The difference between two strains of Ostertagia ostertagi in resistance to morantel tartrate

Abstract

Borgsteede F. H. M. 1988. The difference between two strains of Ostertagia oslertagi in resistance to morantel tartrate. International Journal for Parasitology 18: 499–502. A comparison was made of the efficacy of morantel tartrate against a susceptible laboratory strain and a field strain of Ostertagia ostertagi isolated during the use of a morantel-sustained release bolus. Twenty calves were divided into four groups. Calves of groups 1 and 2 were infected with the susceptible strain and calves of groups 3 and 4 were infected with the other strain. Calves of groups 2 and 4 were then treated after 28 days with 10 mg kg −1 morantel tartrate and slaughtered on day 34. Faecal egg output of calves of group 2 fell to zero after treatment, while calves of group 4 remained positive, though lower than before. After morantel treatment pepsinogen values dropped markedly in calves of group 2, but less in calves of group 4. Worm numbers of calves of group 2 were reduced by 98.5% compared with those of group 1. Calves of group 4 showed a 77.4% reduction in worm burden compared with calves of group 3. It was concluded that it is possible to select an O. ostertagi strain with morantel resistance by the use of a morantel-sustained release bolus.