Last week’s announcement regarding the reintroduction of Startect to the market in Northern Ireland and the availability of opportunities, via veterinary practitioners, for farmers in Ireland has raised discussions about worm control programmes.

Reports from vets and farmers show a variable incidence of worm burdens in recent weeks.

Some highly stocked farms experiencing grass supply issues have encountered higher worm burdens, which is not surprising given weather has provided a good environment for worms to multiply and lambs have been grazing at a lower grass height.

There are significant differences however within and between farms with many faecal egg counts (FECs) returning low worm counts, raising the importance of establishing if there is a need to treat animals before undertaking what may be unnecessary treatments.

Faecal egg counts

The process of carrying out FECs is straightforward. A fresh faecal sample should be taken from approximately 10 lambs selected randomly in a batch. This can be carried out in the corner of a field or yard. Samples should be submitted for analysis promptly as delaying submission will only result in eggs hatching and an inaccurate reading.

Where samples are being posted then they should be adequately sealed and posted following collection, ideally on the same day, to arrive the next day – ie samples should not be posted on a Friday as eggs will hatch while transit is delayed over the weekend.

If samples need to be stored then they should be stored in a refrigerator / cool box. Avoid freezing with exposure to direct sunlight.

A low worm count for strongyles species, which are generally the main threat during the summer months, is 200 eggs per gram (epg) of faeces, a medium count is 250 to 750 epg while a high count is upwards of 750 epg. Treatment is typically warranted at a level of 500 epg. Some farms are working on a system of only treating lambs which are not performing to target (target selective treatment).

The practice of leaving lambs which are performing adequately without treatment will help to maintain a population of susceptible worms but good management and regular performance recording is required to keep on top of any potential issues.

Unfortunately, a high percentage of farms have experienced anthelmintic resistance developing in recent years. On some farms resistance issues are minor while on others, there is significant resistance to the three commonly used anthelmintic classes – Group 1 – BZ; benzimidazoles or white drenches, Group 2- LV; levamisoles or yellow drenches and Group 3 – ML; macrocyclic lactones or clear drenches.


A FEC reduction test can be successfully used to help establish if anthelmintic resistance is an issue.

The practice is similar to a standard FEC count except the lambs from which samples are collected from must be identified.

The samples are submitted as normal and lambs are treated following a sample being taken.

A repeat FEC is collected from the same lambs seven days later in the case of products with a Group 2 – LV active ingredient and 14 days later for lambs treated with a Group 1 – BZ or Group 3 – ML product.

The initial egg count needs to be in excess of 200 epg to draw conclusions and as such the test should be completed where a worm burden is expected or known.

A reduction of greater than 95% of worms shows the product is working effectively while a reduction of less than 95% shows there are resistance issues.

Optimal practices

There are a number of practices that will help to reduce the rate of anthelmintic resistance developing and these should be adopted at farm level.

  • The volume of product administered should follow manufacturers’ guidelines and be determined by the heaviest lamb in the group. Where there is a significant differential then more than one administration rate can be used.
  • Calibrate dosing equipment before beginning and during the process when treating a high number of animals.
  • Ensure lambs are dosed correctly with the product delivered to the back of the tongue. The above steps are critical where carrying out a reduction test.
  • Return animals to the field they were grazing in and not fresh pasture to help maintain a population of susceptible worms.
  • Avoid dosing ewes for stomach worms where there is no demonstrated need. Avoid the inadvertent treatment for stomach worms (e.g. using a combination of injectable products that treat a number of ailments) by only using products that target the condition being treated.
  • Quarantine any animals moving on to the farm and implement a robust biosecurity plan. This typically includes double treatment for worms using a new generation wormer (Zolvix or Startect) and a levamisole -based product.
  • In brief

  • The development of anthelmintic resistance can be slowed down by practices adopted on your farm.
  • Faecal egg counts are a good strategy to identify the need to treat worms from June through to September.
  • Faecal egg count reduction tests will establish if anthelmintic classes are working effectively.