Expression of acquired immunity to a local isolate of Haemonchus contortus by the Nigerian West African Dwarf goat.

The capacity of young Nigerian West African Dwarf (WAD) goats to express good acquired immunity to their native geographic strain of Haemonchus contortus and the correlates of this responsiveness were studied in a laboratory experiment involving forty 7-8 month old kids. A primary immunising infection with 2000 L3 (equivalent to 260-450 L3/kg body weight) with or without challenge on D42 with 2000 L3 resulted in a mild chronic infection with a pre-patent period of 18-20 days and little or no reduction in worm burden between D14 and D56. In contrast, another group (D) of kids, whose immunising infection had been truncated with fenbendazole on D35 and later received similar challenge infection, developed good protection against challenge. Thus, worm burdens were largest in group E (challenge control), larger in group C (primary+challenge) and least in group D. Of the measures of infection used, namely faecal worm egg counts (FECs), circulating eosinophil (EOS) responses, packed cell volume (PCV) and body weight, FEC and EOS responses exhibited marked individual variability, but only FEC (geometric mean of transformed counts) and PCV showed strong correlation with worm burden. There was also a significant negative correlation between FEC and PCV. The size of inoculum used was well tolerated by the kids, as it induced only mild changes in PCV in some goats and no effect at all on body weights. This suggests that the WAD goat may possess a good measure of resistance to the pathogenic effects of its native strain of H. contortus. The wide individual variability in FEC and its strong relationships to worm burden and PCV are pointers to its likely genetic basis. There are, therefore, good prospects for further studies to identify H. contortus resistant genotypes among the WAD goat population.