Complex life-cycles in trophically transmitted helminths: Do the benefits of increased growth and transmission outweigh generalism and complexity costs?

• Published in: Current research in parasitology & vector-borne diseases 2022-01, Vol.2, p.100085-100085, Article 100085
• Main Authors: Benesh, Daniel P., Chubb, James C., Lafferty, Kevin D., Parker, Geoff A.
• Format: Article
• Language: English
• Subjects: Adaptation; Comparative analysis; Complex life-cycle; Graphical Review; Host specificity; Life-history strategy; Trophic transmission
• ISSN: 2667-114X; 2667-114X
• DOI: 10.1016/j.crpvbd.2022.100085

Why do so many parasitic worms have complex life-cycles? A complex life-cycle has at least two hypothesized costs: (i) worms with longer life-cycles, i.e. more successive hosts, must be generalists at the species level, which might reduce lifetime survival or growth, and (ii) each required host transition adds to the risk that a worm will fail to complete its life-cycle. Comparing hundreds of trophically transmitted acanthocephalan, cestode, and nematode species with different life-cycles suggests these costs are weaker than expected. Helminths with longer cycles exhibit higher species-level generalism without impaired lifetime growth. Further, risk in complex life-cycles is mitigated by increasing establishment rates in each successive host. Two benefits of longer cycles are transmission and production. Longer cycles normally include smaller (and thus more abundant) first hosts that are likely to consume parasite propagules, as well as bigger (and longer-lived) definitive hosts, in which adult worms grow to larger and presumably more fecund reproductive sizes. Additional factors, like host immunity or dispersal, may also play a role, but are harder to address. Given the ubiquity of complex life-cycles, the benefits of incorporating or retaining hosts in a cycle must often exceed the costs. [Display omitted] •Comparing helminth species hints at the costs and benefits of complex life-cycles.•Generalism and survival costs in longer life-cycles are weaker than expected.•Longer life-cycles have growth and transmission benefits.•The benefits of a complex life-cycle must often exceed the costs.