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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. No one anthelmintic compound is effective on all parasites so it is common for compounds to be combined to create a broad spectrum dewormer.

Macrocyclic lactones

Macrocyclic lactones or mectins are one chemical class of anthelmintics. There are two types of mectin: avermectin is one of them. At the present time there are two types of avermectin on the market in Australia for horses: ivermectin and abamectin. Milbemycin is the other class of mectin. Moxidectin is currently the only one sold for horses.

All of the mectin compounds have the same mode of action: they interfere with GABA mediated neurotransmission; this disrupts the neuromuscular functions of such parasites, causing paralysis and subsequent death of the parasite.

Mectins have excellent efficacy against the majority of major parasites of horses, they do not, however, treat tapeworms. Resistance to mectins has been reported in ascarids, and this should be taken into consideration if you are targeting this worm. There have also been reports of treatment failures of mectins when used against pinworms. This has not yet been proven to be resistance, but should be kept in mind if pinworms are a problem.

Praziquantel

Praziquantel is a member of the chemical family called the pyrozene family. It’s 100% effective against tapeworms. Praziquantel increases the permeability of the tegument and muscle cells to calcium ions. This results in parasitic muscular contraction and paralysis. Once this is accomplished the parasite is vulnerable to the horse’s immune system, resulting in detachment and disintegration of the tapeworm.

Benzimidazoles

Benzimidazoles or BZs are another type of anthelmintic compound. BZs inhibit the metabolism and cellular division of parasites, effectively starving the worm and inhibiting egg production. BZs treat the majority of worm species, but have no efficacy against tapeworms or bots. There are several forms of BZ available, including oxfendazole, fenbendazole and oxibendazole.

Cyathostome (small strongyle) resistance to the BZ family is widespread throughout Australia and will be found on the majority of Australian horse properties. Before using a straight BZ wormer on your property to treat cyathostomes, you should conduct a faecal egg count to establish if they are still effective. If a faecal egg count has not been done and you wish to use a BZ based wormer, you should use a combination product like Strategy-T that does not have the same resistance issues.

Tetrahydrapyrimidines

Tetrahydrapyrimidines, or THPs, produce paralysis of the parasite by neuromuscular stimulation allowing the parasites to be passed out by the normal action of the gut. THPs treat the majority of worm species but have no effect on bots. There are only two types of THP – pyrantel and morantel.

There have been limited reports of resistance to THPs in cyathostomes. This is not widespread in Australia, but it may still be preferable to use a combination product.

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