Control of sheep flystrike: what's been tried in the past and where to from here

Antennal transcriptome analysis reveals sensory receptors potentially associated with host detection in the livestock pest Lucilia cuprina

BACKGROUND

Lucilia cuprina (Wiedemann, 1830) (Diptera: Calliphoridae) is the main causative agent of flystrike of sheep in Australia and New Zealand. Female flies lay eggs in an open wound or natural orifice, and the developing larvae eat the host's tissues, a condition called myiasis. To improve our understanding of host-seeking behavior, we quantified gene expression in male and female antennae based on their behavior.

METHODS

A spatial olfactometer was used to evaluate the olfactory response of L. cuprina mated males and gravid females to fresh or rotting beef. Antennal RNA-Seq analysis was used to identify sensory receptors differentially expressed between groups.

RESULTS

Lucilia cuprina females were more attracted to rotten compared to fresh beef (> fivefold increase). However, males and some females did not respond to either type of beef. RNA-Seq analysis was performed on antennae dissected from attracted females, non-attracted females and males. Transcripts encoding sensory receptors from 11 gene families were identified above a threshold (≥ 5 transcript per million) including 49 ATP-binding cassette transporters (ABCs), two ammonium transporters (AMTs), 37 odorant receptors (ORs), 16 ionotropic receptors (IRs), 5 gustatory receptors (GRs), 22 odorant-binding proteins (OBPs), 9 CD36-sensory neuron membrane proteins (CD36/SNMPs), 4 chemosensory proteins (CSPs), 4 myeloid lipid-recognition (ML) and Niemann-Pick C2 disease proteins (ML/NPC2), 2 pickpocket receptors (PPKs) and 3 transient receptor potential channels (TRPs). Differential expression analyses identified sex-biased sensory receptors.

CONCLUSIONS

We identified sensory receptors that were differentially expressed between the antennae of both sexes and hence may be associated with host detection by female flies. The most promising for future investigations were as follows: an odorant receptor (LcupOR46) which is female-biased in L. cuprina and Cochliomyia hominivorax Coquerel, 1858; an ABC transporter (ABC G23.1) that was the sole sensory receptor upregulated in the antennae of females attracted to rotting beef compared to non-attracted females; a female-biased ammonia transporter (AMT_Rh50), which was previously associated with ammonium detection in Drosophila melanogaster Meigen, 1830. This is the first report suggesting a possible role for ABC transporters in L. cuprina olfaction and potentially in other insects.

In vitro insecticide resistance patterns in field strains of the sheep blowfly, Lucilia cuprina

The control of the sheep blowfly relies on the use of insecticides. There have been several reports of in vitro and in vivo resistance to the most widely-used flystrike control chemical, dicyclanil. A recent report also described in vitro resistance to imidacloprid in a strain collected from a single property over three consecutive seasons that also showed resistance to dicyclanil. The present study aimed to use in vitro assays to examine five field-collected blowfly strains to determine if this co-occurrence of resistance to dicyclanil and imidacloprid was present more widely in field strains and to also measure resistance patterns to the other currently-used flystrike control chemicals. Each of the strains showed significant levels of resistance to both dicyclanil and imidacloprid: resistance factors at the IC50 of 9.1-23.8 for dicyclanil, and 8.7-14.1 for imidacloprid. Resistance factors at the IC95 ranged from 16.5 to 53.7, and 14.6-24.3 for dicyclanil and imidacloprid, respectively. Resistance factors were up to 8.5 for cyromazine at the IC95. Resistance to dicyclanil and imidacloprid was suppressed by co-treatment with the cytochrome P450 inhibitor, aminobenzotriazole, implicating this enzyme system in the observed resistances. We discuss the implications of the co-occurrence of resistance to dicyclanil and imidacloprid on insecticide rotation strategies for blowfly control. We also discuss the roles of insecticide resistance, environmental factors (e.g. rainfall), operational factors (e.g. insecticide application technique) and other animal health issues (e.g. scouring / diarrhoea) that together will impact on the likelihood of flystrike occurring at an earlier time point than expected after insecticide application.

Complete Mitochondrial Genome for Lucilia cuprina dorsalis (Diptera: Calliphoridae) from the Northern Territory, Australia

The Australian sheep blowfly, Lucilia cuprina dorsalis, is a major sheep ectoparasite causing subcutaneous myiasis (flystrike), which can lead to reduced livestock productivity and, in severe instances, death of the affected animals. It is also a primary colonizer of carrion, an efficient pollinator, and used in maggot debridement therapy and forensic investigations. In this study, we report the complete mitochondrial (mt) genome of L. c. dorsalis from the Northern Territory (NT), Australia, where sheep are prohibited animals, unlike the rest of Australia. The mt genome is 15,943 bp in length, comprising 13 protein-coding genes (PCGs), two ribosomal RNAs (rRNAs), 22 transfer RNAs (tRNAs), and a non-coding control region. The gene order of the current mt genome is consistent with the previously published L. cuprina mt genomes. Nucleotide composition revealed an AT bias, accounting for 77.5% of total mt genome nucleotides. Phylogenetic analyses of 56 species/taxa of dipterans indicated that L. c. dorsalis and L. sericata are the closest among all sibling species of the genus Lucilia, which helps to explain species evolution within the family Luciliinae. This study provides the first complete mt genome sequence for L. c. dorsalis derived from the NT, Australia to facilitate species identification and the examination of the evolutionary history of these blowflies.

The current status and outlook for insecticide, acaricide and anthelmintic resistances across the Australian ruminant livestock industries: assessing the threat these resistances pose to the livestock sector

The Australian ruminant livestock industries are faced with the need to control parasitic infectious diseases that can seriously impact the health of animals. However, increasing levels of resistance to insecticides, anthelmintics and acaricides are substantially reducing the ability to control some of these parasites. Here we review the current situation with regard to chemical resistances in parasites across the various sectors of the Australian ruminant livestock industries and assess the level of threat that these resistances pose to the sustainability of these sectors in the short to long terms. We also look at the extent to which testing for resistance occurs across the various industry sectors, and hence how well-informed these sectors are of the extent of chemical resistance. We examine on-farm management practices, breeding of parasite-resistant animals, and non-chemical therapeutics that may act as short to long term means to reduce the current reliance on chemicals for parasite control. Finally, we look at the balance between the prevalence and magnitude of current resistances and the availability and adoption rates of management, breeding and therapeutic alternatives in order to assess the parasite control outlook for the various industry sectors.

Mitochondrial genomic investigation reveals a clear association between species and genotypes of Lucilia and geographic origin in Australia

BACKGROUND

Lucilia cuprina and L. sericata (family Calliphoridae) are globally significant ectoparasites of sheep. Current literature suggests that only one of these blowfly subspecies, L. cuprina dorsalis, is a primary parasite causing myiasis (flystrike) in sheep in Australia. These species and subspecies are difficult to distinguish using morphological features. Hence, being able to accurately identify blowflies is critical for diagnosis and for understanding their relationships with their hosts and environment.

METHODS

In this study, adult blowflies (5 pools of 17 flies; n = 85) were collected from five locations in different states [New South Wales (NSW), Queensland (QLD), Tasmania (TAS), Victoria (VIC) and Western Australia (WA)] of Australia and their mitochondrial (mt) genomes were assembled.

RESULTS

Each mt genome assembled was ~ 15 kb in size and encoded 13 protein-coding genes, 2 ribosomal RNAs, 22 transfer RNAs and a control region. The Lucilia species mt genomes were conserved in structure, and the genes retained the same order and direction. The overall nucleotide composition was heavily biased towards As and Ts-77.7% of the whole genomes. Pairwise nucleotide diversity suggested divergence between Lucilia cuprina cuprina, L. c. dorsalis and L. sericata. Comparative analyses of these mt genomes with published data demonstrated that the blowflies collected from sheep farm in TAS clustered within a clade with L. sericata. The flies collected from an urban location in QLD were more closely related to L. sericata and represented the subspecies L. c. cuprina, whereas the flies collected from sheep farms in NSW, VIC and WA represented the subspecies L. c. dorsalis.

CONCLUSIONS

Phylogenetic analyses of the mt genomes representing Lucilia from the five geographic locations in Australia supported the previously demonstrated paraphyly of L. cuprina with respect to L. sericata and revealed that L. c. cuprina is distinct from L. c. dorsalis and that L. c. cuprina is more closely related to L. sericata than L. c. dorsalis. The mt genomes reported here provide an important molecular resource to develop tools for species- and subspecies-level identification of Lucilia from different geographical regions across Australia.

Reduced synergistic efficacy of piperonyl butoxide in combination with alpha-cypermethrin in vitro in an insecticide-resistant strain of the sheep blowfly, Lucilia cuprina

Control of flystrike on sheep relies on the use of insecticides. The present study used in vitro assays to examine the potential for increasing the efficacy of synthetic pyrethroids against sheep blowfly larvae using the synergist piperonyl butoxide (PBO). We examined the potency of alpha-cypermethrin (ACP) / PBO combinations against a reference insecticide-susceptible strain (LS) and a field-derived strain showing resistance to dicyclanil and imidacloprid. Co-treatment of the insecticide-susceptible strain with ACP/PBO resulted in increasing levels of synergism as the PBO concentration was increased, with synergism ratios (SRs) of up to 114-fold. Treatment with PBO/ACP combinations at ratios of 20:1 and 5:1 resulted in significant levels of synergism: SRs of 13.5- and 7.6-fold, respectively. However, the levels of synergism were significantly less for the insecticide-resistant strain: SRs of 4.6- and 2.6-fold for the 20:1 and 5:1 ratios, respectively. The resistant strain showed no resistance to ACP when administered alone, however, was 2-fold less sensitive than the LS strain to the toxic effects of PBO alone. This insensitivity to PBO was removed by co-treatment with the P450 inhibitor aminobenzotriazole, suggesting an increased level of P450-mediated metabolism of the PBO in this strain compared to the LS strain, and hence providing a likely explanation for the reduced synergistic efficacy of PBO on ACP toxicity in the resistant strain. While PBO was able to synergise ACP with both of the blowfly strains examined here, the reduced synergistic efficacy observed with the field-derived insecticide-resistant strain lessens the potential usefulness of such a combination for blowfly control in the field.

How Well Does Australian Animal Welfare Policy Reflect Scientific Evidence: A Case Study Approach Based on Lamb Marking

The development and substance of animal welfare policy is subject to a range of social, cultural, economic, and scientific influences that commonly vary within and between countries. Discrepancies in policy can create confusion and mistrust among stakeholders and consumers and limit the ability to create a uniform minimum level of requirements to safeguard animal welfare, as well as create a level 'playing field' for farmers when trading with other jurisdictions. The livestock sector is receiving growing scrutiny globally for real and perceived violations of animal welfare, for example, the practice of mulesing in Australia. This article explores animal welfare legislation within Australia and how it reflects the scientific evidence surrounding routine husbandry practices in sheep, including tail docking, castration, and mulesing. While there is some variation between state and territory legislation, the most notable concern is the lack of enforceable recommendations surrounding the evidence-based use of analgesia and anaesthesia for painful husbandry procedures. The age at which these procedures are recommended to be performed is relatively consistent across Australian jurisdictions, but there is a marked difference compared to international legislation. The global context of animal welfare legislation, public perception, and producer perception of these procedures are also discussed, highlighting the difficulty of creating robust animal welfare legislation that promotes a good standard of welfare that is respected worldwide whilst being practical in an Australian setting given our unique geography and climatic conditions.

Traumatic sheep myiasis: A review of the current understanding

Myiasis, or the infestation of live humans and vertebrate animals by dipterous larvae, is a health issue worldwide. The economic impact and potential threat to animal health and wellbeing of this disease under the animal husbandry sector is considerable. Sheep are a highly vulnerable livestock category exposed to myiasis (sheep strike), due to several unique predisposing factors that attract flies. The successful mitigation of this disease relies on a thorough understanding of fly population dynamics associated with the change in weather patterns and the evaluation of this disease through different branches of science such as chemistry, molecular biology, and microbiology. The present review provides a summary of the existing knowledge of strike in sheep, discussed in relation to the application of volatile organic compounds, metagenomics, and molecular biology, and their use regarding implementing fly control strategies such as traps, and to increase the resilience of sheep to this disease through improving their health and wellbeing.

Resistance to dicyclanil and imidacloprid in the sheep blowfly, Lucilia cuprina, in Australia

BACKGROUND

The sheep blowfly, Lucila cuprina, is a myiasis-causing parasite responsible for significant production losses and welfare issues for the Australian sheep industry. Control relies largely on the use of insecticides. The pyrimidine compound, dicyclanil, is the predominant control chemical, although other insecticides also are used, including imidacloprid, ivermectin, cyromazine and spinosad. We investigated in vitro resistance patterns and mechanisms in field-collected blowfly strains.

RESULTS

The Walgett 2019 strain showed significant levels of resistance to both dicyclanil and imidacloprid, with resistance factors at the IC₅₀ of 26- and 17-fold, respectively, in in vitro bioassays. Co-treatment with the cytochrome P450 inhibitor, aminobenzotriazole, resulted in significant levels of synergism for dicyclanil and imidacloprid (synergism ratios of 7.2- and 6.1-fold, respectively), implicating cytochrome P450 in resistance to both insecticides. Cyp12d1 transcription levels were increased up to 40-fold throughout the larval life stages in the resistant strain compared to a reference susceptible strain, whereas transcription levels of some other cyp genes (6g1, 4d1, 28d1) did not differ between the strains. Similar resistance levels also were observed in flies collected from the same property in two subsequent years.

CONCLUSION

This study indicates that in vitro resistance to both dicyclanil and imidacloprid in this field-collected blowfly strain is likely mediated by cytochrome P450, with Cyp12d1 implicated as the enzyme responsible; however, it remains possible that another P450 also may be involved. A common resistance mechanism for the two drugs has important implications for drug rotation strategies designed to prolong the useful life of flystrike control chemicals. © 2022 The Authors. Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.