Anthelmintic Development
The Attempt at a Solution: Anthelmintics
As horse owners, we must update our knowledge of how and where parasites interact with our horses.
Parasitism Is a Chronic Disease
Parasitism in the horse has become a chronic disease because of our management practices. We have taken horses out of a rural existence and placed them in an urban environment where they are more often exposed to parasite eggs and larvae. It is our responsibility to protect these horses that depend on us for their care. A major step in that responsibility is a good parasite-control program.
Anthelmintics 1890–1940
Turpentine
Boiled linseed oil
Mixtures of asafetida
Areca nut powder
Savin fern extracts
Iron sulfate
Gentian
Bran mash
Tobacco – nicotine
Before 1940 the only compounds used to deal with parasitism were natural substances that had some effect on parasites but also risked some toxicity to the horse. A few worked by increasing movement in the intestines in an effort to expel parasites. Tobacco was popular because nicotine is a mild anthelmintic.
Anthelmintics 1940–1997
Our attempt at trying to rebalance the relationship between horses and parasites, and give the horse some relief, has been the advent of anthelmintics. In the past 50 years, several anthelmintic compounds have been developed. In 1940 it was phenothiazine, in 1950 piperazine. In 1960 benzimidazoles were created, and in 1970 pyrantel was developed. In 1980 came the macrocyclic lactone ivermectin, and in 1997 another macrocyclic lactone, moxidectin, was introduced.
How Anthelmintics Work
All anthelmintics on the market today have their effect on parasites in one of two ways. Either they interfere with the parasite’s energy metabolism or they interfere with their neuromuscular coordination.
Rotational Deworming
Much has been written about rotational deworming. The reasoning is obvious. We need to alternate chemical classes of anthelmintics to reduce the chances of resistance, as well as to maximize the best attributes of each molecule.
Georgraphic differences make it unwise to recommend a single rotational deworming program. Several considerations must be factored in – the area of the country, the climate, risk, and management of each horse. These considerations are best worked out with your veterinarian.
Three-way Rotational Deworming
Benzimidazole
Pyrantel
Macrocyclic lactones (moxidectin, ivermectin)
With the products shown here, we are rotating not only very effective compounds, but also chemical classes: Anthelcide® EQ is a benzimidazole, Strongid® is a pyrantel, and moxidectin and ivermectin are both macrocyclic lactones. Using different brands of dewormers doesn’t ensure that you are rotating chemical classes.
Macrocyclic Lactones
Macrocyclic lactones are one chemical class of anthelmintics. There are two types of macrocyclic lactones. Avermectin is one of them. Today, there is only one avermectin on the market in the United States for horses: ivermectin. Milbemycin is the other class of macrocyclie lactone. Currently, moxidectin is the only one sold for horses.
Mode of Action of Macrocyclic Lactones
All of the macrocyclic lactone compounds have the same mode of action. They interfere with GABA-mediated neurotransmission, causing paralysis and death of the parasite.
Ivermectin and moxidectin are also sold for cattle, swine, sheep and goats. Although there have been no reports of small strongyles developing resistance to these compounds in the horse, there have been many reports of ivermectin resistance in sheep and goats. The possibility of developing resistance exists. Remember, they all have a single mode of action.
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