Genetic resistance of three genotypes of goats to experimental infection with Haemonchus contortus.

A total of 46 weaned kids of three genotypes aged about 4-5 months were used to evaluate the effects of trickle infection with a sheep strain of Haemonchus contortus. A completely randomized 3 x 2 factorial design was used. Factors were genotype (Thai native (TN), 75% TN x 25% Anglo-Nubian (AN) and 50% TN x 50% AN) and parasite (control and infected). The animals were infected with 750 infective larvae (L3) of H. contortus three times a week for 3 weeks, with a total of 6750 larvae. The experiment lasted 9 weeks. Each week animals were weighed, faecal egg counts done and blood examined for haematological and biochemical variables. Twenty-seven kids were slaughtered at the end of experiment for worm recovery. Weight gain of infected animals was lower than those of uninfected controls (P < 0.05). The genotype 50% TN x 50% AN had higher growth rate than TN and 75% TN x 25% AN genotypes (P < 0.05). Eggs per gram of faeces (EPG) were significantly higher in 50% TN x 50% AN kids than in TN (P < 0.0005) and 75% TN x 25% AN (P < 0.0001) kids. There was a large variation in the EPG of individual animals within a genotype. The percent establishment of L3 was 8.2% in TN, 16.97% in 50% TN x 50% AN and 17.91% in 75% TN x 25% AN kids. TN kids had worm counts lower than 50% TN x 50% AN (P < 0.05) and 75% TN x 25% AN (P = 0.07) kids. Infection had a significant effect on packed cell volume (PCV), haemoglobin, total protein and albumin. The decrease in the level of these blood parameters was less in TN kids than in 50% TN x 50% AN and 75% TN x 25% AN kids. There was no significant difference between genotypes in the values of total and differential leucocyte counts and mean corpuscular volume (MCV). It can be concluded that TN goats are more resistant to H. contortus than 50% TN x 50% AN goats. However, under the present experimental conditions, the liveweight gain of 50% TN x 50% AN was higher than the TN goats.