Ostertagiosis and Trichostrongylosis - Sheep

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Ostertagia species are nemotodes that inhabit the abomasum and are responsible for outbreaks of clinical ostertagiosis in lambs. Different species are found in sheep and cattle, but the life-cycle and pathogenesis are very similar. Trichostrongylus species are found in the abomasum and small intestine too also contribute to or may even cause ovine PGE.

Type 1 Ostertagiosis (the predominant form of PGE in sheep) is mainly caused by O. circumcincta. It affects lambs during their first grazing season (July - September). Type 2 Ostertagiosis affects lambs >1 year old after first grazing season (January - May). Trichostrongylosis is caused by Trichostrongylus axei (abomasum) and Trichostrongylus species (small intestine) and affects lambs <1 year old towards the end of the first grazing season (November - December).

There is impaired immunity because of Periparturient Relaxation in Immunity (PPRI) (breeding ewes only), a poor plane of nutrition or management (winter housing reduces antigenic stimulation). This means there will be an increased worm egg output because of increased establishment of L3, increased egg production per female worm and resumed development of arrested larvae.

If the pasture is 'clean' then there will be a periparturient (spring) rise in faceal egg counts in ewes, which will cause an autoinfection peak in L3 and therefore an increased lamb faecal egg count. This will then cause a larger, second peak of L3 on pasture. If the pasture is 'contaminated', then there will be overwintered L3 on pasture. This will be followed by the ewe's periparturient rise in faecal egg count and the lamb's rise in faecal egg count from overwintered larvae. As before, an autoinfection peak will then occur of L3 on pasture and lamb faecal egg count will increase, followed by another peak in L3 on pasture. This indicates that if the pasture is contaminated, it makes little difference to lamb egg burden, due to the effect of the ewes on L3 numbers.

Clinical Signs

In Type 1 Ostertagiosis the most common clinical signs include diarrhoea, weight loss and a reduced appetite. In the type 2 disease, there may be progressive weight loss and intermittent diarrhoea. In Trichostrongylosis, there will be a dark, foul-smelling diarrhoea ("black scour") and weight loss, which are quite characteristic symptoms of the condition.


The clinical signs plus the season are enough to make a preliminary diagnosis. A faecal egg count should be performed. Results will mostly show high numbers of both Ostertagia and Trichostrongylus. However, in type 2 ostertagiosis, the numbers of eggs will be variable.

Bloods should be taken and blood pepsinogen levels measured. This will be elevated in ostertagiosis only, unless there is a large Trichostrongylus axei burden.

If a post mortem examination is performed on an animal that has died of the condition, one will see hyperplastic nodules and a raised gastric pH in the case of Ostertagiosis. There will also often be >10,000 adult worms. In the type 2 disease there will also be a large number of arrested larvae present too. If the sheep died of Trichostrongylus infection, there will be a presence of severe enteritis on post mortem and over 30,000 adult worms can be seen.

Treatment and Control

On a clean pasture, the only source of infection are the ewes. So protocol is to dose ewes around lambing, then turn ewes and lambs out onto clean grazing. On contaminated pasture, ewes and lambs are the source of infection, so still dose ewes around lambing (BUT won't prevent reinfection; use medicated feed blocks or intra-ruminal boluses) and then dose lambs at weaning and move to clean grazing. NOTE: If no alternative grazing available = dose lambs at weaning and repeat on two or three occasions at monthly intervals.

Treatment if there is an outbreak of the disease, is anthelmintic drenching and supportive care should diarrhoea be severe enough to warrant this.

Ostertagiosis and Trichostrongylosis - Sheep Learning Resources
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Blood, D.C. and Studdert, V. P. (1999) Saunders Comprehensive Veterinary Dictionary (2nd Edition) Elsevier Science

Fox, M and Jacobs, D. (2007) Parasitology Study Guide Part 2: Helminths Royal Veterinary College

Merck & Co (2008) The Merck Veterinary Manual (Eighth Edition) Merial

Radostits, O.M, Arundel, J.H, and Gay, C.C. (2000) Veterinary Medicine: a textbook of the diseases of cattle, sheep, pigs, goats and horses Elsevier Health Sciences

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