Annual variation in the growth of Haemonchus contortus in experimental infections of sheep and its relation to arrested development

Abstract

Fifty-six sheep, worm-free birth, were each given a sigle infection of 50 000 Haemonchus contortus larvae at varying at varying times over a 2 year period. They were killed from 13 to 89 days infection and the lengths of both male and female parasites measured. Using a computer, growth curves for each sex were constructed and the relative lengths of the worms in each infection were calculated by dividing their measured lengths by the lenghts predicted by the appropriate curve. Since there was no significant difference between the relation length of male and female worms in an infection, the mean value was used in further analyses. Differences in sex of host and duration of infection had no significant effect on the relative length of parasites. There was a close relation between relative length and the time of year of infection. The relation was cyclical and closely followed a sine curve with a maximum at day 90 and a minimum at day 275 of the year. Arrested development at the 4th larval stage occurred at the time of year risen again. It is suggested that this cyclic change in growth pattern and arrested development is controlled by the seasonal variation in concentration of a substances present in the host’s blood. Parasite relative length was also related to sheep age, but the relationship observed was largely an artifact of the close birth dates of many of the sheep.