Effects of Mycobacterium avium subsp. paratuberculosis infection on serum biochemistry, body weight and wool growth in Merino sheep: A longitudinal study

- Mycobacterium avium subsp. paratuberculosis infection in sheep leads to significant effects on live weight and serum albumin.
- These effects were detectable up to 2 years before the development of clinical signs.
- Mean differences for live weight were highly significant for animals with severe disease.
- These results confirm that significant loss in production occurs well before clinical signs are detected.

Johne's disease (JD) is a chronic enteropathy caused by Mycobacterium avium subspecies paratuberculosis (MAP). The financial impacts of mortalities due to ovine JD (OJD) in Australia are recognised; however, production losses in the subclinical and clinical stages of infection have not been considered. A longitudinal field investigation over 3 years was conducted in 83 MAP-infected sheep within a flock with 14% prevalence of infection, to describe the changes in serum biochemistry, wool growth and live weight associated with MAP. Of the 22 biochemical parameters analysed, only serum albumin levels were consistently and significantly associated (reduced) with the development of Johne's disease (P < 0.05). Live weights were significantly less (up to 5 kg) in sheep with severe histological lesions at 3 years of age compared to controls (P < 0.05). In the subset of sheep that died from OJD during the trial, there were severe effects on live weight measured in the 6-12 months prior to death, although early changes in live weight were only apparent when objectively measured and compared to non-affected cohorts. Wool quality and growth were not consistently related to changes in lesion severity or development of JD. Even under exposure conditions leading to relatively mild within flock prevalence, the negative effects of OJD on live weight were measurable and economically significant; greater impacts would be expected at younger ages in flocks with higher prevalence.