ELISA and fecal culture for paratuberculosis (Johne's disease): sensitivity and specificity of each method

The sensitivity and specificity of the ELISA and fecal culture tests for pa ratuberculosis in dairy cattle are examined. ELISA and fecal culture data f rom seven dairy herds where both fecal cultures and ELISA testing was done concurrently are included. A cohort of 954 cattle including 697 parturient adults, cultured every 6 months from 10 herds followed over 4 years served as the basis to determine fecal culture sensitivity. The fecal culture tech nique utilized a 2 g sample with centrifugation and double incubation. Of t he 954 cattle cohort of all ages (calf to adult) that were fecal sampled on the first herd visit, 79 were culture positive. An additional 131 animals were detected as culture positive over the next seven tests at 6-month inte rvals. The sensitivity of fecal culture to detect infected cattle on the fi rst sampling was 38%. Of the 697 parturient cattle cohort, 67 were positive on the first fecal culture, while an additional 91 adult cattle were cultu re positive over the next seven tests, resulting in a sensitivity of 42% on the first culture of the total animals identified as culture positive. Ani mals culled from the herds prior to being detected as infected and animals always fecal culture negative with culture positive tissues at slaughter ar e not included in the calculations. Both groups of infected cattle will low er the apparent sensitivity of fecal culture, infected dairy herds tested c oncurrently with both fecal culture and ELISA usually resulted in more than twofold positive animals by culture compared to ELISA. The classification of infected cattle by the extent of shedding of Mycobact erium paratuberculosis in the feces helps define the relative proportion of cattle in each group and therefore the likelihood of detection by the ELIS A test. ELISA has a higher sensitivity in animals with a heavier bacterial load, i.e. high shedders (75%) compared to low shedders (15%). Repeated tes ting of infected herds identifies a higher proportion of low shedders which are more likely to he ELISA negative. Thus, the sensitivity of the ELISA r est decreases with repeated herd resting over time, since heavy shedders wi ll be culled first from the herds. (C), 2000 Elsevier Science B.V. All righ ts reserved.