Selective deworming — why the faecal sample replaces the calendar wormer

For decades the firm rule applied: all horses of a herd are dewormed several times a year by calendar. This scheme has paid a high price — the wormers increasingly lose their effect. Resistances against several active substance classes are today documented worldwide, and new anthelmintics are not in sight. It is exactly here that selective deworming sets in.

Few horses carry most of the worms

The technical basis is a remarkable observation: the worm burden is very unevenly distributed in the herd. A large part of the adult horses sheds only few eggs and hardly contributes to pasture contamination, while a minority spreads the main part of the eggs. In many herds 50 to 75 percent of the adult horses are low shedders. To treat them all across the board brings hardly any benefit — but drives the resistance.

Selective deworming turns the principle around: instead of by calendar, deworming is by need. A faecal sample determines the egg count per gram of faeces (EpG), and only horses above a threshold value are treated. Thus a part of the worm population stays untreated and so not exposed to the selection pressure — this so-called refugia is the key that slows the resistance development.

Important: the threshold value is no rigid number, but is set veterinarily and farm-individually; widespread are guidance values between 200 and 500 EpG. Just as important is what the faecal sample does not show: tapeworms and the larval stages of small strongyles are not reliably captured in the simple EpG, and foals as well as young horses follow their own rules. Therefore the selective scheme replaces no professional support, but structures it.

Understanding the resistance — and sparing the agents

Resistance arises where worms survive the active substance and pass on their genes. The most important drivers are known: frequent group treatments, the repeated use of the same active substance class and underdosing. Each of these practices selects out the resistant survivors. The selective strategy reduces the number of treatments and maintains an untreated refugia — both slow the adaptation of the worms.

Whether an agent used still works at all in the respective herd can only be checked, not assumed: a control faecal sample around two weeks after the treatment shows via the egg count reduction whether the active substance has taken effect. This efficacy test belongs in the veterinary support — the selection and dosing of a concrete anthelmintic anyway.

Pasture hygiene is half the battle

The most effective step demands no medicine at all: if the paddocks are regularly dunged off — ideally once to twice per week — that interrupts the infection cycle at the root, because the larvae do not reach the pasture in the first place. In addition, pasture rotation, mowing and clearing as well as co- or after-grazing by ruminants, which take up the horse strongyles without being a host themselves, take effect. A well-considered pasture hygiene lowers the infection pressure often more strongly than any additional wormer.

Selective deworming is thus no austerity programme, but resource protection: it preserves the efficacy of the few available agents for the horses and situations in which they are really needed. Faecal sample instead of calendar, dunging off instead of routine — that is the core of a sustainable parasite management.