higher virulence, but if increased virulence, in the form of host death or reduced motility, decreases the overall transmission rate of the pathogen, the trade-off between transmission and virulence results in an optimal level of virulence for a given host-parasite interaction.; This study investigates the factors involved in the evolution of virulence in worms. The species studied are strongylid nematode parasites that live within the gastrointestinal tract of horses or sheep. The objective of this study is to test the “transmission hypothesis” in an effort to determine whether worm virulence results in a trade-off similar to that suggested for microparasites. Comparison of initial estimates of transmission (an index based on offspring production and survival), and virulence (based on estimates of the number of worms that generally cause host death), suggested that the trade-offs between virulence and transmission differ in parasites with different developmental strategies. There is a direct trade-off between transmission and virulence in parasite species that cause disease as reproductively mature adults (Trichonematidic development), but no relationship between transmission and virulence in parasite species where immature stages of the parasite cause the majority of pathology (Strongylidic development).; The development of a stage-structured demographic model that incorporated seasonal variations in parasite and host biology allowed for a more comprehensive estimate of transmission rate in Trichonematidic worms. In addition, analysis of experimental infections allowed for a better estimate of worm virulence. Regression analysis of virulence and transmission rates for parasites that are phylogenetically closely related and which exhibit similar modes of transmission suggested that there was, in fact, no trade-off between transmission and virulence. Other components of the parasites' biology were also examined, and it was determined that worm egg production is positively correlated with virulence. Although fecundity is highest in the most virulent worms, low survival and development of the infective stages of these organisms resulted in transmission rates that were not significantly different from those of less virulent worm species. These data do not support the transmission hypothesis suggested for microparasites, and suggest that virulence is de-coupled from transmission in the trichostrongylid nematodes of sheep.