Literature Review
1. Introduction 1.1 Cost of Nematodiasis to the Australian Sheep Industry Parasitic infections are arguably the most important and prevalent health risk associated with small ruminant production. Gastrointestinal nematodes (GNs) in particular, pose the largest threat to the health, welfare and productivity of sheep as they are usually highly pathogenic and severely limit the efficiency and profitability of meat and wool production (Barger and Cox, 1984; Larsen et al, 1995; Campos et al, 2009). There are varying estimates of the impact of GNs on the Australian economy. McLeod (1995) estimated the annual cost of intestinal parasitism to be approximately $AUS222 million. This figure incorporated total losses from meat and…show more content… Nematode Burdens and their Effect on Sheep 2.1 Pathophysiology of Key Nematode Species in High Rainfall Areas There are 3 main nematode species which are of prime concern to Australian sheep producers, located within climatic zones receiving over 500mm annual rainfall; these are Trichostrongylus colubriformis (Black Scour Worm), Teladorsagia circumcincta (Brown Stomach Worm) and Haemonchus contortus (Barber's Pole), all belonging to the family Trichostrongylidae. T. colubriformis infection has marked effects on animal health, welfare and the productivity of livestock. Members of this species are typically 4 to 9 mm in length, are primarily found in the anterior small intestine and have a prepatent period of between 15 to 23 days (Anderson, 2000). This nematode severely disrupts the mucosal architecture of the ruminant gut by burrowing into the intestinal wall (Poppi et al, 1986). This mechanical assault subsequently leads to an increase in the nitrogen concentration within the ileum (Poppi et al, 1986 and Bown et al, 1991), which then causes mass sloughing of the intestinal mucosa due to increased cellular turn-over (Holmes, 1985) and rapid leakage of plasma proteins into the gut lumen (Poppi et al, 1986 and Bown et al, 1991). Pullman et al (1989) concluded that infection with T. colubriformis also causes severe atrophy of the intestinal villi, a generalised decrease in villous to crypt ratios and goblet cell hyperplasia. These physiological events result in haemorrhage, oedema, hypoalbuminaemia, hypoproteinaemia and watery diarrhoea or "black scours”, which stains the fleece (Barker, 1975; Barker, 1982). This worm has the capacity to increase mortality in susceptible animals and reduce meat and fleece production by approximately 16% depending on the plane of nutrition and level of host immunity (Johnstone,
