The Disease
Gastrointestinal helminths are major contributors to reduced productivity in sheep in the United Kingdom (Holmes, 1987). Parasitic infection ranges from acute disease, frequently with high rates of mortality, and chronic disease, leading to various degrees of morbidity and premature culling, to subclinical infections, where sheep may appear relatively healthy but perform below their potential. Important gastrointestinal helminth diseases in the United Kingdom are nematodiriasis in lambs and parasitic gastroenteritis in lambs and occasionally older sheep (Armour and Coop, 1991). The principal genera responsible for outbreaks of parasitic gastroenteritis are Ostertagia, Haemonchus and Trichostrongylus.
NematodiriasisNematodirus infection
In the United Kingdom Nematodirus battus is the main species of Nematodirus responsible for severe outbreaks of nematodiriasis in lambs in late spring. Clinical signs appear with the emergence and development of the larval stages in the small intestine. The clinical signs are profuse watery diarrhoea accompanied by lethargy and loss of appetite. The fleece is often dull and rough and the lambs may show the typical 'tucked-up belly' appearance. Weight loss can be rapid, with severe dehydration, and if the infection is untreated mortality can be high.
Three factors determine the epidemiology of the disease.
Accumulation of infection on pasture, therefore, takes place over a number of years, and not within a single season as in parasitic gastroenteritis. The requirement of a critical hatching temperature results in a 'flush' of infective L3 larvae on the pasture. This does not always lead to disease, for if the flush happens early many young lambs ingest insufficient larvae with the pasture, and if the flush is late the lambs are resistant to the larval challenge (Armour and Coop, 1991; Coop, 1989).
Parasitic gastroenteritisStrongylate eggs are passed in the faeces of ewes during the periparturient relaxation of immunity. This period lasts from 2 weeks prior to lambing until 6 weeks post-lambing. They are ingested by lambs, as are overwintered L3 larvae, resulting in multiplication of the worm burden on pasture. The larvae can overwinter in fairly high numbers but decline in number rapidly during April and May, although a few may survive for up to two years.
Eggs deposited in the first half of the grazing season (April - June) are responsible for the potentially dangerous populations of infective larvae which accumulate in the second half (July - September). If ingested before October, the majority of these larvae mature in a few weeks. After October, ingested larvae become arrested in development for several months (Armour and Coop, 1991; Coop, 1989; Thomas and Boag, 1972).
Ostertagia infections cause watery diarrhoea in lambs, accompanied by dehydration, loss of appetite and failure to gain weight. Developing larvae within the gastric glands of the abomasum cause the lumen of these glands to distend and stretch the cellular lining. As a result the mature functional parietal and peptic cells are superceded by undifferential cells (Armour et al., 1966). As the infection progresses, adjacent non-parasitized glands also become affected and their parietal cells replaced by non-functional undifferentiated cells. The pH of the abomasum increases and leakage of macromolecules and protein occurs across the damaged mucosa resulting in hypoproteinaemia and increased concentrations of pepsinogen in the plasma (Coop, 1989).
Due to the haematophagic activities of developing larvae and adult worms in the abomasum, haemonchosis is associated with anaemia. Diarrhoea does not normally occur. Acute haemonchosis results from large intakes of infective larvae, and young lambs rapidly become unthrifty, weak and lethargic. Hypoproteinaemia and oedema occur and mortality is high. During the first three weeks of infection, haematocrit values decline. During this phase the erythropoietic system is not activated to compensate for the blood loss. During the next 6-8 weeks the haematocrit levels hardly decline further, as the erythropoietic system is able to compensate for the loss of blood. During the final stages of the infection the haematocrit values decline rapidly as the iron reserves are depleted and the rate of erithropoiesis cannot cope with the rate of blood loss.
Chronic haemonchosis is due to a more gradual intake of infective larvae and results in a general wasting condition, with sheep becoming unthrifty and emaciated, which resembles a state of malnutrition. Growth rates decline and the fleece may be open and dull (Armour and Coop, 1991).
Trichostrongylus infection
Trichostrongylosis is a chronic wasting disease of hoggs and ewes in early winter. Acute disease, however, can occur in lambs. Clinical features include loss of appetite and loss of condition. Varying degrees of hypoalbuminaemia and hypophosphataemia also occur. Dark coloured diarrhoea is often present in the more severe cases and the fleece may be open and the wool brittle. Developing larvae and adult worms burrow just beneath the surface epithelium of the small intestine causing sloughing and disruption of cells and leakage of plasma protein into the lumen of the small intestine (Coop, 1989).