The effect of Johne's disease on production traits in Romney, Merino and Merino x Romney-cross ewes
New Zealand Veterinary Journal, Volume 54, Issue 5, pp 204-209, Oct 2006
Article class: Scientific Article
Subject Terms: Animal industries, Animal production/wastage, Bacterial, Bodyweight/liveweight/condition score, Breed/breeding, Disease/defect, Farm/farm management, Infectious disease, Integument/skin/wool/hair/fur/feather, Mycobacterial, Reproduction, Twinning/parity, Wasting disease/disorderTaylor and Francis
AbstractAIM: To quantify the effects of clinical Johnes disease on the performance of Romney, Merino and Merino x Romney-cross ewes.
METHODS: The performance of ewes was compared using eight birth cohorts (19711978). Merino and Merino-cross genotypes included New Zealand Merino and Australian Superfine Merino sources. Intensive monitoring of Johnes disease was undertaken over the production years 19751982. Positive diagnostic evidence of Johnes disease was established post mortem from lesions of granulomatous enteritis associated with high numbers of acid-fast bacilli. Over years, data on a total of 2,341 Romney ewes and 1,292 Merino and Merino x Romney-cross ewes were recorded, consisting of annual records of liveweight (LWT), greasy fleece weight (FWT), number of lambs born per ewe per year (NLB), and lifetime productivity of ewes.
RESULTS: A total of 82 (3.5%) Romney ewes and 62 (4.8%) Merino and Merino x Romney-cross ewes were diagnosed with clinical Johnes disease over the 8-year monitoring period, equivalent to 0.9% and 1.2% annual cases for these breeds, respectively, of the ewes present at mating. The percentage of clinical cases (p<0.04) and the age at death from Johnes disease (p<0.02) were lower for Romneys than for Superfine Merinos. The mean age of death from Johnes disease was 3.41 (standard error (SE) 0.06) years, lower than the mean disposal age from the flock of 5.03 (SE 0.02) years for clinically normal ewes (p<0.001). In their final year of production, ewes with clinical Johnes disease had lower LWT by 5.3 kg (10.5% of the mean; p<0.001), lower annual FWT by 0.54 (SE 0.10) kg (14.2%; p<0.001), fewer NLB by 0.15 (SE 0.07) lambs (13%; p<0.05), and lower litter weaning weights by 3.6 (SE 1.3) kg (15%; p<0.01) compared with clinically normal ewes. The size of the production losses associated with Johnes disease depended on the age to which ewes survived. Considering all production years of ewes (up to 8 years), the total weight of lambs weaned by ewes with clinical Johnes disease was 30.9 (SE 3.4) kg lower (46%; p<0.001) than the total from clinically normal ewes.
CONCLUSIONS: Clinical Johnes disease led to significant losses in LWT, FWT, NLB, and in the lifetime production of ewes, amounting overall to a 46% reduction in productivity (p<0.001).
CLINICAL RELEVANCE: Productivity losses from clinical cases of Johnes disease would be of considerable economic importance in flocks with a high incidence of the disease. The lack of good diagnostic tests for Johnes disease in the live animal, and the lack of active surveillance programmes, has made it difficult to establish the true prevalence of Johnes disease in sheep flocks in New Zealand, and its economic consequences.
KEY WORDS: Johnes disease, sheep, productivity, liveweight, fleece weight, lambs born
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