Anthelminthic drugs modulate the acute phase immune response but not the microbiota in wild song sparrows

Here, we used anthelminthic drugs to treat free-living song sparrows (Melospiza melodia) for helminth infections (“de-wormed” birds) and measured markers of systemic inflammation (heterothermia and locomotor activity) in response to an immune challenge with lipopolysaccharide (LPS), a nonreplicating cell-wall component of gram -negative bacteria. Using birds from a population that previously showed high helminth prevalence, we monitored skin temperature and activity remotely using automated radio telemetry in the field. We also collected cloacal swabs from both anthelminthic-treated and control birds to determine whether drug treatment was associated with changes in the cloacal microbiota, and whether the structure of the cloacal microbial community was associated with the severity of birds' immune responses. Because helminths can reduce the severity of inflammatory immune responses in other species, we predicted that in comparison with untreated control birds, de-wormed birds would be more lethargic and display higher fevers when challenged with LPS. Consistent with these predictions, de-wormed birds expressed higher fevers in response to immune challenge. However, all LPS-challenged birds decreased locomotor activity to a similar degree, regardless of anthelminthic treatment. Although the presence of several individual indicator bacterial taxa were strongly associated with anthelminthic treatment, this treatment did not alter overall bacterial alpha- and beta- diversity. Similarly, we did not find evidence that bacterial community diversity influenced the severity of immune responses to LPS. These results suggest that under field conditions, natural helminth infection can reduce the severity of songbirds' thermoregulatory responses (fever) during an immune challenge, without major impacts on internal microbial communities or behavioral responses to infection.