Recent research suggests donkey lice may be developing resistance to commonly used treatments.
Lauren Ellse, Faith Burden and Richard Wall studied the value of pyrethroid-based insecticides on a population of donkey lice. They found a high level of pyrethroid tolerance in the lice tested, and suggested this was likely to reflect the development of resistance.
In laboratory tests, they assessed the effect of cypermethrin* and permethrin** on chewing lice (Bovicola (Werneckiella) ocellatus) collected from donkeys.
They compared its effectiveness with that of diazinon, an organophosphate compound to which the lice were unlikely to have been exposed previously.
Neither permethrin, nor cypermethrin, (at concentrations recommended for use on animals) had any significant effect on mortality of B. ocellatus, the latest issue of Equine Science Update reported. Combining cypermethrin and permethrin with piperonyl butoxide, which may enhance their activity, did not make them any more effective.
In contrast, 0.04 per cent diazinon caused 70 per cent mortality within 4 hours and 100 per cent mortality after 24 hours exposure.
The researchers also found that the pour-on permethrin product was poorly distributed around the body. Twenty-four hours after routine treatment of 10 donkeys with the product , they collected hair tufts from the middle of the back and from the flanks. The tufts taken from the flanks were not significantly insecticidal compared with those taken from the midline where the permethrin was applied.
They comment that exposing lice to sub-lethal doses risks encouraging the development of resistance.
Another study raised the possibility of using essential and non-essential oils in the control of biting lice.
Rose Talbert and Richard Wall examined the toxicity of six plant essential oils to the chewing louse, Bovicola (Werneckiella) ocellatus collected from donkeys.
The six oils assessed were: tea-tree (Melaleuca alternifolia), lavender (Lavandula angustifolia), peppermint (Mentha piperita), eucalyptus (Eucalyptus globulus Labillardiere), clove bud (Eugenia caryophyllata) and camphor (Cinnamomum camphora).
Talbert and Wall found that all oils except camphor showed high levels of toxicity. Fifty per cent mortality was achieved at concentrations below 2 per cent (w/v). Concentrations of 5-10 per cent resulted in 100 per cent mortality. Two essential oil components: eugenol and (+)-terpinen-4-ol, showed similar levels of toxicity.
They suggest that these botanical products may offer environmentally and toxicologically safe, alternative veterinary treatments for the control of ectoparasitic lice.
*DeosectTM, Pfizer Ltd., 5% (w/v) cypermethrin
**SwitchTM, VetPlus Ltd., 4% (w/v) permethrin
Pyrethroid tolerance in the chewing louse bovicola (werneckiella)ocellatus. Veterinary Parasitology.
Lauren Ellse, Faith A. Burden, Richard Wall.
Veterinary Parasitology (2012) 188, 134-139
Toxicity of essential and non-essential oils against the chewing louse, Bovicola (Werneckiella) ocellatus
R. Talbert, R. Wall
Research in Veterinary Science, (2012) 93, 831–835