While drench resistance is widespread in sheep, veterinarian Katie Mason says there’s no room for complacency among dairy farmers.

Pasture-based animals, like we have in New Zealand, are prone to high worm burdens, particularly as young animals, before their natural immunity develops in adulthood.

Most farms depend on strategic use of drenches to control worms.

The three main classes of worm drenches available to treat worms in cattle are: benzimidazoles, mectins and levamisole.

Benzimidazoles are also known as ‘white drenches’. Mectins, as macrocyclic lactones and include products like ivermectin and abamectin. Levamisole is also known as clear drench.

Drench resistance is widespread in sheep. Although less common in cattle, it is important not to become complacent.

When worms are not killed by a particular drench they are said to be drench ‘resistant’ – the worms survive the drench. Worms that are resistant to one class of drench are mostly resistant to the whole family of drenches; all the drenches that fall into that particular class.

Managing drench resistance is complex.

First and foremost monitor the situation on your farm

Monitor the drench resistance situation on your farm by taking faecal samples from young animals (<8-9 months old) 7-10 days after drenching and running FEC (faecal egg count) tests on the faeces.

The presence or absence of worm eggs in the faeces is a good indicator of the effectiveness of the drench. If there are no eggs, the drench has been effective. Growing larvae (young worms) from any eggs found in the faeces is a good way to identify which species are resistant to the worm drench.

Appropriate use of drenches

Slow the development of resistance on your farm by using drenches at appropriate intervals (at least 28 days) and dose. Take care not to underdose (to promote resistance) or overdose (to cause toxicity).

This means you will need to check bodyweights and the dose your drench gun is administering.

Using one particular worm drench for protracted periods can pressurise worms to develop resistance. Your vet may recommend a slow rotation of drench product use, but this alone may simply not be good enough. It is often advisable to use a multi-active product, that contains more than one class of worm drench, as this will broaden the range of worms against which the drench is effective; but other strategies need to be employed.

Creating refugia

One way to slow the development of resistance, is to leave some animals untreated which allows some worms to avoid exposure to drench. This, in turn, allows colonisation of the pasture with susceptible worms and also provides a population of non-resistant worms for the resistant ones to breed with.

This concept is called creating ‘refugia’ by selective drenching. You can also promote susceptible worm populations (and refugia) by drenching the mob and returning them to same infective pasture for a week or two before they go on to ‘clean’ pasture.

Managing pasture contamination

Reducing the number of eggs or larvae (young worms) on the pasture is as important as killing the worms inside the animal.

Consider using adult stock with high worm immunity to mop up the larvae and eggs on the pasture. Rotate paddocks used for calves, not just the ones around the calf shed. Don’t allow stock to graze to very low pasture residuals as this will increase larval intake.

Quarantine drenching

Be vigilant with bought-in stock, as they may bring resistant worms with them.

Similarly, animals that are grazed away from home, may bring resistant worms home. Quarantine drenches may be used – typically a triple active drench – on arrival back onfarm to try to kill all worms and prevent them from populating the home farm.

It’s usually best to set aside quarantine pastures for these animals too. There is no ‘one-size-fits-all’ approach.

Worm species will vary between farms and a drench product should be selected which is most effective at controlling the particular parasites on a particular farm.

Properties with a high proportion of high-risk (young) stock will be particularly challenging to manage. Remember that administering worm drenches to your stock should be only one part of an effective worm management plan.