Environmental temperature influences ophidiomycosis progression and survival in experimentally challenged prairie rattlesnakes (Crotalus viridis).

Ophidiomycosis is a prevalent and intermittently pervasive disease of snakes globally caused by the opportunistic fungal pathogen, Ophidiomyces ophidiicola. Host response has yet to be fully explored, including the role of temperature in disease progression and hematologic changes. This study enrolled twelve adult prairie rattlesnakes (Crotalus viridis) in an experimental challenge with O. ophidiicola at two temperatures, 26°C (n = 6) and 20°C (n = 6). Each temperature cohort included four inoculated and two control snakes. Assessments involving physical exams, lesion swabbing, and hematology were performed weekly. Differences were observed between inoculated and control snakes in survival, behavior, clinical signs, ultraviolet (UV) fluorescence, hematologic response, and histologic lesions. All inoculated snakes held at 20°C were euthanized prior to study end date due to severity of clinical signs while only one inoculated animal in the 26°C trial met this outcome. In both groups, qPCR positive detection preceded clinical signs with regards to days post inoculation (dpi). However, the earliest appearance of gross lesions occurred later in the 20°C snakes (20 dpi) than the 26°C snakes (13 dpi). Relative leukocytosis was observed in all inoculated snakes and driven by heterophilia in the 20°C snakes, and azurophilia in the 26°C group. Histologically, 20°C snakes had more severe lesions, a lack of appropriate inflammatory response, and unencumbered fungal proliferation and invasion. In contrast, 26°C snakes had marked granulomatous inflammation with encapsulation of fungi and less invasion and dissemination. The results of this study identified that O. ophidiicola-infected rattlesnakes exposed to lower temperatures have decreased survival and more robust hematologic change, though minimal and ineffective inflammatory response at site of infection. Ophidiomycosis is a complex disease with host, pathogen, and environmental factors influencing disease presentation, progression, and ultimately, survival. This study highlighted the importance of temperature as an element impacting the host response to O. ophidiicola.

Egg Incubation Temperature Affects Development of Innate Immune Function in Nestling American Robins (Turdus migratorius).

The innate immune system provides important first-line defenses against invading pathogens and is considered especially important for developing organisms. However, we know little about how early-life conditions influence these defenses in wild animals. For oviparous species such as birds, embryonic development occurs in the egg, which can be subject to variation in thermal conditions. There is evidence from cavity-nesting species and species with precocial young that reduced incubation temperatures can result in reduced measures of innate immunity. Whether and how this thermal variation impacts innate immunity for open-cup-nesting species with altricial offspring has not been examined. In this study, we experimentally manipulated egg incubation temperature for American robins (Turdus migratorius) and compared the bacteria-killing ability (BKA) of the nestlings' blood plasma. We collected baseline and poststressor samples on day 7 and day 10 after hatch to gain additional insights into the ontogeny of this immune measure, as well as into whether any changes were linked to levels of the glucocorticoid hormone corticosterone (CORT). We found that nestlings incubated at the low treatment (36.1°C) had significantly reduced BKA compared with nestlings incubated at the high treatment (37.8°C) when controlling for the posthatch nest environment. We also documented a significant reduction in poststressor levels of BKA, as well as an increase in BKA from day 7 to day 10. We found a weak inverse association between CORT and BKA but no other indications that BKA was mediated via treatment-induced variation in CORT. Our results suggest that incubation temperature can affect development of innate immunity in open-cup-nesting passerines.

Incubation temperature impacts nestling growth and survival in an open-cup nesting passerine.

For oviparous species such as birds, conditions experienced while in the egg can have long-lasting effects on the individual. The impact of subtle changes in incubation temperature on nestling development, however, remains poorly understood, especially for open-cup nesting species with altricial young. To investigate how incubation temperature affects nestling development and survival in such species, we artificially incubated American robin (Turdus migratorius) eggs at 36.1°C ("Low" treatment) and 37.8°C ("High" treatment). Chicks were fostered to same-age nests upon hatching, and we measured mass, tarsus, and wing length of experimental nestlings and one randomly selected, naturally incubated ("Natural"), foster nest-mate on days 7 and 10 posthatch. We found significant effects of incubation temperature on incubation duration, growth, and survival, in which experimentally incubated nestlings had shorter incubation periods (10.22, 11.50, and 11.95 days for High, Low, and Natural eggs, respectively), and nestlings from the Low treatment were smaller and had reduced survival compared to High and Natural nestlings. These results highlight the importance of incubation conditions during embryonic development for incubation duration, somatic development, and survival. Moreover, these findings indicate that differences in incubation temperature within the natural range of variation can have important carryover effects on growth and survival in species with altricial young.