Ectoparasite presence is associated with alterations in immune function after an acute stressor in cane toads (Rhinella marina)

Abstract

Parasites can alter host fitness and physiology in multiple ways, including chronic activation of the hypothalamus–pituitary–adrenal/interrenal (HPA/I) axis, resulting in prolonged glucocorticoid release. Such chronic effects may also disrupt the acute stress response mediated by the HPA/I axis, with consequences for immune function. In this study, we investigated if ectoparasites (ticks) are associated with alterations in stress biomarkers and immune function in Florida invasive cane toads (Rhinella marina). We measured corticosterone (CORT) plasma levels and neutrophil to lymphocyte ratio (NLR), two common stress indicators, as well as the bacterial killing ability (BKA), a protein-mediated innate immunity trait, at baseline and after a short-term stressor (24 h restraint). Parasitized toads had higher baseline CORT levels than non-parasitized individuals, although restraint increased CORT and NLR similarly in both groups. Interestingly, only toads with ticks had increased BKA post-restraint. In addition, baseline CORT was negatively correlated with body index (BI) in parasitized toads, suggesting a possible association between tick presence and increased energetic demands. Together, these results indicate that ectoparasites modulate the interplay between stress physiology and immunity in invasive cane toads. Future work should experimentally manipulate tick infection and broaden immune measures to clarify mechanisms underlying parasite–host stress–immune interactions.