Comparison of the resistance to sympatric and allopatric isolates of Haemonchus contortus of Black Belly sheep in Guadeloupe (FWI) and of INRA 401 sheep in France

The resistance of a tropical breed of sheep towards the nematode parasite Haemonchus contortus (He) was compared to that of a European breed after artificial infection with the parasite following a 2 x 2 x 2 experimental design: two isolates (Hc Gua from Guadeloupe versus He Fra from France), two breeds each reared in their native habitats (Black Belly (BB) in Guadeloupe, FWI, INRA 401 reared in France), and two groups of lambs with differing infection status (one naive, i.e. infected for the first time during the experiment, and one primed, i.e. which had previously received doses of 5000 L3 on Days -35 and -32 before the infection during this experiment). Both groups were composed of 10 male lambs, which were 20 weeks of age when they were given a dose of 10,000 L3 of the relevant isolate on Day 0. Resistance was assessed by regular fecal egg counts (FECs) and blood samples, and worm counts were processed on Day 35 p.i. The FEC was lower in the BB than in INRA 401 lambs, lower in the primed than naive groups, and lower in He Fra than in He Gua. The eosinophil count remained unchanged in the naive INRA 401 groups, but increased in all the other groups. The hematocrit fell after infection, but the live weight was not affected by the infection. Worm burdens were 1868 and 5237 in the naive BB and INRA 401 groups, respectively, and 596 and 4835 in the primed BB and INRA 401 groups. The daily production of eggs per He Gua female worm was 1.5-fold that of Hc Fra. The number of worms had no effect on worm length or He in utero egg count in the BB sheep, but a positive and significant correlation between these two traits was seen in the INRA 401 lambs. These findings are indicative of innate resistance, and confirmed the high acquired resistance of the BB sheep to both sympatric and allopatric isolates of Hc. The regulation of worm population and fecundity differed in the BB and INRA 401 sheep. (C) 2003 Elsevier B.V. All rights reserved.