Pneumonic lesions in lambs in New Zealand: patterns of prevalence and effects on production

AIM: To describe temporal and spatial patterns of the prevalence of pneumonia in lambs in three regions of New Zealand, and determine effects on liveweight gain. METHODS: A longitudinal study was conducted in 19992000 on 14 commercial sheep farms, five from the Southland region of the South island, and five from the King Country and four from the Northland regions of the North Island. Pneumonia was known to be a problem in lambs on all farms selected. On each farm, 400 lambs were randomly selected at weaning and allocated at random to one of two groups (Groups I and II). Lambs in both groups were weighed at 4-weekly intervals. The extent of pneumonic lesions in lungs was scored visually at slaughter in 40 randomly selected lambs after each weighing (Group I). Lambs in Group II were selected for slaughter by the farm manager on a commercial basis of liveweight and body condition, then assessed for pneumonic lesions. RESULTS: Prevalence of extensive pneumonic lesions (greater than or equal to10% lung surface area affected) increased from December to March from 1.4 to 10.1% lambs in Southland (n = 1,917), 1.2 to 25.7% in the King Country (n = 1,831), and 4.7 to 21.4% in Northland (n = 1,485). Overall, 42.3% of lambs in both groups had pneumonic lesions (n = 5,233) and in 15.1% of these, greater than or equal to5% of the lung surface area was affected. Daily weight gain in the month prior to slaughter was reduced in lambs for which greater than or equal to20% lung surface area was affected in Group I (p<0.05) but not in Group II. CONCLUSIONS: The association between pneumonic lesions and reduced weight gain was highly significant in lambs from Group I, but not from Group II. Thus, pneumonia had a negative impact on weight gain, and selection by farmers of lambs for slaughter counteracted this association in lambs from Group II. CLINICAL RELEVANCE: Pneumonia occurs commonly as a subclinical disease in lambs in New Zealand. It has no public health significance and is not recorded at slaughter plants nor reported to farmer clients. Effects of the disease on daily weight gain of lambs is considerable and costly for lamb producers.