Efficacy of macrocyclic lactones for the control of larvae of the Old World screw-worm fly (Chrysomya bezziana)

Chrysomya bezziana: a case report in a dog from Southern China and review of the Chinese literature

Chrysomya bezziana is an obligate, myiasis-causing fly in humans and warm-blooded animals throughout the tropical and subtropical Old World. We report a case of cutaneous myiasis due to C. bezziana in a dog from Guangxi province in China. A total of 35 maggots were removed from the lesions. Direct sequencing of the mitochondrial cytochrome b gene showed that the specimen belonged to haplotype CB_bezz02, which was previously reported in Malaysia and the Gulf region. This paper also reviews reported cases of screwworm myiasis from humans and animals in China. Geographical records indicate that the distribution of C. bezziana is expanding, suggesting that an integrated pest management control should be taken into consideration in China.

The comparative efficacy of afoxolaner, spinosad, milbemycin, spinosad plus milbemycin, and nitenpyram for the treatment of canine cutaneous myiasis

BACKGROUND

Canine screw-worm myiasis due to Cochliomyia hominivorax or Chrysomya bezziana has traditionally been treated with extra-label use of ivermectin. The larvicidal activities of nitenpyram and spinosad/milbemycin also have been described, but there have been no reports to describe the efficacy of isoxazolines such as afoxolaner.

OBJECTIVES

To evaluate and compare the efficacy of spinosad, spinosad/milbemycin, milbemycin, nitenpyram and afoxolaner for the treatment of canine screw-worm myiasis.

ANIMALS

Forty client-owned and naturally infested dogs.

METHODS

Cases were randomized into five groups of eight dogs, with each receiving an insecticide at the dose recommended by the manufacturer. Each case was evaluated hourly for 7 h and then again 24 h after treatment. Scores of 0 (no visualized effect), 0.5 (partial eradication) or 1 (complete kill and eradication) were recorded at each observation and compared between groups. Time to complete killing of all larvae was recorded for each dog and compared between treatment groups.

RESULTS

Nitenpyram killed all larvae at 6 h post-consumption and spinosad/milbemycin at 7 h. In groups receiving afoxolaner or spinosad, all larvae were killed within 24 h. For those receiving milbemycin, two cases were still infested with live larvae at 24 h. Shih tzus and their crosses were most commonly affected.

CONCLUSION

Spinosad/milbemycin or nitenpyram seem to be effective drugs for the treatment of canine screw-worm myiasis. Afoxolaner and milbemycin are effective but exhibited slower larvicidal activity. Synergism between spinosad and milbemycin was observed.

Prophylactic and therapeutic efficacy of Australian-registered insecticide formulations against Old World screwworm (Chrysomya bezziana) infestation

OBJECTIVE

To determine the prophylactic and therapeutic efficacy of Australian-registered insecticide formulations against Old World screwworm (OWS) myiases for potential use in screwworm containment and eradication programs.

METHODS

The longevity of protection provided by six insecticidal formulations (subcutaneous ivermectin, doramectin and abamectin, a topically applied aqueous formulation of spinosad, ivermectin controlled-release capsule and a dicyclanil spray-on formulation) was tested using implants of 1st-instar OWS larvae on Javanese thin-tail sheep. Therapeutic efficacy of four formulations (topical ivermectin, chlorfenvinphos/cypermethrin mixture, aerosol spinosad formulation and a formulation containing propetamphos and eucalyptus oil) was tested against 2- and 4-day-old OWS strikes.

RESULTS

Both the ivermectin capsule and dicyclanil spray-on formulation gave 100% protection against screwworm implants for the full 12 weeks of the study. Ivermectin, doramectin and abamectin administered SC all gave 100% protection at 3 days post-treatment, but at 2 weeks the protection had become incomplete. Spinosad dipping did not give complete protection at any time. All four therapeutic treatments gave complete resolution of 2-day-old strikes and topical ivermectin, spinosad and chlorfenvinphos/cypermethrin, but not the propetamphos/eucalyptus oil formulation, gave complete resolution of all 4-day-old strikes.

CONCLUSION

Dicyclanil spray-on and ivermectin capsule formulations, both registered for use in sheep, but not for cattle or other livestock species, gave much longer protection against screwworm implants than the currently recommended SC ivermectin. Pre-emptive action to facilitate rapid deployment of these formulations in the event of a screwworm incursion is urgently needed.

A review on the toxicity and non-target effects of macrocyclic lactones in terrestrial and aquatic environments

The avermectins, milbemycins and spinosyns are collectively referred to as macrocyclic lactones (MLs) which comprise several classes of chemicals derived from cultures of soil micro-organisms. These compounds are extensively and increasingly used in veterinary medicine and agriculture. Due to their potential effects on non-target organisms, large amounts of information on their impact in the environment has been compiled in recent years, mainly caused by legal requirements related to their marketing authorization or registration. The main objective of this paper is to critically review the present knowledge about the acute and chronic ecotoxicological effects of MLs on organisms, mainly invertebrates, in the terrestrial and aquatic environment. Detailed information is presented on the mode-of-action as well as the ecotoxicity of the most important compounds representing the three groups of MLs. This information, based on more than 360 references, is mainly provided in nine tables, presenting the effects of abamectin, ivermectin, eprinomectin, doramectin, emamectin, moxidectin, and spinosad on individual species of terrestrial and aquatic invertebrates as well as plants and algae. Since dung dwelling organisms are particularly important non-targets, as they are exposed via dung from treated animals over their whole life-cycle, the information on the effects of MLs on dung communities is compiled in an additional table. The results of this review clearly demonstrate that regarding environmental impacts many macrocyclic lactones are substances of high concern particularly with larval instars of invertebrates. Recent studies have also shown that susceptibility varies with life cycle stage and impacts can be mitigated by using MLs when these stages are not present. However information on the environmental impact of the MLs is scattered across a wide range of specialised scientific journals with research focusing mainly on ivermectin and to a lesser extent on abamectin doramectin and moxidectin. By comparison, information on compounds such as eprinomectin, emamectin and selamectin is still relatively scarce.

How to test nontarget effects of veterinary pharmaceutical residues in livestock dung in the field

To register veterinary medicinal products (VMPs) as parasiticides on pastured animals, legislation in the European Union requires an environmental risk assessment to test the potential nontarget effects of fecal residues on dung-dwelling organisms. Products with adverse effects in single-species laboratory tests require further, higher-tier testing to assess the extent of these effects on entire communities of dung-dwelling organisms under more realistic field or semifield conditions. Currently, there are no documents specifically written to assist researchers in conducting higher-tier tests or to assist regulators in interpreting the results of such tests in an appropriate context. Here we provide such a document, written by members of the SETAC Advisory Group DOTTS (Dung Organism Toxicity Testing Standardization) with research experience on dung fauna in central and southern Europe, Canada, Australia, and South Africa. This document briefly reviews the organisms that make up the dung community and their role in dung degradation, identifies key considerations in the design and interpretation of experimental studies, and makes recommendations on how to proceed.

Veterinary pharmacovigilance. Part 3. Adverse effects of veterinary medicinal products in animals and on the environment

Like humans, animals may experience adverse effects when treated with medicinal products. These effects may be related to the pharmacological or toxicological properties of the substances used or they may arise because of hypersensitivity. Veterinary medicinal products may also possess the ability to harm the environment. This paper reviews the potential of veterinary medicinal products to cause adverse effects in animals and on the environment.

Pour-on formulation of eprinomectin for cattle: fecal elimination profile and effects on the development of the dung-inhabiting Diptera Neomyia cornicina (L.) (Muscidae)

The plasma and fecal concentrations of eprinomectin were determined in cattle following topical administration at a dose rate of 0.5 mg kg(-1). The maximum plasma concentrations of 12.24 ng ml(-1) occurred 2 d after administration, and eprinomectin remained detectable in plasma 29 d after administration (0.10 ng ml(-1)). The maximum dung concentration of 350 ng g(-1) was observed 3 d after administration and thereafter for at least 29 d (4 ng g(-1)). The amount of drug recovered in dung during this period was 20.50%+/-4.31% of the total administered dose. The effects of eprinomectin against the nontarget dung-feeding Diptera Neomyia cornicina was assessed under laboratory conditions. Feces voided by cattle treated with eprinomectin were associated with high larval mortality during the first 12 d after treatment, with null emergence until day 7. The no-observed-effect concentration for N. cornicina was estimated to be close to 7+/-5 ng g(-1).

Fecal residues of veterinary parasiticides: nontarget effects in the pasture environment

Residues of veterinary parasiticides in dung of treated livestock have nontarget effects on dung-breeding insects and dung degradation. Here, we review the nature and extent of these effects, examine the potential risks associated with different classes of chemicals, and describe how greater awareness of these nontarget effects has resulted in regulatory changes in the registration of veterinary products.

Endectocide exchanges between grazing cattle after pour-on administration of doramectin, ivermectin and moxidectin

Self-licking behaviour in cattle has recently been identified as a determinant of the kinetic disposition of topically-administered ivermectin. In the present study, we document the occurrence and extent of transfer between cattle of three topically-administered endectocides, as a consequence of allo-licking. Four groups of two Holstein cows each received one pour-on formulation of doramectin, ivermectin, or moxidectin, or no treatment. The cows were then kept together in a paddock. Systemic exposure to each topically-administered endectocide was observed in at least five of six non-treated cattle. Plasma and faecal drug concentration profiles in non-treated animals were highly variable between animals and within an animal, and sometimes attained those observed in treated animals. Drug exchanges were quantified by measuring plasma and faecal clearances after simultaneous i.v. administration of the three drugs as a cocktail. Plasma clearances were 185+/-43, 347+/-77 and 636+/-130ml/kg/day, faecal clearances representing 75+/-26, 28+/-13, and 39+/-30% of the plasma clearance for doramectin, ivermectin and moxidectin, respectively. The amount of drug ingested by non-treated cattle attained 1.3-21.3% (doramectin), 1.3-16.1% (ivermectin), 2.4-10.6% (moxidectin) of a pour-on dose (500 microg/kg). The total amount of drug ingested by all non-treated cattle represented 29% (doramectin), 19% (ivermectin), and 8.6% (moxidectin) of the total amount of each drug poured on the backs of treated animals. The cumulative amounts of endectocide ingested by each non-treated cow ranged from 1.3 to 27.4% of a pour-on dose. Oral bioavailability after drug ingestion due to allo-licking was 13.5+/-9.4, 17.5+/-3.5 and 26.1+/-11.1% for doramectin, ivermectin and moxidectin, respectively. The extent of drug exchange demonstrated here raises concerns for drug efficacy and safety, emergence of drug resistance, presence of unexpectedly high residue levels in treated and/or untreated animals and high environmental burdens. Moreover, scientific and regulatory aspects of clinical and bioequivalence trials for topical drug administration in cattle should be explored.

Factors affecting abundance and oviposition rates of a field population of the Old World screw-worm fly, Chrysomya bezziana (Diptera: Calliphoridae)

Sentinel cattle and a grid of swormlure-baited sticky traps were used to monitor a Malaysian population of the Old World screw-worm fly, Chrysomya bezziana Villeneuve. Observations were carried out on an isolated cattle station at monthly intervals during the period August 1996 to June 2000. The number of flies caught was unaffected by weather conditions at the time of trapping, but was positively correlated with the total rainfall and the average daily air temperature prevailing 15-28 days earlier, when trapped flies were still juveniles. Trap catches were biased in favour of females, but daily catch rates of both sexes increased significantly the longer traps were open, suggesting that efficacy was related to the differential volatility of the chemicals comprising swormlure. Oviposition on sentinel cattle occurred mostly in late afternoon or early evening but increased significantly as the wound aged. Oviposition rates were positively correlated with female catch rates, but the relationship was curvilinear, suggesting that fly populations may be subject to some form of density-dependent constraint. Consistent differences in oviposition rates on sentinel cattle at different localities on the cattle station suggested the existence of highly clumped, quasi-stationary populations. Differences in trap catches between traps located in pastoral areas and those sited in nearby oil palm or rubber plantations supported this interpretation of the data. These findings are discussed in relation to the use of the sterile insect technique for the control of screw-worm fly infestations.

Retrospective study of Old World screwworm fly (Chrysomya bezziana) myiasis in 59 dogs in Hong Kong over a one year period

This retrospective study examined the case records of Old World screw-worm fly myiasis in dogs in Hong Kong's New Territories. Myiasis due to OWSWF was identified and treated in 59 dogs at our clinic over a period of 1 year. Signalment, causes of initial wound, and treatments implemented, are reported. All cases recovered from myiasis with a combination of larvae removal and wound debridement, antibiotics, and topical and systemic insecticides. Whilst this parasite is currently exotic to Australia, much of the country is environmentally suitable (tropical and subtropical) for establishment of OWSWF.

Chrysomya bezziana in pet dogs in Hong Kong: a potential threat to Australia

The Old World Screwworm fly (Chrysomya bezziana) has recently been causing aggressive myiasis in pet dogs in Hong Kong, and the Agriculture, Fisheries and Conservation Department of Hong Kong has confirmed the presence of Chrysomya bezziana in the territory. This report describes screwworm infestation in pet dogs presented for treatment at our clinic in the past year. Screwworm fly larvae were found in typically punched out round wounds or embedded in exposed or neglected wounds. Infested wounds had a characteristic strong offensive smell and oozed a sero-sanguineous exudate or frank blood. Removal of the larvae, wound treatment, continuous monitoring for re-infestation, application of insect repellants and protective housing were essential for successful treatment. Because Hong Kong is a source of pet migration or a transit point to Australia, the fly could threaten Australia.

Reductions of non-pest insects in dung of cattle treated with endectocides: a comparison of four products

Pour-on formulations of four endectocide products were compared to assess the effect of faecal residues on insects developing in naturally-colonized dung of treated cattle. In each of three independent experiments, suppression of insects was associated with application of doramectin, eprinomectin and ivermectin, but no effect was observed for moxidectin. When data were combined across experiments to increase sample sizes, suppression of insects was observed for each compound, with the least effect being observed for moxidectin. Based on the number of species affected and duration of suppression, doramectin > ivermectin > eprinomectin >> moxidectin were ranked in descending order of adverse effect. A second set of three independent experiments was performed to assess the effect of endectocide treatment on dung degradation. Delayed degradation was observed for dung of cattle treated with doramectin, eprinomectin and moxidectin in the first experiment. No effect of treatment was detected in the second experiment. An effect of moxidectin was detected in the third experiment, but differences could not be detected with subsequent post-hoc tests. When data were combined across experiments to increase sample sizes, delayed degradation was detected only for eprinomectin. The apparent discrepancy between the low effect of moxidectin on insects versus its effect of dung degradation suggests the confounding action of other unidentified factors. Results of the current study indicate that use of moxidectin is least likely to affect the natural assemblage of insects associated with cattle dung.