Pyrethroid tolerance in the chewing louse Bovicola (Werneckiella) ocellatus

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 (Deosect?, Pfizer Ltd., 5% (w/v) cypermethrin, cutaneous spray) and permethrin (Switch?, 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 4h and 100% mortality after 24h exposure. The addition of a potential pyrethroid synergist, piperonyl butoxide, in combination with 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 (Switch?, 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.     

Confirmation of the occurrence of the chewing louse Bovicola (Lepikentron) breviceps (Insecta: Phthiraptera: Trichodectidae) on alpacas (Lama pacos) in New Zealand

Abstract

AIM: To investigate the cause of classical swine fever (CSF) virus-seropositive animals in a nucleus pig-breeding herd in New Zealand, where porcine circovirus-associated disease had been diagnosed.

CASE HISTORY AND CLINICAL FINDINGS: An exotic disease investigation was undertaken to exclude CSF and porcine reproductive and respiratory syndrome (PRRS) on a nucleus pig-breeding herd comprising approximately 300 breeding sows, 1,000 weaners, and 650 grower pigs. The herd was experiencing poor reproductive performance in sows, and breeding records showed a declining farrowing rate attributable to a single manager. The growing pigs (10–15 weeks old) were experiencing respiratory disease and wasting, and the mortality rate by pen varied between 9 and 20%. Post-mortem changes in affected grower pigs were consistent with circovirus-associated diseases.

DIAGNOSTIC TESTING: Serological screening using an IDEXX-ELISA gave negative results for PRRS virus antibodies, but two grower pigs and one sow tested positive for CSF virus antibodies. These three seropositive animals remained positive to CSF virus, using three commercial ELISA test kits, over 27 weeks. A newly developed virus neutralisation test (VNT), using a New Zealand isolate of border disease (BD) virus, demonstrated that the seropositive pig sera had higher antibody titres to BD virus than to bovine viral diarrhoea (BVD) virus and CSF virus.

PCR performed on tonsil, kidney, ileum and spleen gave negative results for CSF virus, and histopathology on lymph nodes, intestine, lung, kidney, liver and brain showed no evidence of the disease. Virus isolation performed on a number of samples was negative.

CLINICAL RELEVANCE: The seropositive samples for CSF virus found in this investigation were likely to be a cross reaction to a pestivirus other than CSF virus. The finding of a possible endemic pestivirus capable of being transmitted between sheep and pigs on this farm may explain findings from previous serological survey work in New Zealand, and supports experience elsewhere, where BD virus was found to be the predominant ruminant pestivirus infecting pigs. The results show that pestivirus cross reactivity can result in unexpectedly high titres, and that testing with a full set of (local) pestiviruses is necessary to reach the correct conclusion. The investigation has direct relevance where pig herds with a low seroprevalence are encountered during surveillance for CSF.     

Hypersensitivity skin testing of lambs infested with the biting louse (Bovicola ovis)

Abstract Cockle, a defect of lamb's pelts, has recently been shown to be associated with infestation of lambs with the louse Bovicoh ovis. The mechanisms by which infestation with the louse leads to cockle are not known, but immunological hypersensitivity has been suggested. The objectives of our work are to determine if lambs respond immunologically to lice and, if so, whether hypersensitivity occurs. Preliminary results are presented here.     

Hypersensitivity skin testing of lambs infested with the biting louse (Bovicola ovis)

Cockle, a defect of lamb's pelts, has recently been shown to be associated with infestation of lambs with the louse Bovicoh ovis. The mechanisms by which infestation with the louse leads to cockle are not known, but immunological hypersensitivity has been suggested. The objectives of our work are to determine if lambs respond immunologically to lice and, if so, whether hypersensitivity occurs. Preliminary results are presented here.     

Persistence of the sheep body louse, Bovicola ovis, after treatment

SUMMARY Lice were found in samples of baled wool from 198 of 464 flocks treated to eliminate an Infestation in the period July 1988 to June 1990. In 287 flocks the Insecticide was applied as a backline treatment and in 177 flocks a shower dip was used. Of these flocks, 41.5% and 44.6%, respectively, were found to be infested at the following shearing. After adjusting for the accuracy of the test, It was estimated that infestation in 2 consecutive 12-month periods occurred in 52.6% of flocks with a real incidence rate of 27.4%. Using these estimates, it was calculated that in 34.7% of infested flocks treatment did not kill all lice. There were no differences in the proportion of consecutive infestations between flocks treated with any of the 3 chemicals applied as backline treatments. Among flocks treated In a shower dip, 68.4% using coumaphos, 37.8% using diazlnon and 41.5% using cyhalothrin had consecutive Infestations (P < 0.05). The detection of lice in 63.2% of flocks that were treated with magnesium fluosilicate was, in part, attributed to the application of this chemical by one operator.     

Feeding of the chewing louse Damalinia ovis (Schrank) (Phthiraptera:Trichodectidae) on sheep

The possibility that Damalinia ovis ingests intact epidermis could explain economically important irritation behaviour by infested sheep. To test this, feeding lice were observed on sheep by hand lens and macrophotography and on light and scanning electron microscope preparations. Aspects of feeding behaviour were described, particularly an epidermal ingestion posture. Louse ingesta were examined in paraffin and frozen sections. Lipid-covered stratum corneum squames were identified in crop, midgut, rectum and faeces. Nucleated keratinocytes from inner epidermal strata were not seen, neither were wool fibres. Vertical frozen sheep skin sections, with feeding lice cryofixed by liquid nitrogen, showed mandibles engaged in the outer stratum corneum of the epidermis. The epidermal origin of some ingesta was confirmed by confirming lice over sheep whose skin was stained with Oil Red O, whose colour was seen in crop, midgut, rectum and faeces of harvested lice. Lipase was found in louse midgut. It was concluded that sebaceous secretions may form an important component of the D. ovis diet. The relative contribution from loose scurf or from stratum corneum squames was not determined. Mechanical stimuli by feeding lice did not seem to be the source of host irritation. The role of sensitising agents in louse brei filtrates was investigated with inconclusive results. Intradermal injections of louse faeces did not elicit a demonstrable response.     

Treatment of natural infestations of the biting louse (Werneckiella equi) on horses using triflumuron, a benzoylurea derivative insect growth regulator

The horse biting louse (Werneckiella equi) is a common global equine ectoparasite. To our knowledge, benzoyl(phenyl)urea insecticides (triflumuron, diflubenzuron) commonly used as sheep lousicides, have not been evaluated for efficacy against W. equi. The aim of this study was to determine louse control efficacy, general wellness and dermal safety following triflumuron application as a backline pour-on to horses. Two efficacy trials using 25 adult naturally infested lousy horses, and a dermal safety trial using 10 adult louse-free horses were conducted over a 14-month period. Lousy animals were selected by assessment of their lice status prior to treatment. For the efficacy trial, the triflumuron product was applied at a dose of 2.5mg triflumuron per kg bodyweight (1 mL product per 10 kg bodyweight). For the safety study, triflumuron was applied at a 3x clinical dose of 7.5 mg triflumuron per kg bodyweight (3 mL product per 10 kg bodyweight). In our first efficacy trial, 100% lousicidal efficacy was achieved by day 44 post-treatment. In our second trial, no lice were identified on horses by day 71 post-treatment. In the safety trial, no adverse effects were seen. Results of this study demonstrate that the off-label, experimental pour-on application of triflumuron at 2.5 mg/kg bodyweight is convenient, highly effective and safe (at 3x the clinical dose) for the treatment of the horse biting louse, W. equi.     

Resistance to synthetic pyrethroid pour-on insecticides in strains of the sheep body louse Bovicola (Damalinia) ovis

Groups of sheep infested with strains of Bovicola (Damalinia) ovis were obtained from flocks either with a history of failure to control lice with synthetic pyrethroid (SP) pour-on insecticides, or from farms where SP compounds were not used. The sheep were treated according to the manufacturer's recommendations with registered "off-shears" SP formulations. All treatments were applied under ideal conditions with doses calculated on an individual body weight basis and applied to the dorsal mid-line from the base of the neck to the butt of the tail. Treated sheep were kept in pens and maintained in separate groups. The pour-on SP treatments significantly reduced the lice population but failed to eliminate the infestation in 7 of 13 experiments in sheep carrying strains of lice with resistance factors of greater than 4 to at least one of the SP compounds. Failures occurred with all three of the SP pour-ons currently registered for lice control in NSW and with both water-based and organic solvent-based formulations.     

Control of the cattle louse Bovicola bovis with the fungal pathogen Metarhizium anisopliae

The effects of the entomopathogenic fungus Metarhizium anisopliae (Metschnikoff) were evaluated against the common louse parasite of cattle, Bovicola bovis (Piaget) (Trichodectidae: Ischnocera). Two different concentrations and formulations of conidial suspensions were applied to contained populations of adult female lice. In vitro, lice immersed in suspensions of M. anisopliae formulated in 0.03% Tween 80 developed infections; at the highest concentration (1x10(8)conidia ml-1) a mean of 71% (+/-11.52%, 95% C.I.) of lice became infected. Lice exposed to the Tween 80 only in vitro, showed high levels of survival and zero infection. In vivo, fungal conidia were applied to louse populations contained in 7 cm diameter circular arenas glued to the backs of Holstein cattle, maintained in controlled climate conditions. Conidia were formulated in either Tween 80 or silicone oil. The treatment with M. anisopliae resulted in high levels of infection and there was no overall difference between the two formulations in the number of infections observed. At the highest concentration (1x10(8)conidia ml-1) a mean of 73% (+/-15.57%, 95% C.I.) lice became infected. It is concluded that the strategic seasonal use of a fungal pathogen on cattle, applied in early winter, may be of value in suppressing the winter increase in abundance, preventing the population increasing to clinically significant levels.     

Acaricidal activity of plant essential oils against Dermanyssus gallinae (Acari: Dermanyssidae)

The acaricial activity of 56 plant essential oils against poultry house-collected adult Dermanyssus gallinae De Geer was examined using direct contact and fumigation methods. In a filter paper contact bioassay, 100% mortality at 0.07 mg cm⁻² was observed in bay, cade, cinnamon, clove bud, coriander, horseradish, lime dis 5F, mustard, pennyroyal, pimento berry, spearmint, thyme red and thyme white oils, whereas the mortality of these oils was significantly decreased at 0.02 mg cm⁻². In fumigation tests with adult D. gallinae at 0.28 mg cm⁻², cade, clove bud, coriander, horseradish and mustard oils were more effective in closed containers than in open ones, indicating that the effect of these essential oils was largely due to action in the vapour phase. Plant essential oils described herein merit further study as potential D. gallinae control agents.     

A preliminary investigation into resistance to synthetic pyrethroids by the sheep biting louse (Bovicola ovis) in New Zealand

Increasing reports of failure of synthetic pyrethroids for the control of Bovicola ovis on sheep lead to a preliminary investigation into the insecticide resistance status of this pest during 1994. Eighteen strains of lice from North and South Island flocks experiencing failure of synthetic pyrethroid treatments to control lice were exposed to cypermethrin (high-cis) in a treated surface (contact) bioassay. Lice from a group of hermit sheep were chosen as a reference susceptible strain against which resistance of field strains was measured and resistance factors (RFs) calculated. Low to moderate resistance to cypermethrin (high-cis) was demonstrated, with RFs ranging from 1.0 to 12.4.     

An evaluation of two cypermethrin-based pour-on formulations on sheep infested with the biting louse, Bovicola ovis

Two synthetic pyrethroid (cypermethrin) based pour-on insecticide formulations, with high cis/trans isomer ratios (80:20) but differing in their respective active ingredient concentrations and solvent component(s), were applied to sheep infested with the biting-louse, Bovicola ovis. All treated sheep were penned with louse-infested sheep 9,12 and 15 weeks after the insecticide was applied. The 2% cypermethrin formulation achieved a higher level of control than the 1.25% cypermethrin formulation at each challenge interval when applied 12 weeks after shearing. The 2% cypermethrin formulation provided 97-100% control of lice from 4 to 16 weeks after application on sheep shorn 6 or 12 weeks prior to treatment. The 1.25% cypermethrin formulation provided 85% control of lice 4 weeks after application on sheep shorn 12 weeks prior to treatment, the level of control increasing to a maximum of 100% by week 9, and declining thereafter. The 2% cypermethrin formulation may provide a better level of control in long-woolled sheep than 1.25% cypermethrin, by compensating for the diluent effect of lipid.     

Infestation with the sheep body louse (Bovicola ovis) in Merino lines divergently selected for maternal multiple rearing ability

Data were obtained from a population consisting of 160 Merino ewes, as well as 64 male and 75 female progeny of these ewes at the 2-tooth age. The age and sex groups were maintained in separate flocks. The population has been divergently selected from the same base since 1986, either for (high or H line) or against (low or L line) maternal multiple rearing ability. All animals were inspected for sheep lice (Bovicola ovis) during September/October 2002 after a wool growth period of at least 4 months. When expressed relative to mean L-line performance, the advantage in lambs weaned per ewe during the lambing season amounted to approximately 110% in the H line (1.01 vs 0.48; P < 0.01). The proportion of animals on which 1 or more lice were observed were markedly lower in 2-tooth replacement ewes than in replacement rams and mature breeding ewes (0.053 vs 0.625 and 0.531, respectively; P < 0.01). One or more lice were observed on a lower proportion of H-line ewes than on those of the L line (0.413 vs 0.571, respectively; P < 0.10), while a similar tendency was observed in 2-tooth ewe hoggets (0.033 vs 0.143, respectively; P = 0.16). When the mean number of lice on individuals on which 1 or more lice were observed was compared between lines, the average number of lice observed on H-line young rams was lower than in the L line (4.36 +/- 0.85 vs 9.71 +/- 1.84 lice, respectively; P < 0.01). A similar tendency was observed in mature breeding ewes (2.15 +/- 0.38 vs 3.22 +/- 0.42 lice, respectively; P < 0.20). The more highly reproductive H-line ewes were not more susceptible to infestation with B. ovis than ewes of the L line. In fact, available evidence suggest that H-line animals could be more tolerant of B. ovis than those of the L line.     

Increased control of the sheep biting louse Bovicola (Damalinia) ovis with deltamethrin formulated in a fractionated wool grease carrier

The synthetic pyrethroid deltamethrin (DM) containing a trace of [(14)C]-DM was formulated with non-oxidised sterol and wax ester fractions (F1) of wool grease and as the commercial preparation 'Clout-S'. These were applied as a 'backline' strip to sheep immediately after shearing and the concentration of [(14)C]-DM at meridians adjacent to the application strip and at 1/4 and 3/4 of the dorsal-ventral distance was determined. The F1 formulation resulted in significantly greater lateral spread of DM with less remaining at the application site (66+/-8% of dose) 98 days after treatment compared to 'Clout-S' (94+/-3% dose). Autoradiographic examination of treated wool demonstrated that there was more DM in the lower half of the wool staple when formulated in F1 compared to 'Clout-S'. Greater mortality occurred when sheep biting lice Bovicola (Damalinia) ovis were exposed in vitro to wool containing DM from F1 compared to 'Clout-S' treated sheep. In field trials there was increased efficacy against synthetic pyrethroid resistant B. ovis with F1 formulation than with 'Clout-S'. The study has demonstrated that synthetic pyrethroid availability, and therefore efficacy, can be significantly increased when the insecticide is formulated in a 'carrier' with the physicochemical characteristics of wool grease.     

Field efficacy of an ivermectin jetting fluid for control of the sheep body louse Bovicola (Damalinia) ovis in New Zealand

The efficacy of an ivermectin jetting fluid against the sheep body louse, Bovicola (Damalinia) ovis, in long-woolled sheep was investigated in three field trials. In all trials, louse populations in hand-jetted sheep were reduced to non- detectable numbers within 2 weeks of treatment. By comoarison, shorn and unshorn control sheep maintained detectable infestations for the duration of the trials.