Keeping your horse free from worms, by Debbie Archer MRCVS

Though many horse owners are aware of the seriousness of worm infestation and the importance of using wormers (anthelmintics), worms are still a common cause of problems such as diarrhoea, weight loss and colic.

While the types of worms may not have changed, there is an ever-increasing array of worming products from which to choose. There is also the added complication of resistance — in certain locations worms have developed resistance to types of anthelmintic and they are no longer killed by drugs that used to be effective. They are, in effect, "superworms".

For every horse owner, it is vital to understand what types of wormers should be used, how often they should be used and how the development of drug resistance by worms can be slowed down. Sophisticated lab techniques are being used to investigate drug resistance, and in the future it may be possible to develop vaccines against worms.

Different worming chemicals

There are many different trade names for wormers, which can make it difficult to decide which to select. In fact, there are relatively few chemicals currently used to treat horses against worms — the most common being fenbendazole, ivermectin, moxidectin, praziquantel and pyrantel. Trade names may change and new wormers are constantly marketed, so it is helpful to make yourself aware of what the active ingredients are in each.

In addition, some wormers now contain more than one compound. Most are effective against many of the types of worms, and all can be used effectively on a routine basis. However, when choosing wormers specifically to kill tapeworms or encysted small strongyles, it is important to choose an anthelmintic that will work against these parasites. Not all are effective.

There is no single worming programme or strategy that will work for every horse. Your vet will understand the level of risk for your horse and is the best person to advise you on the most effective combination of worming products and appropriate treatment intervals.

How do I know if my horse has worms?

You cannot tell if a horse has worms by looking at it — tests are needed to see which are the most infected. Faeces can be tested for the presence of larvae and worm eggs, but this test will not detect tapeworms and encysted small strongyles. It is cheap and easy to perform and can give a useful indicator whether individual horses have heavy burdens of worms. This is particularly useful when new horses arrive on the premises.

Treating encysted small strongyles

There is no available diagnostic test to determine if horses are carrying large burdens of encysted small strongyles. It is therefore advisable that all horses (especially youngsters) are treated for this type of worm at least once a year.

Testing for tapeworms

A new blood test has been developed to detect whether horses are infected with tapeworms. It measures antibodies to tapeworms in the horse's blood and can tell if horses have low, moderate or high burdens of tapeworm.

Getting the dose right

The use of a weighing scale or a weigh tape to get an accurate idea of a horse's weight is an important part of management for worms, and ensures that the correct dose of anthelmintic is given. Overdosing could be potentially harmful to the horse and wastes money. Underdosing does not kill all the worms and can encourage drug resistance.

Resistance

Worms in some locations have become resistant to benzimidazoles and, more recently, drug resistance to pyrantel has been reported. This is likely to be an increasing problem due to the widespread use of wormers, and it is possible that worms will become resistant to other anthelmintics in the future.

So how can this problem be minimised?

Give the correct dose of anthelmintic (do not underdose)

Minimise the amount of anthelmintics to which worms are exposed, for example, prevent large burdens of worms building up on the pasture and target treatment at horses with known high worm burdens

Avoid giving anthelmintics to horses with zero/low worm burdens.