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Grazing ruminants are affected by a range of internal (e.g. worms, fluke, protozoa) and external parasites (e.g. sheep scab). As a general rule these diseases are most prevalent in young, susceptible stock and usually occur at certain periods in their first grazing season. In contrast, sheep scab represents a threat to stock of all ages. Although most disease occurs in young stock, scab and fluke are primarily an issue in older animals (Moredun Research Institute, 2010).

External parasitism results in poor quality sheep and goat products especially skins and lost income to producers. Common external sheep and goat parasites include ticks, lice, ked and mites. Some parasites feed on blood causing blood-loss anemia, especially in young animals. The result is unthrifty, poor-performing sheep and goats. A regular program of treatment and prevention of external parasites should be an important part of a flock health program. The benefits of an effective external parasite control program include increased comfort for animals, improved performance, and higher quality of products (Sileshe et al., 2010).

In most sheep-raising areas, internal parasites (i.e. worms) are usually the primary disease affecting sheep and lambs (Susan, 2006). Sheep are more susceptible to internal parasites than most other types of farm livestock. Their small fecal pellets disintegrate very easily thus releasing the worm larvae onto pastures (Susan, 2006). They graze close to the soil surface and to their feces. They are slow to acquire immunity. It takes 10 to 12 months for most lambs to develop immunity to parasites. Sheep also suffer a loss of immunity at the time of lambing, which does not restore itself until approximately four weeks after lambing (Susan, 2006). Heavy stocking rates and insufficient pasture rest periods further contribute to the incidence of parasitic disease in sheep and lambs. Internal parasites tend to be much less of a problem under range-type conditions where sheep do not graze the same pasture twice in the same grazing season. They are also less of a problem in arid regions, because parasites require moisture for their development (Susan, 2006).

Parasitism in sheep and goat is a substantial problem plaguing farmers across the nation. As gastrointestinal parasite infection is the most important limiting factor of sheep productivity, parasitism has a highly detrimental effect on the sheep industry (Jones, 2001). Outbreaks are most severe in warm, humid climates; the optimum temperature for larval growth is between 50° and 80° Celsius and the optimal rainfall is at least 5 centimeters. A climate that is too hot or dry can kill most larvae on the pasture. Larvae numbers peak in late winter and early spring. Haemonchus has a life cycle lasting approximately four weeks. When ingested, the larvae travel to the abomasum of the animal, where they burrow into the mucosa and develop into true adults in 21 days (Fitch, 2006). While in the abomasums, female adults can lay over 5000 eggs per day (Scarfe, 2006). Roughly 10,000 adult Haemonchus worms can kill a sheep. The eggs are deposited in feces. After approximately 24 hours, the eggs hatch on grass in pastures and under optimal conditions, become infective in five to seven days (Fitch, 2006).

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