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Sheep and Parasites

The impact of worms on animal production begins as soon as animals are exposed to worm larvae on pasture. Parasitism is second only to nutrition as the most common reason for ill-thrift in young sheep.

Internal Parasites

Almost all sheep have internal parasites. They often don’t cause any disease and sheep can look normal or even in good condition. However if worms get out of control, sheep can suffer from both subclinical disease (signs that can be measured but not seen visually) or clinical disease (signs range from very subtle to obvious, including death).

The visual effects (clinical signs) become apparent when the worm burden becomes high.  It is the non-visual (sub-clinical) effects which accounts for the majority of production loss as they can go un-noticed over a long period of time.


There can be a huge impact on production from the lowest of worm burdens - through reduced appetite, a compromised immune system (refocused to the parasite), damage to the gastrointestinal system and potential protein and blood loss.

Fly Strike

Flystrike is the most important ectoparasitic disease of sheep in New Zealand.


There are three species of blowfly which can initiate flystrike in sheep:

  • Lucilia sericata (common green blowfly)

  • Lucilia cuprina (australian green blowfly)

  • Calliphora stygia (brown blowfly)

Chrysomya rufifacies and other Calliphora spp are secondary invaders.

The normal flystrike season varies slightly between areas, but usually occurs from November to March when it is warm and humid.


  • Faecal and urine staining around the crutch

  • Fleece rot and dermatophilus.

  • Footrot.

  • Head injuries in rams.

  • Wrinkled skin (merino’s).

  • Infected or open wounds.


Lice populations are generally highest in autumn through to late winter and decline in summer. Known as the sheep body lice, biting lice or chewing lice, Bovicola ovis is a 1.5-2.00mm long, yellowish- brown wingless insect. They do not fly or jump. The female lives an average of 4-6 weeks and lays about 30 eggs in her lifetime, on wool fibres. 

Lice are transferred by very close contact between sheep. They tend to spread slowly within a flock,  except when sheep are in poor condition. Well-fed, well conditioned sheep are less susceptible to lice than sheep in poor condition.  A lice-infected ewe will infest her lamb within the first 24 hours, so it is important to ensure ewes are treated before lambing.  If infested ewes cannot be treated for any reason, then lambs should be treated as soon as practical – be that at weaning or shearing.




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Drench Resistance

Drench resistance is caused by internal parasites developing inherited tolerance to commonly used drenches. It results in animals with worms that don’t respond to drenching.

Theoretically, drench resistance occurs once a population of a species of worm can survive a dose of a drench that would have previously killed it. Initially resistant worms are rare in a population of worms.  When a sheep is treated, the resistant worms survive and, if they find a mate, can reproduce.  The resultant offspring are resistant and if they survive as larvae on the pasture and infect another sheep.  Over time, and with continued treatment, the overall resistance level to the treatment within the worm population increases.

Drench resistance is a major concern for NZ sheep farmers, resulting in production losses and increasing costs to control parasites.   Please contact us to discuss your individual flock requirements, or for more information on management click the link below: