Nematodirosis in young lambs

NEMATODIROSIS is caused by the gut parasite Nematodirus battus. Nematodirosis is an important disease affecting young lambs managed on pasture grazed by young lambs the previous year.

Nematodirus battus developing larvae and adults cause profuse diarrhoea in lambs; adult sheep are unaffected

Nematodirus battus developing larvae and adults cause profuse diarrhoea in lambs; adult sheep are unaffected

Economic impact

  • There is rapid weight loss
  • Up to 5 per cent of lambs may die if left untreated for several days
  • Protracted convalescence such that affected lambs may take an extra 4-8 weeks to reach slaughter weight.

Clinical presentation

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
  • Lambs rapidly develop a gaunt appearance with obvious dehydration and condition loss. Deaths occur if left untreated – 5 per cent of lambs to die within a few days.

Diagnosis

  • The appearance of large numbers of lambs with profuse diarrhoea would suggest the possibility of coccidiosis
  • 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).

Diagnosis is based upon the clinical findings and exclusion of other possible causes such as coccidiosis.

Diagnosis is based upon the clinical findings and exclusion of other possible causes such as coccidiosis.

Welfare implications

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 (1-BZ) 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 targeted against Nematodirus battus.

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

Management/Prevention/Control 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 is:

  • 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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