Veterinary Parasitology 189 (2012) 238–249 Contents lists available at SciVerse ScienceDirect Veterinary Parasitology jou rn al h om epa ge: www.elsevier.com/locate/vetpar Research article Exploration of the epidemiological consequences of resistance to gastro-intestinal parasitism and grazing management of sheep through a mathematical model Yan Christian Stephen Mountfort Laurenson a,∗ , Ilias Kyriazakis b , Andrew Barnet Forbes c , Stephen Christopher Bishop a a The Roslin Institute and Royal (Dick) School of Veterinary Studies, University of Edinburgh, Midlothian EH25 9RG, UK b School of Agriculture, Food and Rural Development, Newcastle University, Newcastle upon Tyne NE1 7RU, UK c Merial, 29 avenue Tony Garnier, Lyon 69007, France a r t i c l e i n f o Article history: Received 30 January 2012 Received in revised form 9 May 2012 Accepted 10 May 2012 Keywords: Sheep – nematodes Teladorsagia circumcincta Modelling Epidemiology Host resistance Grazing management a b s t r a c t Predicting the impacts of selection for decreased faecal egg count (FEC) (i.e. host resistance) in grazing ruminants is difficult, due to complex interactions between parasite epidemiol- ogy, management and host responses. A mathematical model including heritable between lamb variation in host-parasite interactions, Teladorsagia circumcincta epidemiology and anthelmintic drenching, was developed and used to (i) address such interactions and their impact on outcomes including FEC, live weight (LW, kg) and pasture larval contamina- tion (PC, larvae/kg DM), and (ii) investigate how grazing management strategies, aimed at reducing host exposure to infective larvae via pasture moves at 40 day intervals, affect these outcomes. A population of 10,000 lambs was simulated and resultant FEC predictions used to assign the 1,000 lambs with the highest and lowest predicted FEC to ‘susceptible’ (S) and ‘resistant’, (R) groups, respectively. The predicted average FEC of the S group was ∼8.5-fold higher than the R group across a grazing season. The R and S groups were then simulated to graze separate pastures (R sep and S sep ); and repeated for 3 grazing seasons to allow pre- dictions to diverge and stabilize. Further, different grazing strategies were superimposed on all groups. PC and average FEC were affected by whether lambs of different resistance genotype grazed together or separately, with differences increasing across grazing seasons. By the third grazing season the average PC of the R sep group was reduced by ∼83%, and the S sep group was increased by ∼240%, in comparison to the whole population average. Aver- age FEC of the R sep group was reduced by ∼40%, and the S sep group increased by ∼46% in comparison to the R and S groups, respectively, whilst drenching had little impact on the proportional differences in FEC between groups. Predicted LW was similar for the R and R sep groups irrespective of anthelmintic treatment, whilst LW of the S sep group was reduced by ∼14% compared to the S group for un-drenched lambs, and by ∼4% for drenched lambs. The differing grazing strategies were predicted to have little impact on FEC or LW, with the exception of the S sep group which was predicted to have a 2 kg increase in LW when drenched and moved to a clean pasture. Together, these results suggest that host genotype has a substantial impact on parasite epidemiology, however the benefits of anthelmintic treatment and grazing management should only be expected for susceptible animals. This supports the use of targeted selective treatment, focussing on susceptible animals. © 2012 Elsevier B.V. All rights reserved. ∗ Corresponding author. Tel.: +44 131 651 9100; fax: +44 131 651 9105. E-mail address: yan.laurenson@roslin.ed.ac.uk (Y.C.S.M. Laurenson). 0304-4017/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.vetpar.2012.05.005