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Endoparasites - Control

Control and Management

Control measures must be adapted to the climatic, topographical, and management conditions of each individual area. Anthelmintic treatment of animals is generally necessary both to reduce worm burdens and to decrease pasture loads of infective eggs and larvae.


Management Practices

Pasture management is very important. Drainage of marshy pasture decreases larval survival, particularly that of Dictyocaulus and Fasciola (not significant in New Zealand), as well as interfering with mosquito proliferation necessary for Onchocerca (not significant in New Zealand) and Setaria (not significant in New Zealand) transmission. Permanent pastures tend to develop thick mats, spaces between the soil and the ground cover which retain high humidity and are ideal for larval nematode development. Mats can be broken by ploughing and reseeding old pasture land. Rotation of pastures or alternate grazing with cattle allows time for the larvae to die. Proper manure disposal will aid in prevention and control. The protection of young susceptible animals is of primary importance in parasite management. The first step is to ensure clean quarters by treating brood mares and moving them to uninfected pastures before foals arrive. Young animals should be checked frequently for worms and treated accordingly.


Anthelmintic Control

Dosing a horse with anthelmintic paste

Chemicals used to control nematodes, cestodes, and trematodes are termed anthelmintics. Anthelmintics may eliminate parasites in a variety of ways; for example, by paralysing them and allowing the host to expel them, by halting their ability to metabolise nutrients and thereby killing them, and by limiting their ability to reproduce. Different pharmaceuticals may act in one or more of these ways. Anthelmintics should have the following characteristics:

  • Dosing a horse with anthelmintic paste
  • Exhibit a high level of toxicity to the parasite, when used under a variety of conditions. Parasites vary greatly in their susceptibility to different drugs, determined by numerous factors, including the strain of parasite present, its life cycle stage, and hypobiotic conditions.
  • Have a wide therapeutic index or margin of safety; that is, the dose of the compound used should be toxic to the parasite without causing any damage to the host.
  • Be easy to administer.

Since the beginning of this century many pharmaceuticals have been used to destroy parasites. The earliest compounds were relatively ineffective, but much progress has been made in recent decades. Cupric sulfate, lead arsenate, tin compounds, preparations of chenopodium oil, and nicotine were some of the first chemicals used to combat parasitic infections, but they either had poor efficacy or were toxic to the host. The use of carbon tetrachloride began in the 1920´s and was a significant improvement in parasite control, especially of trematodes. This compound, however, was quite toxic to the host.

The discovery of phenothiazine in the late 1930´s was the first major breakthrough in the development of anthelmintics. This drug has a wide margin of safety and has provided limited control of Parascaris, Oxyuris, and Trichostrongylus.

A revolutionary advance in the control of internal parasites came with the introduction, in the early 1960´s by Merck & Co., Inc., of thiabendazole, the first of the benzimidazole class of compounds. This product has been extremely successful in the control of a wide range of nematodes. It is effective against the adults and also kills eggs and larvae. It is also very safe. Other benzimidazoles followed the introduction of thiabendazole.

Presently, a number of different generic forms are marketed. The benzimidazoles control most of the equine gastrointestinal worms, including large and small adult strongyles, Parascaris, Strongyloides, and Oxyuris. Strongylus vulgaris larvae may be killed by high doses of some of these compounds. The major drawback to the wide use of these drugs is that certain parasites, particularly the small strongyles, are developing resistance. These compounds work by interfering with the energy metabolism of parasites.

The tetrahydropyrimidines, including the products pyrantel, morantel, and oxantel, were also introduced in the late 1960´s. They have a wide spectrum of activity against nematodes. Parasite resistance to pyrantel has also been reported in horses.

The organophosphorus compounds, such as dichlorvos, haloxon, etc., are used for treatment of stomach bots (Gastrophilus larvae). These compounds are cholinesterase inhibitors, which are toxic to both parasites and the host. Cholinesterase is a compound that plays an essential role in the function of the nervous systems of both parasites and their host animals. Used at the correct dosage, they kill the parasites without damage to the host. Manufacturers´ contraindications indicate that organophosphorus compounds should not be used concomitantly with other cholinesterase inhibiting compounds.

The older anthelmintics had a number of deficiencies. Some of these deficiencies are:

  • Resistant strains of parasites have developed that reduce the effectiveness of some of the better anthelmintics.
  • Some anthelmintics, when administered to horses, cause undue stress and can be toxic. Many anthelmintics have limited spectra of effectiveness and thus several different treatment regimens have to be given if horses are infected by more than one parasite species, as is usually the case.

Few drugs are effective against migrating or arrested larval nematodes.

The introduction of ivermectin (EQVALAN) overcame many of these disadvantages. The compound has a wide margin of safety and can be used in pregnant mares and breeding stallions; it has a very wide spectrum of activity; and is active against all important worms of horses. In particular, ivermectin provides unsurpassed control of the migrating larval stages of the large strongyles.

Additional products containing anthelmintics of the same anthelmintic action family (the avermectin/milbemycin compounds) have become available.


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