The rising threat of anthelmintic resistance

An anthelmintic (or wormer) is a drug used to destroy parasitic worms. Anthelmintic resistance (AR) is when worms become able to tolerate an anthelmintic and so survive a dose of a drug that would previously have killed them.

The resistant worms pass this ability on to their offspring. And as more and more of the resistant worms survive, the rate at which resistance develops increases.

A significant global concern

AR is a growing problem and is affecting agriculture across the world. As worms resist treatment and infection in livestock increases so production decreases. This means farmers lose money and less milk and meat is available to feed the population.

Therefore, AR is something we urgently need to combat. New anthelmintics are being developed – although currently there are few – but we still need to do all we can to slow the rate of resistance.

You can help do this by:

Only worming when necessary

Every time we use an anthelmintic we select for resistance because the susceptible worms die but the resistant ones survive and breed. And the more often we treat the more we add to the problem. However, we need to preserve the susceptible genes as far as possible so they can mix with the resistant ones and slow the rate at which AR develops.

Therefore, we need to move away from the old routine way we used to worm our animals and only worm when necessary. This could be, for example, when animals are young and more vulnerable, at particular times of year or when we are certain our animal is infected.

You can find out if your animal is infected and the extent of the infection by having a worm egg count (WEC) test on a sample of your animal’s faeces. For more information please visit our WEC page.

Use a targeted wormer

Not all products have the same effect on all worms. So to have the best chance of clearing an infection you should use a drug that has been developed to target the specific worm your animal is carrying.

Again, a WEC will help you identify the specific worm that is infecting your animal.

Give the correct dose

Animals are often not given a high enough dose of an anthelmintic. For example, animals may not be weighed accurately or a lower dose is given in order to reduce costs. In addition, different animals species may be given the same dose. However, for example, goats require a much higher dose than sheep as they metabolize drugs differently.

Low dosing can contribute to the increase of resistance as resistant worms are more likely to survive. Therefore, it is vital you give your animals the recommended dose level. If you’re unsure what this is, please ask your vet for advice.

Managing pastures

You can also help slow the rate of AR and reduce the risk of infection by carefully managing your pastures.

For example, where possible graze young stock on a safe pasture i.e. one that hasn’t been grazed for 3 – 6 months, depending on the weather conditions. Rotating young stock ahead of older stock will also reduce the risk of exposure to infective larvae.

The level of risk on a pasture will depend on which animals previously grazed there – for example young stock, older stock or a different species – and the time of year they were grazing. For example, a pasture that was grazed in spring by young animals will be at a high risk of worm infestation in July.

You can determine the level of infection on your pasture by having a pasture larval count. For more information please visit our pasture larval count page.