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Equine Wormers and Worm Resistance

Worms can result in horses suffering from a number of health issues including colic, lethargy, weight loss and serious diseases of the gastro-intestinal tract. It is very important that you conduct an effective worming programme. However, take care not to overuse anthelmintics, the drugs used to control worms, as the parasites can become resistant to them. Be aware that there are several types of worms which can infest horses and so you may require more than one variety of equine wormers to address them.

In order to ensure that worms do not become completely immune to the equine wormers that you use, you need to maintain a refugia. This is a small population of worms which are never exposed to equine wormers. The refugia will dilute the genes of drug resistant worms so that they can be treated. Horses should be wormed only when necessary and you can establish when the time is right by taking worm egg counts (WECs). This is done via a test kit which you can obtain from vets and testing laboratories. Horses with a low worm count should not be treated and every horse should be treated according to its needs – avoid blanket treatments.

There are three worming strategies which you can consider adopting:

Interval Dosing

With interval dosing specific anthelmintics are administered at set intervals throughout the year. Spring worming targets tapeworms and can then be followed by routine worming in the summer and then tapeworm treatment in the autumn. Bots and encysted small redworms can be addressed in the winter. With this strategy you do cover all bases but could treat your horse unnecessarily and risk resistance.

Strategic Dosing

Strategic dosing is the administering of a broad spectrum of equine wormers at specific times of year. This approach can disrupt the life cycles of the worms. However, problems are possible if unseasonal weather leads to early or late peaks in the number of pasture larvae. This can cause your timing to be out.

Targeted Strategic Dosing

This is now the recommended approach as it tackles the issue of resistance. You should conduct a worm egg count (WEC). If the egg count is greater than 200 epg then treatment with anthelmintics is required. Small, developing redworms will not be detected by a WEC and neither will tapeworms. It is wise to treat your horse in spring and again in November with a broad spectrum anthelmintic which also addresses tapeworms. A tapeworm antibody (Elisa) test could be useful. Conduct egg counts in between these times and use equine wormers if the count exceeds 200 epg. Then you can avoid unnecessary treatments.

Treatments

The active ingredients in the various available wormers address different parasites. Wormers are grouped as follows:

• Ivermectin kills large redworms, small redworms, pinworms, large roundworms, lungworms, intestinal and neck threadworms and stomach worms. It can also be effective against bots.
• Praziquantel addresses tapeworms.
• Moxidectin kills small redworms including encysted small redworms, large redworms, pinworms, large roundworms, intestinal threadworms, stomach worms and bots.
• Pytantel kills large redworms, small redworms, and pinworms. An elevated dose can be used to kill tapeworms.
• Fenbendozole kills large redworms and small redworms including encysted stages with an elevated dose. Will also kill pinworms, large roundworms and intestinal threadworms at elevated doses.

If you are in any doubt as to how to proceed then consult your vet for guidance.

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