A stochastic model accommodating the FAMACHA© system for estimating worm burdens and associated risk factors in sheep naturally infected with Haemonchus contortus

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dc.contributor.author Reynecke, D.P. (Dean Peter)
dc.contributor.author Van Wyk, J.A. (Jan Aucamp)
dc.contributor.author Gummow, Bruce
dc.contributor.author Dorny, P.
dc.contributor.author Boomker, Jacob Diederik Frederik
dc.date.accessioned 2011-11-09T11:15:46Z
dc.date.available 2011-11-09T11:15:46Z
dc.date.issued 2011-05
dc.description.abstract A previously developed multiple regression algorithm was used as the basis of a stochastic model to simulate worm burdens in sheep naturally infected with Haemonchus contortus over five consecutive Haemonchus seasons (November to January/February) on a farm in the summer rainfall region in South Africa, although only one season is discussed. The algorithm associates haemoglobin levels with worm counts in individual animals. Variables were represented by distributions based on FAMACHA© scores and body weights of sheep, and Monte Carlo sampling was used to simulate worm burdens. Under conditions of high disease risk, defined as the sampling event during the worm season with the lowest relative mean haemoglobin level for a class of sheep, the model provided a distribution function for mean class H. contortus burdens and the probability of these occurring. A mean H. contortus burden for ewes (n=130 per sample) of approximately 1 000 (range 51 to 28 768) and 2 933 (range 78 to 44 175) for rams (n = 120 per sample) was predicted under these conditions. At the beginning of the worm season when the risk of disease was lowest (i.e. when both classes had their highest estimated mean haemoglobin levels), a mean worm burden of 525 (range 39 to 4910) for ewes and 651 (range 37 to 17260) for rams was predicted. Model indications were that despite being selectively drenched according to FAMACHA© evaluation, 72% of the ewes would maintain their mean worm burden below an arbitrarily selected threshold of 1 000 even when risk of disease was at its highest. In contrast, far fewer rams (27%) remained below this threshold, especially towards the end of the worm season. The model was most sensitive to changes in haemoglobin value, and thus by extrapolation, the haematocrit, which is used as the gold standard for validating the FAMACHA© system. The mean class haemoglobin level at which there was a 50% probability of worm burdens being ≤1000 worms was 7.05 g/dl in ewes and 7.92 g/dl in rams. en
dc.description.sponsorship This work was carried out within the Framework Agreement between the Directorate General for Development Cooperation, Belgium and the Institute of Tropical Medicine, Antwerpen (Belgium – Grant No AG534) with the Department of Veterinary Tropical Diseases of the University of Pretoria Faculty of Veterinary Science, and with the support of the EU “PARASOL” FOOD-CT-2005-022851 FP6 project. en
dc.description.uri http://www.elsevier.com/locate/vetpar en
dc.identifier.citation Reynecke, DP, Van Wyk, JA, Gummow, B, Dorny, P & Boomker, J 2011, 'A stochastic model accommodating the FAMACHA© system for estimating worm burdens and associated risk factors in sheep naturally infected with Haemonchus contortus', Veterinary Parasitology, vol. 177, no. 3-4, pp.231-241. en
dc.identifier.issn 0304-4017 (print)
dc.identifier.issn 1873-2550 (online)
dc.identifier.other 10.1016/j.vetpar.2011.01.042
dc.identifier.other 7003908833
dc.identifier.other N-9314-2014
dc.identifier.uri http://hdl.handle.net/2263/17546
dc.language.iso en en
dc.publisher Elsevier B.V. en
dc.relation.requires Adobe Acrobat Reader en
dc.rights © 2011 Elsevier B.V. All rights reserved. en
dc.subject Haemonchus en
dc.subject Targeted selective treatment en
dc.subject Integrated parasite management en
dc.subject FAMACHA© en
dc.subject.lcsh Anthelmintics en
dc.subject.lcsh Sheep -- Diseases en
dc.subject.lcsh Nematoda -- Control en
dc.title A stochastic model accommodating the FAMACHA© system for estimating worm burdens and associated risk factors in sheep naturally infected with Haemonchus contortus en
dc.type Postprint Article en


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