| Upper respiratory tract disease (URTD) of tortoises is caused by the mollicute Mycoplasma agassizii, and is characterised by nasal and ocular discharge, palpebral edema, and conjunctivitis. Hyperplasia and dysplasia of the nasal passage and cavity epithelia and inflammatory infiltrates are seen histologically. In order to provide data for management decisions and to better understand URTD, I studied uninfected, naturally infected, and experimentally infected tortoises. The pathological and immune responses of tortoises to and transmission of M. agassizii were investigated using clinical and histological observations, culture and polymerase chain reaction (PCR) tests, and an enzyme-linked immunosorbent assay (ELISA). Infection with M. agassizii caused mild to severe damage to mucosal and olfactory nasal epithelia, with increased damage in longer-term infections. Although clinically healthy, ELISA-positive, culture and PCR-negative tortoises may have eliminated the bacteria, when five such animals were examined at necropsy, three were found to harbor M. agassizii in the nasal cavities. When seropositive tortoises were challenged with M. agassizii, a more rapid and more severe clinical response resulted than on initial exposure, and plasma antibody levels began rising more quickly. When uninfected animals were housed with infected individuals, horizontal transmission occurred, probably via direct contact, but possibly via food, water, or fomites. Transmission was more likely to occur from a tortoise that was clinically ill and culture or PCR-positive. There was no discernible transmission when tortoises inhabited pens or entered burrows previously occupied by ill tortoises. There was no demonstrable vertical transmission, although there was transfer of maternal antibodies via egg yolk. The level of antibodies in egg yolk or hatchling plasma was approximately 10-20% of that in maternal plasma. Movement of tortoises during relocation, repatriation, or restocking efforts potentially could transport M. agassizii to previously uninfected sites. Because of the uncertainty involved in determining latent infections, ELISA-positive animals should not be moved to locations with no seropositive individuals. However, they can be used in captive breeding efforts and their offspring released into the wild. Clinically ill animals should not be relocated to new sites, and should not be housed with clinically healthy animals in temporary holding situations, such as on-site relocations, or in captive breeding programs. |
