Haemonchus contortus Infection in Sheep; Effects of Various Levels of Primary Infections on Nontreated Lambs

Abstract

SUMMARY The slotted platform method for raising worm-free lambs proved highly effective, and all lambs given a basic high-protein nutritional regimen survived exposure to as many as 10,000 Haemonchus contortus larvae. Oral inoculation levels of 25,000 H. contortus larvae killed 2 of 5 lambs, and inoculations of 50,000 larvae killed 4 of 5 lambs. Average nematode egg counts of each group of lambs inoculated with a given dose of larvae generally reflected the level of exposure. Lambs 50 to 52 days old and weighing less than 20.4 kg., when given 25,000 or 50,000 larvae died by the beginning of patent infection, whereas lambs more than 55 days old and weighing more than 20.8 kg., usually survived longer or even recovered. Noninoculated control lambs weighing more than 40.8 kg. were almost as resistant to initial inoculations of 10,000 infective-stage larvae as were reinoculated lambs initially exposed to 1,000 larvae and then given 10,000 larvae. Immunity against reinfection occurred in lambs after chemical abortion of the primary infection at the end of 133 days and a subsequent “resting” period of 21 days before reinfection. Maximal resistance against reinfection occurred in lambs surviving the primary infections with 25,000 H. contortus larvae, but apparently inoculation doses of 50,000 larvae partially paralyzed the resistance mechanisms. Iron-deficiency anemia was the most prominent sign of haemonchosis, although noninoculated controls developed a “normal anemia” believed to be due to the demands of rapid growth on the hematopoietic system. Production of erythrocytes was stimulated during the first 7 days of the infection, but hemoglobin synthesis decreased steadily unless recovery occurred. Deaths were apparently due to failure of the hematopoietic system.