Worm control measures: working with clients on effective protocols

Gastrointestinal parasites are ubiquitous in grazing horses and control of them is fundamental to equine health and welfare. However, a major obstacle to client compliance is horses are kept under a variety of conditions, and all too often advice is given about individual horses, rather than the entire population in any given area.

Livery yards probably present the greatest risk from gastrointestinal parasites, due to high stocking densities, mixed and changing populations of horses and an environment where individual horse owners have less control over parasite control programmes due to individual yard policies. Therefore, the veterinarian has an extremely important role to play in trying to ensure effective worming control policies are implemented. Additionally, vets have a vital role to play in ensuring owners are educated on the principles underpinning an effective control policy, including the fact parasites present in low numbers are not deleterious to a horse’s health – and only large numbers of certain stages of specific parasites are potentially life-threatening.

Pathogenic stages of equine gastrointestinal parasites

When considering effective gastrointestinal worm control, it is important veterinarians and horse owners consider the pathogenic stages of each type of parasite.

Large strongyles

This category includes Strongylus vulgaris, S edentatus and S equinus. The adults (Figure 1) live in the caecum and colon and, if present in large numbers, their feeding habits can cause damage to the gastrointestinal mucosa. Of the larvae, only S vulgaris causes clinical problems. Following ingestion from the pasture, the larvae penetrate the gastrointestinal mucosa and enter the small arteries, migrating to the cranial mesenteric artery. Here, the presence of the larvae can result in arteritis and thromboembolic colic. However, the large strongyles, particularly S vulgaris, are now quite rare in managed horses.

Cyathostomes

Approximately 50 different species of cyathostomes have been recorded, although 10 to 12 species account for more than 90% of the burden in horses. They are currently the most common and pathogenetically significant parasite to affect horses around the world¹. Cyathostomes have a direct and non-migratory life cycle, and the infective third stage larvae (L3) are ingested from the pasture and invade the wall of the large intestine.

They then either continue through their life cycle and emerge back into the intestinal lumen before moulting to become adults (Figure 2), or the larval development becomes arrested at the early third larval stage, they remain within the gastrointestinal mucosa until the following spring and the result is mass emergence and clinical disease known as larval cyathostominosis. The trigger for hypobiosis and emergence is not known, but emergence can be triggered by removal of the luminal stages by adulticidal anthelmintic administration².

Ascarids

Parascaris equorum is considered the most important parasite in foals and weanlings. The larvae migrate extensively through the liver and lungs before returning to the small intestine to become adults. High levels of infection have been associated with respiratory symptoms, unthriftiness, rough haircoat, weight loss and small intestinal impaction.

Tapeworms

Adult Anoplocephala perfoliata (Figure 3) are found at the ileocaecocolic junction, where their presence is associated with an increased risk of various types of colic, including spasmodic colic, intussusceptions and ileal impactions.

Pinworms

Oxyuris equi (Figure 4), also known as the pinworm, is a gastrointestinal parasite of the adult horse associated with perianal pruritus, which is caused by the adult female migrating from the right dorsal colon to the anus to lay eggs in sticky clumps on the skin of the perianal region.

Bot larvae

Adult Gasterophilus flies are active in late summer, laying their eggs on the hair of horse legs and head. The eggs hatch and larvae are ingested when the horse grooms itself. The larvae penetrate the tongue or buccal mucosa and wander for several weeks before passing to the stomach via the pharynx and oesophagus.

Once in the stomach, the larvae attach to the gastric mucosa and remain there for 10 to 12 months. The presence of the larvae can cause stomatitis or gastritis.

Summary of potentially pathogenic stages

Effective worm control programmes should ensure adequate control of the following potentially pathogenic stages:

Currently rare

• Adult large Strongyles if in large numbers
• Strongylus vulgaris larvae during migration

Of most concern

• Mass emergence of Cyathostome larvae
• Adult and larval Parascaris equorum
• Adult tapeworms

Cause less severe pathogenic effects

• Adult pinworms
• Gasterophilus larvae

Worm control programmes

Gastrointestinal parasite control programmes should employ strategies to achieve adequate control of worms’ potentially pathogenic stages, but also decelerate further selection for drug resistance, thereby extending, wherever possible, the lifetime of currently effective anthelmintics. Resistance to anthelmintics has been seen worldwide, and reported in all currently available drug classes³.

Recipe-based treatment programmes – based solely on the calendar and with no regard to the medical needs of individual horses, the biology of the parasites or whether the drug is effective against the target parasites – can no longer be justified or recommended⁴. Instead, an effective worm control programme should have several facets, including identification of individual animals with high parasite burdens, identification of anthelmintic resistance on individual premises, targeted use of selected anthelmintics and management strategies to reduce pasture contamination.

Identifying animals with high parasite burdens

It is important veterinarians and owners understand parasites present in low numbers are not harmful to a horse’s health, but a few horses in a herd will have consistently high parasite burdens, so will be the main source of pasture contamination. In any given herd, roughly 20% of horses shed approximately 80% of the total amount of strongyle eggs in the population⁴. Ideally, only those individuals exceeding a pre-chosen threshold (usually 200 eggs/g faeces⁵) should receive treatment with an anthelmintic effective against strongyles. Thus, effective worm control programmes should utilise faecal egg counts (FECs) to focus on identification of these individuals.

FECs are used to determine the strongyle parasite burden in an individual animal. However, it should be remembered strongyle eggs will be detected and this reflects only the adult parasite burden. It will not reflect the larval burden of these parasites, nor the tapeworm, Oxyuris or Gasterophilus burden. Additionally, eggs from various equine strongyles cannot be discerned morphologically, and coproculture with subsequent microscopy is required to differentiate the large and small strongyles.

Ascarid eggs will additionally be detected, but no linear association is seen between the number of eggs per gram of faeces and the number of worms harboured in the small intestine. The false-negative rate of FECs for detecting P equorum infection is considerable. Thus it is a useful qualitative, but not quantitative, diagnostic measure for ascarid adults only in some animals, and not for the migrating larvae.

Tapeworm burden cannot be reliably estimated from a FEC, due to the generally accepted fact cestode eggs are not equally distributed in the faecal matter. This is probably due to the eggs being shed as gravid proglottids detached from the tapeworm. Instead, either serum or saliva antibody ELISA testing can be performed. It is recommended horses are tested every six months and the result interpreted in the light of previous anthelmintic therapy, due to the lingering nature of the antibodies that delays the reduction of optical density (OD) levels after removal of the parasites. Animals with a high OD level should be treated with an anthelmintic effective against adult equine tapeworms.

As the eggs are laid outside the rectum, it is possible FECs will fail to detect the presence of O equi. Instead, a diagnosis is made based on the results of a sticky tape test, where a strip of tape is applied to the skin close to the rectum, removed and then examined under a microscope for the presence of O equi eggs. Affected animals should be treated with an appropriate anthelmintic.

Unfortunately, it should be remembered, therefore, it is only possible to detect some, but not all, potentially pathogenic stages of all of these species of gastrointestinal parasites; for example, there is currently no commercially available test to detect cyathostome larvae. Therefore, strategic treatments targeting these stages of the specific parasites are often warranted.

Available anthelmintics

Four major classes of anthelmintics are used to control gastrointestinal parasites in horses. It is important owners are aware of the different drug classes and that wormers with different trade names do not necessarily contain different anthelmintic drugs.

Benzimidazoles

Benzimidazoles bind to nematode tubulin, preventing glucose uptake leading to starvation. They are theoretically effective against adult and developing larval stages of cyathostomes, adult and larval stages of large strongyles and adult Oxyuris, but there is significant nematode resistance^6,7, including the encysted cyathostome larval stages⁸, and they should only be used following resistance testing. They are not effective against tapeworms. Fenbendazole is effective against Parascaris, but the recommended dosage for removal is 10mg/kg, double that recommended for strongyles and pinworms⁹.

Macrocyclic lactones

The macrocyclic lactones available for use in horses include ivermectin and moxidectin. They act on glutamate gated chloride channels, increasing the chloride current and paralysing the pharynx and somatic (body) musculature of the parasite. They are effective against larval and adult large strongyles, adult and luminal larval stages and developing mucosal larval cyathostomes, Parascaris, Oxyuris and bot larvae, but not tapeworms. Moxidectin has some effect against the mucosal encysted stages of cyathostomes, but not the inhibited stages. However, evidence now details ascarid and cyathostome resistance to both drugs⁹.

Tetrahydropyrimidines

Pyrantel is a nicotinic receptor agonist which can elicit spastic muscle paralysis in parasitic worms due to prolonged activation of the excitatory nicotinic acetylcholine (nACh) receptors on body wall muscle. It is effective against adult tapeworms at double the recommended dose, adult cyathostomes, Parascaris and Oxyuris, but there is ever increasing resistance.

Praziquantel

Praziquantel’s mode of action is not exactly known, but experimental evidence indicates the treatment increases the permeability of the membranes of the parasite muscle cells to calcium ions. The drug thereby induces contraction of the parasites, resulting in paralysis in the contracted state. It is only effective against adult tapeworms in the horse.

Identifying anthelmintic resistance

Veterinarians and owners should be aware of significant resistance among certain gastrointestinal parasites to certain anthelmintic drugs. FEC reduction tests (FECRTs) are used to detect drug resistance on an individual premise. The FEC is determined immediately before, and 14 days after, administration of a specific anthelmintic in a number of horses on a farm – typically at least 5 to 10 animals.

If the FEC is not reduced by at least 90% to 95%, depending on the drug tested, then this is suggestive of drug resistance, and that particular anthelmintic should be avoided. However, this test is not applicable to determining anthelmintic resistance in equine tapeworms.

Pasture management

Pasture management is a key area owners can focus on to guarantee their worming programme is efficient. The principle of pasture control of parasites is to prevent pasture contamination with eggs, thereby preventing the parasite from completing its life cycle in the host. The easiest way to implement this strategy in horses is to prevent horses having contact with contaminated pasture or feed.

Horses that graze pastures are likely to encounter many infective parasite larvae that they will inadvertently ingest as they graze. To reduce the parasitic contamination of pastures, owners should ensure droppings are removed from the pasture at least twice a week, either manually or using a pasture sweeper.

Chain harrowing is only advisable if the animals are removed from the pasture and a period of dry weather ensues, in which the eggs are desiccated, followed by a period of wet weather to wash the faeces into the soil before the animals return.

Other measures that will help reduce the parasite burden include alternating grazing of any particular land with ruminants, as the gastrointestinal parasites are equine specific, rotating pastures to allow paddocks to be rested and desiccation of eggs and larvae to take place and avoiding overcrowding, which can force animals to graze the roughs adjacent to faeces that harbour the highest numbers of infective larvae.

The moving of animals to clean grazing immediately after treatment should also be avoided, as potentially only the resistant parasites will continue to be excreted post-treatment, resulting in spread to the clean pasture.

Additionally, new horses should be handled effectively, namely through quarantine, FEC and anthelmintic treatment, if necessary, before introduction to the pasture to ensure they do not contribute excessively to pasture contamination.

Finally, good stable hygiene, with regular mucking out, is essential to reduce this route of parasite ingestion.