Worms in Sheep

The effects of worms are likely to be most severe and of greatest long-term significance in the first year of life.

Symptoms of the Disease/Condition.

There are two broad types of worms that can affect sheep:

Round worms:

  • Most live in the gut but there are two species that live in the lungs.
  • Cylindrical shape.
  • Range in size up to 7-10mm long, but the Barber’s Pole worm gets up to 20-34mm.
  • Round worms are by far the most important parasites in sheep and they would be the most important of all the animal health challenges faced by sheep farmers.

Tape worms:

  • Live in the gut.
  • Long, flat worms made up of many segments.
  • Can get up to 6 metres in length.

The Impact

The classical effects of worms include diarrhoea, weight loss and death. The Barbers Pole worm is a blood sucker and causes anaemia and death. Such severe cases are still relatively common because many farmers underrate the likelihood of their animals suffering from worms. They may be using a drench that the worms have become resistant to or they may be using an effective drench but the sheep are rapidly reinfected due to a high level of pasture contamination.

The more common economic impact of worms is related to sub-clinical levels of infection. Decreased appetite, decreased feed conversion, and mild weight loss are almost impossible for farmers to observe without regularly weighing their sheep.

Most farmers would not see a 50g/day drop in weight gain in their lambs. In a mob of 2,000 over 100 days the lost opportunity is equivalent to 10,000 kgs of live weight. At $2.50/kg the loss is worth $25,000. For most farmers this will not end up being a direct loss as they will just keep the lambs for longer but that means additional costs such as more pasture eaten, extra animal health, and more chance of lambs getting facial eczema or pneumonia.

If the lambs are ewe replacements it may mean they are well below their target two-tooth mating weight the following year. This could have an impact on their life-time performance.

Always consider worms as the cause of decreased weight gains and weight loss in any age group of sheep.


A faecal egg count can provide a reasonable assessment if there are adult worms present and they are more reliable than in cattle. However, sheep with worm problems can have a zero faecal egg count, usually when there are heaps of immature larval stages being ingested.

Management and Control

There is a wide range of anthelmintics available for use in sheep.

Managing worms at the farm level is an extremely complex issue. It requires a sound understanding of the worm life cycle and the interactions between the worm, host, and environment.