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Ectoparasites - Control

The problems resulting from 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. Cattle 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 cattle in chemical solutions contained in large concrete vats, a process called dipping. In many countries, it is compulsory by law to dip cattle when they are relocated from parasite-infested areas to "clean" country. In other countries compulsory dipping of all cattle 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 must be added to maintain the proper concentration of chemicals.

The next compound used for dipping was the gamma isomer of benzene hexachloride, introduce in the late 1940´s. In the 1950´s the organophosphates were introduced as dipping agents. The first on the market was diazinon, developed by Ciba Geigy.

 
An in ground cattle dip   Using a pour on insecticide

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. One method of treatment, know as a pour-on, is effective against cattle grubs. 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 cattle if used incorrectly or if used at the same time as an anthelmintic having similar cholinesterase-inhibiting action. Insecticide-impregnated eartags are being utilised to control ectoparasites. Resistance to certain synthetic pyretharoids and organophosphates have been reported. The use of systemic parasiticides, eg. Avermectins is becoming widely accepted.

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