Estimating genetic differences in natural resistance in Rhön and Merinoland sheep following experimental Haemonchus contortus infection.

Genetic parameters of natural resistance were estimated in Rhön and Merinoland (German Merino) sheep following experimental infection with Haemonchus contortus. A total of 133 Rhön and 244 Merinoland lambs descending from 5 and 6 rams, respectively, were evaluated. Each helminth-naive lamb was orally infected with 5000 infective third-stage larvae (L(3)) of the nematode H. contortus at 12 weeks of age. Faecal egg counts (FEC) and haematocrit values were measured in all lambs at 16 and 20 weeks of age. Seventy-nine Merinoland and 29 Rhön male lambs were slaughtered immediately after the second sampling and worms were collected. Mean worm burden was calculated and the length of adults worms from an aliquot was measured.FEC of Rhön sheep was higher compared with Merinoland sheep (P<0.01). H. contortus L(3)-larvae specific antibody (IgL) level was higher in Rhön sheep (P<0.05). However, no differences in haematocrit, worm burden and IgG antibody values could be found between the breeds. Heritabilities for log FEC (+/-S.E.) were 0.0 and 0.07 (+/-0.07) for the first sample in Rhön and Merinoland sheep, respectively. Values for the second sample were higher in both breeds (Rhön 0.35+/-0.14, P<0.05; Merinoland 0.17+/-0.07, P<0.05). Corresponding heritabilities for haematocrit were higher in Merinoland (0.56+/-0.20 and 0.51+/-0.27) compared with Rhön (0.29+/-0.12 and 0.08+/-0.13). Heritabilities for worm burden were high in Rhön (0.54+/-0.2) and low in Merinoland (0.06+/-0.14 and 0.11+/-0.15). Estimated values for IgL were between 0.13 (+/-0.11) for the first sample in both breeds and 0.30 (+/-0.18) for the second sample in Rhön sheep. Corresponding heritabilities for IgG were not different from 0.0 in both breeds (P>0.05). Positive phenotypic correlations were estimated for IgG and IgL values in both breeds (P<0.01). IgG was significantly (P<0.05) and positively correlated with worm burden in male Merinoland and IgL with worm burden in male Rhön sheep.