COMPARISON OF RESISTANCE OF 4 GENOTYPES OF RAMS TO EXPERIMENTAL-INFECTION WITH HAEMONCHUS-CONTORTUS

Fifty-five rams aged about 18-24 months weighing 30-42 kg were used in this study. Ten rams of each of four genotypes, S (Sumatra), H1 (50% Sumatra-50% Virgin Island), B1 (50% Sumatra-50% Barbados Blackbelly), E1 (50% Sumatra-50% Java Fat-tail), were infected orally with a newly isolated strain of Haemonchus contortus. Each animal received 2000 inf ective larvae 3 times week(-1) for 3 weeks, with a total of 18000 larv ae. Fifteen rams belonging to Sumatra and its crosses with Virgin Isla nd were used as uninfected controls. Peak egg counts were observed on Day 35 for genotype B1 and on Day 42 for genotype H1 and S. In genotyp e E1 a slow but consistent increase in EPG continued until Day 49 when the experiments terminated, Overall faecal egg counts at all sampling dates were not statistically different between genotypes (P > 0.05).T here was a large variation in the EPG of individual rams within a geno type, The overall average EPG (geometric means) of individual rams wit hin a genotype ranged from 3 to 1028 for B1, 4 to 261 for E1, 7 to 311 9 for H1 and 9 to 506 for S. The analysis of packed cell volume (PCV) for four genotypes and controls from all sampling times showed signifi cant differences (P < 0.05). The overall mean PCV was highest in S (31 .1) and lowest in H1 (28.4), The ranking of four genotypes for PCV was S > E1 > B1 > H1. The decrease of PCV during the course of infection was highly significant for all genotypes of infected rams (B1, E1, H1, S) (P < 0.01). PCV of the control group did not exhibit much change d uring the course of the experimental period. Weight gain of infected r ams was lower than those of uninfected controls (P < 0.5) but there wa s no significant difference between the four genotypes of infected ram s. Individual variation within genotype in susceptibility to infection s was generally more important than between genotype differences. Two major conclusions of the present study are: (1) The imported breeds wi th higher body weight, namely the Barbados Blackbelly and Virgin Islan d, may be used in cross breeding to increase the body size of local Su matra sheep. (2) Based on the faecal egg counts it is possible to iden tify the animals for use in selective breeding programmes for higher r esistance to H. contortus.