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Endoparasites - Control

 

Control of Parasites

Sheep raised on wet pastures often have heavy parasite loads. As mentioned earlier, moisture and warmth are usually necessary for the development of infective roundworm larvae. These conditions promote the buildup of dangerous pasture populations of Strongyloides by allowing the free-living life cycle.

Moisture on the pasture may also increase sheep 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 tyre tracks fill with water. If pastures are drained, the snails die off, and sheep are less likely to become infected with liver flukes.

 

Management Practices

Parasitic disease can be avoided by exercising good livestock management. Essential strategies are 1) to prevent the accumulation of large numbers of infective larvae on pasture, and 2) to limit infections by keeping sheep, especially the young, off contaminated pastures during times when numbers of larvae are high. Generally, this can be accomplished by manipulation of grazing schedules and the strategic use of anthelmintics to kill internal parasites. The goal is to provide safe pastures that do not contain a dangerously high number of larvae. Infected animals must be treated before they are permitted to graze such areas. These protective measures are directed especially at the young, more susceptible animals.

The greatest parasite threat to lambs is the high numbers of larvae from the post-parturient rise in egg output from ewes. This threat to the young may be reduced by treating ewes with anthelmintics before and after lambing and by treating lambs at weaning time. Development of controlled release capsules has greatly assisted this goal. It is most helpful to move the lambs to a safe pasture immediately after treatment. In this way, they will carry few egg-producing adult parasites to the new grazing area.

Another strategic time to treat lambs is in the autumn as infections are developing from the second peak of larvae on pasture. At this time, animals should again be moved to a new grazing area.

Producing a safe pasture requires an absence of animals for up to 12 months. During this time, larvae will die off in great enough proportions to reduce the risk of infection to sheep. Often the expense of leaving land idle for so long is prohibitive. Another option is alternative grazing, which refers to rotating the type of livestock that graze on each pasture. For example, sheep and cattle may be alternated on a given area. Parasites of cattle are generally not parasitic for sheep. While the cattle graze, larvae of the sheep parasites are dying. A notable exception to the safety of alternate grazing is cattle infected with Haemonchus, which may cause serious disease in sheep.

If sheep are fed supplementary feed, the ration should be well balanced, with adequate vitamins and minerals. This will strengthen the animals' resistance and build up their iron reserves. Weight losses caused by parasitic infection may thus be minimised.

 

Anthelmintic Control

 
Injecting sheep with a parasiticide   Using an automatic drenching gun to dispense a liquid parasiticide

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 anthelmintics 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 residual toxic substances in the host that are harmful 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.

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

A revolutionary advance in the control of internal 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 was effective against the adults and also killed eggs and larvae. It was also very safe. Other benzimidazoles followed the introduction of thiabendazole.

In the late 1960's, levamisole was introduced by Janssen Pharmaceutical in Belgium. This product is effective against many nematode parasites of sheep and was more effective than most other currently available products against the lungworm (Dictyocaulus).

The organophosphorus compounds, including dichlorvos, haloxon and many others, are effective against nematodes in the upper gastrointestinal tract. However, they have little effect on worms in the large intestine. 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. However, if other organophosphorus compounds are used to treat external parasites, the two used at the same time may be sufficient to harm the sheep. Currently organophosphate compounds have only limited use in New Zealand for the control of internal parasites.

Chlorinated hydrocarbons have been used to kill cestode and trematode parasites. The first compound used was carbon tetrachloride. This was followed by hexachlorophene and a variety of other compounds. However, because of many residual problems that arose with the chlorinated hydrocarbons, these products have been withdrawn from use in most, if not in all, countries.

The salicylanilide group of compounds such as rafoxanide is very effective against flukes.

Classical anthelmintics have a number of deficiencies. Some of these deficiencies are:

  • Resistant strains of parasites have developed which reduce the effectiveness of some of the better anthelmintics.
  • Some anthelmintics when administered to sheep 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 sheep are infected by more than one parasite species, as is usually the case.
  • With the exception of the avermectin/milbemycin group few compounds are consistently effective against arrested or migrating larvae nematodes.

           

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