High-level ivermectin resistance in a field isolate of Haemonchus contortus associated with a low level of resistance in the larval stage: implications for resistance detection

A. Kotze; R.J. Dobson; K.L. Tyrrell; P.A. Stein

doi:10.1016/S0304-4017(02)00200-5

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High-level ivermectin resistance in a field isolate of Haemonchus contortus associated with a low level of resistance in the larval stage: implications for resistance detection

Journal article

Peer reviewed

High-level ivermectin resistance in a field isolate of Haemonchus contortus associated with a low level of resistance in the larval stage: implications for resistance detection

A. Kotze, R.J. Dobson, K.L. Tyrrell and P.A. Stein

Veterinary Parasitology, Vol.108(3), pp.255-263

2002

DOI: https://doi.org/10.1016/S0304-4017(02)00200-5

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Abstract

The IVPro isolate of Haemonchus contortus was isolated in 1999 after significant numbers of the parasite survived an ivermectin capsule treatment of grazing sheep acquiring a natural infection at Prospect, NSW, Australia. The isolate shows high-level resistance to ivermectin (faecal egg count is unaffected by ivermectin oral treatment at 0.2mgkg-1). The larval LC50, as assessed by larval development assays (LDAs), is only approximately two-fold higher than several susceptible isolates, making it difficult to detect the resistance using larval LC50 as an indicator. However, the isolate shows the presence of a small proportion of the population (<20%) able to develop at significantly higher drug concentrations than the susceptible isolates. Hence, if the IVPro and susceptible isolates are compared at the LC99 level, the IVPro isolate is readily identifiable as resistant. This degree of distinction at the LC99 allows the IVPro isolate to be identified as resistant by simply observing the highest drug concentration in the development assay at which some larvae develop relative to the susceptible isolates. Assessing the development assay using this criterion allows a distinction between IVPro and the susceptible isolates equivalent to 10-fold differences in drug concentration, greatly increasing the likelihood of detecting the resistant isolate in routine resistance tests. This study highlights the need to examine this aspect of LDAs in order to detect the type of resistance displayed by IVPro H. contortus.

Details

Title: High-level ivermectin resistance in a field isolate of Haemonchus contortus associated with a low level of resistance in the larval stage: implications for resistance detection
Authors/Creators: A. Kotze (Author/Creator)
R.J. Dobson (Author/Creator)
K.L. Tyrrell (Author/Creator)
P.A. Stein (Author/Creator)
Publication Details: Veterinary Parasitology, Vol.108(3), pp.255-263
Publisher: Elsevier BV
Identifiers: 991005544604407891
Copyright: © 2002 Elsevier Science B.V
Murdoch Affiliation: Murdoch University
Language: English
Resource Type: Journal article

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Citation topics: 1 Clinical & Life Sciences; 1.163 Parasitology - General; 1.163.1022 Anthelmintic Resistance
Web Of Science research areas: Parasitology; Veterinary Sciences
ESI research areas: Plant & Animal Science