Pyrethroid tolerance in the chewing louse bovicola (werneckiella)

Equine pediculosis is a significant health and welfare issue, particularly in elderly and chronically debilitated animals. Currently infestation is controlled predominantly using topically applied pyrethroid insecticides, allowing limited scope for the rotation of drugs and increasing the risk of selection for resistance. Here the insecticidal efficacies of two pyrethroid-based products against the louse Bovicola (Werneckiella) ocellatus collected from donkeys were examined in vitro. The products were cypermethrin (DeosectTM, Pfizer Ltd., 5% (w/v) cypermethrin, cutaneous spray) and permethrin (SwitchTM, VetPlus Ltd., 4% (w/v) permethrin, pour-on). The pyrethroid efficacy was contrasted with that of the organophosphate diazinon, since the louse populations examined were unlikely to have had prior exposure to this compound. The efficacy of diluted pure permethrin, the excipient, butyl dioxitol and the synergist piperonyl butoxide in the presence of the pyrethroids, were also considered. At the concentrations recommended for animal application, neither 4% (w/v) permethrin, nor 0.1% (w/v) cypermethrin had any significant effect on the mortality of B. ocellatus and neither induced significantly more mortality than an acetone-only control. In contrast, 0.04% diazinon caused 70% mortality within 4 h and 100% mortality after 24 h exposure. The addition of a potential pyrethroid synergist, piperonyl butoxide, in combinationwith cypermethrin and permethrin, resulted in no significant increase in mortality.

It is concluded that the population of lice tested display a high level of pyrethroid tolerance which is likely to reflect the development of resistance. Twenty-four hours after routine treatment of 10 donkeys with a pour-on permethrin product (SwitchTM, VetPlus Ltd., 4% (w/v) permethrin, pour-on) hair tufts taken from their flanks were not significantly insecticidal compared with hair from the midline application site, implying a low level of insecticide distribution. Such a distribution pattern is likely to create an insecticide concentration gradient over the body and further facilitate selection for resistance.

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Efficacy of anthelmintics in horses and donkeys in Ireland: an in vivo and in vitro study

Nagwa Elghryani
Theo De Waal
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Strongyles are the most important parasite group infecting equids. Management of these parasites has relied on intensive use of anthelmintics, however, resistance has developed against all drug classes and is becoming a major practical problem in many countries. Resistance to the benzimidazole (BZ) group is geographically widespread and resistance to pyrental has also been reported. Today the macrocyclic lactones (ML) class of drugs has become the most commonly used drug, but evidence of emerging resistance (i.e. shortened egg reappearance period (ERP)) has been identified in many countries. A variety of tests are available to monitor anthelmintic efficacy but most of them are expensive, laborious and time consuming. The aim of this project was to determine the efficacy of anthelmintic drugs used in eight equine groups in Ireland. The anthelmintic efficacy was determined by calculating the percentage reduction in the faecal egg count (FEC) between the group mean at Day 0 and Day 14 post-treatment (FECRT). In addition FECs were also calculated at two week intervals for up to 16 weeks after anthelmintic drug administration to determine the ERP for BZ, ivermectin and moxidectin. ERP was defined when the group arithmetic mean FEC exceeded 10% of the group arithmetic mean FEC at Day 0.The larval development assay (LDA) was used to detect resistance to BZ in two groups of horses and the larval migration inhibition assay (LMIA) was also performed to measure the sensitivity to ivermectin in two groups of donkeys and moxidectin in two horse farms and two groups of donkeys. The results of FECRT indicate BZ-resistance on both farms; FECR of 86% and 61%, an ERP of only two weeks and the EC50 for the LDA of 0.3 and 0.7 µg/ml, respectively. While MLs were still effective in all cases with a FECR >95% and the EC50 for the LMIA ranging from 0.06 to 0.38 µg/ml the ERP ranged from only 4 to 10 weeks. Overall the results from this study indicate that BZ was ineffective but both ivermectin and moxidectin are still effective in all groups. However, the reduced ERP results for the MLs would suggest that these products are less effective compared to label claims - a shortened ERP is believed to be an early indicator of resistance.

Not published as conference proceedings
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