REVIEW Small ruminant resistance against gastrointestinal nematodes: a case of Haemonchus contortus Hafiz A. Saddiqi & Abdul Jabbar & Muhammad Sarwar & Zafar Iqbal & Ghulam Muhammad & Mahrun Nisa & Aasif Shahzad Received: 10 December 2010 / Accepted: 27 July 2011 / Published online: 14 August 2011 # Springer-Verlag 2011 Abstract Gastrointestinal nematode (GIN) infections are a common constraint to small ruminant industry throughout the world, and among those, haemonchosis has its own significance. Control of GIN primarily relies on the use of anthelmintics, but this approach has become less reliable due to the development of resistance in GINs against commonly used anthelmintics and an increased consumer demand for environmentally friendly animal products. These issues have stimulated investigations to find alterna- tive sustainable control strategies, which are less reliant on anthelmintic input. One of such strategies is breeding of small ruminants for their resistance to the GINs. The susceptibility and resistance of animals to GIN infections varies within and between breeds. Various parasitological, biochemical and immunological parameters are employed to evaluate natural resistance status of animals both in natural pasture and artificial infections. The immune mechanisms responsible for resistance are not completely understood, but it has a significant effect in inherited resistance. Relatively resistant or tolerant animals show better local and generalised immune response as compared to susceptible. Immune response against GINs is influenced by many physiological factors. Determination of specific genes linked with host resistance will provide a valuable approach to find out the molecular mechanism of host resistance to GINs. Resistance has been reported to reduce pasture contamination, which in turn reduces re-infection and thus the requirement of the frequent anthelmintic treatments. The efficiency of control can be increased through objective and accurate identification of genetically tolerant individuals by natural and artificial infections with GINs. Complete resistance is the ultimate solution, but this has generally been ignored as a commercial reality. This paper reviews the published reports on natural resistance in small ruminants and discusses the prospects of developing small ruminants, which could be resistant to GINs. Introduction Gastrointestinal nematodes (GINs) pose a major threat to the productivity of small ruminants, and the economic impact inflicted by these parasites is quite substantial throughout the world (Barger and Cox 1984; Larsen et al. 1995; Campos et al. 2009). For example, losses due to GINs are over AUD 400 million annually only in Australia (Sacket et al. 2006). A number of GINs (e.g. Haemonchus spp., Trichostrongylus spp. Teladorsagia spp., Cooperia spp. Nematodirus spp. and Oesophagostomum spp.) are H. A. Saddiqi (*) Department of Zoology, Government College University Faisalabad, Faisalabad, Pakistan e-mail: abubaker_79@yahoo.com H. A. Saddiqi : A. Jabbar : Z. Iqbal Department of Parasitology, University of Agriculture, Faisalabad 38040, Pakistan M. Sarwar : M. Nisa : A. Shahzad Institute of Animal Nutrition and Feed Technology, University of Agriculture, Faisalabad 38040, Pakistan G. Muhammad Department of Clinical Medicine and Surgery, University of Agriculture, Faisalabad 38040, Pakistan A. Jabbar Department of Veterinary Science, The University of Melbourne, 250 Princes Highway, Werribee, Melbourne, Victoria 3030, Australia Parasitol Res (2011) 109:1483–1500 DOI 10.1007/s00436-011-2576-0