Observations on the development of Haemonchus contortus in young sheep given a single infection.

Worm free lambs 2-4 months old were given a single infection of 50,000 Haemonchus contortus larvae. The changes in packed red cell volume were compared with those of uninfected sheep and the infected sheep were divided into two groups according to the rate of change of packed cell volume. In group 1, packed cell volume fell sharply from the 10th day of infection and the sheep eventually died. In group 2, the fall was less marked and the packed cell volume returned to normal after about 40 days. The rates of increase of parasite body length and dry weight were compared for the two groups. Parasites in group 1 sheep grew more slowly than their group 2 contemporaries. Accurate planning of metabolic studies on H. contortus was made possible by using this information to predict the course of an infection and the size of the worms. Parasite development was also measured in sheep for which the packed cell volume was not recorded. Computer analysis showed that adult worm length increase did not follow a simple growth pattern starting from the last ecdysis, but consisted of a rapid elongation followed by a simple negative exponential increase. Dry weight increase also followed a negative exponential during the second phase. Measurements of dry weight as a percentage of wet weight indicated that the rapid elongation was possibly linked with cell enlargement by the uptake of water.