PROTECTIVE IMMUNITY TO THE BLOOD-FEEDING NEMATODE HAEMONCHUS-CONTORTUS INDUCED BY VACCINATION WITH PARASITE LOW-MOLECULAR-WEIGHT ANTIGENS

Partially purified low molecular weight antigens obtained by gel filtr ation of whole worm homogenates or total adult excretory-secretory (ES ) products were tested in a vaccination experiment to determine their ability to induce protective immunity against Haemonchus contortus in sheep. Sheep were challenged with 20 000 infective 3rd-stage larvae. O ne animal in the low molecular weight vaccinated group showed no prote ction against H. contortus, whereas the 4 other sheep in this group sh owed a mean reduction of 99 . 9 % in faecal egg counts and of 97 . 6 % in abomasal worm burden compared to the non-vaccinated controls and t he adjuvant controls. The ES-vaccinated sheep showed a 32 . 2 % reduct ion in parasite egg production and a 63 . 7 % reduction in abomasal wo rm counts. Analysis of the humoral immune responses revealed no signif icant differences in antibody recognition of putative protective antig ens between the protected and non-protected vaccinated animals. Howeve r, a marked lower lymphocyte proliferation response was found in non-p rotected sheep.