Tapeworms are internal parasites that live in the intestinal tract of horses. The most common species found in the UK is Anoplocephala perfoliata. Studies have estimated that 51-69% of horses in the UK and Ireland are infested with tapeworm. But how do we know? And can you reliably check if your horse is one of them?

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What problems can they cause?

Horses with high tapeworm burdens are at an increased risk of colic; particularly a type of colic known as “caecocolic intussusception”, where the caecum becomes trapped in the colon, a condition that requires surgery to correct and carried a very poor prognosis. Others may suffer from weight loss or repeated episodes of medical colic. Although they are an uncommon cause of weight loss or colic, tapeworm testing often forms part of a diagnostic work up for ill thrift or chronic colic so that affected animals can be managed.

Tapeworms do not spend their whole lives in the horse, they are reliant on a forage mite to complete their lifecycle. Unlike other internal parasites that produce eggs that are passed in faeces, tapeworm eggs are found inside body segments called proglottids. These proglottids are passed in faecal material, from which the eggs are released and eaten by forage mites. 

The larvae then live and grow in these forage mites; which are in turn ingested by horses grazing pasture and in turn grow, develop into adults, and produce eggs of their own. Faecal worm egg counts are very useful in measuring internal parasite burdens by counting roundworm eggs. But it cannot accurately identify tapeworm as the eggs are released intermittently and are often protected by a proglottid layer. We need to rely on a separate test for tapeworm. 

So how do we test for tapeworm?

Two tests are commercially available, one uses saliva and the other blood. The blood test has been available for many years, but the blood sample must be taken by a veterinary surgeon. The saliva test, however, can be performed by an owner, thus reducing the need for a veterinary visit. 

The tests measure antibody levels to the tapeworm. So in some cases, it is possible that the horse has been treated successfully for tapeworm, but antibodies remain for several months in the blood or saliva. This is why it is important to know the timing of treatment and when it relates to testing in order for your vet to interpret the significance of the test. Once treated, horses can easily become reinfested. 

The results will typically say low, borderline or moderate/high, rather than a specific number

The clinical signs associated with tapeworm infestation can be correlated with the level of burden, with low levels unlikely to produce clinical disease. These tests have been validated by a number of sources and both have been found to be effective at identifying infested animals. False negative results (when the test fails to identify a burden) are uncommon. 

It is recommended that higher risk herds, with new animals, documented moderate or high levels or poor pasture management, be tested every 6 months and treated accordingly. For those at low risk, testing every 12 months, usually in the autumn is likely to be sufficient. 

However…

The test is not useful in foals pre-weaning as they will have maternally derived antibodies in the blood. This means that they may return a positive test based on the fact that their mother has a tapeworm infestation. 

Why don’t we just use a dewormer instead of testing?

Currently we have very few choices when it comes to anthelmintics suitable for horses. Many of these drugs are already starting to lose efficacy due to widespread emerging resistance in parasites. The two main drugs used for tapeworms are Praziquantel and Pyrantel. They are available without prescription (with the exception of praziquantel without the addition of another drug). However, it is still important to consult your veterinary surgeon about the suitability of these drugs for your specific circumstances. There is already a suspicion that tapeworms may have developed a level of resistance to praziquantel. And many helminths have developed a resistance to pyrantel or to moxidectin, another drug frequently used in combination with praziquantel. 

It is very difficult to create new drugs and it has been many years since the most recently developed drug was first marketed. It is of utmost importance that we continue to preserve these drugs for as long as possible to ensure that they remain effective. The best way of doing this is by only treating horses that need to be treated. This will reduce the amount of these drugs entering the environment. With a combination of good pasture management and regular testing to identify affected animals are the best ways of achieving this. 

Treatment does not just involve the administration of a suitable dewormer

Pasture management is critical for success. One very small study carried out by EquiSal demonstrated that when pasture management was used in combination with drug therapy, 90% of horses had a reduction in burden as evidenced by a repeat test, within 10 weeks. This figure increased to 100% after 12 weeks. In addition to this, new horses joining the herd should be tested, quarantined and treated as necessary before joining the herd. 

In summary, testing for tapeworm antibodies is a useful method to reduce the amount of anthelmintic drugs used in our horses. Reducing the deworming frequency will save money. And it will also help to reduce the risk of the development of resistance to these valuable drugs. A proactive approach will also help to reduce the risk of horses suffering from problems associated with an undiagnosed tapeworm burden. Your veterinary surgeon will also be able to advise on all aspects of parasite control and herd management.

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