University of Wisconsin–Madison

Lambing affects PCR results in sheep with subclinical paratuberculosis

Journal of King Saud University –Science. In press corrected proof available online 11-FEB-2019. This is an Open Access article.

Mataragka et al. from the Agriculture University of Athens and Kapodestrian University of Athens (Greece) reported intriguing findings regarding paratuberculosis in sheep. Their novel study explored the effects of parturition (lambing) on detection of MAP infections using IS900 real-time PCR. The study was done in a single flock of sheep that had a history of many animals testing positive for MAP infections by culture or PCR. All animals deemed pregnant were enrolled in the study. Feces was collected from animals at 5 different time periods: 4–15 and 1–3 days before lambing (FP1/FP2), immediately before and 4-15 days after lambing (FP3/FP4), and before mating roughly 6-7 months later (FP5).

The results demonstrated significant differences in rate of PCR positivity based on sampling time: FP1 = 38.3% (23/60), FP2 = 38.3% (31/81), FP3 = 34.1% (29/85), FP4 = 40% (34/85), and FP5 = 21.3% (16/75). The proportion of the animals that reacted positively to real time PCR at one, two, three or four sampling periods was 47.1% (33/70), 27.1% (19/70), 15.7% (11/70), and 8.6% (6/70) of the positive reactors, respectively. Animals testing PCR-positive at more than one sample collection had lower Ct values (indicative of higher levels of MAP shedding in feces) than animals testing PCR-positive only one time. Interestingly, only one sheep was IS900-PCR-positive at all five sample collections (FP1-5) corresponding to 1.4% (1/70) of PCR reactors. Interestingly, this was the only sheep that tested positive by ELISA (IDEXX laboratories). It was a 3-year old ewe and clearly in the most advanced stages of a MAP infection.

In conclusion, when testing sheep with subclinical paratuberculosis, the period of 4–15 days postpartum is the best time to test for paratuberculosis using IS900 real-time PCR. The use of ELISA is not recommended due to its low diagnostic sensitivity.

Take-home messages:
1. Real-time IS900-PCR is far superior to ELISA (blood tests) for paratuberculosis in sheep.
2. Testing fecal samples collected 4-15 days after lambing provides somewhat greater diagnostic sensitivity (ability to detect MAP-infected animals).
3. Testing a single fecal sample, regardless of time relative to lambing, will not detect all MAP-infected animals. Thus, testing annually is recommended.

Comment: Testing multiple fecal samples from an individual will likely increase diagnostic sensitivity even more than the timing of the collection of a single sample relative to lambing. Zoological institutions in the U.S. when testing individual animals for paratuberculosis are commonly collecting fecal samples on 3 different days and then having the laboratory test a pool of these samples by real-time PCR as a means of enhancing the ability to detect MAP-infected animals.