DEWORMING NOTES FOR HORSE OWNERS: Nov 2006

Effective deworming of your horse, pony or donkey is essential for maintenance of its general health and well-being. In addition to causing loss of body condition, high parasite burdens may lead to episodes of inappetance, diarrhoea, colic, breathing problems and even death. Parasitic syndromes can be very difficult to treat once diagnosed, and prevention is clearly the best option on both welfare and economic grounds.

The major parasitic cause of death of horses in Scotland used to be the large redworm, the larvae of which cause damage to the blood vessels supplying the intestines. However, since the widespread use of ivermectin in the last 20 years, this parasite has declined and been replaced in importance by tapeworm and the small strongyles (‘small redworm’ or cyathostomes). Tapeworm has been shown to be responsible for outbreaks of colic in specific livery yards. The parasite causes colic as it changes the motility of the gut. Horses contract tapeworm by eating infected forage mites.

Cyathostomes are contracted from other infected horses. Horses infected with cyathostomes shed huge quantities of eggs in their droppings, which develop into infective larval stages. If the droppings are not picked up, millions of these larvae will be present on horse pasture by the end of a typical wet Scottish summer. Once eaten, the larvae make their way into the large intestine of the host, where they burrow into the gut lining. After reaching this site, the larvae may remain encysted for several months before emerging in massive numbers. Emergence of the larvae causes serious inflammation in the colon, often resulting in weight loss or diarrhoea and signs of chronic abdominal pain. The classical presentation of cyathostome-related disease in this country is of severe diarrhoea in yearlings and young horses either in early autumn or early spring. Young stock and old horses (over 15 years) are particularly susceptible to accumulation of large numbers of this parasite, probably due to poor immunity. Other presentations include ill thrift, intermittent colic and chronic soft droppings in older horses.  

An important fact to note in cyathostome infections is that the egg count in the droppings may be negligible, even if there are thousands of larvae encysted in the wall of the large bowel. In young horses, the clinical signs of diarrhoea may appear before the parasites have reached the egg–producing stage of the life cycle. Appearance of larvae in the faeces may also be sporadic. This makes it difficult for vets to confirm the presence of a high cyathostome burden, unless lots of larvae are detected in the droppings. Scientists are developing blood tests to aid in this, but they are not yet commercially available.

For the horse owner, the life cycle of the cyathostome presents some specific problems:

• It is difficult to tell if a new horse with an unknown deworming history is carrying a heavy cyathostome burden
• Young horses and old horses particularly should be dewormed carefully. It is important to prevent these age groups from picking up sufficient numbers of larvae to result in clinical signs of disease

Although drug resistance is increasing among horse parasites, the main reasons for failure of deworming programmes are more likely to include some of the following:

• Under-dosing of the horse, either due to under-estimation of weight or spitting out of some of the dewormer
• Importation of new horses to premises that have not be dewormed sufficiently as youngsters, causing contamination of pasture
• Insufficient frequency of deworming. If there is a high density of horses on the pasture, and droppings are not picked up, then significant numbers of larvae will accumulate on the pasture more quickly. Horses on these pastures will need to be dewormed more frequently to prevent signs of disease from developing
• Failure to deworm all co-grazing horses at the same time. All horses on a specific pasture should be dewormed at the same time or numbers of larvae on the pasture will still continue to rise.

Some practical general tips include the following:

• Use a weigh tape before dosing horses. Whilst an average Thoroughbred in training weighs around 450 kg, a heavy cob may weigh 650 kg and an adult Clydesdale will tip the scales at 750 kg
• Smear the paste on the tongue to prevent spitting out
• Arrange with other owners, or livery yard manager, that all horses on the premises are dewormed at the same time
• New horses should be isolated and not turned onto shared pasture until 2 days after completion of deworming. If deworming history is not known, a 5-day course of fenbendazole (Panacur) is recommended, with either ivermectin (eg Eqvalan) or moxidectin (eg Equest) on day 6. Faecal egg and larval counts may also be beneficial in new horses with an unknown deworming history.
• The required frequency of deworming depends on the product being used, as some are more potent than others e.g. Moxidectin (Equest) is generally only required at 14-week intervals, whilst ivermectin (Eqvalan) is required at 10-week intervals. If using Panacur alone, it should be given at 6-weekly intervals.
• Measurement of eggs in the faeces of all horses on the premises can be useful when developing a deworming strategy as it is known that certain horses will be high carriers ie 20% of horses account for 80% of the eggs produced. Ideally, these horses should be targeted with deworming treatment. 

As there are now so many dewormers on the market it can be confusing deciding which to use. Although there are many brands, there are only 5 main drugs in wide use:

• pyrantel
• praziquantel
• fenbendazole (or related compound)
• ivermectin (or related compound)
• moxidectin.

Therefore, make sure to read the ingredient list for the product, and follow the recommended dosing interval. The best approach is to use one specific drug such as ivermectin for one year, and then change to a different drug, such as moxidectin, the following year. This will minimise the rate of development of resistance among parasites on the pasture.

Not all drugs kill all the common parasites. For example, ivermectin and moxidectin are not effective against tapeworm. So, if using these dewormers, it is recommended that a dose of pyrantel or praziquantel is given in Spring and Autumn to reduce tapeworm numbers. Similarly, pyrantel or praziquantel administration is recommended in young foals as the roundworms present in these youngsters are more susceptible to these compounds. Foals should be dewormed for the first time at 3 months of age, and then monthly, until starting on the adult deworming programme at 6 months of age.
 
Once encysted, the cyathostomes are particularly difficult to get rid of, as the cysts protect them from the effects of deworming treatment. However, daily treatment for several days can be effective, as killing off larvae in the gut encourages encysted larvae to emerge. Once they have emerged they are more susceptible to deworming treatment. Given the high incidence of cyathostome-related disease observed in Lanarkshire this year, a 5-day course of fenbendazole (Panacur/Panacur Guard) is recommended for horses beneath the age of 3 or over 15 years, to be given in addition to the normal dewormer at the beginning of Autumn and again at the beginning of Spring. This should aid in reducing larval numbers below that likely to result in signs of disease.

Example deworming programmes can be found here. We would be happy to assist in designing a specific deworming programme for your yard or to answer specific questions. Needless to say, one of our aims is to reduce the incidence of cyathostome-related disease in this area during the next few years!