Farmers using faecal egg count (FEC) analysis to monitor the need to treat lambs for worms are being cautioned about the possibility of a spike in worm egg counts where rain follows a prolonged period of dry weather. UK-based sheep vet Fiona Lovatt says farmers in such situations need to be in a position to act quickly.

“[The] worm risk will spike within a week after the rain. This means if you are monitoring worm egg counts on a fortnightly basis and have only just taken a count before rain arrived, don’t assume that all is OK and wait for another couple of weeks before checking again, as it could be too late then.”

Initially, because there will be more infective larvae, you may not see high worm egg counts for two-three weeks after the rain

Fiona advised farmers who were seeing high worm (strongyle) egg counts earlier in the year to be especially cautious of the risk. This is due to dry weather delaying worms from completing their life cycle and leading to the possibility of a high number of eggs lying dormant in dung on pasture.

“When the rain arrives, these will hatch out into infective larvae very quickly, completing their lifecycle. Initially, because there will be more infective larvae, you may not see high worm egg counts for two-three weeks after the rain. However, that doesn’t mean to say your animals aren’t infected.”

For this reason, Fiona is advising farmers to monitor growth rates and performance, along with faecal egg count analysis, for optimum protection.

Faecal egg counts

The aforementioned situation is relevant to a smaller cohort of farmers, as the majority of sheep farmers in Ireland and Northern Ireland are located in areas that have received some level of rainfall in recent weeks. As such, these farmers should continue with their worm control programme, with faecal egg count analysis remaining a useful aid in determining the need to treat lambs.

Relying on aspects such as lambs scouring can be dangerous, as research has shown that lambs can be scouring without possessing any worm burden, and likewise, lambs can possess a worm burden without scouring.

A faecal egg test can be carried out by flocking sheep and collecting a pooled fresh sample of faeces from about 10 lambs randomly selected from the group. The sample is sent to a laboratory or business offering the testing with a result returned anywhere from the same day of delivery to a couple of days later. This practice will determine if lambs need worm control treatment.

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 medium-to-high categories in the early summer months, a worm treatment is generally advised. Where the egg count is below 500epg, it is recommended to delay treatment and depending on the time of year and circumstances, carry out another faecal egg count after one to two weeks.

Reduction test

If the aim is to establish the efficacy of an anthelmintic class, then a faecal egg count reduction test needs to be carried out. The faecal collection process should be repeated seven days later if testing for resistance to levamisole or yellow drenches and 14 days post-treatment if testing for resistance to benzimidazole (white drenches) or macrocyclic lactones (clear drenches including ivermectin).

If the result is >95%, it is assumed that there is no problem with anthelmintic resistance, while if the result is <95%, it is likely that there is an issue with resistance, provided the recommended dosing guidelines were followed during treatment.