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

Research award
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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.
  1. Detected high levels of tolerance to permethrin ((SwitchTM, VetPlus Ltd, 4% (w/v) and cypermethrin (DeosectTM, Pfizer Ltd., 0.1% (w/v) consistent with resistance development.
  2. A strong seasonal pattern of louse infestation was identified, with the highest number of donkeys with lice recorded in winter (Oct-Mar). Significantly more very young (<4 yrs) and very old (>30 yrs) donkeys had lice than middle aged animals.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). 
  3. 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 . Excoriation consistent with hair fibre shortening was indicative of pediculosis and the amount of this type of rubbing a donkeys coat displayed was positively correlated with louse burdens. However, more severe dermal rub lesions, such as alopecia, showed no association.
  4.  Clipping was found to have no significant effect (P=0.15) on louse abundance 2 weeks or more after clipping in the winter;  however, in the summer months clipping did have a detrimental effect on louse populations.
  5. Tea tree and lavender essential oils were investigated and identified as clinically and statistically significant (P<0.05) methods of louse control used at 5% concentration as a topical grooming spray.

The population of lice at the sanctuary have a high level of tolerance to the pyrethroid insecticides previously used for their control. In vitro contact assays showed that 4% permethrin and cypermethrin resulted in <30% louse mortality after 24 h exposure. This level of insecticide tolerance may account for the treatment failures reported at the Sanctuary in recent years and highlights the need for alternative louse control strategies.

Detection of lice can be aided by targeting animals most at risk from infestation. Indeed, this study found a convex quadratic relationship between donkey age and probability of carrying lice, with the elderly and young most at risk. 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.

As is the case with many species of Phthiraptera in temperate climates, the prevalence and abundance of Bovicola ocellatus displays a strong seasonal pattern, with >80% of animals infested in the winter months. However, a year round management protocol maybe advisable as summer clipping appears to have a detrimental effect on louse populations. 

 In addition, these preliminary findings indicate 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.