Agar gel immunodiffusion test for diagnosis of clinical paratuberculosis in cattle.

During a 19-month period, the agar gel immunodiffusion (AGID) test was used as an aid in differential diagnosis of paratuberculosis in 51 cattle with a history of chronic weight loss and/or chronic diarrhea. Thirty-three cattle were AGID test-positive for paratuberculosis. Twenty-eight cattle (87.5%) yielded Mycobacterium paratuberculosis from bacterial culture of feces. Four cattle were confirmed as having paratuberculosis on necropsy. One had a negative fecal culture but was lost to follow-up. Thus, 32 of the 33 AGID test-positive cattle (96.9%) were confirmed as paratuberculous by culture or necropsy. Of the 18 cattle that were AGID test-negative, 17 (94%) also were culture-negative after 12 to 20 weeks' incubation of the culture tubes. Direct fecal smears from 29 confirmed paratuberculous cattle were examined. Twenty-three (79.3%) had smears positive for M paratuberculosis. Fifteen culture-negative cattle were examined by direct smear; the results were negative for all.