1College of Veterinary Medicine, Iowa State University, Ames, IA, USA; 2Zoological Society of San Diego, Center for Reproduction of Endangered Species, Department of Pathology, San Diego, CA, USA; 3National Animal Disease Center, USDA, Agricultural Research Service, Ames, IA, USA; 4Department of Microbiology, Immunology, and Preventive Medicine, College of Veterinary Medicine, Iowa State University, Ames, IA, USA
A presumptive diagnosis of avian tuberculosis can be made when characteristic histopathologic lesions and acid-fast bacilli are observed in avian tissue samples. However, a definitive diagnosis requires isolation of the causative organism, a process that can take several weeks to complete. The purpose of this study was to determine if formalin-fixed, paraffin-embedded avian tissues could be tested by polymerase chain reaction (PCR) to reliably and rapidly diagnose avian tuberculosis. Ninety-seven samples of formalin-fixed tissues collected over a 14-yr period (1983–1997) from both definitive and presumptive cases of avian tuberculosis in captive exotic birds were examined. The primers used for PCR amplified a 180 base pair fragment of 16S rRNA which is a sequence shared by Mycobacterium avium subspecies avium and Mycobacterium avium subspecies paratuberculosis. All positive samples were presumed to be due to the presence of M. avium subspecies avium rather than M. avium subspecies paratuberculosis.
PCR testing detected the sequence in 26 of the 97 samples (27%). Of the 17 samples that were culture positive for M. avium and were known to have been fixed in formalin for 4 wk or less, 11 tested positive by PCR (65%).
Some of the negative PCR results may have been due to the degradation of nucleic acid in the samples. Nucleic acid degradation may be caused by a variety of factors, including prolonged fixation in formalin. This study demonstrates that PCR can be a rapid indicator of the presence of M. avium subsp. avium in formalin-fixed, paraffin-embedded tissues; however, the low sensitivity the test demonstrated in this sample set may limit its practical use as a diagnostic tool.