Fitness of indirectly transmitted pathogens: restraint and constraint

Area of Science:

• Parasitology
• Evolutionary Biology
• Disease Ecology

Background:

• Many important pathogens utilize multihost life cycles, but research often overlooks host exploitation patterns and pathogen fitness across these diverse hosts.
• Existing theoretical models predominantly focus on directly transmitted microparasites, leaving gaps in understanding complex multihost pathogen dynamics.

Purpose of the Study:

• To investigate the correlated fitness responses of pathogen genotypes across different host species within a multihost life cycle.
• To examine the relationship between infection intensity and virulence in an intermediate host for a multihost pathogen.

Main Methods:

• Utilized artificial selection experiments on Schistosoma mansoni lines, selecting for high or low infection intensity in the intermediate host.
• Assessed pathogen fitness in both intermediate and definitive hosts to evaluate trade-offs.

Main Results:

• Demonstrated a strong inverse correlation between pathogen fitness in the intermediate host and fitness in the definitive host.
• Found that high infection intensity in the intermediate host was associated with decreased, not increased, virulence.

Conclusions:

• Genetic constraints significantly influence pathogen fitness trade-offs across hosts, impacting polymorphism maintenance in natural populations.
• Findings have critical implications for understanding the evolution of parasite virulence, host specialization, and the effectiveness of host-directed control programs.