Teladorsagia Circumcincta Michael Stear¹*, David Piedrafita², Sarah Sloan¹, Dalal Alenizi¹, Callum Cairns¹, Caitlin Jenvey¹
Total Page：16
File Type：pdf, Size：1020Kb
WikiJournal of Science, 2019, 2(1):4 doi: 10.15347/wjs/2019.004 Encyclopedic Review Article Teladorsagia circumcincta Michael Stear¹*, David Piedrafita², Sarah Sloan¹, Dalal Alenizi¹, Callum Cairns¹, Caitlin Jenvey¹ Abstract One of the most important parasites of sheep and goats is the nematode Teladorsagia circumcincta. This is com- mon in cool, temperate areas. 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 ani- mals. 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. Introduction Teladorsagia circumcincta is a nematode that parasitises Morphology sheep and goats. It was previously known as Ostertagia circumcincta and is colloquially known as the brown Adults are slender with a short buccal cavity and are [7] stomach worm. It is common in cool temperate areas, ruddy brown in colour. The average worm size varies such as south-eastern and south-western Australia and considerably among sheep. Females range in size from [8] the United Kingdom. Teladorsagia davtiani and Telador- 0.6 to 1.2 cm with males typically about 20% [7] sagia trifurcata are probably phenotypic variants smaller. (morphotypes) of T. circumcincta.[1] This parasite is re- sponsible for considerable economic losses in sheep,[2][3][4] and is believed to cause severe losses in Life cycle goats although there is a relative dearth of research in The life cycle is relatively simple. Male and female goats. 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 Taxonomy to feed and are the infective stage. Following ingestion, Teladorsagia is a member of the subfamily Ostertagi- iniae in the family Trichostrongylidae and the superfamily Trichostrongyloidea.[5][6] Ostertagia, Haemonchus and Graphidium are closely related taxa that are usually assigned to separate subfamilies. 1 AgriBio, La Trobe University, Australia 2 Faculty of Science and Technology, Federation University, Australia *Author correspondence: [email protected] ORCID: MS 0000-0001-5054-1348 DP 0000-0001-8692-990X CJ 0000-0002-4052-4330 Licensed under: CC-BY Figure 1 | A scanning electron micrograph of Teladorsagi cir- Received 09-11-2018; accepted 24-04-2019 cumcincta. C.M. Enterocasso, CC BY-SA-2.0 1 of 6 | WikiJournal of Science WikiJournal of Science, 2019, 2(1):4 doi: 10.15347/wjs/2019.004 Encyclopedic Review Article third-stage larvae rapidly moult within 48 hours into among individuals are relatively well understood for T. fourth stage larvae (L4) which develop in the gastric circumcincta compared to most parasites. 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.[9] The num- Clinical signs and diagnosis ber of eggs in the uterus of mature females can vary from less than 10 to more than 60[9] and is heavily de- Essentially all grazing animals are exposed to infection pendent on the host immune response.[10] The number and most animals will carry some nematodes either as of eggs produced per day by an adult female worm has adults or arrested early fourth-stage larvae or both. For been estimated as ranging from 0 to approximately disease control, the aim is not to diagnose infection but 350,[10] with longer females laying more eggs.[10] to identify animals or flocks that are sufficiently heavily infected to show reduced production, decreased animal welfare or parasitic gastroenteritis. Heavily infected an- Epidemiology imals are relatively protein deficient. Clinical signs include reduced appetite, poor growth performance, In most countries, infection with third-stage larvae re- weight loss and intermittent diarrhoea.[18] In addition to sumes in the spring and is triggered by the production clinical signs, faecal egg counts and the timing of infec- of large numbers of eggs by periparturient females.[11] tion are used to identify severely affected animals.[7] The key trigger appears to be a relative protein defi- However, lambs with very high numbers of worms pro- ciency in the ewe[12] and the periparturient rise can be duce very few eggs; there are strong density- prevented by feeding supplementary protein.[13] In the dependent effects of worm number on egg produc- stereotypical pattern, egg production (assessed by fae- tion.[19] cal egg counts in the lambs) rises till midsummer then declines.[14][15] The development of free-living nema- todes is influenced by temperature and moisture and Genetics there is considerable variation within and among among years in the number of infective larvae available The heritability of a trait in the narrow sense is the pro- for ingestion. Consequently, the pattern of egg produc- portion of inherited variation divided by the total [20] [21] tion during the year also varies among years.[15] The variation. It determines the response to selection. number of infective larvae gradually dies down at the Faecal egg count is widely used to identify and select end of the grazing season when grass growth also de- animals that are relatively resistant to nematode infec- [22][23][24] clines. The end of the season varies among countries tion. Selection is most advanced in Australia depending upon their climate and is also likely to vary and New Zealand where the dominant nematodes are from year to year within countries depending upon local Haemonchus contortus or a mixture of T. circumcincta weather but is poorly documented. For example, in and Trichostrongylus colubriformis. Scotland, the season usually ends about late October. For naturally infected Scottish Blackface lambs grazing There is considerable variation among animals in faecal fields contaminated with predominantly T. circumcincta egg counts[16] and much of this variation is genetic in eggs, the heritability of faecal egg count was not signif- origin.[17] The sources of variation are dynamic and their icantly different from zero for the first two months after relative importance changes over the course of the birth then rose to approximately 0.3 at the end of the [25] year. Other sources of variation, apart from inherited grazing season . Similar heritability estimates were [26] effects include maternal and common environmental obtained for a flock of Texel sheep. This heritability effects and effects specific to each individual, including is similar to the heritability of growth rate in beef cattle variation in intake and non-additive genetic effects.[16] and milk production in dairy cattle and indicates the The gender of the lamb, type of birth (single or twin), feasibility of selective breeding for resistance to natural [26] date of birth and intensity of early exposure to nema- predominantly T. circumcincta infection. tode infection have relatively minor but still important Genetic variation in faecal egg count in lambs following effects.[16] The inevitable variation between the ob- natural infection is predominantly due to genetic varia- served count and the true faecal egg count also tion influencing worm size and fecundity rather than contributes to the observed variation.[16] Both the natu- worm establishment and survival or larval inhibition.[8] ral history of infection and the sources of variation Indeed, the heritability of adult female worm size, 2 of 6 | WikiJournal of Science WikiJournal of Science, 2019, 2(1):4 doi: 10.15347/wjs/2019.004 Encyclopedic Review Article which is closely related to fecundity, has been esti- causal rather than acting as a marker for some other re- mated at 0.6[8] whereas the heritability of worm sponse. IgE activity against third-stage larvae seems to number was only 0.14.[8] The high heritability of worm be most important.[43][44][9] The number of molecules size is one of the highest heritabilities known for a fit- recognised by IgE seems to be relatively small; two-di- ness related trait. mensional Western blotting identified only 9 molecules, although there is evidence for at least two There has been a lot of effort devoted to identifying the other allergens.[45] For comparison, 155 molecules were causal polymorphisms that underlie resistance to infec- recognised by IgA on third-stage larvae.[46] tion.[27][28] Two genomic regions in particular have shown associations in a number of different studies: the DRB1 locus of the major histocompatibility com- plex[29][30][31][32][33] and the interferon gamma locus.[34] Pathology Linkage disequilibrium in sheep is quite high compared The immune response appears to be responsible for to humans and the causative loci need not be in these much of the pathology following nematode infec- regions but are probably in or around these regions. tion.[18] Immune suppression abolishes many of the There are a number of regions which have shown sug- clinical signs.[47] gestive (nearly but not quite statistically significant) associations in more than one study[28] and some of Infection causes a relative protein deficiency that leads these are probably harbouring causal polymorphisms to reduced weight gain or even weight loss. This due in [48][49] for nematode resistance. part to a reduction in appetite. There is also a loss in digestive efficiency. Lesions in the epithelial barrier Many of the putative candidate genes influence the im- allow a loss of protein and in addition protein is diverted mune response and genetic variation seems to to tissue repair and immune and inflammatory pro- predominantly influence immune responsiveness to in- cesses.