Overcoming macrocyclic lactone resistance in Haemonchus contortus with pulse dosing of levamisole.

The in vivo effect of dosing levamisole as a pulse release within an ivermectin (IVM) controlled-release device (CRD) was simulated by periodic dosing of levamisole to Haemonchus contortus-infected sheep already treated with an IVM CRD. The rationale for this treatment combination arises from the need to find alternative approaches to the treatment of gastrointestinal parasites in livestock in the face of increasing levels of anthelmintic resistance which is now widespread in Australia. Thirty merino sheep (4 months of age) were infected weekly with a mixture of susceptible and ivermectin resistant H. contortus beginning at Day 28. Twenty eight days after first infection, groups of 10 sheep were treated with IVM capsules alone, IVM capsules and an oral dose of levamisole (LEV) at Days 50 and 100 or no treatment. At pre-determined intervals, up to 126 days after treatment, faecal worm egg counts (FWEC) were determined and development rates of infective larvae (L3) cultured in faeces were measured. Haematological parameters and drug concentration in plasma were measured throughout the 100-day release period of the controlled-release device. Sheep were slaughtered at Day 135 for estimates of total worm burden. FWEC of sheep treated with IVM+LEV declined (99.9% reduction) after administration of oral LEV and were suppressed until Day 98. There was a significant difference (p<0.0001) in worm counts at slaughter between groups. The results demonstrate the potential advantage of combining a pulse of short-acting drug into the long-acting anthelmintic capsule to provide better parasite control than that achieved from the existing CRD treatment when IVM-resistant worms were present.

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.

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 0.2mg kg(-1)). The larval LC(50), 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 LC(50) 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 LC(99) level, the IVPro isolate is readily identifiable as resistant. This degree of distinction at the LC(99) 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.