Equine Worming Update

essential for your horse's health

Worming your horse is an ever-changing subject, which can be very confusing.

Signs of worms in your horse can vary from no signs of disease to very dramatic disease such as colic from impactions, gas filled intestines, rupture of intestine, intussusception (where one part of the intestine prolapses into the lumen of the adjacent part of intestine), death of part of the intestine due to damage from the worms, diarrhoea and weight loss. Other less common signs of worms can be ulcers, coughing, poor coat, decreased weight gain, itchy skin and urticaria.

Diagnosis of a worm infestation in your horse can also be difficult. It cannot be based just on the signs listed above as they can be signs of other types of disease and so are not specific to worms. Worm egg counts from a sample of faeces from your horse are very good when we get a positive result but a negative worm egg count could mean the worms in your horse are not mature enough to produce eggs yet; are not a type of worm that produces eggs in large quantities or eggs of a type easily seen when the faeces sample is analysed. Tapeworms and cyathostomes are examples of worms that are difficult to diagnose on faecal analysis. There is a blood test (an ELISA) available to look for tapeworm. The blood test gives a result which tells us the level of infection (low, moderate or high) but a high result goes down very slowly after treatment, work suggests it can take 12 to 16 weeks to go from a high to low result on the blood test. However re-exposure from the pasture in this time will stop the result from going to low.

It is therefore much better to introduce a treatment strategy for internal parasite control in your horse. The most effective thing you can do is reduce your horses exposure to worms by minimising over-grazing and regularly removing dung from the pasture. If this is not possible rotation of the pasture or mixed species grazing horses with cattle or sheep will reduce the worms on your pasture. The next best strategy is to target anthelmintic treatments (wormers) only to the horses with a worm infestation. This requires regular worm egg counts on all horses and tapeworm ELISA blood tests from all horses.

Often these control measures are impossible to introduce and so we rely on the use of anthelmintics (wormers). This does not mean you should not still pick up the dung and reduce over-grazing of the pasture. Grass harrowing of pasture in dry weather will desiccate (dry out) worm eggs and larvae so killing them, so it is a good aid but should never be done in damp weather as this would spread the worm eggs and larvae around the field.

The common worms that horses in the UK get are: -

o Large Strongyles (Strongylus vulgaris, Strongylus edentatus, Strongylus equines);
o  Small Strongyles (Cyathostomins – more than 50 known species, Strongyloides westeri);
o Ascarids (Parascaris equorum – most problematic in foals);
o Tapeworms (Anoplocephala perfoliata, Anoplocephala magna, Anoplocephaloides mamillana)
o Lungworm (Dictyocaulus arnfieldi).

Essentially their life cycles are similar. Eggs are passed out in the dung, larvae hatch from the eggs and crawl up grass. This grass in eaten by the horse and the larvae develop inside the horse into adult worms who lay eggs and the cycle continues.

Infectivity of the worms depends on a multiple of factors,
• weather is very important -the larvae need warn damp conditions to hatch and crawl up the grass;
• Age of horse – young, old and sick horses have less natural resistance to worms;
• Pasture management – horses naturally will not eat near their dung thus in the wild they would not pick up many worm as the worm larvae cannot crawl far. If the pasture is over crowded then especially horses lower in the pecking order will eat near dung and can pick up worms.

The larvae can survive on the pasture if environmental conditions are good (warm and moist) but most infection on the pasture is in the form of eggs that are very long lasting. This means that pastures need to be left without horses on for at lest a year to be clear of infection.

There are five chemical classes of anthelmintics licensed for use as wormers in horses.

 Moxidectin (e.g. Equest),
 Ivermectin (e.g. Vectin),
 Pyrantal embonate (e.g. Strongid P),
 Fenbendazole (e.g. Panacur)
 Praziquantel (e.g. Equitape).

Praziquantel and double dose Pyrantal embonate are the only ones that will kill tapeworms.

Moxidectin and a five-day course of Fenbendazole are the only ones that will kill cyathostomes when they are encysted (especially during the winter, cyathostome larvae burrow into the wall on your horses intestine and hibernate in a little cyst over the winter to hatch in spring.)

The other worms should all be killed by moxidectin, ivermectin, pyrantal embonate and fenbendazole.

Lungworm is rarely seen nowadays and is very effectively treated with either moxidectin or ivermectin.

The problem with using drugs is that resistance to them develops over time. Some studies put the level of resistance to fenbendazole at 80-90% and pyrantel embonate at 30-50%. A single report of ivermectin resistance in Lincolnshire has been published, with no reports of moxidectin resistance to date. We need to use drugs in a controlled manner to try and reduce resistance development and so preserve the effectiveness of them. How to use drugs in such a way as to minimise resistance developing is a controversial subject. There have been examples of resistance developing as a direct result of the incorrect use of cattle anthelmintics in horses, so we always advise the use of equine licensed products. Everyone worming a horse should use a weight tape so the dose given is accurate.

Our advice at Parkside Equine is to isolate and treat all new horses with moxidectin and praziquantel and keep boxed for 48-72 hours. Then if a monitoring and targeted program is not suitable then use one class of anthelmintic (ideally moxidectin or ivermectin) for every worming, rotating every two to three years between moxidectin and ivermectin. In addition dose high-risk horses (those with evidence of tapeworm infestation or previous tapeworm-associated colic signs) every 6 months, and low-risk horses annually in the autumn with either Praziquantel or double dose Pyrantal embonate.

Summary

1) Reduce over-grazing of pasture.
2) Remove dung from pasture, ideally daily but weekly in colder months and two to three times per week in the warmer months if daily is not possible. Grass harrowing is helpful but should never fully replace collection of dung as it requires particular weather conditions to be effective.
3) Rotate pasture with other livestock or arable land on a two to three year rotation. If this isn’t possible mixed grazing with cattle or sheep may reduce pasture burden and resistant populations of worms.
4) Carry out faecal worm-egg counts every three to six months and only treat with anthelmintics if strongyle egg counts are greater than 200-250 eggs per gram. Tapeworm ELISA (blood test) should be performed every six to twelve months in high-risk horses and every 12-18 months in low risk horses. Treatment should be administered if moderate to high exposure is diagnosed on the ELISA.
5) If the testing above is not possible, then selection of anthelmintic groups should be on your done in conjunction with us and tailored to your individual situation. Anthelmintics should be administered at appropriate dosing intervals for the drug used (for example every 13 weeks for moxidectin, eight weeks for ivermectin, six to eight weeks for pyrantel embonate or fenbendazole.) Double dose pyrantel embonate or praziquantel should be administered in late autumn or early winter (October or November) for treatment of tapeworm infestation.
6) If horses on the same premises are to be wormed regularly, all animals should be wormed at the same time with the same product. Ideally, animals should be stabled for 48 hours after anthelmintic administration.
7) All new horses should be isolated when arriving at new premises and be treated with anthelmintics that are effective against small strongyles and tapeworm. They should be stabled for 48-72 hours after administration.