A parasite prevention plan cannot rely on dosing of wormers alone! Pasture management is a more effective way of keeping low infestation, as unnecessary drug dosing can lead to worm resistance.
Worming regimes should be individually tailored and faecal examination enables us to identify most types of egg-laying parasites present in the intestinal tract at the time of testing. However this means migrating or immature parasites and encysted worms could well be missed. So a zero egg worm count doesn’t rule out the presence of parasites!
Pasture management tips:
- Avoid overstocking of horses. Ideally rotate (or share) fields with cattle or sheep.
- Remove droppings from field (at least twice a week) and avoid harrowing/spreading (infesting areas with fresh manure).
- Horses that are lower in the pecking order will be forced onto poorer grazing (may have higher worm burden).
- Worm new arrivals with Eqvalan or give 5 day course of Panacur equine guard, then double dose of pyrantel on the sixth day. Keep new horses in for 48 hours before mixing to allow ovicidal effects time to take place.
- Worm horses 48 hours before moving to new pastures.
- If possible, rest pasture for 12 months (or from autumn until mid-summer the following year). For mares with foals, weanlings and yearlings, rest pasture for at least five months from February onwards (most winter larvae will have died by June).
- Perform an egg worm count twice a year (a 4g faecal sample is required).
- Tapeworm, encysted small red worm and bots cannot be detected by a faecal worm egg count, so it is still necessary to worm your horse for these parasites.
- Do not aim for a zero count – frequent dosing stops young horses developing natural functional immunity and also encourages drug resistance in worms!
- Use a weigh tape or weighbridge to ensure your horse is getting an effective dose.
- Use the maximum interval if grazing is kept clean – use the minimum interval for crowded conditions, sick horses etc.
- Some worms are targeted strategically at specific times of the year. If there is no resistance, ivermectin, pyrantel, benzimidazoles and moxidectin can be used the rest of the year.
- Rotate wormers not by brand name but by drug class to help prevent resistance.
- If resistance is suspected you should ask your vet to carry out a faecal egg count before and after treatment to check its effectiveness (7 day interval between samples for pyrantel, 10-14 day interval for fenbendazole and 21 day interval for ivermectin) in a representative number/age range of animals with at least 200 eggs per gram on first sample – less than 80% reduction would be suggestive of resistance.
Although there is a great variety of commercial brands there are actually only a limited amount of chemical compounds (active ingredients). This means we need to be careful; only judicious use will prevent us from facing a generation of resistant superworms in the future!