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Ectoparasites - Control

The problems caused by parasite worry and the transmission of disease make control of external parasites essential. Elimination of external parasites from a geographic area is difficult if the parasites involved are capable of living for prolonged periods independent of the host. Ticks, for example, can live in the environment for up to 2 years without feeding. Sheep in areas where many ticks exist are continually reinfested, and treatment must therefore be repeated at frequent intervals.

The control of ectoparasites has been practiced with mixed results over the last 60 years. An extensively used method has been immersion of sheep in chemical solutions either contained in large concrete vats or by spraying the sheep with the chemical. This process is called dipping. In many countries, it is compulsory by law to dip sheep when they are relocated from parasite-infested areas to "clean" country. In other countries compulsory dipping of all sheep at frequent intervals has been enforced.

The first chemicals used for dips were arsenicals. These chemicals were suspended in the water and each animal that passed through the dip removed some of the chemical. This process is termed stripping the dip, and fresh chemicals need to be added to maintain the proper concentration of chemicals.

The next compound used for dipping was the gamma isomer of benzene hexachloride, introduced in the late 1940's. In the 1950's the organophosphates were introduced as dipping agents.

Certain organophosphates, which are absorbed by the animal and circulated throughout its body, were introduced in the 1960's. These are termed systemic compounds. In some cases, their use has provided relatively easier treatment methods than dipping. The organophosphorus compounds are cholinesterase inhibitors. Cholinesterase is a compound that plays an essential role in the function of the nervous systems of both parasites and their host animals. The organophosphates can therefore be dangerous to sheep if used incorrectly or if used at the same time as an anthelmintic having similar cholinesterase-inhibiting action.

Blowflies are best controlled by keeping sheep unattractive to the blowfly. All areas of the sheep must be kept as dry as possible. The wool around the tail of ewes in shorn 2 or 3 times a year. This process is known as crutching.

Wrinkled Merino sheep may be treated by the Mules' operation. The loose skin on the hind legs below and on either side of the tail is cut away in two crescent-shaped cuts. When these heal, the resultant scar tissue stretches the skin tight, and with crutching it will remain dry.

Dipping or "jetting" with organophosphate insecticides will help kill maggots before a strike is established. Jetting is the use of high-pressure jets of liquid insecticide to a specific part of the sheep, such as around the tail or the centre of the back, where water might remain in continuously wet weather.

Ectoparasites of sheep include arachnids and insects. They are responsible for serious economic losses throughout the world.

Ticks and flies tend to be most prevalent in summer months in warm areas, while mites, keds, and lice are more frequently encountered in winter months in cooler climates. Damage caused by ectoparasites includes worry, wool damage, and the transmission of disease organisms.

Ectoparasites can develop resistance to chemicals used for their control. Insecticides previously used were marginally effective and are labor intensive in their application. Descriptions of economically significant ectoparasites of sheep are listed in the following section.

           

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