Tapeworms belong to a group of parasites known as cestodes (or flatworms). Three species of tapeworms affect horses in the UK, the most common of which is Anoplocephala perfoliata (1). Tapeworms are short and flat in appearance, growing up to 8cm long and 1.5 cm wide. They consist of a small round head which attaches the worm to the wall of the gut, and a body formed of a chain of egg containing segments known as proglottids.
Adult tapeworms release egg-containing body segments into the host horse’s intestine. These are passed in the faeces and disintegrate, releasing large numbers of eggs into the environment. Eggs are eaten by tiny forage (oribatid) mites, which can be present in very large numbers on pastures. Over a period of approximately two to four months, these eggs develop within the mite into an infective intermediate stage of the tapeworm. As the horses graze, they will inevitably ingest the mites, releasing the infective stage of the worm into the intestines. The immature worm migrates to the ileo-caecal junction (the join between the small and large intestines). Here it develops into the adult form which, after one to two months, starts to produce egg filled segments, continuing the life cycle.
Effects of tapeworm infestation
For many years tapeworms were considered to be harmless to the host horse. The arrival of more reliable methods for (a): identifying horses infected with tapeworm, and (b) determining severity of parasite burden (see diagnosis below) has changed this perception of tapeworm infections. In a study of UK horses, researchers discovered that infection with the most common species of tapeworm, Anoplocephala perfoliata, increases the risk of two types of colic-spasmodic colic and ileal impaction colic. It also demonstrated that the heavier the tapeworm burden, the higher the risk of these types of colic.1 Although the level of risk should be kept in perspective, effectively minimising tapeworm burden should decrease the risk of tapeworm associated colic.
Diagnosing tapeworm infection can be tricky in horses. Clinical signs are not a reliable indicator, as horses can have tapeworm burdens and appear clinically healthy. Unfortunately, counting tapeworm eggs in a faecal worm egg count (FWEC) is also unreliable, as the eggs are encased in the tapeworm segment, rather than being distributed evenly throughout the droppings. Also, egg filled tapeworm segments are only shed intermittently. As a result, a negative result on a FWEC does not necessarily mean a horse is free of tapeworm.
Fortunately we can use a blood sample to test for levels of tapeworm infection- a method known as the tapeworm antibody test. A horse that has a tapeworm infection will produce antibodies to the tapeworm, which can be detected in the blood. Instead of giving the exact number of tapeworms, the test indicates the intensity of infection, the level of antibodies indicates whether the burden is low, medium or high.
While the blood test is useful for assessing whether your horse has been infected with tapeworms in the past, it is not a reliable indicator of response to treatment. This is because it takes at least 16 weeks for the antibody levels to return to normal after the worms have been killed (2).
As well as the appropriate use of worming treatments (see below), it is important to manage pastures carefully to minimize the parasite burden that horses are exposed to. The aim is to prevent contact with faecally contaminated pasture or feed. This could be achieved by regular removal of faeces from pasture, prevention of over-stocking, resting heavily grazed pasture and rotating pasture with other species-eg sheep or cows. There are two different approaches to tackling tapeworm. Treatment can be targeted following a blood test, or administered within an annual worming program.
With targeted treatment, a decision is made on (a): whether to treat, and (b): with what product. This is based on the horse’s worming history alongside combined FWEC and blood test results. Targeted treatment identifies horses which are at risk of high levels of worm infection. This helps to ensure that the most appropriate wormers are only used when necessary, thus reducing the risk of worms becoming resistant to treatment.
Using this method, horses are blood-tested for tapeworms once a year. Those with a high burden are treated after the test and then again six months later. The blood sample is then repeated the following year. New horses entering the yard should have a blood test and a FWEC to determine whether worming is required, or be routinely treated for both tapeworms and roundworms and stabled for 48 hours, before they are turned out with others.
Also new to the market is a DNA saliva test which has been developed to work like a blood test but in fact uses a simple saliva swab. For more information click this link: http://www.wormers-direct.co.uk/equisal-horse-tapeworm-test-kit/
In some yards, targeted worming may not be suitable or practical. In these instances, all grazing horses should be treated for tapeworms at least once yearly, and every six months in high-risk animals or those shown to have high levels of infection1.
There is little evidence of infection being linked to seasonality, so timing of tapeworm treatment is not critical. As risk is higher after prolonged periods of grazing, horses are often treated at the end of a summer out at grass.
It is important to be aware that not all wormers kill tapeworms. The parasites can mainly be treated with one of two drugs — praziquantel or pyrantel. Fortunately both drugs are very effective and cause death and detachment of the tapeworms from the gut wall. They are available as single products or in combination with other drugs that target different types of worms.
In summary, careful pasture management combined with correct testing and treatment will ensure that tapeworms are effectively controlled. thus reducing risk of associated disease.
1. Proudman C. and Matthews J. Control of intestinal parasites in horses. In Practice (2000) 22: 90 – 97
2. Tapeworm Antibody Test, Diagnosteq, University of Liverpool, http://www.liv.ac.uk/diagnosteq/tapeworm_test.htm. Page retrieved 4th April 2014.
Wormers-direct.co.uk are indebted to Elanco the makers of Strongid P paste for this article