Evaluation of the phenotypic performance of a Red Maasai and Dorper double backcross resource population: indoor trickle challenge with Haemonchus contortus.

Six F(1) Dorper (D) x Red Maasai (R) rams were mated to both D and R ewes to produce backcross lambs. These six double backcross resource families are being analysed to identify quantitative trait loci that may be controlling resistance to gastro-intestinal (GI) nematode parasites, mainly Haemonchus contortus. After assessing the phenotypic performance of the lambs following exposure to natural infections, the surviving lambs were drenched and moved indoors for an artificial challenge with H. contortus along with straightbred D and R lambs. A total of 1317 lambs were included in the analyses consisting of 523 3/4D, 580 3/4R, 87 D and 127 R. The D lambs were significantly and consistently heavier than R lambs and 3/4D were significantly heavier than the 3/4R lambs. The difference between the backcrosses was about half of that between the straightbreds. Resistance was assessed in terms of faecal egg counts (FEC) and total worm counts (TWC) at necropsy while packed cell volume (PCV) was used to assess resilience to weekly oral doses of 2500 infective larvae of H. contortus. No significant breed differences were observed for log transformed FEC (LFEC). A significant breed difference in PCV was recorded. The backcrosses had the higher values and while no differences were observed between the straightbreds, 3/4D had significantly higher PCV than the 3/4R. Despite the absence of breed differences in FEC the R and the 3/4R had significantly fewer worms than the D and the 3/4D. The D had significantly longer worms than the R and the 3/4D had significantly longer worms than the 3/4R. Worms recovered from D had more eggs than those recovered from R. Similarly worms from 3/4D contained more eggs than those from 3/4R. Thus, on a breed basis the breed with more worms had longer worms. In contrast, when, in a small part of the experiment two doses of larvae were used to check for any breed by dose interactions, worms from the low dose (and hence fewer worms) animals were longer. We postulate that in fast growing hosts like the D, worms also have a better potential for growth and reproduction than in hosts that have less potential for growth. The overall correlation coefficient between PCV and LFEC was -0.67 while that between LFEC and LTWC was 0.72. From these results it is clear that the R do not respond to the artificial challenge the way they do to natural infection suggesting that phenotyping of R and R cross lambs for purposes of selecting those that are resistant or susceptible is best done under natural challenge.

Resistance of Galla and Small East African goats in the sub-humid tropics to gastrointestinal nematode infections and the peri-parturient rise in faecal egg counts.

A study was carried out from 1994 to 1996 to compare the resistance to naturally acquired gastrointestinal (GI) nematode infections (predominantly Haemonchus contortus) of 88 Galla and 114 Small East African (SEA) goats (does) in the sub-humid coastal region of Kenya. Live weights (LWT), blood packed cell volume (PCV) and faecal egg counts (FEC) were recorded each year at mating, 3-4 months after mating, 1-2 weeks before kidding and 1-4 months after kidding. There was a significant increase in FEC and a decline in PCV in lactating does compared to non-lactating does over the peri-parturient period. This peri-parturient rise in FEC occurred in both breeds but was more marked and persistent in the Galla than in the SEA. The SEA does were more resistant to GI nematode infections than Galla does as shown by their significantly lower FEC and higher PCV at all sampling times over the reproductive cycle, but this breed difference was particularly marked over the lactation period.

Evidence for multiple anthelmintic resistance in sheep and goats reared under the same management in coastal Kenya.

Four experiments, two with sheep and two with goats, were carried out to determine the efficacy of ivermectin, fenbendazole, levamisole, closantel and some of their combinations by faecal egg count reduction tests. In the first experiment, injectable ivermectin, oral ivermectin, fenbendazole and levamisole were tested in 6-month-old lambs, and their reduction percentages were 77%, 13%, 42% and 92%, respectively. In the second experiment, with yearling sheep, the reduction percentages were 35% for injectable ivermectin, 32% for fenbendazole, 99% for levamisole, 48% for closantel, 92% for injectable ivermectin combined with fenbendazole, 99% for injectable ivermectin combined with levamisole, and 100% for fenbendazole combined with levamisole. In the study with 18-month-old goats given the same dose rates as those recommended for sheep, the reduction percentages were 73% for injectable ivermectin, 25% for fenbendazole, and 78% for levamisole. Another group of 14-month-old goats was treated with dose rates 1.5 times those recommended for sheep and the reduction percentages were 93% for levamisole, 92% for injectable ivermectin, and 97% for a combination of levamisole and ivermectin. In all experiments with sheep and goats the gastrointestinal nematode parasites identified by larval cultures were Haemonchus contortus, Trichostrongylus spp. and Oesophagostomum spp. The gastrointestinal nematodes of both sheep and goats on this farm are resistant to ivermectin and fenbendazole, whereas levamisole is still effective in sheep, but not in goats. The results are discussed in relation to the farm as a source of breeding stock to smallholder farmers and its potential to spread anthelmintic resistance.