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de-worming 2

De-worming: Part 2 – Anthelmintic Resistance

In brief:
Resistance to the anthelmintics (drugs) used to de-worm horses is increasing, and is made even worse by poorly planned de-worming programmes and overuse/misuse of de-wormers. There are limited drug families available, and it is believed that once resistance has developed then it is irreversible. Therefore strategic de-worming combined with other control measure is essential to ensure horses are protected from colic, ill-thrift, and other problems caused by high burdens of internal parasites.

Anthelmintic resistance: what is it and is it important?
Anthelmintics are the drugs we use to remove internal parasites (such as worms and bots) from horses, but the parasites themselves can become resistant to the drugs in the same way that ‘super-bugs’ in hospitals are becoming resistant to antibiotics.

When we use an anthelmintic, it doesn’t kill off every single worm in the body of a horse. The worms that survive treatment may have been protected from the drug (e.g. larvae that are protected by cysts or are still travelling round the body) OR they may be resistant i.e. that particular drug cannot kill them. Surviving worms are the ones that go on to lay eggs and start the next generation, so some of the new generation will also be resistant to the drug. Eventually, just about all of the worms inside the horse become resistant and the drug no longer protects the horse i.e. the worms can make the horse sick.

Old de-worming programmes used to recommend treating horses every 6-8 weeks all-year-round, in order to control the larvae of Stronglyus vulgaris (large redworms) which used to be a major problem in horses. This practice was successful in reducing S. vulgaris damage, but it has since been found that frequent treatment sped up the process of resistance by other parasites e.g. cyathostomes (small redworm).

There can be no doubt that resistance to de-wormers is a growing problem, and if this process continues then we will be back to the ‘old days’ when there was no effective treatment i.e. horses dies from worm damage. Old veterinary textbooks refer to doses of arsenic and similar compounds to treat worms -- they may have killed the worms but think of the damage they did to the horse in the process!

Modern drugs have only been around since the 1970’s, and it costs millions of £ and decades of research to find and develop a new effective drug, so it is important to look after the ones we have.

What things can be done to reduce the risk of anthelmintic resistance?

  1. Correct dosage - The dose of drug that is given to a horse or pony is based on its weight, therefore the larger the horse the more it needs. If the horse doesn’t get enough de-wormer for its size, then the drug won’t work as efficiently and therefore more worms will survive the dose.
    Common reasons for under-dosing are:
  • Horse’s weight is underestimated.
  • Some of the drug is dropped out of the horse’s mouth when the de-wormer is given.

Using weigh scales is the most accurate way to work out how much the horse weighs, but the use of calculations based on various measurements, or the use of special horse ‘weigh-tapes’ also gives a good estimate. Guessing the weight by looking at the horse is the least accurate.

  1. Strategic dosing – Over-use of drugs causes resistance, therefore using drugs only when needed will help delay resistance. Research has shown that the largest number of eggs contaminating the pasture come from a small percentage of horses, and that there are also seasonal fluctuations in egg and larvae production. Therefore it is not necessary to de-worm all horses all year round.

To get the best success out of a strategic dosing program, the plan must be worked out based on knowledge of the lifecycles of the parasites, the limitations of the tests available, and the individual circumstances of the area and property involved. De-worming programmes should be based on:

  • Identifying and treating the horses shedding the most roundworms and tapeworms; via repeated WEC’s and saliva tests at the correct time of the year, and at the correct interval after last treatment (see part 1).
  • Targeted use of drugs against parasites not detected in those tests, at appropriate times of the year and based on circumstances.
  • Designing separate de-worming programmes for horses under three years of age, as these have different parasites as the main problem (e.g. Parascaris equorum).
  • Combining any program with other control measures that do not involve drugs (See below).
  • Testing to ensure that the parasites on a farm/establishment have not become resistant to the drugs being used.

Strategic dosing can also save money, as treatment is only given if needed.

  1. Take advantage of the Refugia

The refugia is the amount of worms and eggs (either in the horse or on the pasture) that are NOT resistant to the drugs, and therefore they dilute the resistant parasites. It is believed that the larger the refugia, the longer it will take for drug resistance to build up.

  • Don’t dose and move. It used to be thought that it was best to dose a horse and then move it to ‘clean’ (ungrazed) pasture. However, this is no longer recommended because there are few worm eggs on new pasture to dilute resistant eggs from worms that survive de-worming.
  • Don’t treat horses unnecessarily. Horses shedding less than 200epg do not need treating for some types of worms. Most of the roundworm eggs are shed by a small percentage of shedders, so allowing other horses to shed some non-resistant eggs helps dilute the resistant ones.
  • Reduce treatment frequency when there are low levels of eggs on the pasture e.g. during very cold weather or droughts. Treating when the pasture refugia is low will increase the percentage of resistant eggs on the pasture even though the total number is reduced.
  • Only use larvacidal drugs (those anthelmintics that kill the larval stages too instead of only the adults) as part of a strategic plan. Larvicidal drugs reduce the non-resistant refugia inside the horse, but are essential at some times of the year to prevent damage to the horse.
  1. Reducing egg/larvae numbers on pasture
  • Poo-picking – The droppings are removed from the pasture or pens on a regular basis; also known as manure collecting.

Research has shown that regular thorough removal of droppings can be as effective as anthelmintic use for reducing pasture contamination from endoparasites. It also has the advantage that it does not involve drugs (and therefore doesn’t increase resistance) and it removes resistant eggs from the pasture before they are ingested by the horse.

To be effective, poo-picking must be done every 3-4 days, as the parasite can become infective again in approximately 4 days in ideal conditions (although it can be weeks if conditions are not ideal). The more dropping that is left on the pasture, the more eggs that are left behind.

  • Cross-grazing with other species – the majority of horse endoparasites do not survive in species such as cattle or sheep, so these can be used to ‘clean’ the pasture and remove resistant eggs and larvae. Note that liver fluke, although uncommon, can transfer to horses (from cattle or sheep) in areas where it is prevalent.
  • Resting pasture- some egg types can survive for long periods in suitable conditions, and can last through the winter, therefore the time that the pasture needs to be rested for will depend on the type of parasite and the weather conditions. Just resting the pasture for a few weeks is unlikely to make a difference.
    Eggs are most susceptible to drying (e.g. hot dry summer conditions), but also can be killed by freeze/thaw cycles (such as frequent frosts and warm days). Harrowing pasture in dry conditions can help kill the eggs, by breaking up and drying out the droppings, but if the conditions are not dry enough then harrowing will just spread the eggs across the pasture instead.

Drug resistance can be readily increased by:

  • Underdosing
  • Dosing frequently when there is no need (either groups or individuals)
  • Not having a planned de-worming programme based on the parasite life-cycles
  • Using drugs that are no longer effective, without realising
  • Not using other control measures alongside a de-worming programme
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Many sources will provide information on designing a de-worming programme. Some of these have a commercial interest in selling de-wormers and/or test kits; therefore, it is important to know the basics of how programmes work and why tests are not 100% accurate in order to make an informed decision about when to test and when to de-worm.