Small intestinal worm is mostly found in the cooler parts of Australia. It is 10 – 15mm long and coiled close to the wall of the small intestine. It is not a major parasite.

Find out more about small intestinal worm:

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

 Small intestinal worm life cycle

The adult female in the small intestine lays eggs, which are passed out in the dung.

If the weather is warm and moist the eggs hatch into first stage larvae (L1). These moult or shed their skin (cuticle) to become 2^nd stage larvae (L2). Second stage larvae (L2) undergo an incomplete moult to become 3^rd stage larvae (L3) which are the infective larvae. L3 larvae retain the old L2 cuticle, which provides a protective sheath and nutrients until the larvae are ingested by a sheep. However this cuticle prevents L3 larvae from feeding on bacteria.

All larval stages (except L3s) feed on bacteria in the dung pellet until they either die or escape from the dung pellet onto the pastures.

Under ideal environmental conditions, development from egg to L3 takes around seven days, but can be as long as five weeks if conditions depending on warmth and moisture.

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.

 Location of small intestinal worm in the sheep

These worms live in the small intestine of the sheep.

 Effect of small intestinal worm on the sheep

Does not usually cause disease, but may contribute to the severity of disease in mixed parasite infections. Small intestinal worm does not cause any characteristic symptoms in infected sheep. They may worsen the effects of small brown stomach worm and black scour worm when found in mixed infections with these other worms.

At post mortem examination numbers of small intestinal worms are usually small and the worms do not cause any specific lesions in the intestinal lining.

 Diagnosing small intestinal worm

Small intestinal worm is diagnosed by conducting a worm egg count and a larval culture.

 Treatment of small intestinal worm

All broad spectrum drenches are effective. No drench resistance has been reported.