Clinical signs

Ill thrift, weight loss

Heavy infections cause a profuse watery scour which can lead to dehydration

In yearlings and adult cattle a heavy infestation with Ostertagia roundworms can damage the fourth stomach resulting in loss of weight, reduced milk production and diarrhoea.


There are 3 types of roundworm found in Australian dairy cattle (Ostertagia ostertagia; Trichostrongylus axei and Cooperia oncophora). Most animals carry some roundworms, but tend to develop immunity as they grow older and show few clinical signs if they are lightly infected.

Roundworms produce huge numbers of eggs which are passed out in cow manure and hatch into tiny larvae. Dry, hot conditions destroy many larvae while cold weather slows down their life cycle. The worm larvae go through several life stages while living on grass, soil and in pats of cow manure, before becoming infective. When eaten by cows, these larvae develop into adult worms and find their way to the stomach and intestines.

Animals likely to be affected

Recently weaned calves grazing heavily contaminated paddocks. Occasionally worm numbers build up in adult animals and may cause type 1 or type 2 ostertagiosis, which usually affects only a small proportion of a mob.

Other diseases with similar signs

See Calf scours. Other causes of diarrhoea in adult cattle and yearlings, including coccidiosis, yersiniosis, salmonellosis, BJD, BVD.

Confirming the diagnosis

Worm egg counts on manure. Laboratory tests to rule out other causes of scours.

Risks to people

Cattle roundworms do not pose any risks to people.


Roundworms can be treated with 3 types of cattle drench, some of which are formulated as pour-on applications. Regular use of the same drench has the potential to select worms that are resistant, so drenches should be used strategically.

Risk factors

• Grazing recently weaned cattle on pastures that are heavily contaminated with infective worm larvae
• Short intervals between grazing episodes
• Cold wet conditions


The aim is to expose stock to a low level of roundworms that do not harm them while allowing immunity to develop. Use grazing management to avoid the build-up of very high numbers of infective worm larvae on pastures. Identify safer paddocks for young vulnerable calves, such as those used for cutting hay or silage.

If grazing programs are not sufficient to control roundworms, use strategic drenching to reduce impact on vulnerable animals. Ask your vet to help plan a drenching program to ensure that costs are minimised, benefits are maximised and there is no build-up of resistance to drenches.

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