An experiment was conducted to examine the ability of Saanen kids to develop an effective resistance to re-infection by Haemonchus contortus. Kids were divided into three groups. Those in Groups I and III had received colostrum at birth while those in Group II were removed from their dams at birth in order to deprive them of colostrum. All kids were reared in the absence of parasites. At the start of the study they were 10-14 weeks of age. Each animal was given 350 infective larvae of H. contortus per kg liveweight during weeks 0, 6 and 17. During week 16 all kids in Group III were treated with levamisole to remove infections prior to re-infection. Faecal nematode egg counts (FEC) were determined at weekly intervals throughout the trial. All animals were killed to determine worm burdens 8 weeks after the final dosing with larvae. FEC of all groups peaked 5-6 weeks after each infection. Re-infections at weeks 6 and 17 were followed by continued declining egg counts in all three groups. Although worm counts for the three groups were not significantly different, establishment rates based on total larvae given after anthelmintic treatment were higher in Group III (15.2%) than in Group I (7.0%) or Group II (6.0%) even though both had remained untreated. Female worms dominated males when infections were superimposed (Group I - 1:0.42; Group II - 1:0.44) however, female and males were found in roughly equal proportions when infections were removed by anthelmintic prior to challenge at 17 weeks (Group III = 1.06:1). These data suggest that, in Saanen kids, recruitment into the population is affected by repeated exposure to H. contortus in the presence of an existing worm burden. One explanation is that the natural death rate may be similar to the recruitment rate however, given the similarity in worm numbers and the continuing decline in FEC across treatments, worm fecundity would have to have been depressed dramatically by some mechanism in order to effect this result. Further work is on-going to resolve these and other issues.
Proceedings of the New Zealand Society of Animal Production, Volume 53, , 201-202, 1993
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