Deworming Horses - Selectively or Systematically?

There is not one right way, but we have taken a closer look at the treatment options.

I. Worms

The grazing season is slowly coming to an end and once again the question of the giving a worm cure arises. Resistance to worm medicines, altered housing conditions, feed quality and other factors affect the required deworming of the horse. How often should a horse be dewormed? Are there any alternative deworming methods? Do all the horses of a property really need to be dewormed? Are feces samples used to investigate worm infestations? What medication is given when and against which worms is the medication useful? While the worm treatment used to be thoughtlessly given four times a year, the issue has now become a very complex topic.

Worms are endoparasites and can be found in every horse. However, important is the concentration of worms per horse. A small number is not hazardous to health and is considered normal. It is important to prevent a mass infestation or to treat it properly. The parasites to be considered especially in horses are the small and large strongyles, roundworms, tapeworms, dwarf-nematodes, pinworm, and the larvae of the ox-warble fly.

Over the years of periodic deworming, more and more resistance against the common worm remedies have developed, which has led to a rethinking in the deworming process. There have always been and are worms that are immune to the worm remedies. If the horse gets dewormed always the same way, the non-immune worms die, but the immune worms can continue to multiply. Holistic deworming becomes increasingly difficult if the number of resistant parasites increases.

Table 1) Traditional deworming intervals with the risk of increased resistance development



Active Substance

Before the Start of the Grazing Season

Small and Large Strongyles, Roundworms, Dwarf-Nematodes

Ivermectin, Pyrantel

During the Grazing Season


Moxidectin, Ivermectin, Pyrantel

At the End of the Grazing Season

Round- and Tapeworms

Praziquantel, Ivermectin


Roundworms and Tapeworms and Larvae of the Ox-Warble Fly

Ivermectin, Praziquantel, Moxidectin


II. Proper Prophylaxis and Alternatives

With proper prophylaxis, a mass infestation can already be well counteracted. Above all, the hygiene on the grazing land, paddocks and within the box is important. Regular cleaning can already reduce the risk of infections. Overgrazing by too many horses should also be avoided. Ideally, a maximum of two horses is expected per hectare (1000m²) grazing area, this area should be changed regularly. Also, silage or lack of food quality cause the generation of worms and infestation of horses. Despite all measures, a worm infestation cannot be ruled out and should be treated. Modern medicine is seeking alternative methods of fighting parasites, due to the increased resistance.

- Selective Deworming

The principle of selective or contemporary deworming is relatively simple. In order to avoid deworming of the entire stable, feces-samples are collected, those get examined in the laboratory for parasitic eggs. Only horses that pass 200 or more eggs per gram of feces should be dewormed. A regular examination of the feces is of crucial importance, since not every horse dropping has to have the same amount of eggs. However, this check only applies to strongylid. If other parasites are detected in a horse, the entire stable must be treated.

Various studies on parasite infestation have found that deworming is necessary for only 30 percent of adult animals. The mere fact that all worm remedies burden the sensitive horse organism should contribute to a rethinking of deworming. This type of selective deworming seems at first glance much more complex than a total deworming of all horses but contributes significantly to the health of the animals.

The horse droppings should be examined at regular intervals to obtain a complete overview of the worm infestation. Different droppings from different heaps of one day are collected, because the worm concentration can be very different. Even after the treatment, it is essential to check the effectiveness of the medication so that it can be re-dosed if necessary. Over time, the control intervals can be extended, ultimately reducing costs to the point of the strategic deworming - but without the unnecessary burdening of the horse with chemicals.


  • Change preparations for deworming frequently
  • Precisely dose the worm agent; to dose as precisely as possible order a horse scale
  • Only deworm when it is really necessary

III. Signs of a Worm Infestation

Depending on the species, the worms infect different organs of the horse. Therefore, the symptoms can be very different and have different effects. A massive worm infestation can be recognized through the slimming down of the affected horse, also the fur looks blunt and dull. Other signs may include an increased number of colics, severe diarrhea, constipation, frequent flatulence or scales on the tail pulp. Also, inflammation of the gastric mucosa or gastric ulcer may be caused by parasitic infestation, as many of the worm species attach to the intestinal and gastric wall very tightly. The parasites can also have negative effects on the respiratory tract. Coughs, nasal discharge, and respiratory sounds are often symptoms of lungworm infestations.

Most parasites are absorbed from the grazing land via food. Only the ox-warble fly has its own tactics: She lays her eggs on the horses front legs, who in turn lick their own legs frequently. This is how the eggs and therefore larvae get into the sensitive horse organism. ox-warble fly eggs are easily recognizable and should be removed immediately from the horse's legs.

- Alternative Options?

There are many different alternative deworming agents, for example, high-dosed herbs, certain kind of feed, globules or Bach flower essences. Such additives have the great advantage of not burdening the horses' organism unnecessarily, but they are merely used to assist in its defense reaction. However, there is no guarantee about the effectiveness, which is why medical worm remedies should always be used. But because of this necessity, the horse owner should pursue the best possible prophylaxis and only deworm when it is really necessary.