ABSTRACT
Nematodes that parasitize the GI tract of ruminants and trypanosomes in the blood can have a major impact on nutrient intake and metabolism (see reviews by Coop and Holmes,[1] van Houtert and Sykes,[2] and Holmes et al.[3]), leading to impaired productivity such as reduced weight gain, soft-tissue deposition, reduced skeletal growth, reduced milk and wool production, and reduced capacity for draft work. Two main effects of parasitic infection have been documented: changes in feed intake and changes in the efficiency of feed utilization. The impact of each will be influenced by the species of parasite, the intensity and duration of the infection, and the immune and nutritional status of the host. The majority of experimental studies of GI parasitism have been conducted in growing animals exposed to infections with a single species of nematode. However, in field situations, infection with a number of species of nematodes is more common and may involve both the abomasum and small intestine. Steel et al.[4] and Sykes et al.[5] have shown that the effects on growth rate
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of mixed infections are multiplicative rather than additive, in which case the impact of GI helminth infections in field situations may be significantly greater than that shown by experimental infections with single species. Similarly, trypanosome-infected ruminants are frequently also infected with GI helminths, some of which may exacerbate the anemia caused by the protozoan parasite (e.g., Haemonchus contortus).
