Insecticide resistance in lice at The Donkey Sanctuary: strategies to mitigate its effects

Research award
Start date
End date

Prospective questionnaire and sample collection, and in vitro laboratory assay and analyses.

  1. To identify the presence of insecticide resistance in donkey lice.
  2. To identify risk factors for pediculosis.
  3. To develop a useful tool for diagnosing and estimating louse burden.
  4. Developing targeted selective treatment.
  5. Develop novel methods for controlling lice in donkeys.

A strong seasonal pattern of louse infestation was observed, with the highest number of donkeys infested in winter (Oct-Mar); with more than 80% of the animals were infested in the winter months. A significant age variation in lice infestation was also observed; young and old donkeys of age less than 4 years and greater than 30 yrs, respectively, were found infested with more lice than middle aged animals. The axilla and supraorbital fossa were identified as being the sites most commonly populated by live lice in both housed and outdoor donkeys (P<0.05). In addition, the presence of lice eggs in the first 2cm of the coat is a good indicator of an active infestation. Donkeys’ hair length was positively correlated with the presence of lice (P<0.05) but not with the number of lice each donkey carried (P=0.1). Excoriation consistent with hair fibre shortening was indicative of pediculosis and the amount of this type of lesion was positively correlated with louse burdens. However, more severe dermal rub lesions, such as alopecia, showed no association. Clipping in the winter was found to have no significant effect (P=0.15) on louse abundance examined after 2 or more weeks clipping; however, in the summer months clipping had a detrimental effect on louse populations. The efficacy study revealed a high level of tolerance to permethrin ((SwitchTM, VetPlus Ltd, 4% (w/v) and cypermethrin (DeosectTM, Pfizer Ltd., 0.1% (w/v) consistent with resistance development. In vitro contact assays showed that 4% permethrin and cypermethrin resulted in less than 30% louse mortality after 24 h exposure. On the other hand, tea tree and lavender essential oils were identified as clinically and statistically significant (P<0.05) methods of louse control used at 5% concentration as a topical grooming spray.


The study showed high lice infestation in donkeys, season and age of the animals being the main risk factors for having high infestation. A convex quadratic relationship between donkey age and probability of carrying lice was found, with the elderly and young were at higher risk. Detection of lice can be aided by targeting animals most at risk of infestation. In addition, excoriation consistent with light coat abrasion is an indicator of louse presence. However, overreliance on donkey characteristics and appearance is likely to lead to misdiagnosis of pediculosis. For accurate detection, examination of the coat should be thorough and include the most commonly infested louse predilection sites namely the axilla and supraorbital fossa. Although more animals are found infested in winter months, a year round management protocol may be advisable, as summer clipping appears to have a detrimental effect on louse populations.

The insecticide efficacy trial showed that the population of donkey lice at the Donkey Sanctuary developed a high level of tolerance to the pyrethroid insecticides previously used for their control. Such a high level of tolerance may account for the treatment failures reported at the Sanctuary in recent years and highlighted the need for alternative louse control strategies. The preliminary study made to address this indicated that tea tree and lavender essential oils provide a clinically useful level of B. ocellatus control and with further research and refinement they could form the basis of future louse control regimes in donkeys.