Comparing control strategies for parasitic gastroenteritis in lambs grazed on previously contaminated pasture - a network modeling approach

In practice, protection against ovine ostertagiasis in sheep-management systems is generally achieved by anthelmintic treatment and pasture switching. This paper uses a network model to describe the dynamics of the life-cycle of the parasite when ewes and lambs commence spring grazing on pasture which is contaminated. The model enables the efficacy of several recognised control strategies, involving various frequencies of drug application, to be assessed in regulating the adult worm burden of the lamb. The results indicate that dosing ewes at lambing does not provide adequate control. If lambs only are dosed 6 times between May and September significant control is achieved. Combining ewe and lamb treatments only marginally enhances the level of control obtained by the treatment of lambs alone. However, the most effective method of control was to dose lambs 3 times in May-June and on moving lambs to clean pasture at weaning.