Parasiticide resistance in flies, lice and ticks in New Zealand and Australia: mechanisms, prevalence and prevention.

This review outlines the history of parasiticide resistance in the principal ectoparasites of sheep and cattle in New Zealand and Australia, blowflies, buffalo fly (Haematobia irritans exigua), sheep biting louse (Bovicola ovis) and cattle ticks, and discusses recent changes in their response to insecticides and acaricides. Mechanisms of resistance and evaluation methods are described, with ways in which insecticide resistance can be avoided or ameliorated also discussed. Resistance in sheep blowflies (Lucilia cuprina; L. sericata) to organophosphates and benzoylphenyl urea compounds is widespread in Australia and New Zealand, but there are lesser concerns about a pyrimidine carbonitrile product as well as cyromazine and macrocyclic lactone actives which still offer the promise of long-term protection. In Australia the effectiveness of synthetic pyrethroid and benzoylphenyl urea products against the biting louse of sheep have been reduced by widespread resistance but effective temephos, macrocyclic lactone, imidacloprid and spinosyn-based products are now available. Pyrethroid-resistant sheep lice are also present in New Zealand. Buffalo fly remains a significant problem in Australia and control relies heavily on insecticide use. Resistance to synthetic pyrethroids is widespread and organophosphate resistance common, but less evenly distributed. There is no evidence of resistance to acaricides used against the New Zealand cattle tick, Haemaphysalis longicornis and experience in Japan with the most commonly available active, flumethrin (a synthetic pyrethroid), suggests that this three-host tick is, in the short term, likely to remain susceptible. The same cannot be said for Rhipicephalus australis (formerly Boophilus microplus) which, in some strains, is highly resistant to many of the active ingredients in the acaricides used against it. A formamidine, a benzoylphenyl urea and macrocyclic lactones still show efficacy although some resistance is being detected to macrocyclic lactones in a few strains and the occurrence of resistance to the benzoylphenyl urea active in Brazil is ominous for Australian producers. Insecticide resistance is a price paid for improved livestock health, welfare and cost-effective animal production, but it can be managed through judicious and targeted use of insecticides, consideration of refugia, and observance of the principles of integrated pest management. The use of traps, biological control agents and judicious husbandry all have a role to play, but strict adherence to the correct application of insecticides and recognition of their capabilities and limitations is also required. Alternative methods of ectoparasite control and recommendations for their integration into preventative programmes are discussed briefly in this review.

Survival advantage of cyromazine-resistant sheep blowfly larvae on dicyclanil- and cyromazine-treated Merinos.

OBJECTIVE: To determine whether a cyromazine-resistant strain of Lucilia cuprina was able to establish strikes sooner than a susceptible strain on cyromazine- or dicyclanil-treated sheep. METHODS: Groups of 7 sheep were treated with cyromazine or dicyclanil in accordance with label directions. Beginning 5 weeks after treatment, 5 sheep from each group were challenged by implantation of neonate larvae belonging to the cyromazine-resistant strain 'Nimmitabel-selected' and the susceptible blowfly strain 'Field 2011' according to standard larval implant technique. The implant sites were alternated between the shoulder, mid-back and rump within the treatment zone. Similarly, implants of the resistant and susceptible strain larvae were alternated at each challenge such that the strain implanted on the left side of the sheep at one challenge was implanted on the right side at the next. Challenges were conducted at 3-weekly intervals until the susceptible larvae formed strikes on at least 2 of the 5 sheep in a treatment group or until 29 weeks after treatment. RESULTS: Sheep treated with cyromazine or dicyclanil were protected from flystrike by the cyromazine-susceptible strain for periods consistent with, or longer than, the registered product label claims. The cyromazine-resistant strain created strikes several weeks sooner after treatment than did the susceptible strain. Accordingly, the protection periods provided by cyromazine and dicyclanil against the resistant strain were reduced from 14 and 18-24 weeks to <8 weeks and <11 weeks, respectively. CONCLUSION: Resistance, even in the pure-breeding resistant strain, was not so severe as to cause treatment failure with cyromazine or dicyclanil, but was sufficient to reduce the protection period provided. It is recommended that producers adopt management practices that minimise the development of resistance to these and other compounds.

Control of the sheep blowfly in Australia and New Zealand--are we there yet?

The last 50 years of research into infections in Australia and New Zealand caused by larvae of the sheep blowfly, Lucilia cuprina, have significantly advanced our understanding of this blowfly and its primary host, the sheep. However, apart from some highly effective drugs it could be argued that no new control methodologies have resulted. This review addresses the major areas of sheep blowfly research over this period describing the significant outcomes and analyses, and what is still required to produce new commercial control technologies. The use of drugs against this fly species has been very successful but resistance has developed to almost all current compounds. Integrated pest management is becoming basic to control, especially in the absence of mulesing, and has clearly benefited from computer-aided technologies. Biological control has more challenges but natural and perhaps transformed biopesticides offer possibilities for the future. Experimental vaccines have been developed but require further analysis of antigens and formulations to boost protection. Genetic technologies may provide potential for long-term control through more rapid indirect selection of sheep less prone to flystrike. Finally in the future, genetic analysis of the fly may allow suppression and perhaps eradication of blowfly populations or identification of new and more viable targets for drug and vaccine intervention. Clearly all these areas of research offer potential new controls but commercial development is perhaps inhibited by the success of current chemical insecticides and certainly requires a significant additional injection of resources.

Response to laboratory selection with cyromazine and susceptibility to alternative insecticides in sheep blowfly larvae from the New South Wales Monaro.

OBJECTIVE: To determine the significance of cyromazine resistance in blowfly larvae from a region where resistance has been previously confirmed and to measure susceptibility of the cyromazine-resistant blowflies to ivermectin and spinosad. METHODS: Blowfly larvae from five properties located within 5 km of where cyromazine-resistant Lucilia cuprina were detected in 2011 were tested for resistance to cyromazine. The original cyromazine-resistant 'Nimmitabel' strain was reared for 13 generations on homogenised liver containing cyromazine at a concentration lethal to susceptible larvae. Larval development bioassays in which batches of neonate larvae were transferred onto homogenised bovine liver containing cyromazine or dicyclanil were used to determine whether the level of resistance had increased. The sensitivities of the 'Nimmitabel' strain to ivermectin and spinosad were also measured. RESULTS: Cyromazine-resistant larvae were identified in samples from four of the five properties near to the site of the original resistance detection. The 'Nimmitabel' strain responded to laboratory selection by becoming more resistant to cyromazine (8×) and to dicyclanil (3×). No larvae died after feeding on homogenate containing cyromazine at a concentration lethal to susceptible larvae. There was no cross-resistance to ivermectin or spinosad in the 'Nimmitabel' strain. CONCLUSION: The modest increases in resistance to cyromazine and dicyclanil following laboratory selection suggest that unless the situation worsens considerably, it is unlikely that this resistance will have a significant effect on flystrike protection provided by these insecticides if applied as directed on product labels. Moreover, cyromazine-resistant larvae remained susceptible to ivermectin and spinosad.

Cyromazine resistance detected in Australian sheep blowfly.

OBJECTIVE: To investigate the cause of cyromazine failure to protect lambs from flystrike. METHODS: Lucilia cuprina larvae from a Nimmitabel (New South Wales) population associated with failure of a cyromazine spray-on to protect lambs from flystrike were compared with larvae from a susceptible field strain and a reference susceptible laboratory strain in laboratory bioassays. Batches of neonate blowfly larvae were transferred onto homogenised bovine liver containing varying concentrations of cyromazine or dicyclanil and the numbers of larvae pupating and completing development were recorded. RESULTS: Based on the ability of larvae to complete development on liver homogenate containing 1 mg/kg cyromazine, the phenotypic frequency of resistance in the Nimmitabel population was estimated to be approximately 4%. Compared with a susceptible field strain, the Nimmitabel population was 3-fold more resistant to cyromazine and twice as resistant to dicyclanil at the LC95 level (lethal concentration killing 95% of larvae). In the laboratory, the Nimmitabel strain responded to sequential exposure of larvae to food containing cyromazine by becoming more resistant. Resistance to cyromazine was incompletely dominant, giving resistant larvae a survival advantage over susceptible types over a relatively narrow range of cyromazine concentrations. CONCLUSION: Cyromazine resistance was detected in a field population of L. cuprina. Low-level cross-resistance to dicyclanil was also confirmed. Until more is known about the resistance, the prudent recommendation to control flystrike by this blowfly population is topical treatment with ivermectin.

Effectiveness of a non-surgical alternative to the Mules operation in sheep.

OBJECTIVE: To measure changes to the perineal bare area, local tissue reaction and healing responses of young sheep, following intradermal administration of cetrimide and polyvinylpyrrolidone (PVP), with and without ethanol, to the breech and tail. METHOD: A needle-less injector was used to deposit formulations containing 40 g/L cetrimide and 30 g/L PVP (group 2) or 20 g/L cetrimide, 30 g/L PVP and 15 g/L ethanol (group 3), within the dermis of the tail and the region surrounding the perineal bare breech area of groups (N = 8) of Merino weaner sheep. The dimensions of the perineal bare area (length, width and diagonal distances left and right) and tail width were recorded before and at intervals after treatment for 60 days. Observations of swelling and bruising and scab formation at the treatment sites were recorded for up to 35 days after treatment. Rectal temperatures were monitored for up to 35 days after treatment and bodyweight for up to 60 days after treatment. An untreated control group (group 1) was included. RESULTS: Comparison of day -3 and day 35 measurement data showed that both treated groups had significantly (P < 0.05) wider breech bare areas compared to the untreated controls and that group 2 sheep had significantly (P < 0.05) longer breech bare areas compared to group 3 sheep or to the untreated controls, which were not significantly different. At this time scabs were still firmly in place on many treated sheep. At day 35 there was no increase in tail bare area caused by either treatment. By day 60 there was no significant difference between the treated and control groups in either the breech or tail regions indicating that the changes present at day 35, were not permanent. Mean weight gain in the groups throughout the 60-day interval was unaffected by treatment. Intradermal treatment was associated with a significant elevation in body temperature. This effect lasted for 3 days and was associated with signs of discomfort and depressed appearance in at least some of the treated sheep. Bruising was mild to severe in all treated sheep within two days of treatment but was not evident in any sheep by day 21. Mild to moderate swelling was also associated with treatment but was not uniform across sheep in the groups. The tail of one sheep was severely swollen for several days. Swelling remained obvious in most treated sheep until day 14 but was not present at day 21. CONCLUSION: Under the conditions of this study intradermal injection of cetrimide had no permanent effect on bare area measurements on the breech or the amount of wool-bearing skin on the tail. It also caused signs of discomfort and pain that raise welfare concerns.

Control of endoparasites in horses with a gel containing moxidectin and praziquantel.

A gel formulation containing moxidectin (20 g/kg) and praziquantel (125 g/kg) reduced the geometric mean faecal strongyle egg count in horses to below 100 eggs per gram of faeces (epg) for at least 12 weeks despite their being exposed continuously to reinfection from pasture grazed by treated and untreated horses. The geometric mean egg count of horses treated with a proprietary paste containing abamectin (3.7 g/kg) and praziquantel (46.2 g/kg) increased steadily from six weeks after the treatment, peaking at over 820 epg after 12 weeks. Relative to the efficacy of the abamectin/praziquantel treatment, the reduction in mean faecal egg count compared with the pretreatment counts was significantly (P<0.05) better in the horses treated with moxidectin and praziquantel from eight weeks after the treatment. Both products eliminated tapeworms from horses in a non-invasive modified critical trial.

Effectiveness of a mixture of cyromazine and diazinon for controlling flystrike on sheep.

OBJECTIVE: To determine whether there is any need for woolgrowers to add diazinon to cyromazine jetting fluid to control active flystrike on sheep. METHOD: Larval implants were placed on 143 sheep and allowed to develop for 2 days. Groups of 47 or 48 of these struck sheep were jetted with cyromazine at 1000 mg/L, diazinon at 400 mg/L, or a mixture of the two. After treatment, the implants were assessed daily for 3 days for the presence of feeding larvae. RESULTS: When checked 1 day after treatment, there was a 62% reduction in flystrike in the diazinon treated sheep compared to only 12% reduction on the cyromazine treated sheep. Flystrike reduction on the sheep jetted with the mixture of cyromazine and diazinon was 69%. Three days after treatment flystrike control was 97 to 99% in the latter two groups. CONCLUSION: Diazinon provided a faster resolution of strikes but by 3 days after treatment there was no significant difference in the number of implants remaining active on the sheep treated with cyromazine or with the mixture of cyromazine and diazinon.

Increased susceptibility to Bacillus thuringiensis associated with pyrethroid resistance in Bovicola (Damalinia) ovis (Phthiraptera Mallophaga: possible role of monooxygenases.

A comparison of the toxicity of Bacillus thuringiensis (Berliner) to pyrethroid-susceptible and -resistant strains of Bovicola (Damalinia) ovis showed that B. thuringiensis and pyrethroid toxicity were inversely related. The B. ovis strain with an apparent monooxygenase-mediated pyrethroid resistance mechanism was significantly more susceptible to B. thuringiensis. Susceptibility to B. thuringiensis in both the pyrethroid-susceptible and -resistant strains was significantly reduced after administration of the monooxygenase inhibitor piperonyl butoxide. Susceptibility to B. thuringiensis in the pyrethroid-susceptible strain was significantly enhanced after administration of a monooxygenase inducer, sodium phenobarbital. These results suggest that monooxygenases may be important in increasing the toxicity of B. thuringiensis against B. ovis. We discuss the potential use of B. thuringiensis to control pyrethroid-resistant lice.

Resistance and the control of sheep ectoparasites.

For about 100 years Australian woolgrowers have used a variety of chemicals to control blowflies, lice and other ectoparasites of sheep. While the chemicals have changed considerably the application technology has not changed very much at all. Chemicals registered for use on sheep have paralleled the evolution of synthetic insecticides with the unfortunate consequence of the development of resistance in the Australian sheep blowfly, Lucilia cuprina, following closely behind. Organochlorine (dieldrin) resistance peaked at about 70% in 1958 when unacceptable residues in meat and wool forced their withdrawal. Organophosphate (OP) resistance appeared in 1965. With no alternative insecticide classes until 1979, OP resistance reached near fixation levels by the early 1970s and has remained unchanged. OP resistance has reduced the protection period from over 16 weeks to about 6 weeks. Moreover, resistance has decreased the effectiveness of many flystrike dressings to unacceptably low levels. OPs are still very effective against sheep body lice, Bovicola ovis but control is hampered by inadequate application via plunge or shower dipping. Synthetic pyrethroid (SP) pour-on products were released in 1981 but resistance developed by 1985 and many woolgrowers were unable to eradicate lice with pour-on products. Highest Resistance Factors at this time were only about 26 x but this was sufficient to prevent pour-ons working efficiently. By 1991 a population from Hartley in NSW was found to be 642 x resistant to cypermethrin with side-resistance conferred to the other SPs. SP resistance was partially suppresible by piperonyl butoxide but field trials suggested that the resulting improvement in efficacy was not sufficient to be commercially attractive. OPs remain very effective if applied correctly and the release of ivermectin and 2 benzoylphenyl urea products significantly improves the prospects for resistance management. However the increasing environmental concern about the persistence of chemical residues in wool has stimulated interest in biological control of sheep lice by Bacillus thuringiensis.

Efficacy of dressings for killing larvae of the sheep blowfly.

Resistance to organophosphorus (OP) insecticides in the Australian sheep blowfly has decreased the larvicidal effectiveness of several popular products used as dressings for flystrike. Laboratory bioassays in which near full-size Australian sheep blowfly larvae were immersed in flystrike dressings at registered concentrations for times ranging from 5 to 180 s indicated that none of the products was completely effective in killing highly OP-resistant larvae. Several products performed poorly, even against a susceptible population. Effectiveness did not always reflect the concentration of active ingredient. For example, the products considered to be the most, and least effective overall, contained 0.036% propetamphos but were formulated very differently. Larvicidal efficacy is important in terms of minimising injury to stock but also in the management of insecticide resistance. In situations when the degree of resistance is known, it will be possible to make recommendations for the most cost-effective treatment of flystrike. In the meantime, there appears to be a clear advantage for woolgrowers to use a propetamphos-based flystrike jetting product to dress flystrike lesions.

Pyrethroid resistance in Australian field populations of the sheep body louse, Bovicola (Damalinia) ovis.

Synthetic pyrethroid (SP) resistance has developed in Australian field populations of the sheep body louse, Bovicola (Damalinia) ovis. Laboratory bioassays were used to measure the susceptibility of lice to cypermethrin and the other registered SPs. Results of these bioassays indicated resistance to cypermethrin, deltamethrin, cyhalothrin and alphacypermethrin. So far, high-level resistance has been diagnosed in only a few strains. The toxicological responses of these strains were clearly separated from those of the majority of louse strains tested. Furthermore, these strains had survived immersion in commercial SP dips. The level of resistance described in some strains was sufficient to cause pour-on products to fail despite the fact that the LC50s of these strains fell within the normal range of field responses.

Chemical control of Ornithonyssus sylviarum on caged layer hens.

Manual application of aqueous solutions of malathion, carbaryl and permethrin controlled northern fowl mite, Ornithonyssus sylviarum Canestrini and Fanzago, on caged layer hens for at least 118 days. Azamethiphos sprayed manually provided acceptable control for a shorter period. Machine application, particularly of azamethiphos, but also malathion, provided lesser control. The addition of surfactant increased the wetting ability of the sprays but interfered with the efficacy of azamethiphos.

Evaluation of dressings to aid healing of mulesing wounds on sheep.

Five proprietary and one experimental dressing were compared with no treatment in their ability to aid healing of mulesing wounds in over 1900 young lambs. Healing was judged as the completeness of shrinkage of the mulesing cuts and the condition of scabs on the new skin surface. When assessed at 21 d it was found that treatment with Heriots Crown Wound Powder or Coopers Mulesing Powder offered a significant advantage over leaving the wounds untreated. Neither aqueous organophosphate washes, Defiance nor Defiance containing 0.08% chlorfenvinphos offered any healing advantage over controls. However, washing the wounds with an aqueous organophosphate solution aided healing more than the Defiance-based dressings. It was considered that the powders or the washes encouraged quick scab formation either by creating a dry covering (powders) or by washing away blood and allowing fast drying of the wound. The Defiance-type dressings slowed healing by keeping the wound moist for up to 10 d, but healing was not significantly different to the untreated group by 21 d.

Monitoring and selection of resistance to pyrethroids in the Australian sheep blowfly, Lucilia cuprina.

Field and laboratory populations of the Australian sheep blowfly, Lucilia cuprina (Wiedemann) (Calliphoridae), were surveyed by bioassay for possible resistance to the synthetic pyrethroids, a group of insecticides under development for blowfly control. A normal distribution of LC50 values was found using deltamethrin, the test pyrethroid, with no indication of specific resistance despite widespread use of deltamethrin on sheep to control the sheep body louse, Damalinia ovis (Schrank) (Trichodectidae). There was no cross-resistance to deltamethrin from existing organophosphate (OP) resistance nor from previous use of DDT. Selection with deltamethrin on a combined field strain, CSF85, increased the LC50 gradually over the first twenty generations until it stabilized at approximately 25x that of the unselected CSF85. This laboratory-induced resistance extended to other pyrethroids, cypermethrin (16x), cyhalothrin (25x) and cycloprothrin (10x), and increased the existing resistance of CFS85 to the OP diazinon (11x) and the carbamate, butacarb (83x).

Simulation of fly-waves to assess the ability of diflubenzuron to protect sheep against flystrike by Lucilia cuprina.

Sheep were exposed to mass-released gravid females of the sheep blowfly, Lucilia cuprina, in a fly-proof animal house at various times after treatment with the insecticide diflubenzuron (1000, 1500 and 2500 ppm a.i.). Untreated sheep were similarly exposed as controls, while sheep treated with either diazinon (400 ppm a.i.) or cyromazine (1000 ppm) were used as standards for comparison. Before exposure, groups of sheep were wetted by simulating rainfall in the animal house in order to increase their susceptibility to flystrike by L. cuprina. In one trial, sheep jetted with either diflubenzuron or cyromazine (both at 1000 ppm) were protected against flystrike for at least 110 days. At 1500 ppm, diflubenzuron performed significantly better with no bodystrike occurring in the group until the end of the trial at 170 days. Under more severe fly pressure in a second trial diflubenzuron at concentrations up to 2500 ppm provided the same protection as diazinon (approximately 56 days), but performed significantly better thereafter. No cross-resistance to diflubenzuron was found in diazinon-resistant field populations of L. cuprina in laboratory bioassays. The data show that diflubenzuron would be suitable as a prophylactic treatment for flystrike.