Virbac Australia

Health Care

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The development of worm resistance

Anthelmintic resistance is a real concern in Australia with increasing horse worms becoming resistant to chemicals. Resistance occurs because a very small percentage of worms always had a genetic ability to survive chemical treatment. A strategic approach to worming horses is necessary to manage and/or prevent chemical resistance.

What is resistance?

Resistance is the ability of worms in a population to survive treatments that have generally been effective against the same species and stage of infection.

When worms develop resistance to one type of wormer they are normally found to be resistant to other wormers of the same type or family. For example, if worms became resistant to moxidectin there is a very high likelihood that they would also be resistant to ivermectin and abamectin.

horse worms resistance key

A horse is dewormed

A horse is dewormed

If some worms survive they are resistant

If some worms survive the treatment they may be resistant to the chosen wormer.

pasture contamination with resistant worms

Those worms reproduce resulting in pasture contamination with resistant worms.

The horse eats a mixture of resistant and susceptible worms

The horse eats a mixture of resistant and susceptible worms.

continual treatment same family wormer resistant worms will increase

With continual treatment with the same family of wormer the numbers of resistant worms will increase.

The wormer fails as resistance levels increase

The wormer fails as resistance levels increase.

Anthelmintic families

Four key chemical groups used for worming

1. Macrocylic Lactones (MLs or ‘Mectins’): Macrocyclic lactones or mectins are the most widely used chemical class of anthelmintics. They are very useful to control a wide range of worms. There are two types of mectin: avermectin is one of them. At the present time there are two types of avermectin on the market in Australia for horses, that is ivermectin and abamectin. Milbemycin is the other class of mectin. Moxidectin is currently the only one sold for horses.

2. Benzimidazoles (BZ’s or ‘azoles’): BZs include drenches ending in ‘a-zole’, treat the majority of worm species, but have no efficacy against tapeworms or bots. There are several forms of BZs available, including oxfendazole, fenbendazole and oxibendazole. Small stronglyes have become widely resistant to BZs when used on their own.

  • Oxfendazole
  • Oxibendazole
  • Fenbendazole

3. Praziquantel (active against tapeworms only): Praziquantel is a member of the chemical family called the Isoquinolines family. It’s highly effective against tapeworms but has no effect on any other worms.

4. Tetrahydropyrimidines (THPs): Tetrahydrapyrimidines, or THPs, treat the majority of worm species but have no effect on bots. There are only two types of THP – pyrantel and morantel.

No one worming compound is effective on all parasites, especially with developing resistance, STRATEGY-T and EQUIMAX ELEVATION are the leading examples of combination wormers that combine a BZ or mectin respectively with pyrantel. Pyrantel needs to be a key component of young horse programs.

Factors affecting resistance

Several factors can influence how quickly resistance to a particular anthelmintic will develop.

  1. Frequency of worming treatment

    High frequency dosing is an important risk of selection for resistance.

  2. Dosing and efficacy of treatment

    Under-dosing or using an ineffective anthelmintic will increase the development of resistance.

  3. Refugia population

    (In refugia) literally means ‘taking refuge’ or ‘hiding’ and refers to the population of worms not exposed to or affected by treatment. The larger the refugia population the slower resistance will develop.

  4. Overgrazing and overstocking

    This can contribute to an increase in the number of resistant worms on a property.

Too frequent use of chemicals when not needed and underdosage of chemicals are the major ways that worm resistance to chemicals is being increased. Resistance tends to occur first in the worm species that is the hardest for a particular chemical to control e.g. roundworm resistance to mectin chemicals. Worming too frequently will ensure only resistant worms survive, as the most susceptible worms are killed, leaving the resistant worms to reproduce. Under-dosing is most commonly caused by incorrect weight estimation, incorrect method of administration, spitting out of the product and splitting of doses between horses.

Worms in refugia remain susceptible to a worming treatment. This means that the higher the proportion of worms in refugia is, the more slowly resistance will develop. To maximise refugia of susceptible worms a Gold Standard approach to worming is needed where horses that do not need treatment are not wormed as frequently. To do this faecal egg count testing is required.  Pasture management practices can have an impact on this as well, for example if grass is scarce or paddocks are overstocked, horses are forced to graze close to their manure and therefore consume more larvae. In this way overgrazing and overstocking can cause a rapid increase of resistant worms in the horse. Other factors such as length of parasite lifecycle, their egg production capabilities as well as a horse’s immunity levels can influence resistance to anthelmintics.

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