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A parasite is a living organism which lives at the expense of another living organism, known as the host. The parasite gains an advantage from the host, usually without providing any compensation to the host.

Endoparasites live within a host and must obtain nutrients from that animal in order to survive and reproduce. The host's ability to thrive, or even survive, is often decreased as a result of the parasite's presence. Many parasites are specific to a species of animal. For example, Oesophagostomum radiatum, the nodular worm, infects only cattle, whereas another species of the same genus, Oesophagostomum venulosum, infects sheep and cattle. Other parasites, such as Fasciola hepatica, the liver fluke, may infect several different animal species including cattle, sheep, horses, rabbits and goats. Each species of parasite has a predilection for a specific location within the host animal. The common names of many worms reflect location of the adult parasites. Two examples are the large stomach worm or Barberspole worm (Haemonchus contortus), and the stomach hairworm (Trichostrongylus axei).

Parasites that feed or live on the body surface of a host animal are called ectoparasites. Most of them are arthropods, that is, invertebrates with jointed legs and an exo-skeleton. Arthropod ectoparasites fall into two classes, arachnids and insects.

Ectoparasite infestation affects the health of the host animal in several ways. The sheep may be so preoccupied with the itching and irritation caused by ectoparasites that feeding is irregular, and, consequently, the host fails to thrive. Such parasite worry is a problem in almost all infestations. Animals may become emaciated and susceptible to various other sicknesses such as bacterial and viral diseases. Loss of blood resulting from heavy infestations can be serious and cause unthriftiness and loss of condition, greatly reducing income from a flock. Perhaps the most serious aspect of ectoparasite infestation is the ability of some ectoparasites, particularly ticks, to transmit other serious diseases, including louping ill (rickettsial fever), which is a serious disease of sheep in some parts of the world.

Various parasites are found wherever sheep are raised. The worldwide incidence of parasites in sheep and their economic importance are greatly influenced by the geographic location, season of the year, and climatic conditions.

Significant concentrations of sheep

In very general terms endoparasites tend to be a more serious cause of production losses in warmer, moister areas, while ectoparasites probably cause the greatest losses in drier areas. However, there is considerable overlap in the incidence of internal and external parasites. The effects of both types of parasites acting together on sheep flocks are greater than the damage caused by either type of parasite by itself. For example, a flock that is heavily infested with lice will be more severely affected by a sudden buildup in roundworm parasites than would an animal that is free of ectoparasites.

Because of the compound effects of parasites, the use of a broad-spectrum parasiticide will provide more effective treatment than the use of a product that kills only one group of parasites. Additionally, any reduction in frequency and total number of parasiticide treatments will produce savings in labour costs.

On a worldwide basis, the heaviest losses in the sheep industry are caused by the intestinal roundworms. Haemonchus is most important in warm, wet areas, while Ostertagia and Trichostrongylus are especially prevalent in temperate areas. The lungworm, Dictyocaulus, can cause serious damage in sheep flocks in areas of heavy rainfall. The nodular worm, Oesophagostomum, has always been very difficult to control because larval worms in nodules in the large intestine are resistant to most anthelmintics. Liver flukes parasitise sheep in wet areas with swampy niches that provide habitats for the intermediate snail host of Fasciola. They reduce wool production and weight gain, and livers from infected animals are condemned at slaughter.

One of the most significant external parasites of sheep is the sheep blowfly, particularly damaging in Australia, Africa and New Zealand. Sheep struck by the blowfly frequently die. If they survive, the quantity and quantity of their wool is severely reduced.

Ticks are important parasites of sheep. However, losses caused by ticks are much heavier in cattle than in sheep. Mites, particularly Psoroptes, cause mange in sheep in some areas, but well enforced control measures have eliminated this parasite from many sheep-raising areas including New Zealand. The sheep ked is an obvious parasite when it is present in a flock.

Most parasites have complex life cycles. When these life cycles are coupled with climatic conditions, there is a definite seasonal incidence of each parasite particular to each region in which it occurs. This seasonal incidence of parasites is a very important factor to consider when planning a total parasite control program. For that reason the accompanying charts show the approximate variations in activity of some of the major parasitic groups in different climatic regions.

Internal organs showing predilection sites


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