Effects of level of larval intake on the productivity and physiological and metabolic responses of lambs infected with Trichostrongylus colubriformis

Abstract

The interrelationships between level of exposure to Trichostrongylus colubriformis infection, production loss, and the physiological and metabolic changes associated with the development of the disease were studied over 24 weeks in groups of weaner lambs on larval dosing regimes (LDR) of either 0, 300, 950, 3000, 9500 or 30,000 infective larvae per week. During the first 12 weeks faecal egg counts generally accorded with the level of larval dosing, but thereafter declined, and at 24 weeks worm counts showed that most animals were resistant to parasite establishment. Liveweight gain over the 24-week infection period was significantly depressed only on LDR 3000, 9500 and 30,000 being, respectively, 67, 61 and 49% of the gain on LDR 0. These differences reflected reduced rates of gain, or, on LDR 30,000, loss of weight, during the first 12 weeks; rates of liveweight gain were similar on all LDR between 12 and 24 weeks. Wool growth rate was significantly depressed on LDR 3000, 9500 and 30,000, the maximal effect being recorded during weeks 8-12, when the growth rate was reduced to 79, 47 and 44% respectively of the rate on LDR 0. Thereafter, wool growth rate on these three LDR increased and by weeks 20-24 had returned almost to the level in uninfected sheep. Food consumption was significantly depressed on LDR 3000, 9500 and 30,000, the maximal effect occurring between weeks 8 and 12 when food consumption was 71, 61 and 44% respectively of that on LDR 0. By weeks 20-24 food consumption on these LDR had returned to the level in uninfected sheep. On LDR 30,000 there was a significantly increased excretion of nitrogen (g/100 g nitrogen intake) in faeces during the 8th, 12th and 24th weeks of infection and in urine during the 4th, 8th and 12th weeks. Nitrogen retention (g/100 g nitrogen apparently digested) was significantly reduced on LDR 30,000 during the 4th, 8th and 12th weeks. Enteric loss of plasma was significantly increased on LDR 950 and above, this effect occurring earlier and being more pronounced with increasing level of larval dosing; maximal losses were recorded during the 12th week when on LDR 30,000 sheep were losing approximately four times as much plasma into the gut as on LDR 0. The development and degree of hypoalbuminaemia in infected sheep paralleled the development of enteric plasma leakage, significantly reduced albumin levels being recorded on LDR 950 and above during the 12th week. By the 24th week both enteric plasma loss and plasma albumin concentration in sheep on these four LDR had returned to values similar to those in uninfected sheep. Measurements of albumin kinetics indicated that rates of albumin turnover and irreversible loss were increased in infected sheep only during the early stages of infection. Depletion of the albumin pool was accompanied by a significantly reduced rate of irreversible albumin loss during the 12th week on LDR 3000 and above. Measurements of the RNA/DNA ratio in liver and muscle at 4, 10 and 17 weeks indicated that protein synthesis in these tissues was significantly depressed only on LDR 30,000. Catabolism of muscle protein declined concomitantly with reduced food consumption.