Advice on Worming Horses

Roundworms in horses

Roundworms that infect the intestines of horses are an important cause of disease and are a major welfare issue if not properly controlled
Several types of worm infect horses: the most important are the cyathostomins
Virtually all grazing horses are exposed to these worms and animals will often be infected all of their lives
It is important that these worms are controlled using appropriate deworming treatments
Because of the problem of resistance to wormers, it is almost as important to not over-use these drugs as using them for control
Resistance to all three available classes of drug has been recorded
As no new products are being developed, it is important that effectiveness of currently available drugs be maintained for as long as possible
Worming programmes should be used that control parasite burdens, but at the same time, aim to retain the effectiveness of currently available drugs

The cyathostomin life cycle.

L3 = third stage larvae; EL3 = early (or inhibited) L3; DL = developing encysted larvae. EL3 and DL are immature worm stages that live in the large intestinal wall.

Clinical impact

Most horses have sub-clinical infections, i.e. no outward signs of disease
A few horses can develop larval cyathostominosis due to large burdens of EL3 and DL in the intestinal wall
The clinical signs of larval cyathostominosis include weight loss, oedema, colic and diarrhoea. Unfortunately, this disease is fatal in up to 50% cases
EL3 and DL can persist in the gut from months to years

Worm transmission in groups of horses

Horses vary in susceptibility
Most animals control infection well and this is reflected in low or negative faecal egg counts (FEC). A few harbour high levels of infection and will have moderate to high FECs (i.e. >500 eggs per gram, EPG)
Relative susceptibility is maintained through life so that animals that excrete many eggs do so from foal- through to adult-hood
Because most worms are present in a few horses, only a relatively small number of individuals contribute the bulk of pasture contamination. This is central to the design of control programmes that minimize selection pressure for drug resistance
During autumn/winter (in the UK), EL3 and DL comprise the majority of the burden. This means that horses can have substantial levels of infection but these worms are not detectable by FEC analysis - this must be taken into consideration when designing year round control

Drugs for cyathostomin control

benzimidazoles (BZ: e.g. fenbendazole)
tetrahydropyrimidines (THP: e.g. pyrantel salts)
macrocyclic lactones (ML: e.g. ivermectin and moxidectin)
all drugs are effective against egg producing adult worms (in drug sensitive populations)
Avoiding large EL3 and DL burdens is essential in preventing cyathostomin-associated disease. Only some dewormers have licensed efficacy against EL3 and DL worms – these are moxidectin (MOX) and fenbendazole (when administered over 5 consecutive days)

Drug resistance

Overuse of wormers has resulted in widespread drug resistance
Resistance to BZ is geographically widespread
Resistance to THPs occurs, especially in the US
Resistance to both BZ and THP within single populations occurs
Evidence of reduced effectiveness is starting to appear for IVM and MOX
Drug resistance is an inherited characteristic, passed between worm generations via the genome
Movement of animals carrying resistant worms between establishments encourages flow of resistance carrying genes meaning that, without appropriate quarantine procedures, inherited resistance traits are spread
Once resistance is present it does not lost from worm populations, even over decades

How drug resistance develops in roundworms

Responsible approaches to roundworm control in horses

Surveys over the last decade indicate that most horses are wormed too frequently and that vets are not getting involved in parasite control
Horse owners should work with vets in designing control programmes. This is to ensure that

the health and welfare of the horse population is monitored
drug treatments are applied sensibly to avoid resistance

A sound knowledge of past control is required to identify previous over-use of drugs and to obtain information on potential levels of contamination
Treatments should be administered to reduce pasture contamination with eggs when animals are grazing, keeping in mind the way cyathostomin populations are dispersed in horses.
It is important to perform FEC analysis to identify which horses make the most contribution to contamination.
Leaving some animals untreated reduces selection pressure for resistance and will reduce the total number of treatments (and costs)
Treatments should be avoided following a move to ‘clean’ grazing

A targeted approach to wormer treatments based on FEC analysis

Before performing FEC analysis keep in mind the ERP for each drug - this will determine how soon after the last treatment that testing will be of use. The ERP for commonly used drugs is:
pyrantel and BZs - 4 weeks
IVM - 8 weeks
MOX - 14 weeks
Eggs can reappear quicker in younger animals/those with large burdens
The threshold EPG value on which to base treatment varies from 200-500 EPG. This should be selected on the basis of grazing and worming history of the horses sampled
Note that regular FEC analysis will identify horses most susceptible to infection and therefore most at risk of disease
Ideally, this approach should be supported by awareness of population drug sensitivity as assessed by a faecal egg count reduction test (FECRT).

FECRTs

These are performed by assessing FECs on the day of treatment and then repeating FEC analysis ~10-14 days after administration of drug
Contact your veterinary surgeon for getting this done

Good management

All control programmes should combine drug therapies with good general management
avoid overstocking
remove faeces at regular (i.e. weekly) intervals
administer wormers based on the weight of each animal
Ensure all newcomers are quarantined and treated appropriately. Current best practice is to treat with MOX, preferably combined with praziquantel for tapeworms. A 2-week post treatment FEC can be performed to ensure that the horse is not introducing resistant worms.

Selection of drugs for control

Because of the degree of BZ resistance and emerging PYR resistance, it is difficult to rotate different drug classes each year
Where there is sensitivity to >1 drug class, an annual rotation programme can be practiced
For example, one could institute one year of ML treatments, with IVM used for treating egg-producing adult worms (based on FEC analysis) in spring through to autumn. This could be combined with a MOX treatment for EL3/DL in late autumn/early winter (all horses). An IVM treatment in the spring/summer could be combined with praziquantel for anti-tapeworm therapy.
In year 2, treatments for egg-producing adult worms could be done using pyrantel (use once in the year at a double dose for tapeworm control). The levels of resistance that have been reported for BZ are such that it is probably best that, should BZ be considered, effect of treatment be monitored by FECRT, especially where BZ have been used a lot before.
Annually rotating the class used for EL3 and DL treatments in autumn can be problematical and, in the absence of FECRT data to indicate sensitivity to BZ, MOX is best used for this purpose.

The future

A number of researchers are investigating development of tests that will enable earlier detection of drug resistance and hence help us avoid it at an earlier stage. The Horse Trust has been funding this research for a number of years.