It is common in cool, temperate areas, such as south-eastern and south-western Australia and the United Kingdom.

[1][2] Ostertagia, Haemonchus and Graphidium are closely related taxa that are usually assigned to separate subfamilies.

[6] The number of eggs in the uterus of mature females can vary from less than 10 to more than 60[6] and is heavily dependent on the host immune response.

In most countries, infection with third-stage larvae resumes in the spring and is triggered by the production of large numbers of eggs by periparturient females.

[14][15] The development of free-living nematodes is influenced by temperature and moisture and there is considerable variation within and among years in the number of infective larvae available for ingestion.

[15] The number of infective larvae gradually dies down at the end of the grazing season when grass growth also declines.

[16] Both the natural history of infection and the sources of variation among individuals are relatively well understood for T. circumcincta compared to most parasites.

Clinical signs include reduced appetite, poor growth performance, weight loss and intermittent diarrhoea.

[18] In addition to clinical signs, faecal egg counts and the timing of infection are used to identify severely affected animals.

[21] Faecal egg count is widely used to identify and select animals that are relatively resistant to nematode infection.

[22][23][24] Selection is most advanced in Australia and New Zealand where the dominant nematodes are Haemonchus contortus or a mixture of T. circumcincta and Trichostrongylus colubriformis.

For naturally infected Scottish Blackface lambs grazing fields contaminated with predominantly T. circumcincta eggs, the heritability of faecal egg count was not significantly different from zero for the first two months after birth then rose to approximately 0.3 at the end of the grazing season.

There are a number of regions which have shown suggestive (nearly but not quite statistically significant) associations in more than one study[28] and some of these are probably harbouring causal polymorphisms for nematode resistance.

The regulation of adult worm size and fecundity is strongly and consistently associated with IgA activity especially against fourth-stage larvae.

[42] The other major mechanism of resistance to T. circumcincta is the IgE and mast cell mediated control of worm establishment and survival.

A plausible explanation is that infection in the small intestine inhibits reabsorption of proteins lost in the abomasum.

Anthelmintic treatment is cheap, simple and effective but is threatened by the development of drug resistance in parasite populations.

The criteria to decide which animals are to be treated and the size of the untreated population are matters of active research.

[57] Grazing management is cheap, simple and at least some elements are widely used[63] but is seldom very effective in isolation and can be impractical.

[62] Biological control with spores of the fungus Duddingtonia flagrans has recently been licensed in Australia and can be very effective[64] but requires regular, possibly daily, feeding.

[57] Non-protein sources such as urea are effective[48] and relatively cheap but great care would be needed to avoid toxicity with high doses.

Selective breeding can be very effective[24] and almost completely eliminate the need for anthelmintic treatment but requires specialist expertise.

[58] This article was adapted from the following source under a CC BY 4.0 license (2019) (reviewer reports): Michael J Stear; David Piedrafita; Sarah Sloan; Dalal Alenizi; Callum Cairns; Caitlin J Jenvey (24 April 2019).