Nematode Infections of Domestic Animals: Gastrointestinal Infections

Abstract

Anthelmintics have been used for many years to treat clinical outbreaks of gastrointestinal (GI) helminthiasis. With the advent of modern drugs that are both broader in spectrum and much less toxic than many of the early compounds, large-scale prophylactic use of anthelmintics has increased enormously. At its simplest, prophylactic use of drugs involves treatment of susceptible animals at intervals frequent enough to prevent accumulation of heavy parasite burdens. This can be expensive and is not ideal, since generally the greater the use of a drug, the more likely it is that resistance to it will arise. In many situations, knowledge of the epidemiology of the disease permits strategic timing of drug treatments which, in combination with certain management practices, makes for a reduction in the number of treatments required. Efficient control programs for grazing cattle and sheep are based on knowledge of seasonal fluctuations of infective (L₃) larval stages on pasture. In temperate countries in the northern hemisphere, sufficient L₃ overwinter on pasture to infect susceptible animals grazing the following spring, culminating in fresh infections and in eggs being deposited in the feces; however, these burdens are seldom sufficient to cause a clinical effect. Most noningested overwintered L₃ die off by late June, and the buildup of pasture L₃ that occurs in midsummer is primarily attributable to hatching of eggs deposited the same year. Thus, the treatment of young cattle with anthelmintics 3 and 6 weeks after commencing spring grazing in April or May will markedly reduce subsequent pasture contamination. If treatment is delayed until July and is combined with removal to pasture not grazed by cattle that year, there will be no need for further treatments until the end of the grazing season.