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Metabolic consequences of intestinal parasitism

Clinical and subclinical intestinal parasitism is characterized by impaired production (Sykes & Coop, 1976; Steele, 1978) with poor growth rates in young animals and loss of body weight in older animals. Sheep also incur severe reductions in wool growth. A number of comprehensive reviews prepared over the last decade characterize and contrast the magnitude of such impairment across a range of intestinal roundworms and consider the factors which contribute to this problem (Armour & Ogbourne, 1982; Bremner, 1982; Sykes, 1983; Holmes, 1986; Parkins & Holmes, 1989; Poppi et al. 1990). Laboratory-based experiments designed to investigate the aetiology of different infections have followed two distinct approaches. They have involved administration of either a massive single dose of parasite larvae (e.g. 30 000-100 000; Steele, 1972; Roseby, 1973), or smaller doses of larvae given daily, or perhaps two to three times per week, in a so-called ‘trickle infection’ approach (Sykes & Coop, 1976; Jones & Symons, 1982). The latter regimen was adopted to more closely simulate the way animals incur infestation under practical conditions and it has provided a clearer picture of the aetiology and the subsequent immunological resistance which accompanies any subclinical condition. It has also shown the way in which the severity of infection relates to dose rate of larvae and nutritional state of the animal. As the present review will address mainly the metabolic consequences of subclinical parasitism, it will relate predominantly to experimental data obtained from animals subjected to ‘trickle infection’, mainly with the intestinal roundworm Trichostrongylus colubriformis, where very few clinical symptoms other than parasite eggs in the faeces and high eosinophil counts in the blood are seen.