Thin-necked intestinal worm

(Nematodirus spp)

Thin-necked intestinal worm occurs in most of the major sheep and goat production areas of Australia, but is mostly an issue in the winter rainfall districts. It is very resilient and its free-living stages on pastures can survive severe winters and dry conditions.

Nematodirus are relatively long worms with a slender anterior end that is frequently coiled. The female worm in the small intestine is 15–23mm long and lays 25–30 eggs per day.

Unlike most roundworms, Nematodirus develop over a period of two months or more to third stage larvae (L3) while still in the eggshell. The combination of eggshell and L3 sheath make it able to survive desiccation and cold.

Further ecological information on worms and their control:


The small intestine.


Death, lethargy and collapse, weight loss, damage and inflammation of the gut resulting in diarrhoea (scouring).

While thin-necked intestinal worms can cause disease in adult sheep, it is not very common as sheep develop a strong immunity, but in contrast, adult goats do not develop such a strong immunity and disease is possible. Most damage is caused by the immature worms in the small intestine.

Serious problems tend to be in young sheep in cool regions or after dry periods when sheep graze short, green feed. Heavy infections can cause diarrhoea in lambs. These worms do not cause any specific lesions in the lining of the intestine, although there may be mild inflammation.

In young lambs and kids, a worm egg count of over 200 eggs per gram (epg) would require treatment.


The only accurate way to diagnose worm infections before productivity losses have occurred is to conduct a WormTest (worm egg count). The results allow you to make the best choice of drench for the situation. The eggs of thin-necked intestinal worm are very large and are easily identified in a worm egg count.

Visual signs only occur after significant production loss has already occurred. Also, these signs can occur with other parasites and diseases.


There are many options to treat for this worm and your choice will depend on:

  • the current size of the burden of this worm
  • what other worms are also present and in what proportion
  • which drenches are effective on your property and the length of protection you are seeking
  • the likely worm-risk over the next few months
  • the likely level of worm contamination on your pastures
  • the class of sheep or goat affected and their susceptibility to worms
  • the last drench group/s you used on this (and other) mobs
  • the time until these sheep or goats are sold/slaughtered and the withholding period and export slaughter interval of drenches you might use

Your decision can be assisted by using the Drench Decision Guide, a simple tool that considers some of the points above.

You can also review the Drench pages on this site to find out specific information about drenches, including their drench active, drench group, length of protection, which worms they treat, dose rate, withholding period, export slaughter interval and manufacturer.

Note: only a few drench types are registered for use in goats.

The negative impact of this worm can also be reduced through browsing and grazing management strategies and by using one of the integrated worm control programs that have been developed for different regions across Australia.

Paid Advertisement