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 filtration 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. One animal in the low molecular weight vaccinated group showed no protection against H. contortus, whereas the 4 other sheep in this group showed 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 the adjuvant controls. The ES-vaccinated sheep showed a 32.2% reduction in parasite egg production and a 63.7% reduction in abomasal worm counts. Analysis of the humoral immune responses revealed no significant differences in antibody recognition of putative protective antigens between the protected and non-protected vaccinated animals. However, a marked lower lymphocyte proliferation response was found in non-protected sheep.