Three infectious diseases seen commonly in exotic pet practice are best diagnosed by using immunoassays. Mycoplasma pulmonis, along with other pathogens, causes chronic respiratory disease in rats but is difficult to grow in the laboratory. Moreover, cultures do not detect 25% to 30% of infections. Serology (enzyme-linked immunosorbent assay [ELISA] or immunofluorescence assay [IFA]) and polymerase chain reaction using tracheal or nasal swabs are the recommended diagnostic methods, although young rats may remain seronegative for up to 4 months after exposure. Pasteurella multocida is involved in 55% of cases of rhinitis ("snuffles") in rabbits, but isolation and identification are difficult. Testing may yield both false-negative and false-positive results. In older rabbits, serology is useful only in ruling out infection with P. multocida because of false-positive results. The best way to diagnose infections with P. multocida is to combine ELISA with PCR on tracheal or nasal swabs. Encephalitozoon cuniculi infects rabbits and can present in renal, ocular, or neurologic forms. Definitive diagnosis is difficult because many seropositive rabbits show no clinical signs and immunoassays are not specific. ELISA or IFA may be used in these cases, but other diagnostic findings must be considered in evaluating these results. A seronegative result in a sick rabbit indicates that other differential diagnoses must be considered. Unlike commercial ELISAs, which tend to be highly sensitive, laboratory animal diagnostic laboratories test for high specificity. Therefore, a second immunoassay (most often immunofluorescence) is usually run on samples that fall between the higher cutoff point for normal and a second, lower cutoff point to confirm or negate the initial test result.
COMMENTARY: Most of this article concentrates on the diagnosis of the causative organism of each of 3 diseases seen commonly in exotic animals. A large section focuses on operating characteristics of laboratory tests in general. Very good graphs clearly depict the relationships between reliability and accuracy, as well as sensitivity and specificity in laboratory testing, thereby providing an excellent overview of diagnostic test concepts. This discussion also helps readers understand the difference between laboratory animal testing, in which false-positive results can mean the destruction of an entire research colony, and commercial testing, in which false-negative results can mean litigation against a manufacturing company. The article also lists recommended animal diagnostic laboratories in the United States and throughout the world.
Application of laboratory animal immunoassays to exotic pet practice. Donnelly TM. Exotic DVM 8:19-26, 2006.