Reducing the worm threat

This causes nematodirosis which is an important disease affecting young lambs managed on pasture that have been grazed by young lambs the previously year. So it is worthwhile planning ahead to get the most out of your hill lambing flocks this season, by trying to apportion some clean grazing for them.

As many hill flocks do not have many option, flockmasters should carefully plan their control measures to keep the threat from the disease sown to a minimum.

Its economic impact can be severe - it causes rapid weight loss and up to 5% of lambs may die if left untreated, even for just a few days. Further loss is caused by the protracted convalescence of affected lambs, which may take an extra four to eight weeks to reach slaughter weight.

Only lambs are affected by nematodirosis - ewes do not show disease. Clinical signs include profuse diarrhoea during the late spring and early summer months; faecal staining of the wool of the tail and perineum; lambs are dull and depressed, and stop sucking; they can rapidly develop a gaunt appearance with obvious dehydration and condition loss.

Deaths occur if left untreated - and studies show that 5% of lambs die within a few days.

Your vet will have to carry out a differential diagnosis, so that other causel diseases of having large numbers of lambs with profuse diarrhoea, such as coccidiosis, can be ruled out. Sudden death of young lambs without signs of previous illness could also be caused by pasteurellosis and pulpy kidney.

Diagnosis is based upon clinical findings of severe diarrhoea in lambs grazing infested pasture. Faecal worm egg counts are not always helpful because acute disease is caused by developing larvae and adults before they start laying eggs (pre-patent period).

The welfare implications are only too apparent. Profuse diarrhoea in severe infestations may cause death and severe condition loss. While blowfly are not normally active in May in the UK, fleece contamination can persist to attract flies later in the grazing season.

Treatment:

Outbreaks of nematodirosis should not happen because the risk factor of prior grazing by young lambs is well established - consult your veterinary surgeon and update your veterinary flock health plan.

Lambs should be immediately treated with an anthelmintic and moved from infested pastures whenever possible;

Nematodirus battus is susceptible to drugs from all groups of anthelmintic currently available, including the group 1 benzimidazole anthelmintics;

As there is resistance to group 1 benzimidazole (BZ) anthelmintics in many other worm species, it makes sense to target their limited efficacy against nematodirus battus.

It must be recognised that injectable moxidectin has no efficacy against nematodirus battus. Oral moxidectin (drench) must be used if this drug is to be targeted against nematodirus battus.

However, there is no persistent action of moxidectin against nematodirus after drenching, unlike other gut parasites.

Prevention measures:

Prevention is based upon avoidance of pastures grazed by lambs during the previous grazing season because adult sheep are resistant to infection and only lambs produce significant numbers of eggs.

Anthelmintic administration to lambs grazing pasture grazed by lambs during the previous season should be:

Guided by forecasts based upon environmental temperature whereby high risk follows prolonged cold spring weather;

Typically, for lambs born from mid-March onwards in 'normal risk' years anthelmintic treatments are given three weeks apart during May;

In 'high risk' years, three anthelmintic treatments are given extending the drenching period into June.

Over-use of anthelmintics has led to resistance in certain classes of drugs - an integrated approach to parasite control is the best approach and must be planned for the whole farm as part of the veterinary flock health plan.

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