Transmission of Mycoplasmal Upper Respiratory Disease in the Desert Tortoise (Gopherus agassizii)

Mycoplasma agassizii has been identified as a cause of disease, and a possible cause of declines in wild populations of desert tortoises (Gopherus agassizii) (USFWS 1994). In Clark County in Nevada, more than 3,230 federally protected desert tortoises were euthanized over the course of 17 years, as a means to reduce transmission of upper respiratory tract disease (URTD) to a wild population. Immunological tools have been developed to diagnose an immune response to the pathogen. However, there remains a paucity of information on the conditions required to transmit the disease, and how the immune response influences transmissibility. This study is the first large-scale, longitudinal investigation of transmission of pathogen in this host/pathogen complex. Reported here are the results of a three-year, semi-natural field experiment in which serological tests and clinical signs are used to determine transmission of M. agassizii among desert tortoises. We found that appearance of disease does not require exposure to sick tortoises. The appearance of clinical signs does not predict a tortoise seroconverting. There are no differences in disease between females and males. Tortoises emerge from brumation immunologically challenged, and for most tortoises, there is a lag period of eighteen months between clinical signs and seroconversion