Horses can contract infections with various types of worms, also known as internal parasites. These worms can cause considerable damage in the horse’s body and therefore affect its health. One way to combat worms is to deworm your horse with a deworming agent. To be able to do this as effectively as possible, you can have a fecal test done. 

But before we explain what a fecal test is, it is useful to know how worms live. Most worm species have similar life cycles. The female worms settle in the horse’s gastrointestinal tract and lay their eggs here. These eggs are then excreted with the horse’s manure. As soon as the manure ends up on the pasture, the eggs hatch and the larvae develop. The larvae then crawl out of the manure and move to the grass. A horse takes a bite of the grass on which the larvae sit and the larvae end up in the gastrointestinal tract. Once inside the horse, the larvae develop into adult worms. It differs per worm species where and how this development takes place. Then the adult worms will lay their eggs again and the whole cycle starts again. 

So you can imagine that, without our interference, this process continues in a vicious cycle and the horse’s worm burden increases and increases. To put a stop to this cycle, you can deworm your horse. The big problem with this is that worms can become resistant to certain deworming agents. Resistance means that the worms become insensitive to a certain substance, they have built up an immunity. If a worm species has built up resistance to all existing deworming agents, this would mean that the infected horse can no longer be treated. It is therefore important to prevent resistance as much as possible. You can do this by deworming horses only when they have a significant worm burden and by giving the correct dosage of deworming agent. In addition, not every agent is suitable for every type of worm. Some worm species have a different life cycle, which must be broken in a different way. It can therefore be very difficult to give one standard deworming policy. 

One way to limit resistance and implement an effective deworming policy is to have a fecal egg count done. During the fecal test, the number of worm eggs in the horse’s manure is examined under a microscope. This is indicated as EPG (number of eggs per gram). The number of eggs and the type of eggs then determine whether treatment is necessary and, if so, by what means. When doing a fecal test, you collect fresh manure from the horse in a sealable plastic bag. You then take the manure to your vet or send it to a specialized laboratory. They examine the manure and give you advice based on the results. There are also worms that do not leave eggs in the manure. Blood tests are required to show whether these worms are present in the horse. 

Fecal tests and deworming are not the only things you can do to combat worms in your horse. Hygiene and pasture management also play a major role in this. As described above, the larvae crawl out of the manure onto the grass. By mucking out the field or paddock at least twice a week, you limit the chance of contamination. It can also be beneficial to alternate grazing with cows or sheep. They will eat the worms, which cannot survive in their bodies, breaking the worm’s life cycle. You may assume that almost every horse in for example the Netherlands will regularly have a worm infection. Regular tests are therefore very, very necessary!