Two species of nodule worm occur in Australia – Oesophagostomum columbianum in the summer rainfall areas and Oesophagostomum venulosum in winter rainfall areas.

Oe columbianum is much more pathogenic than venulosum. Oe columbianum larvae cause 'nodules' in the large intestine and colon of the sheep. Oe venulosum is rarely pathogenic.

Modern drenches have made nodule worm quite rare, but even low levels of infestation can cause economic loss through damage to the intestinal wall, making the intestines useless for sausage casings.

It is a large (20mm) white stout worm found in the large intestine and colon of sheep.

Image kindly supplied by Associate Professor Nick Sangster, University of Sydney.

Find out more about nodule worm and large bowel worm:

• Life cycle
• Location in the sheep
• Effects on the sheep
• Diagnosing nodule and large bowel worms
• Treatment

 Nodule worm life cycle

Nodule worms live in the large intestine of the sheep and cause damage to the lining of the gut. Damage to the gut from larval nodule worms (specifically Oe columbianum) causes a condition called 'knotty gut' and nodules.

The adult female lays 5000 – 12,000 eggs per day, 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 3rd 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 five weeks, depending on warmth and moisture. The L3 larvae of Oe columbianum are susceptible to cold conditions and die in frosty sub-zero temperatures. Oe venulosum is more cold tolerant.

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 large intestine. In the large 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 nodule worm in the sheep

Both species of 'nodule' worm are found in the large intestine of the sheep. Only Oe columbianum causes nodules in the gut. They are large (20mm), stout, creamy coloured worms.

Graphic kindly supplied by Associate Professor Nick Sangster, University of Sydney.

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 Effects of nodule worm on the sheep

Nodule worm (Oesphagostomum columbianum), if present in numbers, causes severe disease.  Sheep, particularly weaners, show ill-thrift, often standing with a characteristic humped back, lose condition, become weak and scour intermittently.

Nodule worms damage the lining of the intestines as they mature from larvae to adult worms, making them useless for valuable by-products such as sausage skins and specialised thread for surgical purposes. 

In sheep infected with nodule worm, the nodules formed around the developing larvae in the intestinal wall are clearly visible to the naked eye. Nodules in the small intestine are small and gritty, and those in the colon larger with soft centres. The colon is reddened, thickened and oedematous and adult worms are clearly visible.

Large bowel worm (Oesophagostomum venulosum) may cause production losses in young sheep if present in large numbers, but rarely disease. Some experimental infections have resulted in scours and ill-thrift. Large bowel worm does not form nodules or any specific lesions in the intestines, but worms are clearly visible in the blind gut (caecum) of infected sheep.

It generally does not cause a problem for adult sheep.

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 Diagnosing Nodule worm

The only accurate way to diagnose nodule worm infestations before productivity losses have occurred is to conduct a worm egg count and larval culture and identification.

Conducting worm egg counts diagnoses worm infestations before they cause productivity losses and allows producers to plan treatment rather than react to a crisis.

To accurately identify the worm species present, the sheep dung sent to a laboratory is processed and put in an incubator to hatch the eggs. The resulting larvae are identified.

By knowing the worm egg count and the species present, a producer can make the best choice of drench for the situation.

Nodule worm can be diagnosed during a post mortem, when 'nodules' and the worms are seen.

Traditionally, sheep producers relied on visual signs of nodule worm infestation such as scouring and ill-thrift to diagnose infestations.

The problem with this diagnostic method was that, by the time the sheep showed these signs, the infestation was high and significant productivity losses had already occurred.

Producers who conduct regular worm egg counts, know the species present and the drench resistance status of their properties have healthier sheep and drench less.

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 Treatment of Nodule worm

There are no reports of drench resistance in nodule worm. All broad spectrum drenches will remove them.

The key to effective treatment is early diagnosis through worm egg counts and larval cultures.