APPROPRIATE TARGETING OF WORMING THERAPY IN HORSES
When they were first introduced in the 1960s, anthelmintics were primarily targeted at the large strongyles (namely Strongylus Vulgaris), with frequent administration recommended to eliminate the infection. Although the populations of large strongyles have been thanks to this significantly reduced, an unintended consequence of regular anthelmintic administration lead to the development of cyathostomins (small strongyles) that were resistant to available anthelmintics. Cyathostomins are now recognised as a major cause of potentially fatal disease, when mass eruption of larval stages occurs.
Despite this, and despite development of resistance to all the licensed classes of anthelmintics, surveys of horse owners indicate the majority of owners continue to administer anthelmintics on a regular basis, regardless of need.
In two recent investigations, 84% and 82% of horses routinely treated with anthelmintics had negative faecal worm egg counts (FWECs) and therefore did no warrant treatment. It has been estimated targeted worming should reduce anthelmintic use overall by 75% by adopting targeted worming strategies. Studies have not only confirmed targeted programmes of worm control reduce pasture contamination with strongyle eggs, but they also reduce the cost of worm control.
ASSESSING NEED FOR TREATMENT - COMMON WORMS AND TESTS AVAILABLE
The adult worms live in the large intestine and suck blood, hence the red colour. They lay eggs, which pass out in the droppings, develop into infective larvae that are ingested by the horse during grazing. There are two types:
Large red worms (Strongylus) - lives in the large intestines with one developmental phase spent outside the large intestine, in the blood vessels supplying the digestive system. These vessels can become blocked, leading to colic.
Diagnosis: The burden can be effectively estimated using Faecal worm egg counts (FWECs)
Treatment: Majority of current wormers targets this species, although there is growing resistance to many of these.
Small red worms (Cyathostomins) - These are only a few millimetres long and just visible to the naked eye. Larvae taken in during grazing develop in the wall of the large intestine and emerge when mature to form adult worms. If ingested in the late autumn some may become inhibited in the wall and then emerge in waves in spring causing sudden weight loss and sometimes diarrhoea. In severe infestation this can be fatal. Encysted larvae also cause damage to the large colon walls, which can affect colon peristalsis and can lead to recurrent bouts of colic.
Diagnosis: The burden can be effectively estimated using Faecal worm egg counts (FWECs). Unfortunately there is no test currently available to diagnose the encysted larvaes (serologic tests are currently being developed, but not yet available).
Treatment: Moxidectin is the only product that may be relied on in the treatment of larval cyasthomosis (therefore use of this wormer should be restricted to these cases, and not being used for un-targeted worming!)
TAPEWORMS (Anoplocephala perfoliata)
These small flat worms live in the small intestine, principally at the junction of the lower small intestine and the caecum. Eggs are passed in the faeces and ingested by a small mite on the grass. Further development occurs in the mite that is in turn inadvertently eaten by a grazing horse. Adult tapeworms then develop in the horse. They can cause spasmodic colics (about 20% of spasmodic cases are believed to be tapeworm associated) or impaction of the lower small intestine, causing surgical colic.
Diagnosis: Eggs can be seen during Faecal worm egg counts (FWECs), but since they are not excreted on a daily basis, negative results do not rule out infestation. There is a blood test and a new saliva test available to check for antibodies, which will confirm the presence of the tapeworms and a need for targeted worming. It takes about 4 months for these antibodies to reduce, so it’s not recommended to perform these tests within 4 months of worming.
Treatment: Praziquantel and double dose of Pyrantel are the only two products effective against tapeworms.
ASCARIDS (ROUNDWORMS) (Parascaris Equorum)
These are parasites only of foals and weanlings because older horses become naturally immune. The eggs are taken in while grazing, hatch in the stomach and the larvae travel via the liver and lungs before settling down in the small intestine and developing into adult worms, which may be very large. The adults lay eggs and the cycle continues. the clinical signs of ascarid infections in foals and weanlings are failure to grow, poor body condition, coughing and/or colic due to small intestinal obstruction.
Diagnosis: The burden can be effectively estimated using Faecal worm egg counts (FWECs).
Treatment: Majority of current wormers targets this species, although there is growing resistance to some. Eggs survive many years on pasture, therefore youngsters (foals and weanlings) should be wormed in regular intervals, starting from 8 weeks of age (n.b. moxidectin cannot be given until age of 4 months)
PINWORMS (Oxyuris Equi)
These worms live in the rectum of horses and females come out of the anus to lay the eggs, which leads to tail rubbing and anal irritation.
Diagnosis: “Sticky tape test” in which sticky tape is applied around the anus and examined under a microscope for the presence of O. Equi eggs. However, this test is not always reliable (eggs not shed daily), so a diagnosis is often made on the basis of evidence of pruritus and self-trauma of the perineal region.
Treatment: Due to overusing of the wormers there is now significant resistance and none of the wormers available is now 100% effective. Therefore, specific intensive targeted worming will be undertaken if the horse is severely affected by these worms (they don’t often cause any real problems though). Mechanical removal of the eggs from the anal area is an important part of the treatment.
These are reddish grubs found attached to the wall of the stomach and small intestine during the winter. The grubs detach in spring and are passed in the faeces, where they develop into bot flies. the flies during the summer lay their eggs on the horse’s legs. These are licked off by the horse, swallowed and in turn develop into grubs in the stomach and small intestine.
The clinical signs of bot infection are annoyance of grazing horses by bot flies attempting to lay eggs on their legs, and veryrarely colic (principally caused by the species that attaches to thesmall intestine rather than to the stomach)
Diagnosis: Presence of the eggs on horse’s legs
RESISTANCE IN THE UK
Overusing of wormers lead to growing resistance; this resistance is heritable (the worm's ability to survive is passed onto its offspring) and thought to be irreversible, even if selection pressure is removed. Therefore, it’s absolutely essential that we start using wormers responsibly.
Recent studies showed widespread across the world resistance to benzimidazoles (= fenbendazole (e.g. Panacur) and menbendazole (e.g. Telmin)). Resistance to fenbendazole (Panacur) within the UK was found between 24% and 100% depending on the geographic area; this includes 5-day course regime.
Worldwide, resistance to pyrantel has been slower to develop, which may be related to less intensive use compared to fenbendazole or lack of efficacy against larval stages, both of which reduce the selection pressure.
Depending on the establishments tested, the resistance to pyrantel in the UK was found between 0 and 13% (higher in professional training and breeding facilities).
Ivermectin has been used widely since the 1990s and impending resistance is a major concern, with recent studies reporting inadequate effect in Thoroughbred racing populations. There are also increasing numbers of reports of macrocyclic lactone (i.e. ivermectin and moxidectin) resistance in Europe and other parts of the world.
Moxidectin resistance has been reported in two populations of donkeys at The Donkey Sanctuary in Devon.
Regular Faecal worm egg counts (FWECs)
The results of Faecal worm egg counts (FWECs) provide an indication of which horses are contaminating the pasture and to what extent. They are straightforward and inexpensive to perform and enable anthelmintics to be administered in a targeted manner - based on need rather than routine.
The results of FWECs are most useful when all horses in the population are tested together. They should be performed at the start of the grazing season and horses with a FWECs of more than 400 eggs per gram are treated. Testing will be then repeated two to three months later (interval influenced by the level of burden and anthelmintic used) - you will be advised on appropriate interval when results are reported.
This way only the horses responsible for pasture contamination (usually small number of culprits) will be targeted and treated, resulting in reduction of worms in this population without a risk of increasing anthelmintic resistance.
In populations where exposure is low and negative FWECs results are repeatedly obtained, the interval between deworming may be extended beyond the typical three months. Many horses will go the entire grazing season without developing a high FWEC and do not need to receive any anthelmintic treatment.
Strongylus Cyathostomins Tapeworm Ascarid Oxyuris equi
Tapeworm testing (saliva or blood test)
This should be ideally performed twice yearly (spring and autumn) to check for the tapeworm burden. Only horses with positive test would be treated, hence preventing further increase of resistance (we have only two products effective!).
Targeted worming for encysted larvae (cyathostomins)
Administration of moxidectin at the end of the grazing season is an effective means of reducing the burden of encysted larvae and its use should be reserved for this purpose.
Targeted strategies may not be appropriate for populations in which there are large numbers of young horses, frequent movement of horses or if there is poor compliance and, on such premises, a programme of routine, but strategic dosing utilising FWECs to reduce anthelmintic use should be considered.
Also, it's advisable to de-worm and quarantine (for 72 hours) any new horse arriving to your premises before they are allowed to your pastures.
Cyathostomin-related disease is a major cause for concern and resistance to the major classes of anthelmintics is an increasing problem.
We should not seek to eliminate all worms, but to target therapy to prevent disease while maintaining a population of parasites that receives minimal exposure to anthelmintics (“refugia”). In addition to limiting anthelmintic use, refugia should be maintained by limiting stocking densities, co-grazing with other species, “dose and move” strategies and regular removal of faeces from pasture.
The negative effect of regular interval dosing of wormers has been proven showing the rise in overall resistance to all classes of wormers and mainly increasing number of resistant cyathostomins, which are now a major cause of potentially fatal disease. Switching to targeted control when wormers are used only when needed, can help to prevent further resistance (as they‘ve been shown to reduce anthelmintic use by 75%); it’s also very cost-effective in addition to having ethical and environmental benefits.
Equine premises vary hugely and it is important that veterinary advice tailored to each individual premise is obtained. What is appropriate for a small stud farm, for example, may be wholly inappropriate for a large livery yard with changing population of horses.
If you would like any further information or to plan the right worming regime for your horse, please contact us.