Busting The Jargon

Helminth: a parasitic worm.

Individual approach: All horses respond differently to the same circumstances so it’s imperative to assess every horse independently as well as of a part of the group in which it is kept, when you plan your worming tactics.

Resistance: This is when a parasite population previously controlled by a drug is no longer susceptible to that drug. The active ingredient of the horse wormer kills the sensitive parasites in the population, but those parasites not affected go on to create new generations of resistant parasites. Frequent dosing or under-dosing can cause resistance to occur, as can the effectiveness of the wormer. Under-dosing means that parasites which are only partially susceptible to the drug may survive and reproduce instead of being killed as expected with the correct dose. Dosing correctly and only when necessary is the key to managing the issue of resistance in our horses.

Rotation: This refers to the now outdated practice of using a different class of drug each time you worm your horse to try and prevent resistance to any one wormer ingredient. Rotation alone will not prevent the onset of resistance as there are not enough drug classes to rotate to enable this method to work. Reducing frequency of treatment and selectively treating those horses that need it is more important than rotating a product every time through the grazing season.

Refugia are the worms that, for whatever reason, remain unexposed to the wormer used at each treatment. These worms importantly contribute to the next generation of worms and may then be susceptible to the next wormer used. They helpfully counter the population of worms generated from resistant stock and it is thought that they are important in slowing the development of resistance. The largest and most important source of refugia is parasites on the pasture and those parasites in horses that are not treated. A smaller source is any stages not treated by a wormer (i.e. larval stages).

Routine worming: Worming at pre-set intervals throughout the year – now largely regarded as outdated as it can encourage resistance to occur.

Strategic worming: Worming with a specific wormer at certain times of year, that have been identified as the optimum times to treat for these particular parasite burdens, for example treating for ESRW in the autumn.

Strongyles: the scientific name for redworms. They come in small and large varieties.

Targeted or selective worming: Worming to treat for a specific type of worm in only those animals with a demonstrated need. For strongyles this is most effectively done following a faecal worm egg count (FWEC) from spring through to early autumn and a specific antibody test in late autumn/early winter.