HORSE DEWORMERS – do you know what you’re treating with?
Deworming drugs are called anthelmintics. Helminth = any type of worm, and ant = against, so anthelmintic = drug that kills worms.
There are many classes of anthelmintics available for treating animals, however in Australia, there are only a handful of anthelmintics approved for use in horses. Below is a table with short descriptions of these drugs.
It is important to note that anthelmintics can either be broad-range, or narrow-range. Broad-range anthelmintics will kill a wide range of worm types (hence broad-range), will narrow-range generally only target one or two species/groups of worms.
In Australia, there are two main classes of broad-range anthelmintics approves for use in horses: avermectins and Benzimadazoles.
The avermectin class includes ivermectin, abamectin and moxidectin. Not only will these drugs kill all roundworms but they are also insecticides and therefore will kill bot-flies as well. Avermectins are fat-soluble compounds, and as such are present for the longest time in the body after treatment. It has been claimed that moxidectin will remain in the body at high enough levels after treatment to prevent re-infection for up to two weeks.
Benzimadazoles, or BZs, include fenbendazole and oxfendazole. These are often marketed as ‘rotational’ (rotational in quotation marks because rotation is a myth – There’s a blog post on the website if you’d like to see why). BZs aren’t as broad-range as the avermectins. While they will treat for all round-worm types, they are not insecticides and so will not kill bot-flies. Due to the mode of action of BZs, they are usually prescribed for removal of ascarid infection over the avermectins. Unlike the avermectins, BZs do not remain in the body for a long time after dosing, and so reinfection occurs almost straight away.
While the Tetrahydropyrimadines are also considered broad-range, these drugs are not as effective as the avermectins or BZs, and as such you cannot purchase them individually – they are always sold in combination with either an avermectin or a BZ.
The last group on the list is the Pyroxenes, of which we only have praziquantel available for horses in Aust. Praziquantel is considered narrow-range, as it only treats for tapeworm. However: this is important as tapeworms are flatworms (cestodes), while most other worm types (strongyles, pinworms, ascarids) are roundworms (nematodes). Again, praziquantel is only sold in combination with an avermectin or a BZ, usually marketed as a ‘plus-tape’.
If you only worm once a year, it’s best to pick a drug combination that will hit as many parasites as possible - the best choice for this is a dewormer that is a combination of an avermectin (to treat for bot flies and strongyles/roundworms) and praziquantel (to treat for tapeworms). These are often marketed at "... plus tape".