The acanthocephalan parasite, Corynosoma enhydri, uses sea otters (Enhydra lutris) as definitive host. Despite high prevalence and abundance in southern sea otters (E. l. nereis), sublethal impacts of infection on otter health are unknown. Parasites are an integral part of ecosystem structure and functioning. Many affect host behavior, reproduction, predation, or prey preference. Parasites can suppress host immune response, facilitate secondary bacterial, viral, or parasitic infections, and influence concurrent microbial infection. Acanthocephalan infections can have significant effects on host metabolism, digestion, nutrient absorption, and energetics. Because high metabolic expenditures and resource limitations are known to affect southern sea otter populations, even subtle effects of infection could negatively impact individual health and population recovery. In this review we summarize reports of host–parasite‐environmental interactions for helminth infections and discuss how these same attributes could manifest in southern sea otters and coastal food web dynamics. Based on these data, investigation of potential sublethal impacts of C. enhydri is warranted. We recommend expanding understanding of the significance of parasites in marine ecosystems beyond associated mortality. Future investigation into the ecological implications of sublethal effects are essential to fully grasp parasite impacts on host populations, and predict potential additive effects with climate change in ecosystem functioning.

Parasites, by definition, have a negative effect on their host. However, in wild mammal health and conservation research, sub-lethal infections are commonly assumed to have negligible health effects unless parasites are present in overwhelming numbers. Here, we propose a definition for host health in mammals that includes sub-lethal effects of parasites on the host's capacity to adapt to the environment and maintain homeostasis. We synthesized the growing number of studies on helminth parasites in mammals to assess evidence for the relative magnitude of sub-lethal effects of infection across mammal taxa based on this expanded definition. Specifically, we develop and apply a framework for organizing disparate metrics of parasite effects on host health and body condition according to their impact on an animal's energetic condition, defined as the energetic burden of pathogens on host physiological and behavioural functions that relate directly to fitness. Applying this framework within a global meta-analysis of helminth parasites in wild, laboratory and domestic mammal hosts produced 142 peer-reviewed studies documenting 599 infection-condition effects. Analysing these data within a multiple working hypotheses framework allowed us to evaluate the relative weighted contribution of methodological (study design, sampling protocol, parasite quantification methods) and biological (phylogenetic relationships and host/parasite life history) moderators to variation in the magnitude of health effects. We found consistently strong negative effects of infection on host energetic condition across taxonomic groups, with unusually low heterogeneity in effect sizes when compared with other ecological meta-analyses. Observed effect size was significantly lower within crosssectional studies (i.e. observational studies that investigated a sub-set of a population at a single point in time), the most prevalent methodology. Furthermore, opportunistic sampling led to a weaker negative effect compared to proactive sampling. In the model of host taxonomic group, the effect of infection on energetic condition in carnivores was not significant. However, when sampling method was included, it explained substantial inter-study variance; proactive sampling showing a strongly significant negative effect while opportunistic sampling detected only a weak, non-significant effect. This may partly underlie previous assumptions that sub-lethal parasites do not have significant effects on host health. We recommend future studies adopt energetic condition as the framework for assessing parasite effects on wildlife health and provide guidelines for the selection of research protocols, health proxies, and relating infection to fitness.

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Acanthocephalans are common intestinal parasites of marine mammals, the most widespread of which is the genus Corynosoma. In this study, parasite infrapopulations of two closely related species of Corynosoma were examined: Corynosoma enhydri from sea otters (Enhydra lutris) in Alaska (n = 12) and California (n = 19), and Corynosoma strumosum from seals in Germany (n = 22). Prevalence of C. enhydri was 100% in Californian otters, with a mean abundance of 30, and 83% in Alaskan otters, with a mean abundance of 232. In seals, C. strumosum had a prevalence of 65%, with a mean abundance of 33. Female C. enhydri dominated both Californian (82%) and Alaskan (79%) infections, while, in seals, female C. strumosum made up 68% of the parasite population. Reproduction rates for C. enhydri, with 16% (California) and 18% (Alaska) of females mated, were low compared to C. strumosum in seals, of which 40% of females were mated. Habitat selection also differed significantly between the two species. Corynosoma enhydri was found most frequently in the second and third fifths of the small intestine, while C. strumosum was found most frequently in the fourth. The differences in habitat selection and prevalence analysed in this study may be related to a trade-off between growth and reproduction between the two species.