Atlas of livestock parasites
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Ostertagia circumcincta



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Ostertagia circumcincta, Ostertagia trifurcata, Ostertagia lyrata
Brown stomach worm

General Description:
Brownish, thread-like nematodes up to 12 mm in length. Males are smaller with relatively straight, slender spicules.

Life Cycle:
Typical direct nematode life cycle. Hatched larvae become infective in about 6 to 7 days and may survive on pasture for up to 4 months. Infection is by ingestion of these third-stage larvae. The minimal prepatent period is about 17 days, but many larvae enter the wall of the abomasum and become dormant (hypobiosis). Development to the adult stage may resume at any time; some larvae require up to 3 months for this process.


Geographical Distribution:
Widespread in moist, temperate regions of the world, especially where climates are cooler than where Haemonchus is prevalent.

This nematode causes serious losses by producing weight loss and decreased wool production.

Effect on Host:

Larvae in the glands of the abomasal wall cause irritation and erosion of cells. Continual, rapid replacement of damaged cells prevents cellular maturation. Consequently, the abomasum develops a lining of immature cells that are not joined together well and that do not secrete hydrochloric acid and the enzyme pepsinogen, two important elements of digestion. As a result, fluid leaks between cells into the gut. The pH of the gut contents rises from pH 2 to pH 7, resulting in poor digestion and increased growth of bacteria. The gut becomes more inflamed. Fluid absorption, a major function of the intestine, is blocked, causing profuse, watery diarrhea. Albumin is an important blood protein, of which large amounts may be lost in fluid leaked into the gut. Lost albumin must be replaced from the diet. This is difficult to achieve since improper digestion prevents adequate absorption of nutrients in the intestine. Ostertagiasis is worsened by the fact that infected animals lose their appetite and do not feed. Weight loss occurs: animals may lose up to 20 percent of their body weight in 7 to 10 days. Death may follow.

Diagnostic Information:
Strongyle-type eggs appear in feces.

Pasture management can reduce damage by stomach worms. Overstocking should be avoided, and lambs should be weaned early and separated from ewes. Minimize contamination of feed and living quarters with manure. Anthelmintic treatment of infected animals with chemicals effective against larvae stages at appropriate times is important. A diet with sufficient nutrients, especially iron, is essential to help sheep overcome the effects of Ostertagia infection.


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