Parasitic worms (i.e. helminths) commonly infect multiple hosts in succession before reproducing. At each life cycle step, worms may fail to infect the next host, and this risk accumulates as life cycles include more successive hosts. Risk accumulation can be minimized by having high establishment success in the next host, but comparisons of establishment probabilities across parasite life stages are lacking. We compiled recovery rates (i.e. the proportion of parasites recovered from an administered dose) from experimental infections with acanthocephalans, cestodes, and nematodes. Our data covered 127 helminth species and 16,913 exposed hosts. Recovery rates increased with life cycle progression (11, 29, and 46% in first, second, and third hosts, respectively), because larger worm larvae had higher recovery, both within and across life stages. Recovery declined in bigger hosts but less than it increased with worm size. Higher doses were used in systems with lower recovery, suggesting that high doses are chosen when few worms are expected to establish infection. Our results indicate that growing in the small and short-lived hosts at the start of a complex life cycle, though dangerous, may substantially improve parasites’ chances of completing their life cycles.

We compiled data on helminth infectivity from literature sources. Specifically, we recorded parasite recovery rates from experimental infections (i.e. the number of worms recovered / the number of worms administered). Characteristics of each host-parasite system, such as larval worm size and host mass, were obtained from other databases. More details on how data were compiled, processed, and analysed can be found in our manuscript (Froelick et al. 2021. Comparative analysis of helminth infectivity: growth in intermediate hosts increases establishment rates in the next host. Proceedings B), in the metadata associated with the data files, and in this GitHub repository: https://github.com/dbenesh82/comparative_helminth_infectivity.

Helminth recovery rates are in two csvs ("recovery_rates_infection_level.csv" and "recovery_rates_condition_level.csv"). The first file contains recovery rates at the individual host level whenever possible, whereas the second file summarizes recovery rates at the condition level. In the latter case, individual hosts exposed under the same experimental conditions (i.e. same dose and time of dissection) were pooled to make recovery rates more comparable across studies. Taxonomic biases were explored by calculating the proportion of family-level species diversity included in our data. These proportions are in "taxonomic_representation.csv". Column names in all csv files are described in the accompanying metadata files. Finally, the phylogeny used for analyses has also been included ("parasite_tree.nex").