There are a growing number of flocks identifying worm resistance to the three commonly used anthelmintic classes; benzimidazoles/1-BZ (white drenches), levamisole/2-LV (yellow drenches) and macrocyclic lactones/3-ML (clear group including ivermectin, moxidectin and doramectin).

Resistance to an anthelmintic class is identified if the treatment fails to kill at least 95% of worms present. The level of resistance varies hugely across farms, with some farms finding that upwards of 90% of worms have been killed by a treatment, whereas on the worst affected farms, the efficacy of treatment can be as low as 30% to 40%, or worse. In such a scenario, treatments are having very little effect on enhancing animal health.

The quicker resistance is identified, the more opportunities there will be to put a treatment programme in place in conjunction with your vet to try and prolong the lifetime use of these products and also protect anthelmintic classes which are effective, including the relatively new classes, monepantel/4-AD or orange drench and spiroindole/5-SI or purple wormer.

Opportune timing

Resistance can be identified by carrying out a faecal egg count reduction test. This essentially involves carrying out two faecal egg counts one or two weeks apart depending on the anthelmintic class used. The first faecal egg count (FEC) will determine the number of worm eggs that are present at the time of dosing.

Now is generally a good time to carry out a FEC reduction test as there is typically a high worm burden built up on pasture over the summer.

A word of caution is needed here, as some areas experiencing drought conditions may experience a significant increase in worm burden following forecast rain.

Tests will need to be strategic to avoid such a scenario influencing worm burdens in the second FEC.

Recommended procedure

The first faecal egg count needs to take place when there is a high to medium worm burden and coincides with treating lambs. Where there is doubt, the ideal scenario is to carry out a preliminary FEC test. This will add cost, but you can be sure the reduction test is accurate.

This test will not, however, suffice as the first test and a FEC needs to be taken from 10 lambs when they are being treated. The lambs from which faeces is collected need to be clearly identifiable, as it is these lambs that faeces will be collected from for the repeat FEC.

This must take place seven days following treatment in the case of levamisole-based products and 14 days post-treatment in the case of benzimidazoles or macrocyclic lactones.

If the result is less than 95% reduction in the worm count, there is not considered to be a problem with anthelmintic resistance ,while if the result is more than 95%, it is likely that there is an issue with resistance, provided the recommended dosing guidelines were followed during treatment.

Treatment guidelines

Table 1 provides a guide to interpreting low, medium and high faecal egg counts in lambs. It is presented in terms of eggs per gram for the main pathogenic roundworm species in Ireland. Where strongyle worm counts are in the categories of medium to high in the early summer months, then a worm treatment is generally advised.

Where the egg count is below 500epg, then it is recommended to delay treatment, and depending on the time of year and circumstances, to carry out another FEC after one to two weeks.

This two week period may need to be reduced in areas experiencing drought, as mentioned previously, as there could be a significant release of larvae which have entered dormancy to survive.

Key factors

There are a number of other measures that are key to reducing the rate at which resistance develops in a flock.

These are summarised as follows;

  • Never dose mature ewes for worms unless there is a demonstrated need (e.g hogget ewes that have reared lambs or sheep which have a compromised immune system or identified the presence of Haemonchus Contorus).
  • Do not use combination products that lead to inadvertent treatment for worms.
  • Implement a rigid quarantine programme for any purchased sheep.
  • Follow manufacturers guidelines and ensure the recommended volume of product is administered by dosing to the recommended weights and ensuring dosing equipment is calibrated and working correctly.