Studies on Susceptibility Influence of Age on the Schistosoma of Sheep to Infestation

Twenty-eight Dorper wethers, allocated according to age into 4 groups of 7 animals each, and 1 group of 7 Merino wethers, were compared for susceptibility to Schistosoma mattheei infestation. The group mean ages of the Dorper sheep varied from 5-61 months and their live mass from 25-66 kg, while the Merinos were 8 months old and had a mean mass of 19 kg. Despite the marked differences in the age and live mass of the Dorper sheep and the inclusion of 2 breeds in the experiment, no statistically significant differences were found in cercariae which failed to penetrate the sheep, the mean percentage development of cercariae to adult worms, worm distribution in the mesenteric and gastric radicles of the portal vein and the pulmo nary arterial system, and worms not removed by perfusion. Significant differences between groups (5~~ significance level) were found, however, in the number of worms recovered from the hepatic portal system, and in the worm sex ratio. On 3 occasions the total number of eggs excreted per female schistosome in the mesentery per 24 hours differed significantly between groups, but each time a different group or groups of sheep were responsible for the variation which was probably due, therefore, not to the age or breed of the sheep, but to daily variations in individuals. Highly significant differences occurred in the infectivity of the 6 cercaria! pools used for infestation in spite of standardized collection and handling of the cercariae. Possible reasons for this are discussed and a solution suggested. Frequent egg counts (5 per sheep per week) were done during the first 25 days of patency. until the sheep were slaughtered. Schistosome ova were detected in the faeces of only I /18 sheep examined on Day + 43 after infestation, and 3/ 17 on Day --44, whereafter this increased rapidly to 15/34 on Day + 45, 25 /33 on Day + 46, etc. A highly significant correlation was found between the total worm egg excretion in the faeces of the sheep per day and the numbers of female schistosomes in the mesentery. especially shortly after the onset of egg production .