Seroepidemiology of upper respiratory tract disease in the desert tortoisein the western Mojave Desert of California

Several factors have combined with art upper respiratory tract disease (URT D) to produce declines on some population numbers of desert tortoises (Goph erus agassizii) in die western USA. This study was designed to determine th e seroepidemiology of URTD in a population of wild adult tortoises at the D esert Tortoise Research Natural Area (DTNA) study site in Kern County (Cali fornia, USA). Prior to initiation of the study, there was a dramatic declin e in the number of individuals in this population. At each individual time point, samples were obtained from 12 to 20 tortoises with radiotransmitters during winter, spring, summer, and fall from 1992 through 1995. During the course of the study, 35 animals were sampled at one or more times. Only 10 animals were available for consistent monitoring throughout the 4 pr perio d. Specific antibody (Ab) levels to Mycoplasma agassizii were determined fo r individual tortoises by an enzyme linked immunosorbent assay (ELISA) test . Specific Ab levels were not influenced by the gender of the tortoise. Lev els of Ab and distribution of ELISA+, ELISA- and suspect animals were not c onsistently affected by season within a single year or for a season among t he study years. Significantly more tortoises presented with clinical signs in 1992 and 1995, The profile of ELISA+ animals with clinical signs shifted from 5% (1992) to 42% (1995). In 1992, 52% of tortoises lacked clinical si gns and were ELISA-. In 1995, this category accounted fur only 19% of torto ises. Based on the results of this study, we conclude that URTD was present in this population as evidenced by the presence of ELISA+ individual anima ls, and that the infectious agent is still present as evidenced by seroconv ersion of previously ELISA- animals during the course of the study. There i s evidence to suggest that animals may remain ELISA+ without showing overt disease, a clinical pattern consistent with the chronic nature of most myco plasmal infections. Further, there are trends suggesting that the clinical expression of disease may be cyclical. Continued monitoring of this populat ion could provide valuable information concerning the spread of URTD in wil d tortoise populations.