Thin necked intestinal worm occurs in most of the major sheep production areas of Australia and is mostly an issue in the winter rainfall districts. It can cause scouring in young sheep.

It is very resilient and survives severe winters and dry conditions.

Find out more about thin necked intestinal worm:

• Life cycle
• Location in the sheep
• Effects on the sheep
• Diagnosing thin necked intestinal worm
• Treatment

 Thin necked intestinal worm life cycle

The female worm in the small intestine lays 25 – 30 eggs per day. These pass out in the dung.

The larvae develop to 3^rd stage (L3) in the eggshell over a period of two months or more. The combination of eggshell and L3 sheath make it able to survive desiccation and cold. It survives winter in large numbers.

Heavy dews and rain release the L3 from the dung pellet onto the pasture. L3 larvae increase the chance of being ingested by sheep by responding to light and temperature. As the pasture is warmed by sunlight and in the presence of moisture (dew/rain) the L3 migrate up the grass blades where they are most likely to be eaten. On cold nights they move down to the base of the grass.

When the L3 are ingested, compounds in the gut of the sheep stimulate the larvae to rapidly (usually within 30 minutes) complete the second moult (exsheathment) and start moving towards the small intestine. In the small intestine the larvae burrow into the wall of the intestine and undergo another moult to become an immature worm. When these mature, they mate and the cycle starts all over again.

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 Location of thin necked intestinal worm in the sheep

Thin necked intestinal worms live in the small intestine.

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 Effects of thin necked intestinal worm on the sheep

Heavy infections of thin necked intestinal worm can cause diarrhoea in lambs. Thin necked intestinal worm can be a serious problem in young sheep in cool regions or after dry periods when sheep graze short, green feed.

In sheep with heavy infections, tangled masses of worms are found in the lower part of the small intestine. These worms do not cause any specific lesions in the lining of the intestine. There may be mild inflammation.

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 Diagnosing thin necked intestinal worm

The eggs of thin necked intestinal worm are very large and are easily identified during the course of a worm egg count.

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

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 Treatment of thin necked intestinal worm

Broad spectrum, long or short acting drenches will remove these worms. There have been reports of eggs of thin necked intestinal worm still present after treatment with BZ drenches. Levamisole is very effective.