I am sure you’re all familiar with the recent local and international discussion in the animal and human health sectors regarding antimicrobial resistance; drench resistance has been around for even longer. Worms aren’t quite as clever as bacteria and don’t acquire resistance by changing, but in fact there are some genetic variations within the worm populations that just mean they are resistant to the various chemicals we use to kill them – annoying isn’t it?! But the more pressure we put on our animals and farms (higher stocking rates, shifting stock from property to property), the higher the chance that drench resistance will limit the performance of your stock.

Drench Resistance

Drench resistance refers to the ability of a worm species to survive a drench treatment. As susceptible worms are killed by the drench, only resistant worms are left to breed with other worms that also survived the drench. Once the resistant worms are abundant enough to be noticed by the farmer, managing them is difficult. One way to slow the development of resistance is to allow some worms to avoid exposure to drench, providing non-resistant worms for the resistant ones to breed with. This concept is called refugia, which will be discussed more in a later publication.

The fastest way to acquire drench resistance on your farm is by importing it, through buying stock or your own stock returning from grazing. This risk can be mitigated by using an effective quarantine drench on arrival and placing these animals on quarantine pasture for two to three days post arrival. To be considered effective against a broad range of worm species, the drench must contain at least two drench “families”.

Multiple Active drenches

Three drench families exist. These are macrocyclic lactones (ML), also known as the “-mectin” drenches (ivermectin, abamectin, eprinomectin). The benzimidazoles, also known as “white” drenches, despite the fact they come in a range of colours (albendazole, fenbendazole). The third family contains levamisole, which is a “clear” drench.

We recommend drenching youngstock with a product that contains multiple actives. This refers to products containing at least two drench families. For example, levamisole and ivermectin (clear and –mectin), but NOT ivermectin and eprinomectin (-mectin and –mectin).

Worms that are resistant to abamectin will develop side-resistance to ivermectin. This is why we must consider the families rather than the individual drenches. But this side resistance does not occur between different drench families. One of the best ways to slow the development of drench resistance is to used multiple families at the same time which we call multiple active drenches.

How do we detect resistance in a population?

If we are worried that worms are not being killed by a particular drench product, the first step is to carry out a faecal egg count (FEC) 7 to 10 days after drenching. As adult cattle have the ability to suppress worm egg output, calf faeces produces more meaningful results. The presence or absence of worm eggs is a good indicator of the effectiveness of the drench. If there are no eggs, the drench has been effective and further testing is not required. If you would like to check the effectiveness of your drench, consult your Vet to arrange a FEC.

However, if eggs are still present following drenching, the next step is a larval culture. Larval culture enables us to determine which worm species are surviving the drench treatment. For example, if there are lots of worm eggs present following a ML drench (e.g. ivermectin) in calves, it is likely the eggs are predominantly Cooperia. Cooperia is a worm that may cause reduced growth rates, but is considered less pathogenic (less dangerous) than other worms, for example Ostertagia. Drench resistant Ostertagia have been identified in the Waikato, and these worms are highly pathogenic (i.e. infections can kill cattle). So it rapidly becomes important to know which worm species we are dealing with and this can vary from farm to farm.
Managing drench resistance is a complex task. If you would like advice on this topic or are concerned about which worm species may be causing ill thrift in your youngstock, talk to your local Anexa Vet.