Pyrethroid tolerance in the chewing louse Bovicola (Werneckiella) ocellatus.

This research investigates the decreasing effectiveness of pyrethroid-based insecticides against lice found on donkeys, suggesting possible resistance development within the lice population. The article includes comparative analysis with an organophosphate pesticide, diazinon, and a potential pyrethroid synergist, piperonyl butoxide.

Study Overview

• The study investigates the reduced efficiency of pyrethroid-based pesticides in controlling lice infestations, primarily Bovicola (Werneckiella) ocellatus, on donkeys. The focus is on two products, cypermethrin and permethrin.
• This issue is a concern because it limits the options for drug rotation, increases the likelihood of resistance development, and poses health and welfare risks to the animals.

Research Methodology

• The research uses an in vitro evaluation of the efficacies of the two pyrethroid-based products and compares them with the effectiveness of diazinon, an organophosphate pesticide.
• The selection of diazinon for comparison stems from the assumption that the lice population has had minimal previous exposure to it, making it a suitable choice for this study.

Research Findings

• Results showed that neither permethrin nor cypermethrin could significantly impact the mortality rate of B. ocellatus at the recommended concentrations for animal application. These pyrethroid products did not cause more mortality than an acetone-only control, suggesting a high degree of tolerance among the lice population.
• In contrast, diazinon demonstrated high efficacy, resulting in 70% mortality within four hours and a 100% mortality rate after 24 hours of exposure.
• The addition of a potential synergist, piperonyl butoxide, to the pyrethroid solutions did not improve the pesticides’ performance, indicating possible pyrethroid tolerance.

Implications and Conclusions

• The study concludes with the implication that B. ocellatus lice populations have developed pyrethroid resistance, making the commonly used pesticides ineffective.
• Furthermore, the research observed that hair tufts taken from the flanks of treated donkeys demonstrated no significant insecticidal activity compared to hair from the central application site. This observation highlights the likelihood of an insecticide concentration gradient across the body, potentially prompting resistance development.
• The findings call for further research into alternative lice control strategies and the need to manage pesticide use effectively to mitigate resistance development.