CASE NOTES
Know your screw-worm flies
Narelle Sales, Microbiology and Parasitology - Research, NSW DPI, Elizabeth Macarthur Agricultural Institute, Menangle, NSW
Posted Flock and Herd July 2023
Introduction
There appears to be an ever-increasing number of exotic threats to Australian livestock industries. However, the only parasitic insect that is on the Northern Australia Quarantine Strategy (NAQS) list of target pests, diseases and weeds is the Old World Screw-Worm Fly (OWSWF), Chrysomya bezziana.1 This flying obligate parasite is considered a serious threat, not just to Australia's agricultural productivity and profitability, but also to our wildlife and human populations. C. bezziana is present in areas of Papua New Guinea bordering the Torres Strait and throughout much of Indonesia, Malaysia and the Philippines. This close proximity increases the probability of entry via northern Australia. Previous modelling has determined the ability of the OWSWF to survive all year round in Australia's tropical regions but it would be restricted in other regions during months which are either too dry and/or cold.2 Despite these seasonal restrictions, simulations have determined that C. bezziana could establish permanently in areas as far south as Sydney if introduced.3 Because of the high degree of risk and the cost associated with treatment and eradication of OWSWF, surveillance by fly trapping and targeted myiasis monitoring is conducted across the Top End in the Northern Territory, Queensland and Western Australia and is known as the Screw-Worm Fly Surveillance and Preparedness Program (SWFSPP).4
There is another destructive and costly myiasis fly, Cochliomyia hominivorax, commonly known as the New World Screw-Worm Fly (NWSWF) because it is found in the tropical areas of the Americas. This parasite was successfully eradicated from the southern states of the USA and the risk of entry to Australia is not considered as great as that of the OWSWF. In fact two cases of human myiasis have been detected in Australia on travellers from South America.4 For this reason the SWFSPP also includes exclusion of C. hominivorax.
In NSW, introduction of these parasites could occur through our ports and airports on goods, animals and humans or by moving southward following entry through Northern Australia. It is most likely that a District Veterinarian would be called upon to examine myiasis as the adult fly looks very similar to a number of other blowflies if they are observed at all in the field. There is an AUSVETPLAN for Screw-Worm Flies5 (version 5) which outlines the nationally agreed response strategy to be followed in the case of an incursion. Recent advances in multiplex PCR diagnostics mean that updates to section 2.5.4 Laboratory tests will be required when validation is completed by Queensland Department of Agriculture and Fisheries.
Notification is mandatory via Emergency Animal Disease Hotline 1800 675 888 and the following information is required when reporting:
- Full name and contact phone number
- Suspected pest or disease you are reporting
- Location of the suspected pest or disease (including any relevant property identification code)
- Details of any significant biosecurity impact and any other requested information.
- Samples should be collected and sent for identification.
However, there are a number of morphological characteristics of this pest and other general information which may be of interest to the District Veterinarian.
Figure 1
Life cycle
There are taxonomic keys for the identification of all life stages and the multiplex PCR that is currently under validation will identify adults and larvae.
Myiasis
The common clinical signs of screw-worm fly induced myiasis include:
- Constant licking, biting or kicking at the wound and restlessness
- Fever, lethargy
However, there are also very characteristic signs which include:
- A cavernous, foul smelling lesion up to 10-12cm in diameter
- Progressive liquefaction, necrosis and haemorrhage at the wound site
- Entry into the body cavity if left untreated
- Secondary bacterial infection, anaemia, hypoproteinaemia and fluid loss often lead to death of the animal.6
Figure 2 .Distinctive Wound caused by Chrysomya bezziana
Screw-worm fly surveillance and preparedness program. ( nt.gov.au )
Procedure for submission collection
- Confine the animal for sampling and photograph the wound
- Gently flush the wound with clean running water to wash off any secondary-strike maggots (if available)
- Using tweezers collect up to 10 maggots from deep in the wound and place in a small sturdy container
- Pour hot water (just off the boil) over the maggots and wait for two minutes for it to kill them
- Using tweezers, transfer the maggots to a small container with a screw-top lid and cover the maggots (1cm) with either 70% ethanol or another alcohol based product, for example hand gel/sanitiser
- Screw the lid onto the container firmly to close it and place the container into a zip-lock bag with some absorbent paper.
Some interesting and distinctive features of OWSWF7
Eggs
- White eggs (1.25mm long and 0.26mm wide) laid in parallel and firmly cemented to each other compared to creamy coloured eggs loosely clumped in random patterns.
Figure 3. Egg Masses
Larvae
- Smooth maggot with spines that are spaced singly - not in rows
- Spines are large with distinctive 'rose thorn' shapes which prevent backwards motion
- Anterior spiracles with 4-6 lobes
- Extremely prominent and powerful mouth hooks
- Posterior spiracles have a darkly pigmented, open peritreme, with an indistinct button.
Figure 4. 3rd Instar larva of Chrysomya bezziana with inset of interior spiracles, followed by image of posterior spiracles
Identification Studies on Chrysomya Bezziana Larvae Infested Sheep in Eastern Region, Saudi Arabia ( ijsr.net )
Geoff Brown (2022) Old World Screw-Worm Fly Workshop Darwin - Larval Identification.
Figure 5. Rose thorn shaped spines on 3rd instar larva of Chrysomya bezziana
Adults
- C. bezziana has a gold not a white 'face'
- Then identification becomes more difficult due to the amount variation in the colour and size of C. bezziana which is dependent on the original geographic source of the population
- Examples of the morphological features used to differentiate species include the colour of the posterior squama, the comparative length of the anterior spiracles, presence of cheek indentations and the demarcation of the eye facets.
Figure 6. White faced Lucilia sericata
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Figure 7. gold faced Chrysomya bezziana
Geoff Brown (2022) Old World Screw-Worm Fly Workshop Darwin Adult Identification
Conclusion
The visual identification of flies and larvae can be a very rapid process when performed by an experienced operator. Another method is currently used to check the thousands of flies that are routinely trapped during the Northern Australian surveillance program. A real time PCR, capable of detecting an individual C. bezziana in 1000 flies with 95% accuracy, was developed8 and subsequently enhanced.9 However, the primers and probe set for this PCR has proved to be expensive with manufacture and supply taking up to 3 months. As a result, a multiplex PCR, which also detects other species, including the NWSWF C. hominivorax, has been developed and is currently being validated at the Queensland Department of Agriculture and Fisheries Biosecurity Sciences Laboratory.
While the morphology of C. bezziana adults and larvae is impressive we should not forget the risk they pose to Australia's economy, native animals, livestock and vulnerable Australians. Risks from exotics like this make the quarantine and surveillance programs imperative.
For a range of animal health and biosecurity resources access Animal Health Australia's website. For example animalhealthaustralia.com.au
Acknowledgement
Mr Geoff Brown, Senior Principal Technical Officer, for Queensland Department of Agriculture and Fisheries kindly provided the presentation that was delivered at the District Veterinarians Conference Wagga Wagga (2-4 May 2023). Geoff produced and presented this training at the Animal Health Australia 'Screw-worm Fly Surveillance and Preparedness Entomology Workshop 2022' held at Berrimah Veterinary Laboratories, Makagon Rd, Berrimah, NT 0828. Geoff Brown is the Reference Entomologist on Screw-worm Fly in Australia.
References
- www.agriculture.gov.au (Accessed 14/5/2023)
- Atzeni MG, Mayer DG and Stuart MA (1997) Evaluating the risk of the establishment of screwworm fly in Australia Australian Veterinary Journal 75(10):743-745
- Mayer DG, Atzeni MG, Butler DG et al. (1994) Biological simulation of a screwworm fly invasion of Australia. Queensland Government and Commonwealth of Australia, Brisbane
- Monitoring for screw-worm fly animalhealthaustralia.com.au (Accessed 15/04/2023)
- AUSVETPLAN manual for screw-worm fly animalhealthaustralia.com.au (Accessed 15/4/2023)
- Guide to screw-worm fly www.business.qld.gov.au (Accessed 15/04/2023)
- AHA (2020) A Manual for the Identification of Screw-worm Fly. Fourth Edition Animal Health Australia, Canberra. (Revision of A Manual for the Diagnosis of Screw-Worm Fly by JP Spradbury 1991)
- Jarrett S, Morgan JAT, Wlodek BM, Brown GW, Urech R, Green PE and Lew-Tabor AE (2010) Specific detection of the Old World screwworm fly, Chrysomya bezziana, in bulk fly trap catches using real-time PCR Medical and Veterinary Entomology 24:227-235
- Morgan JAT and Urech R (2014) An improved real-time PCR assay for the detection of Old World screwworm flies Acta Tropica 138 (Supplement):S76-S81