Black scour worms occur in all sheep production districts of Australia. Trichostrongylus colubriformis and Trichostrongylus vitrinus are the main species causing disease in Australia. Generally T. colubriformis occurs in the warmer summer rainfall areas and T vitrinus occurs more frequently in winter rainfall areas.

T. vitrinus is considerably more pathogenic than T. colubriformis, meaning that it needs to be treated at lower egg counts. Regular worm egg counts are essential for successful management.

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

Find out more about black scour worm:

• Life cycle
• Location in the sheep
• Effects on the sheep
• Diagnosing black scour worm
• Treatment

 Black scour worm life cycle

Adult female black scour worms lay 100 – 200 eggs per day. Black scour worms live in the first three metres of the small intestine of the sheep and cause damage to the lining of the gut. 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 2nd 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 seven days, but can be as long as five weeks, depending on warmth and moisture. The L3 larvae of black scour worms are extremely resistant to desiccation and cold. 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.

It takes around 21 - 28 days from when a sheep ingests a worm larvae to when worm eggs appear in dung samples.  It is important to remember this for 2 reasons:

• If the sheep are treated with a fully effective short acting drench, they can be used to "clean" paddocks for weaners (smart grazing) for the 21 - 28 day period without fear of recontamination of the paddock.
• If the sheep are tested for worms using worm egg counts, you may think that the sheep are free from worms when, in fact, they could have picked up a considerable burden of larvae.  If you think the sheep could be picking up worms aftr drenching, test at about 4 - 6 weeks, when egg laying has commenced and you will have a much better idea of the rapidity and scale of the reinfection.

Naturally, if a sustained action drench was used, the period from drench to eggs in the dung is longer, being the length of action plus the 21 - 28 days.

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 Location of black scour worm in the sheep

Black scour worms are found in the first three metres of the small intestine.

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

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 Effects of black scour worm on the sheep

Black scour worms damage the lining of the first three metres of the small intestine.

Sheep with heavy infections of black scour worms rapidly lose condition, and develop scours, which may be black. The animal may dehydrate and may die.

Animals with fewer worms are unthrifty and daggy. Mixed infections of small brown stomach worm (Ostertagia) and black scour worm are more lethal than infection with only one of the worms.

In animals that die from heavy infections of black scour worm the lining of the intestines is swollen, has blood spots on it and is covered with excess mucus. The worms are very small and difficult to see. Lymph nodes near the intestines are enlarged. The carcase is emaciated.

In animals with light worm burdens infection will not be obvious at post mortem because the intestine will appear normal and the worms are too small to see easily with the naked eye.

A subclinical infestation of black scour worms can reduce weight gains by 35%, wool growth by 10% and milk production by 20%.

The images below dramatically demonstrate the damage caused by black scour worms.  Absorbtion of nutrients is dependent on having a normal gut wall.  When it is damaged by parasites, absorbtion of nutrients is severely restricted, with consequent negative impacts on the animal.

Images of the damage caused by black scour worms

These images were kindly supplied by Dr Ian Beveridge, University of Melbourne

Normal small intestine

Small intestine damaged by black scour worm

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 Diagnosing black scour worm

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

Conducting worm egg counts diagnoses 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.

A condition known as 'hypersensitivity scours' occurs in districts where black scour worms and brown stomach worm are found. The cause of this scouring is an allergic reaction of the gut wall to the presence of the immature worms. Sheep may cease scouring without any treatment.

Traditionally, sheep producers relied on visual signs of black scour 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 and deaths had often 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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 Treating black scour worm infestations

Drench resistance in black scour worms is very common in all major sheep production areas of Australia. Often the worms are resistant to several drench groups. It is essential to have drench resistance test data in order to choose an effective drench, i.e. one that will kill at least 95% or more of the resident worms.

Treatments for black scour worm are all broad spectrum or mid spectrum. Most are short acting. However, injectable moxidectin gives seven days protection against incoming larvae.  Ivermectin or BZ slow release capsules give 100 days protection, but it is important to do a resistance test before using them to ensure that they are effective against the range of worms present on the property.

A number of combination drenches are also available. If you have any doubts about which drench is the best for your situation, you should seek professional advice.