WHILE we are all preoccupied with the Covid-19 pandemic in the human population, it is easy to forget that endemic diseases in our horses continue to occur as usual.

It is at this time of year, with an increase in temperatures and the availability of a naïve population of foals and young stock at grass, that gut worms begin their annual cycle of infection. For many years the availability and effectiveness of anthelmintic drugs (dewormers) meant that the issue of worms could be easily dealt with. However, as a consequence of our overreliance of these drugs, resistance to them has developed in most worm species which means that these treatments are less, or in some cases, not at all effective.

Here in the Irish Equine Centre (IEC) we are seeing more and serious cases of parasitism in horses and this year already we are seeing an increase in worm egg counts.

There are a number of worms which can infect horses, the most important of which are mentioned here along with some points relevant to their occurrence and control:

Threadworms

Threadworms (Strongyloides species) affect foals from birth to approximately six months of age and may cause diarrhoea in foals from 10 days onwards. The larvae of these worms can pass from the mare during suckling.

Roundworms

Roundworms or ascarids (Parascaris species) are large spaghetti-like worms which mostly affect young stock less than 18 months of age. Migration of the immature roundworms through the lungs can lead to coughing and a nasal discharge while infection with adult worms in the small intestine can lead to ill-thrift with a pot-bellied appearance. In extreme cases the adult worms may result in impaction and rupture of the small intestine. Most significantly from a control point of view, ascarid eggs can survive in the environment for years and infection can be transmitted in the stable as well as at grass. There are reports of resistance to ivermectin and moxidectin in roundworms.

Large redworms

Large redworms or large strongyles (Strongylus species) can cause ill-thrift and weight loss in horses. However, more severe disease is due to the effects of the migrating immature worms in the body. These can damage major blood vessels, the liver and other tissues resulting in severe disease in horses of any age but most frequently in young horses. It takes six months for these worms to mature from the egg to adult and not all dewormers are active against the migrating larval stages.

Small redworms

Small redworms or small strongyles (Cyathostomes species) are the major worm type in almost all horses and are a common cause of poor growth and weight loss but again more significant disease is seen as a result of the activity of their larvae. In contrast to those of the large strongyles, these do no migrate through the body but instead burrow into the wall of the intestine where they form tiny cysts. The emergence of the larvae from these cysts can damage the gut and, when large numbers of larvae emerge at the same time, can result in a life-threatening condition called larval cyathostomiasis. These worms can complete their life cycle in as little as eight weeks but can remain encysted in the gut for up to two years. Very few drugs are effective against the encysted larval stage.

Tapeworms

Tapeworms can affect horses of all ages and cause poor growth, weight loss and possibly colic in severe cases.

Remember that even in the absence of overt disease, gut worms can affect the uptake and utilisation of nutrients in your horse which may subsequently impact growth and performance.

FACTORS such as the age and type of stock, stocking density, pasture management and deworming history will influence the number and type of worms which are present in your horses.

For that reason, it is not possible to draw up one-size-fits-all treatment or control programmes. For example, a single adult riding horse kept on its own on extensive pasture will require a very different control program from a group of riding horses in a livery yard.

Equally, a farm with two mares and foals cross-grazing with cattle will require a different plan from a farm with 20 mares and foals but no cattle. Stocking density is one of the key determinants of worm levels and is particularly an issue where a number of horses share access to a limited number of paddocks, even for a limited time, as happens in many racing yards.

Despite these differences there is one fundamental starting point for any investigation of a worm problem or development of a control program and that is faecal worm egg count testing. The worm egg count, as the name suggests, is a count of the number of worm eggs per gram of faeces. The principle of the test is that worm eggs will float to the surface of a dense salt solution from where they can be counted.

It is a relatively simple but incredibly useful test when carried out properly but it is critical to use an experienced laboratory to ensure the validity and repeatability of results.

Three dung balls, each from separate areas of a fresh dung pile should be submitted for testing in order to account for variation in egg shedding within faeces. A fresh sample is necessary to ensure that eggs have not hatched before they are counted.

Several purposes

Depending on how it is used, this test can serve a number of purposes. Firstly, it can be used to determine what type of worms are present in your horses; this is particularly useful where there is a suspicion that roundworms or threadworms may be causing disease in foals or yearlings for example. As the test detects worm eggs, it relies on the presence of adult egg-laying worms and is not of use where worm larvae only are present.

The majority of worm eggs shed in faeces are strongyle eggs and small strongyle eggs in particular. The main use of the worm egg count is to identify adult horses with a high egg count which need to be treated to prevent further pasture contamination. When thinking about worm control in horses, remember that the horse contaminates the pasture and the pasture contaminates the horse.

If you identify the horses which are shedding the most eggs and treat these more frequently you can reduce the number of eggs on pasture for other horses to pick up. There is still some debate over the number of eggs above which adult horses should be treated, with cut-off values ranging from 250-750 eggs per gram of faeces.

The third use of worm egg count testing is to determine the effectiveness of any deworming treatment. A worm egg count is carried out before treatment and repeated again two weeks after treatment. Depending on the dewormer used, you would expect to see at least a 90% reduction in egg count on the second test. This is referred to as the faecal egg count reduction test and should be carried out on five to 10 horses to ensure accurate results.

For those who take worm control seriously, repeated egg counts are carried out every week from two weeks after treatment to determine the point at which eggs reappear in the faeces. This standard egg reappearance period (ERP) varies between drugs (four weeks for benzimidazoles, six to eight weeks for ivermectin and 12 weeks for moxidectin) but if it is shorter than this it indicates resistance is emerging.

The use of the worm egg count to monitor the effectiveness of treatment and the emergence of resistance should be used especially by those who deworm frequently, such as yards with a lot of young horses or intensively managed farms.

Once the worm egg count has established the parasite situation in a yard or farm, control measures can then be considered. The use of effective dewormers will always be a part of any control program which, along with dung removal from paddocks, can help to prevent the horse from contaminating the pasture.

Further measures to prevent the pasture from infecting the horse will also be necessary and these include cross-grazing with cattle or sheep, taking a crop of grass off the paddocks or resting paddocks for extended periods.

Designing an effective worm control program can be particularly challenging in overused and horse-sick pastures. There is no single or simple action that will control worms in horses but with some effort significant improvement can be made.