Worm control for your horse

Parasitic worms that are able to survive treatment with different classes of wormer are becoming an increasing problem all over the world, and New Zealand is no exception.  If we don't use the drugs that we have wisely, we may find ourselves with no effective drenches. 

Over the last few years there have been some changes in the drenches available and updated recommendations for the best ways to use them. As a result, there is some confusion about how we should manage our horses to prevent worm problems.  Here are some tips for worm control. 

Pasture management:

• Reduce the number of horse worms on the pasture by:
• Picking up droppings twice a week
• Mowing roughs (the long grass where the horses defaecate) reduces the number of parasites on pasture
• Co-graze or alternate grazing with sheep or cattle
• Keep stocking rates low
• Avoid moving horses onto fresh pasture soon after worming. Ideally, move onto fresh pasture when Faecal Egg Counts (FECs - see bottom of article) are low or zero
• Harrowing is not advisable as a means of worm control in the Manawatu (not hot and dry enough to be effective)

When to drench?

Adult horses:

• Cyathostomins (small redworm) are the most common and most dangerous worms in adult horses
• Worming all horses within the group encourages resistance. It's better to identify and treat only horses with a high FEC
• FECs should be done periodically (e.g. three times a year), or at the highest risk time of year (autumn)
• Combination drenches containing drugs from at least two different classes e.g. a macrocyclic lactone (ML) and a benzimidazole (BZ) such as "Equitak Excel"; or an ML and morantel, such as "Ammo" should be used as a first-line treatment for horses with epg >200 through the year
• FECs cannot detect tapeworm or cyathostomin larvae that burrow into the gut wall. These should be targeted once a year with praziquantel and moxidectin respectively, in the winter. Available in combination e.g. "Ultramox", "Equest Plus"
• Dose correctly according to bodyweight
• Bot eggs (small yellow oval eggs on the coat during the spring and summer) should be removed by grooming

New arrivals:

• Combination drench and then ideally FEC two weeks later to confirm that there are no resistant parasites. In the meantime, it is best if the horse is stabled or yarded, so any resistant worms don't get onto the pasture
• If this is not possible, give a combination drench and then turn out onto the "wormiest" paddock so that any resistant worms that survive the drench will be "diluted" with the resident (hopefully susceptible) worms

Foals:

• Mares should be treated with a combination drench just before they are due to foal to reduce transmission of parasites from mare to foal
• Parascaris equorum (roundworm) is the most dangerous internal parasite of foals from one to six months of age.
• ML (both ivermectin and moxidectin) resistance has been demonstrated in Parascaris equorum in numerous countries including anecdotal reports in NZ
• Treat foals at 10-12 weeks of age and thereafter when FEC > 200epg. A combination (e.g. "Equitak Excel": abamectin, oxfendazole, praziquantel) would be a logical choice
• Foals develop strong immunity to Parascaris equorum at about six months of age, and can be managed in the same way as adult horses from then on

Faecal Egg Counts

Totally Vets charges $13.50 for the first sample and $3 for each additional sample.  This means five horses can be tested for the price of one tube of "Equitak Excel". We need about a tablespoon of fresh manure to do an egg count.