Quantitative studies of ovine haemonchosis. 3. The interpretation and diagnostic significance of the changes in serial egg counts of Haemonchus contortus in a sheep flock.

An examination of the level and distribution of egg counts of H. contortus within an extensively managed flock of Merino sheep in south-east Queensland was carried out on three occasions over a ten week period. During this time the geometric mean egg count increased from 527 to 1331 eggs per gram, and although 26.3% of the flock were responsible for this increase, clinical evidence of haemonchosis was not observed. Examination of the distribution of counts supported the hypothesis that counts followed a negative binomial distribution. The values obtained for "k" (the index of over-dispersion), ranged from 0.5 to 1.4. However, on the two latter examinations, counts showed a divergence from the expected pattern (P = 0.015; P = 0.014). Further examinations of flocks at higher mean levels of count are required to establish if the trend towards a more normal distribution were maintained. The evidence from the present study and from outbreaks previously reported, suggested that in extensively managed sheep, the level of variance remained high as the mean egg count increased. It therefore appeared likely that in outbreaks of acute haemonchosis under grazing conditions, only a small proportion of the flock may become seriously affected.