Chronic disease in the Mojave desert tortoise: Host physiology and recrudescence obscure patterns of pathogen transmission

Abstract

A seminatural, factorial-design experiment was used to quantify dynamics of the pathogen <i>Mycoplasma agassizii</i> and upper respiratory tract disease in the Mojave desert tortoise (<i>Gopherus agassizii</i>) over 2 years. Groups of initially healthy animals were separated into serologically positive (seropositive), seronegative, and artificially infected groups and paired into 23 pens. We found no evidence of long-term immune protection to <i>M. agassizii</i> or of immunological memory. Initially seronegative, healthy tortoises experienced an equal amount of disease when paired with other seronegative groups as when paired with seropositive and artificially infected groups-suggesting that recrudescence is as significant as transmission in introducing disease in individuals in this host-pathogen system. Artificially infected groups of tortoises showed reduced levels of morbidity when paired with initially seronegative animals-suggesting either a dilution effect or a strong effect of pathogen load in this system. Physiological dynamics within the host appear to be instrumental in producing morbidity, recrudescence, and infectiousness, and thus of population-level dynamics. We suggest new avenues for studying diseases in long-lived ectothermic vertebrates and a shift in modeling such diseases.