Don’t let wormer resistance lead to poor performance

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The presence of gastrointestinal worms in lambs can reduce growth rates by up to 50 percent, even when no obvious clinical signs are present.

If resistant worms comprise a small proportion of the population, they may not cause obvious problems, but a gradual decline in performance can occur, particularly with over-use of wormers, which speeds up the selection of resistant worms on a farm.

Good growth rates are vital to efficiency and profitability for sheep farmers and any reduction in this can have devastating effects on the bottom line.

The greater level of anthelmintic (drench) resistance in a flock, the fewer the number of options available to farmers for worm control. Once wormer resistance is clinically obvious in a flock it is very difficult to then slow down the rate of resistance development. It is worth noting that the problem will only become apparent on farm when the efficacy of a product drops to less than 50%. Early detection of resistance is therefore crucial.

A quick indication of the efficacy of an anthelmintic can be gauged by testing faecal samples from 10 sheep, seven or 14 days after dosing depending on the wormer used (seven days for the 2-LV class or yellow drenches, 14 days for the 1-BZs or white drenches and the 3-MLs or clear drenches/injectables/pour-ons), this is called a drench check.

Anthelmintic resistance is diagnosed by utilising a faecal egg count reduction test, taking samples pre and post dosing. If the number of eggs per gram (epg) in faeces are reduced by less than 95% when the pre-dosing level was greater than 150epg, it is likely that lambs have failed to respond to an anthelmintic treatment.

An important weapon in the battle against under-recognised wormer resistance is the role of ‘refugia’.

On any sheep farm, there are two populations of worms:

o those present in treated sheep

o those that are free living on the pasture or in untreated sheep.

It is the second group of worms, unexposed to treatment, which is referred to as the ‘refugia population’. The greater the population of worms in refugia, the greater the dilution of resistant worms among anthelmintic susceptible worms, delaying the rate of resistance development.

In order to maintain the refugia population on farm, and slow down development of resistance, farmers should follow the following guidelines.

1) Part flock treatment

Leaving a proportion of flock untreated (for example 10%) before moving them to a low-contamination pasture will be sufficient to provide a large enough dilution effect to delay the development of anthelmintic resistance, providing the anthelmintic used is highly efficacious.

2) Delay the ‘move’ after the ‘dose’

By allowing sheep to become lightly re-infected with susceptible (untreated) worms from the contaminated pasture prior to moving them to a clean pasture, resistant worms will be diluted. Sheep should be kept on infected pasture for a minimum of 4-7 days to allow re-infection. This time will depend on variations in climatic conditions, susceptibility of the sheep to worms and number of infective larvae on pasture (i.e. for sheep under one year old on highly infected pasture, four to seven days should be enough).

A pro-active way to reduce the likelihood of introducing resistant parasites on the farm is quarantine treatment of all in-coming sheep. This includes sheep purchased from other flocks and sheep coming home from grazing on other farms or common land.

The chosen anthelmintic should be likely to remove both resistant and susceptible worms. One option would be to use a dual active wormer (e.g. STARTECT®) as a single drench.

After dosing, sheep should be kept off pasture for 24-48 hours until any worm eggs present in the gut have passed out in the faeces. Sheep should then be turned out to dirty or contaminated pasture to minimise the impact of any worms that survive treatment on the farm’s anthelmintic resistance status.