Anthelmintics or worm doses are medicines used to control parasite infections in livestock on Irish farms.

In recent years, many farmers have noticed these worm dose medicines have become less effective in treating the parasites.

According to Animal Health Ireland’s (AHI’s) monthly newsletter, this is because parasitic worms in cattle and sheep have developed resistance against them.

It can be said that anthelmintic resistance occurs when worms survive a dose of wormer that would normally be expected to kill them.

According to AHI, worm resistance against wormers is inherited by the worm’s offspring and certain management practices can increase the risk of it becoming a problem on a farm.

With each wormer treatment given to animals, susceptible worms are killed and the resistant worms survive to lay eggs, increasing the population of resistant worms on the farm.

The more frequently worming treatments are carried out, the greater the risk is of anthelmintic resistance increasing.

There are three main classes of wormers used against stomach/gut worms in cattle.

The three main types of cattle and sheep wormers used on Irish farms are as follows:

  • White drenches: (1-BZ) Benzimidazoles and probenzimidazoles (which include albendazole, fenbendazole, oxfendazole, mebendazole) also known as ‘white drenches’ but can differ in colour;
  • Yellow drenches: (2-LV): Also known as Levamisole;
  • Clear drenches: (3 ML): Macrocyclic lactones (which include abamectin, doramectin, ivermectin, moxidectin).

Once worms are resistant to a medicine in a particular class, then all products in that class will be less effective.

For example, if a specific brand of clear drench no longer seems to be working, switching to another brand of clear drench will probably make no difference.

It is always best to test if the wormers that are used on-farm are still effective.

A farmers’ local veterinary practitioner can advise on how best to test and which products to use.