02138naa a2200301 a 450000100080000000500110000800800410001910000200006024501830008026000090026352012620027265000140153465000110154865000100155965000350156965000130160465000150161765000100163265000170164265300180165965300130167765300130169070000170170370000200172070000160174070000180175677300620177417881741996-04-18       1980    bl ---       0--  u    #d1 aSANGSTER, N. C.  aTrichostrogylus colubriformis and Ostertagia sp resistant to levamisole, morantel tartrate and thiabendazolebinfectivity, pathogenicity and drug efficacy in two breeds of sheep.  c1980  aMerino and Border Leicester cross Merino sheep, mine months old, were infected with 10,000 third stage larvae of both Trichostrongylus colubriformis (PF4) and Ostertagia cir- cumcincta/O trifurcata (PF5), known to have varying degrees of resistance to levamisole, morantel tartrate and thiabendazole. Crossbred sheep carried heavier Ostertagia sp worm burdens but there was no difference in susceptibility between the two breeds of sheep to infection with T colubriformis. The anthelmintic efficacy of thiabendazole, at 50 mg/kg, against T colubriformis was 81.8 per cent and 92.4 per cent for Merinos and crossbred respectively while levamisole, at 6.75 mg/kg, was 12.3 per cent and 18 per cent effecti- ve. Thiabendazole removed 92.3 per cent and 83.8 per cent of Ostertagia sp in Merinos and crossbreds respectively. However, worm burdens in levamisole treated sheep were not significantly different from controls. No significant breed differences were found in 24 h faecal egg outputs. It is suggested athat breed differences previously described in four- month-old sheep may have been due to differences in the rate of development of immune responsiveness. This disparity was no longer detectable in the immunologically mature sheep used in this study.  ainfection  alarvae  asheep  aTrichostrongylus colubriformis  aCarneiro  aInfecção  aLarva  aResistência  aOstertagia sp  aParasita  aParasite1 aKELLY, J. D.1 aWHITLOCK, H. V.1 aGUNAWAN, M.1 aPORTER, C. J.  tResearch in Veterinary Sciencegv.29, n.1, p.26-30, 1980.