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.
A horse is dewormed
If some worms survive the treatment they may be resistant to the chosen wormer.
Those worms reproduce resulting in pasture contamination with resistant worms.
The horse eats a mixture of resistant and susceptible worms.
With continual treatment with the same family of wormer the numbers of resistant worms will increase.
The wormer fails as resistance levels increase.
Four key chemical groups used for worming
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.
Several factors can influence how quickly resistance to a particular anthelmintic will develop.
High frequency dosing is an important risk of selection for resistance.
Under-dosing or using an ineffective anthelmintic will increase the development of resistance.
(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.
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.
Choose the correct worming strategy