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The Institute of Food and Agricultural Sciences (IFAS) is an Equal Opportunity Institution authorized to provide research, educational information and other services only to individuals and institutions that function with non-discrimination with respect to race, creed, color, religion, age, disability, sex, sexual orientation, marital status, national origin, political opinions or affiliations. For more information on obtaining other UF/IFAS Extension publications, contact your county’s UF/IFAS Extension office. U.S. Department of Agriculture, UF/IFAS Extension Service, University of Florida, IFAS, Florida A & M University Cooperative Extension Program, and Boards of County Commissioners Cooperating. Nick T. Place, dean for UF/IFAS Extension. Gastrointestinal infections are the main health problem for grazing sheep in the southern United States (US). The humid environmental conditions from this region are ideal for survival and growth of Haemonchus contortus, the most pathogenic gastrointestinal parasite. Infections with H. contortus are characterized by weight loss, anemia, reduced performance, and if untreated, death of susceptible individuals (Miller et al. 1998). In the US, anthelmintics have been used for the control of gastrointestinal nematodes of sheep for more than 30 years, with considerable development of anthelmintic-resistant populations being an unfortunate consequence. Alternative practices to control gastrointestinal nematode infections have been targeted to address anthelmintic resistance. One promising alternative approach is breeding for resistance to gastrointestinal nematode infections. Resistance to these infections is most likely based on inheritance of genes that are important in the expression of host immunity. Several breeds of sheep around the globe are known to be relatively resistant to gastrointestinal parasites. Using resistant sheep breeds exclusively or in crossbreeding programs could lead to improved resistance to nematode infection. The main benefits of using selection to enhance parasite resistance in sheep breeding programs are that genetic change is a permanent attribute of that animal, its performance is improved, and infectivity of pastures is decreased as a result of fecal egg reduction, leading to additional benefits for the susceptible and young sheep grazing in the same flock. The Florida Native sheep, commonly known as Florida Cracker sheep, is one of the oldest breeds of sheep in North America. However, this heritage breed is now on the endangered list of sheep breeds. To ensure genetic diversity and long-term survival of the breed, protection of the current genetic stock is critical and conservation efforts are required to promote its breeding and production.

The Genetics of Resistance to Gastrointestinal Parasites in Florida Native Sheep