On the relationship between body condition and parasite infection in wildlife: a review and meta‐analysis

Abstract

Body condition metrics are widely used to infer animal health and to assess costs of parasite infection. Since parasites harm their hosts, ecologists might expect negative relationships between infection and condition in wildlife, but this assumption is challenged by studies showing positive or null condition–infection relationships. Here, we outline common condition metrics used by ecologists in studies of parasitism, and consider mechanisms that cause negative, positive, and null condition–infection relationships in wildlife systems. We then perform a meta‐analysis of 553 condition–infection relationships from 187 peer‐reviewed studies of animal hosts, analysing observational and experimental records separately, and noting whether authors measured binary infection status or intensity. Our analysis finds substantial heterogeneity in the strength and direction of condition–infection relationships, a small, negative average effect size that is stronger in experimental studies, and evidence for publication bias towards negative relationships. The strongest predictors of variation in study outcomes are host thermoregulation and the methods used to evaluate body condition. We recommend that studies aiming to assess parasite impacts on body condition should consider host–parasite biology, choose condition measures that can change during the course of infection, and employ longitudinal surveys or manipulate infection status when feasible.

Parasite infection leads to widespread glucocorticoid hormone increases in vertebrate hosts: A meta‐analysis

“…We thus broadly defined ‘parasites’ based on a general consumer‐resource framework (Lafferty et al, ), that is a natural enemy that feeds intimately on one host (Lafferty & Kuris, ), which is also consistent with other meta‐analyses involving parasites (e.g. Sánchez et al, ). Subsequently, use of the term ‘pathogen’ hereon will specifically refer to bacteria and viruses as a sub‐category of parasitism based on their host effects and transmission requirements whereas ‘parasite’ encompasses all natural enemies under this consumer model, including those which may be distinguished as either macro‐ or microparasites (see definitions in Lafferty & Kuris, ).…”

“…Such effects are likely mediated by increases in host mortality, reductions in condition or fecundity, or shifts in reproductive effort and other life‐history traits (e.g. Agnew, Koella, & Michalakis, ; Forbes, ; Sánchez et al, ). Parasites can exert these effects by being energetically costly to their hosts, either directly or via immune activation (Barber, Wright, Arnott, & Wootton, ; Brace et al, ).…”

City sicker? A meta‐analysis of wildlife health and urbanization

“…We further divided health metrics based on original study methodology (Table 1). We classified whether body condition was measured using qualitative scores, raw quantitative measures, or size-adjusted quantitative measures (Sánchez et al 2018). We categorized stress measures as glucocorticoid concentrations, heterophil-to-lymphocyte ratios (baseline or in response to a stressor), or other measures (eg oxidative damage, blood glucose).…”

Parasites of seabirds: A survey of effects and ecological implications

“…Effects of certain parasites and pathogens may also be subtle and difficult to measure with respect to the impact of other environmental factors, and choosing the appropriate traits to measure requires clear information on the biological system (Sánchez et al, 2018). For example, the effects of a parasite may only be detectable during times of physiological stress (e.g.…”