Thanks in part to modern worm medicine, the health of our animals has increased enormously in the last 50 years. Unfortunately, the development of resistant gastrointestinal worms against these agents is causing more and more problems. To avoid further increases in resistance, a different, sustainable, way of worm control is needed.

Sustainable worm control has three objectives:

  • Make animals suffer as little damage as possible from gastrointestinal worms
  • Limit the chance of recontamination and, lastly, only use dewormers when actually necessary.
  • If we are not careful with our veterinary medicines, it is not inconceivable that effective wormers will no longer be available in the foreseeable future.

The life cycle of gastrointestinal worms

The following are the different stages in the life cycle of gastrointestinal worms.

  • The female gastrointestinal worms in the gastrointestinal tract lay eggs, which end up on the pasture with the manure.
  • The eggs hatch and infectious larvae develop through a few intermediate stages.
  • Infectious larvae crawl from the manure to the surrounding grass.
  • During grazing, the infectious larvae are absorbed by the animals.
  • In the intestinal system, infectious larvae develop through a few intermediate larval stages into adult worms. This development phase differs per worm species. Bloodworms, for example, encapsulate in the intestinal wall while roundworms make a long journey through the liver and lungs. The adult worms in the intestines lay eggs again, after which the life cycle starts again.
  • In horses, the developed larvae cause the most damage. In sheep and goats it is mainly adult worms that cause the most problems

Dewormers and resistance

Resistance of gastrointestinal worms to wormers is increasing rapidly. One or more worm species has developed resistance to most registered worm medicine in the Netherlands. The most important risk factor for the development of resistance is the (too) frequent treatment of all animals in one herd, with the same type of wormer. A way of worm control that is still common for many horse and sheep owners. Underdosing is also a major risk factor. To ensure the effectiveness of wormers for the future, wormers should only be given when necessary. In addition, it is important that the correct worm medicine is supplied in the correct dosage. .

Sustainable worm control in horses

Gastrointestinal worm control should target both the worm population in the horse and the larval development stages outside the animal in the pasture. We start with the various options for control on the pasture.

Worm control outside the horse

Remove manure
Every meadow contamination starts with an egg on the meadow. If horses can be prevented from contaminating the pasture with manure, many worm infections are prevented. By removing manure at least twice a week, the meadow infection is greatly reduced. Removing manure is the basis for effective and sustainable worm control.

As long as no horses enter a pasture, the contamination of a pasture will decrease because the infectious larvae can only survive for a limited time. In dry summer weather, worm larvae survive only a few months, while in a mild winter the larvae can survive for more than 6 months. Many larvae die off in severe frost, such as at the beginning of this year. Unfortunately, not every horse owner has enough land available to graze the animals to clean pastures.

To drag
The manure pile with a warm and humid microclimate acts as a reservoir for worm larvae. By dragging the meadow in dry summer weather, eggs and larvae are removed from the protected environment of the dung heap. Many larvae die quickly due to UV rays and high temperatures. When towing under cold, damp weather conditions, the meadow infection increases sharply.

Contamination on the pasture is reduced by using the pasture as hay land. The larvae, which are on the grass blades, die during the drying of the grass. The mowed meadow is then clean.

Other grazers
Almost all gastrointestinal worms are host-specific and unique to an animal species. A horse gastrointestinal worm does not infect sheep or goat and vice versa. If horses and small ruminants graze together or alternately on the same pasture, this results in a clear decrease of the infection pressure on the pasture, for both animal species.

No grazing
If horses do not go to pasture or only have outdoor access in a sand paddock, the risk of contamination with bloodworms and tapeworms is virtually absent. Roundworms and pinworms can complete their life cycle in the house. Hygiene measures therefore remain important under these circumstances.

Tackle worms in the horse

Developing larvae cause the most problems in horses. Unfortunately, modern dewormers are mainly effective against adult worms. Only moxidectin has limited effectiveness against larvae. The most important result of deworming is therefore the docking of the egg excretion and the prevention of recontamination of the pasture. Detecting adult worms is easy by detecting the eggs they produce with manure research.

Treat only when you have to
Research by, among others, Veterinary Parasitological Laboratory ‘Het Woud’ shows that 75% of adult horses have no or very low worm counts when the owner would like to deworm. In addition, there are also a small number of horses that repeatedly have high worm counts despite regular worming. Unfortunately it is not visible from the outside which animals these are. These horses can be identified and de-wormed in a targeted manner with manure research.

Selective deworming not only makes the environment cleaner but also limits the use of dewormers and thus halts the development of resistance.

Manure research in horses

Whether a horse needs to be treated can therefore only be determined by manure research. By having the manure examined 3-4 times a year, a horse can be de-wormed with the correct wormer.

Manure testing can also be used to determine the effectiveness of a treatment. After treatment, egg secretion should stop after a few weeks. It is wise to determine the effectiveness of any treatment (if any) at least every two years. Horse gadfly larvae and pinworms cannot be detected with manure studies. The importance of both worm types is limited and specific treatment is hardly ever necessary. Tapeworm eggs are rarely found and the value of a negative fertilizer test result is limited. The advice is to treat a horse for tapeworms once a year.

Sustainable worm control in sheep and goats

Also in sheep and goats, the control of gastrointestinal worms should focus on the worms in the animal and on the larval development stages outside the animal on the pasture. In worm control in sheep, the emphasis is on the lambs. Yearlings and ewes have often built up sufficient resistance to gastrointestinal worms. NB; where you read sheep you can also read goats.

Worm control on the meadow

Removing sheep manure from the pasture has the same effect as with horses. In practice, this proves to be difficult to implement, and regular grazing to clean pastures is very effective in preventing worm infections in lambs. A clean pasture is a pasture on which no sheep have walked for at least three months. The principle of good pasture management is to bring lambs to clean pastures and the lambs to pasture before these clean pastures become infectious and potentially dangerous. If infected lambs are kept on a clean plot for longer than 3 weeks, there is a high risk of (re) infections. Please note in the months of July, August, September this only takes 2 weeks. If uninfected lambs are put on a clean plot, it will take 6 to 7 weeks before the animals have to be grazed again. Worm control through regular grazing is of course only possible if there are sufficient clean meadows. In practice, this must be more than 6 lots. If there are only 2 plots, it is wise to reserve at least one clean plot for the weaned lambs.

If it is known that there have been Nematodirus infections (only occurring in lambs) it is important to avoid pastures that have been grazed by young lambs in the previous year. As with horses, haying and alternating grazing with other animal species are also important in reducing of the infection pressure.

Manure research in sheep
Monitoring of egg shedding in the animals is essential in this preventive approach to helminth infections. By having the manure examined regularly, the sheep farmer gets a good picture of the infection in the herd of lambs and the infection pressure on the pasture. Based on the results, the sheep farmer can decide whether or not to treat the flock and with what means. Good times to have manure testing carried out on lambs are around weaning (end of June) and in the summer 3 to 4 weeks after clean lambs have been moved to a possibly unsafe pasture, or 6 to 7 weeks after the same lambs have been moved to a safe pasture have been transferred. Regular manure surveys make it possible to reduce the number of treatments in a season, which is so important in preventing worm resistance. For ewes, a manure survey is recommended around the mating season in October. A standard treatment of ewes at this time is often not necessary. Because almost all ewes excrete large quantities of worm eggs around lambing and thus contaminate the spring meadows, a standard treatment around lambing or before shearing in the spring is important.

Manure testing is of little use for acute liver fluke. If suspected, contact your vet immediately. In addition to manure research, it is of course important to regularly assess the condition of the herd and the possible presence of pale mucous membranes, which is an indication of a serious Haemonchus infection.

In conclusion

By tackling worm populations outside the animals and deworming your horses, sheep and goats only on the basis of manure research, sustainable worm control is possible. Not only are your animals exposed to less wormers, but resistance development is also slowed down and the effectiveness of the resources available to us is guaranteed for the future.

Source: NML Health; Wicher Holland, veterinarian / parasitologist

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