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The relationship between size of intake of infective larvae of Ostertagia circumcincta and faecal egg output by lambs was examined in two experiments in four-month-old Suffolk cross lambs dosed five times per week for 10 weeks. The first experiment compared two strains, Weybridge and Moredun, at two levels, 400 and 4000 per dose. The second experiment compared two levels, 40 and 400 per dose, of Moredun larvae. The group mean output of worm eggs was almost unaffected by the 10-fold reduction in intake from 4000 to 400. The further reduction from 400 to 40 reduced egg output by a factor of 2.5.

Faecal egg counts were determined twice weekly for two groups of four-month-old worm free cross Suffolk lambs experimentally infected with 100 and 320 Ostertagia circumcincta larvae, five times per week for 20 weeks. The group mean egg count in both groups rose to a maximum of approximately 500 eggs per gram during weeks 12 to 14 and had fallen to less than half of that value at slaughter. There was no statistical difference in egg output between these two groups or between them and a comparable group of lambs receiving 4000 Ostertagia circumcincta larvae per day. No significant changes in live weight gain or serum pepsinogen levels were observed in the lambs given 100 and 320 larvae per day.