There are a number of internal parasites that can infect a horse, the most important worms including the following:

Large strongyles

Also known as bloodworms, they are about 10 – 20mm in length and red or grey in colour. They have a long migratory lifecycle of 6 months and are active blood feeders, typically causing anaemia in horses. They are highly pathogenic when they accumulate in the cranial mesenteric arteries. Here they can cause an arterial blockage (thrombosis), damaging the intestine and resulting in possible colic and death.

Small strongyles

Small strongyles or cyathostomins are also commonly known as small redworms. They are generally 5 – 15mm in length and they have a short lifecycle of 6 - 8 weeks, making them highly prevalent and common in horses in Australia. They feed on gut lining and material in the large intestine and can cause mild ulceration, weight loss and diarrhoea. Part of their success is that their larvae (L3) can encyst in the intestinal wall for several months, protecting themselves against many worming treatments. Small strongyle larvae cause severe damage when large numbers emerge from the intestine (called larval cyathostominosis), which can result in signs of diarrhoea, poor condition, colic and possibly death in horses. Only Moxidectin is registered and proven to be effective against encysted small strongyles in one single treatment dose.

                                                        Fig.1 Encysted small strongyles in the intestinal mucosa

                                                        Fig. 2 Adult small strongyles

Ascarids

Ascarids are also known as large roundworms and are of significance in younger horses less than 2 years of age. They are generally white in colour and can be up to 30cms in length. They have a migratory lifecycle of 10 – 16 weeks, travelling through the bloodstream, liver and lungs before residing in the gut as adults. Common clinical signs include respiratory signs, lung damage and more commonly, poor growth and sometimes colic and death. Ascarids have very resilient eggs so it is possible for foals to be infected from one season to the next, as the eggs remain viable in the environment for years.

                                                           Fig.3 Adult ascarids dissected from a 3yo mare

Pinworms

Pinworms can vary from 10mm long (males) up to 100mm long (females) and are white to grey in colour. They are unique in that females migrate to the rectum to lay their eggs outside of the anus of the horse in a gelatinous substance. This can cause itchiness and restlessness, and result in hair loss and possible wounds at the base of the tail as the horse tries to seek some relief by itching themselves. Pinworms predominantly affect younger, stabled horses, mainly as horses can rub their tails (and consequently eggs) into feed bins where they will later ingest them.

Tapeworms

There are three species of tapeworm; however Anoplocephala perfoliata is the most harmful. They are typically 8 – 25cms in length, but can be longer and are found in the small intestine and stomach. Their lifecycle requires the orbatid mite as an intermediate host, which horses ingest whilst grazing on grass, hay or grain. A tapeworm infection can cause signs of weight loss, anaemia, ulceration and colic in high numbers. Praziquantel is the most effective anthelmintic active that targets all three species of tapeworm.

                                                            Fig.4 Tapeworms of about 2cms in length

Stomach bots

The bot fly is not only an annoyance to horses in its environment, but they can also cause harm to horses internally. Typically in the warmer months of August to May, adult female bot flies lay their eggs on the horse’s coat and the eggs are then ingested by the horse. Moisture activates the eggs to moult into larva in the mouth of the horse, causing damage to the tongue and gum tissue, before migrating to the stomach where they develop over a 8 to 12 months. Here they can cause ulceration and lesions and in large numbers possibly colic. In late spring when temperatures start rising, larvae detach and pass out in the faeces to pupate in the soil for a number of weeks before they develop into adults.

                                                             Fig.5 Stomach bot larvae

The other aspect of internal parasite control is treating horses with anthelmintics (wormers) to remove worms internally. There are a number of treatments available made up of the following chemical groups;

• Macrocyclic lactones – provides broad spectrum protection
• Benzimidazoles (white drenches) – typically known as the rotation group
• Tetrahydropyrimides (clear drenches) – more narrow spectrum
• Praziquantel – specifically for tapeworm control

Worming treatments can be made up of one or more of these chemical groups in a number of different formulations including pastes and gels, liquids, granules and stomach tube drenches.

Resistance management

Resistance is an inherited trait where worms have the ability to avoid the effects of drugs that are usually effective against them. Anthelmintic resistance is reported in horses in Australia so it is important that we use these treatments strategically so we don’t encourage worms to develop resistance. There are three important steps we can take to reduce the development of resistance:

• Using faecal egg tests to determine when treatment is necessary to avoid unnecessary drenching
• Ensuring the correct dose is given
• Rotating your treatments and using combination products to ensure that you are not over-using one chemical group

Faecal egg tests

Faecal egg count (FEC) tests measure the number of worm eggs present in a fresh sample of manure, reflecting the egg-laying adults present in the horse. This can be used to estimate the level of worm burden and species present to identify when a horse requires a treatment and also identify if a treatment program is effective. There are some limitations with FEC tests though; they cannot measure larvae or encysted stages of parasites, nor those parasites that don’t shed eggs in manure. However it is still a highly valuable test to identify when you are best to treat and whether resistance may be occurring. A faecal egg count of less than 200epg suggests a light parasite load, where as a high faecal egg count of 500—1,000epg suggests a high parasite load. A medium to high FEC suggests the worming interval is too long and treatment is required. We recommend a veterinarian or animal health advisor provide advice on how to accurately interpret FEC results as other factors such as age, clinical signs, immunity, seasonal conditions, paddock management and stocking rate need to be considered as well.

Correct dosing

It is essential to ensure the correct dose is given, as under-dosing will result in the treatment providing a sub-lethal dose. In this instance, the treatment may not be a 100% effective and this can promote the development of resistance by exposing worms to the treatment without actually killing them.  A useful way to ensure that you are giving the correct dose is to use a girth weigh tape or preferably large animal scales if these are accessible. Otherwise you can measure the horse’s girth and length (from point of shoulder) using a measuring tape and estimate the horse’s bodyweight using the weight estimation formula:

Weight (kg) = (girth (cm) x girth (cm) x length (cm))/ 11877

Once you know your horse’s bodyweight, always round up at least 50kg to determine the correct treatment dose required.  

Rotation strategies

Establishing a rotation program is an important step in delaying the onset of resistance. There are many similar products on the market, often containing the same actives, so it is important to be informed about what’s in a product and when best to use it. Unfortunately rotating brands doesn’t mean you are always rotating a treatment. Use your allwormers when the season favours high worm pressure and use your more narrow spectrum or rotational treatments at times when worm pressure is lower.

To decipher the difficulty in working out which product you should be using when, Bayer has put together an all year round rotation strategy recommendation for you. This is an easy-to-follow treatment calendar designed to target the most prevalent parasites at the right time according to their seasonal prevalence.

Fig.6 - ROTATION WHEEL

It is always recommended to conduct FEC to determine how frequently you should be worming and to determine your resistance status. If you have identified a resistance issue on your property, it is recommended to seek advice from a veterinary or animal health advisor on a tailored program.