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Endoparasites - Control

Control

Cattle raised on wet pastures often have heavy parasite loads. As mentioned earlier, moisture and warmth are usually necessary for the development and survivability of infective roundworm larvae. These conditions promote the buildup of dangerous levels of worm larvae on pasture.

Moisture on the pasture may also increase cattle fluke infection by allowing the proliferation of intermediate hosts. Fasciola hepatica, for example, requires an amphibious snail as intermediate host. These snails require small, temporary bodies of water like those left when hoofprints or tire tracks fill with water. If pastures are drained, the snails die off and cattle are less likely to become infected with liver flukes.

Management Practices

Livestock management can be an important part of parasite control by reducing the probability of infection, especially in young susceptible animals. Pastures should not be overstocked, as cattle have more chance of eating infective larvae in crowded pastures. Separating young cattle from adults as early as possible reduces the incidence of infection since older animals are a source of infection. Whilst leaving an area free of animals for a substantial period (ie. Several months) allows infective larvae to die off without being replenished by eggs from infected hosts it is not a particularly practical recommendation but emphasises the point that any practice which reduces larval contamination of pasture is useful.

Drinking water should also be provided in troughs to avoid the use of fluke-contaminated ponds. Finally, if cattle are fed supplementary feed, the ration should be well-balanced with adequate vitamins and minerals. This will strengthen the host´s resistance and help to reduce iron-deficiency anaemia. Weight losses from parasites may thus be minimised.

Young grazing beef calves fed good nutrition and effective parasite control to attain their growth potential in the first year of life. Well-conducted field trials measuring weight gains possible under New Zealand conditions demonstrate this potential well. Trial results from these studies are available from Merial.

In New Zealand sub clinical parasitism is widespread in the beef industry but can be overcome with regular anthelmintic treatment using effective products.

Anthelmintic Control

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, thereby killing them; and by limiting their ability to reproduce. Different chemicals may act in one or more of these ways. An ideal anthelmintic would have the following characteristics:

  • Exhibit a high level of toxicity to the parasite but not to the host, 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, the time of year, and the weather conditions.
  • Have a wide therapeutic index or margin of safety.
  • Be easy to administer to be of practical use.
  • Not leave tissue residues in the host that are not acceptable to humans who consume meat or milk from the treated animal.

Since the beginning of this century many chemicals 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.

This 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, when introduced, provided limited control of Haemonchus, Oesophagostomum, Ostertagia, and Trichostrongylus.

A revolutionary advance in the control of internal nematode parasites came with the introduction, in the early 1960´s, 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, eggs and some immature stages. Because of low levels of residue, it is approved in many countries for use in dairy cows at freshening. Other benzimidazoles followed the introduction of thiabendazole. Presently a number of different benzimidazoles are marketed worldwide.

In the late 1960´s, levamisole was introduced by Janssen Pharmaceutical in Belgium. This product is effective against most nematode parasites of cattle and is effective against the lungworm (Dictyocaulus). It is marketed as an injectable, pour-on or oral drench product. It has been reported to cause adverse effects in stressed cattle, especially dehydrated cattle.

The organophosphorus compounds, including dichlorvos, haloxon, and trichlorfon, have their greatest effect against nematodes in the upper gastrointestinal tract. However, they have little effect on Oesophagostomum radiatum. These compounds are cholinesterase inhibitors, which may be toxic to both the parasites and the host. (Cholinesterase is a compound that plays an essential role in the function of the nervous systems of animals). Used at the correct dosage, they kill the parasites without damage to the host. Manufacturer´s contraindications indicate that organophosphorus compounds should not be used concomitantly with other cholinesterase inhibiting compounds. Organophosphorus compounds have only limited use in New Zealand for the control of internal parasites.

Chlorinated hydrocarbons have been used to kill trematode parasites. The first compound used was carbon tetrachloride. This was followed by hexachlorophene and a variety of other compounds. However, because the chlorinated hydrocarbons leave residues in milk and meat, these products have been withdrawn from use in most, if not all, countries.

The salicylanilide group of compounds such as rafoxanide is very effective against flukes, some roundworms and nasal bots.

The avermectin/milbemycin family of molecules (eprinomectin, ivermectin, doramectin, moxidectin) are extremely efficacious and consistent against the common nematode parasites of cattle including the arrested or migrating larval stages of nematodes. The marketed products of this family are widely used but each has different features which set them apart from each other.

Some of the existing anthelmintics have a number of deficiencies which are listed as follows:

  • Some anthelmintics when administered to cattle 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 the cattle are infected by more than one parasite species, as is usually the case.
  • The administration of liquid (drench), paste, or solid (tablet type bolus) forms is very labour intensive, especially if cattle have to be rounded up and treated at frequent intervals. Recently developed pour-on formulations are convenient and less labour intensive. Controlled or sustained release bolus forms are convenient and effective because they release a controlled dose of anthelmintic for long periods and can provide effective parasite control over an entire season.
  • With the exception of the avermectins few compounds are consistently effective against arrested or migrating larval nematodes.

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