To transmit or not to transmit, that is the question

Frequent transmission to new hosts selects pathogens for increased virulence. Transmission depends not only on the occurrence of opportunities but also on whether parasites exploit them. Several taxa of ectoparasites lack a specialized transmissive stage (infectious propagules) in their life cycle; instead, all their mobile stages (nymphs or larvae, and adults, but not the eggs) are competent for transmission. These parasites must therefore determine, at different developmental stages, whether to disperse to a new host or remain on the current one. Using realistic variables from an intensively studied system of host birds and parasitic lice (feral pigeon/slender pigeon louse), models were developed to represent vertical (parent-to-offspring) transmission. Within this framework, the present analysis examined whether dispersal was age-dependent and explored the evolutionary rationale for such dependence. The objective was to search for the decision strategy that maximizes the number of infested hosts at the end of the breeding season, rather than maximizing the number of parasite individuals. Previous field studies suggested that avian lice were more likely to transmit during the nymphal stages than during the adult stage. The present findings supported this notion, demonstrating that the prevalence of infestation in future host generations could be enhanced through nymphal transmission rather than adult transmission.