A is for anthelmintics

Drenches, which are more formally known as anthelmintics, are available under an array of brand names, but what's important is the active ingredient - or ingredients.

There are, in fact, only a comparatively modest number of drench families available to us. Each has its strengths and weaknesses and it is useful to understand them.

Most drenches work by paralysing the parasite. This generally has one of two effects. Some will let go their grip on the gut and pass out of the animal. If this isn't the case, the paralysed parasite will be unable to feed, runs out of energy, and dies.

Parasites have no fat reserves so if they're unable to feed they quickly die.

The reason we need to be familiar with the drenches at our disposal is that not all have equal effectiveness. Some target only certain larval stages of some parasites and may not even be able to kill some at all.

Let's start with the stars in the line-up, the so-called macrocyclic lactones which we know better as ivermectin and moxidectin. They are highly effective against gut parasites and also deliver a killer blow to lice and other skin-based parasites.

Parasite control would much trickier without this family batting for our side.

The benzimidazoles are made up of fenbendazole, oxfendazole and oxibendazole. They are effective agents but tend to be at their best in courses delivered over several days.

The tetrahydropyrimidines comprise pyrantel pamoate and pyrantel tartrate. They are fast-acting but kill only adult parasites, meaning worm populations recover rather more quickly than other drenches, given that the maturing larvae survive the dosing.

Piperazine is not commonly used in horses these days and would normally be administered by a vet using a tube into the horse's stomach. In some part of the world it may be available to be administered as a feed supplement.

The last drenching agent is praziquantel, which is an important player because it is the only agent horse owners have in their arsenal which is effective against tapeworms. It is normally sold as part of a combination drench with ivermectin or moxidectin.

It you want to get a hit against tapeworms, you need to be reading the label to ensure the product contains praziquantel.

» Next: Mankind's worming arsenal