Nematodirosis is a parasitic scour of lambs, caused by Nematodirus battus. The scour is a characteristic yellow-green colour.

Life cycle

Nematodirus differs from other worms in that the larvae develop in the excreted eggs, so that the eggs from one lamb crop go on to infect the subsequent year’s lamb crop.

Warm weather is required to get the eggs to hatch.

Therefore, a warm spell following on from a period of cold weather can result in huge exposure to the young lambs.

If this change from cold to warm weather coincides with a period where lambs’ intake of grass is increasing, then there can be significant outbreaks of nematodirus scours, as is currently a growing risk.

Risk factors

As nematodirus can strike so quickly with devastating effects, it is important to assess the risk each year and take prompt action to dose lambs for the worm.

The more of the risks listed below coincide, the greater the threat posed by nematodirus.

Therefore, preemptive dosing should be administered.

  • The key risk factor is that last year’s lamb crop grazed the pasture, so that the pasture has a worm burden waiting for the warm weather to hatch.
  • Lambs between six and 12 weeks of age where their grass intake is significant. If ewes are milking poorly, then younger lambs may be affected.
  • A warm spell preceded by a cold snap, which would facilitate large numbers of eggs hatching at the same time.
  • Lambs under stress, eg triplets, foster lambs, pet lambs, lambs from hoggets and elderly ewes.
  • Concurrent infection with coccidiosis.

    • Treatment

    White drenches are generally recommended.

    Complications of infection

    Mortality is common among batches of affected lambs. A less dramatic but more insidious complication is stunting, where lambs don’t thrive properly as a result of damage done to the gut by the worms. This will delay finishing at the least, and often stunted lambs cannot be finished properly.