Let’s get physiological: circulating vitamin A levels predict the presence and severity of coccidiosis infection in House Finches (Haemorhous mexicanus)

There are many environmental, life-history, and physiological predictors of disease in animals, but we need more studies that examine multiple factors simultaneously to better understand comparative risks of infection in wildlife. The House Finch (Haemorhous mexicanus) is a North American songbird that has served as a model ecological system for assessing avian disease predictors and responses. Among its various diseases is coccidiosis, an intestinal parasitic infection that is more common in urban finches and in males with drab plumage. Here, we examined additional nutritional–physiological predictors of coccidiosis presence and severity in House Finches, with an emphasis on plasma concentrations of lipid-soluble vitamins (retinol, or vitamin A; tocopherol, or vitamin E) as well as blood glucose and ketones. We found that plasma retinol concentration significantly predicted the presence of coccidiosis; coccidia-infected finches had lower levels of retinol than those without coccidiosis. We also found an effect of the sex*retinol level interaction on coccidiosis severity; retinol levels were lower in more-infected females, but no such pattern was found in males. Overall, we demonstrate a unique link between retinol and intestinal disease status in a wild bird species; to date, such a relationship had only been uncovered in domestic chickens. Mechanistically it is possible that, due to the antioxidant and immunosupportive properties of retinol, animals deficient in retinol are more susceptible to coccidial infections, or that, as in poultry, infection directly lowers retinol levels; experimental manipulations are needed to disentangle these possibilities and to understand why the negative relationship between retinol and coccidiosis severity exists in females but not males.