This week, Horse and Hound magazine looked at tapeworm in horses, coming up with some interesting developments of it over the past couple of years.
Once thought relatively benign, the tapeworm is now known to be a potential danger. Of the three tapeworm species affecting horses in the UK, the most common is Anoplocephala perfoliata. The pale-coloured parasite has a flat, segmented body and measures around 5-8cm in length once fully grown. Unlike other worms in the horse, the tapeworms develop inside an intermediate host – the microscopic forage mite. Horses become infected when they eat hay or grass containing these mites, which are themselves infected with tapeworm larvae. These then develop into adult tapeworms inside the horse’s intestines.
Once laden with eggs, segments of the tapeworm’s body detach and are passed out of the intestines in the horse’s droppings. The eggs are released into the pasture where ingested by forage mites. The life-cycle takes from three to six months to complete. Owing to our temperate climate, the UK’s horses are particularly susceptible to tapeworms. Some horses are more prone to infection than others. In fact, 80% of intestinal parasites reside within just 20% of the equine population. While young and geriatric horses are at greater risk from heavy burdens, tapeworms can lurk within any animal. What’s worrying is than an apparently healthy-looking horse can be harbouring a large build-up of these potentially lethal visitors.
Blocking the intestines
Adult tapeworms can be found at the narrow intersection between the small and large intestine called the ileocaecal junction. Here, they attach themselves in clusters to the mucosal lining of the digestive tract, using four suckers located on their head. If they build up in large numbers at this junction, tapeworms can cause inflammation that alters the movement of the intestines.
It is now known that infection with tapeworms is associated with an increased risk of colic, particularly spasmodic colic (a type I have experienced when my own horse got it at a competition – very worrying! ) or a blockage at this narrow junction known as ileal impaction. Another risk is intussusception, in which one portion of the gut telescopes into another and becomes stuck. Surgery is sometimes necessary.
Research at the University of Liverpool revealed that 80% of ileal impaction cases and 22% of spasmodic colic cases were due to tapeworms. Ensuring your horse is free of tapeworms will therefore reduce the risk of colic.
A ticking time-bomb
Testing for tapeworms can be tricky. Horses can have large tapeworm burdens and appear healthy on the outside. Counting the tapeworm eggs in a faecal worm count (FWEC) is unreliable. Egg numbers are usually low and encased in the segmented tapeworm part rather than distributed evenly throughout the droppings. A negative result on a FWEC does not therefore mean your horse is free of tapeworm.
Instead of a FWEC, we can use a blood sample for infection, a method known as the ELISA or tapeworm antibody test. A horse with a high level of tapeworm infection will produce a large number of antibodies, which can be detected in the blood. The test indicates a broad level of intensity, rather than tapeworm numbers. The amount of antibodies will indicate 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 around four to five months for the antibody levels to return to normal after the worms have been killed. Therefore the test only needs to be carried out once a year.
Horses with evidence of a high tapeworm burden on a blood sample are likely to be susceptible to infection with tapeworms in the future. The should be treated a second time that year, before being retested 12 months later.
With targeted treatment, your vet will combine information on the horse’s worming history with FWEC and blood test results to decide whether he needs worming – and the most appropriate treatment if this is necessary. Targeted treatment can reduce the likelihood of worms becoming resistant to treatment and identifies horses at risk of high levels of worm infection. 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 on a yard should have a blood test or a FWEC, or be treated for both tapeworms and roundworms, before they are turned out with others. In some yards, targeted worming may not be suitable nor practical. In these instances, it is best to treat for tapeworms every six months.
Tapeworm infection is not linked strongly to seasonality, so the time of year that treatment takes place is not particularly important. A risk is higher after prolonged periods of grazing, however, it seems wise to treat at the end of a summer out at grass.
It is important to be aware that not all wormers kill tapeworms. The parasites can be treated with one of two drugs – praziquantel and pyrantel. The latter must be at a double dose. Fortunately, both drugs are very effective and cause death and detachment of the tapeworms from the gut wall within 24-48hr following treatment. They are available as single products or in combination with other drugs that target different types of worms. Parasite populations on the pasture can be reduced by regular removal of droppings, resting grazing and rotating sheep and cattle on to horse fields.
To conclude, careful pasture management combined with correct testing and treatment will ensure that tapeworm infection is controlled. For me, having moved around to different yards, I found it very interesting that not once were my horses, or new horses coming onto the yard I was at, were testing for tapeworm. Upon moving, you are subject to the current worming cycle that yard operates on, meaning that if new incoming horses are infected with tapeworm, the chance of them spreading is increased if they are turned out immediately with other horses.
Hope you found this article interesting, more next week!