Immunology and Disease in the Mojave Desert Tortoise (Gopherus agassizii)
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The motivation for this study was to gain a better understanding of the possible effects of upper respiratory tract disease (URTD) and Mycoplasma agassizii (an etiological agent of URTD) in the Mojave desert tortoise, a federally-threatened population. We hope to influence future ecological studies as well as conservation strategies in the management of wild populations of the desert tortoise. Specifically, we (1) reviewed the entire published literature of URTD in Mojave desert tortoises, (2) measured aspects of a generalized acquired immune response in desert tortoises in a controlled environment, and (3) conducted a range-wide survey of URTD and the seroprevalence of M. agassizii in Mojave desert tortoises. (1) In the review of the literature we challenge the view that M. agassizii causes consistent levels of morbidity and/or mortality across the Mojave Desert. Instead, URTD may be described more accurately as a context-dependent disease. We summarize new evidence of relatively high levels of natural antibodies to M. agassizii in desert tortoises, which suggests possible problems of conventional diagnostic tests and a possible tortoise immune mechanism of defense against M. agassizii. Partly because of the problems with diagnostic testing, we recommend abandoning policies to euthanize tortoises that test positive for an immune response to M. agassizii. Based on this review, we question management strategies aimed solely at reducing Mycoplasma spp in desert tortoise populations, and advocate a more careful consideration of extrinsic factors as an additional, potential cause of disease. (2) We induced an acquired, humoral (antibody) response in desert tortoises, via immunization with ovalbumin (OVA). We immunized tortoises both before and after hibernation, and observed a gender-by-season interaction in the ability of desert tortoises to make an induced immune response. We observed relatively high levels of pre-existing natural antibody to OVA in all tortoises, and levels varied among individuals. There was a significant, negative relationship between an animal's natural antibody level and the maximum increase in acquired antibody levels. There was a significant, positive relationship between the magnitude of long-term elevations in OVA-specific antibody levels and maximum increase in acquired levels. This experiment suggested that both natural and long-term elevations in acquired antibody levels may be important elements of the tortoise immune system, with possible influences on the ecology and evolution of host-pathogen interactions. (3) We focused our range-wide survey on population-level analyses (n = 24), and tested for associations among the prevalence of URTD, seroprevalence to M. agassizii, genetics of tortoise populations, mean annual winter precipitation, and mean number of days below freezing. We detected significant associations between mean number of days below freezing and both the prevalence of URTD and the seroprevalence to M. agassizii. Furthermore, we detected a significant association between mean levels of natural antibody and seroprevalence to M. agassizii. Genetics of tortoise populations was associated with mean levels of natural antibody. We propose hypotheses, concerning possible ecological and evolutionary dynamics of the desert tortoise - M. agassizii system, based on these associations and specific recommendations for future research to test these hypotheses.