Brown stomach worm

Teladorsagia circumcincta is a nematode that is one of the most important parasites of sheep and goats. It was previously known as Ostertagia circumcincta and is colloquially known as the brown stomach worm. It is common in cool, temperate areas, such as south-eastern and south-western Australia and the United Kingdom. There is considerable variation among lambs and kids in susceptibility to infection. Much of the variation is genetic and influences the immune response. The parasite induces a type I hypersensitivy response which is responsible for the relative protein deficiency which is characteristic of severely infected animals. There are mechanistic mathematical models which can predict the course of infection. There are a variety of ways to control the infection and a combination of control measures is likely to provide the most effective and sustainable control. Teladorsagia is a member of the subfamily Ostertagiiniae in the family Trichostrongylidae and the superfamily Trichostrongyloidea. Ostertagia, Haemonchus and Graphidium are closely related taxa that are usually assigned to separate subfamilies. Teladorsagia davtiani and Teladorsagia trifurcata are probably phenotypic variants (morphotypes). Adults are slender with a short buccal cavity and are ruddy brown in colour.  The average worm size varies considerably among sheep. Females range in size from 0.6 to 1.2 cm with males typically about 20% smaller. The life cycle is relatively simple. Male and female adults breed on the abomasal surface. Eggs are passed in the faeces and develop through first (L1), second (L2) and third stage larvae (L3). Third stage larvae are unable to feed and are the infective stage. Following ingestion, third-stage larvae rapidly moult within 48 hours into fourth stage larvae (L4) which develop in the gastric glands. About 8 days after infection, the young adults emerge from the gastric glands, mature and breed. The prepatent period can be as short as 12 days. The number of eggs in the uterus of mature females can vary from less than 10 to more than 60 and is heavily dependent on the host immune response.  The number of eggs produced per day by an adult female worm has been estimated as ranging from 0 to approximately 350, with longer females laying more eggs.   It is responsible for considerable economic losses in sheep. It is believed to also cause severe losses in goats although there is a relative dearth of research in this species. 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. The key trigger appears to be a relative protein deficiency in the ewe and the periparturient rise may be prevented by feeding supplementary protein. In the stereotypical pattern, egg production (assessed by faecal egg counts in the lambs) rises till midsummer then declines. The development of free-living nematodes is influenced by temperature and moisture and there is considerable variation within and among among years in the number of infective larvae available for ingestion. Consequently, the pattern of egg production during the year also varies among years. The number of infective larvae gradually dies down at the end of the grazing season when grass growth also declines. The end of the season varies among countries depending upon their climate and is also likely to vary from year to year within countries depending upon local weather but is poorly documented. For example, in Scotland, the season usually ends about late October. There is considerable variation among animals in faecal egg counts and much of this variation is genetic in origin. The sources of variation are dynamic and their relative importance changes over the course of the year. Other sources of variation, apart from inherited effects include maternal and common environmental effects and effects specific to each individual, including variation in intake and non-additive genetic effects. The gender of the lamb, type of birth (single or twin), date of birth and intensity of early exposure to nematode infection have relatively minor but still important effects. The inevitable variation between the observed count and the true faecal egg count also contributes to the observed variation. Both the natural history of infection and the sources of variation among individuals are relatively well understood for T. circumcincta compared to most parasites. Essentially all grazing animals are exposed to infection and most animals will carry some nematodes either as adults or arrested early fourth-stage larvae or both. For disease control, the aim is not to diagnose infection but to identify animals or flocks that are sufficiently heavily infected to show reduced production, decreased animal welfare or parasitic gastroenteritis. Heavily infected animals are relatively protein deficient. Clinical signs include reduced appetite, poor growth performance, weight loss and intermittent diarrhoea. In addition to clinical signs, faecal egg counts and the timing of infection are used to identify severely affected animals. However, lambs with very high numbers of worms produce very few eggs; there are strong density-dependent effects of worm number on egg production. The heritability of a trait in the narrow sense is the proportion of inherited variation divided by the total variation. It determines the response to selection. Faecal egg count is widely used to identify and select animals that are relatively resistant to nematode infection. 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.