Myiasis Producing Flies

Introduction

Myiasis is the parasitism of living animals by dipteran larvae. Myiasis can be obligatory or facultative (optional) and is described as cutaneous, nasal or somatic.

Oestridae

Dermatobia hominis

Calliphoridae

Pollenia rudis (Calliphoridae) - Richard Bartz, Munich - Wikimedia Commons

Lucilia cuprina - Wikimedia Commons

Blowfly Head Close Up - Martin Pot - Wikimedia Commons

The Calliphoridae family are facultative parasites

Cause blowfly strike

If the fly lays eggs on an animal, the animal is said to be blown

The damage the larvae cause to the animal is known as strike

Worldwide distribution

Affects sheep mostly
- Rabbits can also be affected

Species of veterinary importance in Europe

Lucilia sericata; Greenbottle

Phormia terra-novae; Blackbottle

Calliphora erythrocephala; Bluebottle

Calliphora vomitoria; Bluebottle

Species of veterinary importance in the Tropics

Lucilia cuprina; South Africa and Australia

Chrysomya spp.; Africa, Asia and Australia

Wohlfahrtia; Fleshfly

Recognition of Adults

Medium sized flies under 10mm long

Metallic sheen to abdomen
- Colour depends on species

Clear wings

Recognition of Larvae

Smooth maggots

10-15mm long

Spiracles and stigmatic plates on the tail can be used for species differentiation

Life cycle

Eggs laid in wounds, soiled fleece and on carrion
- Females attracted by the odour emitted
- Clusters of yellow-cream eggs laid

Larvae hatch 24 hours later and crawl down onto the skin
- Feed rapidly
- Grow rapidly
- Moult twice before becoming full maggots
- Process takes 1-2 weeks

Larvae fall to the ground to pupate

Adult emerges in under 1 week (during summer)

The female reaches sexual maturity after a protein meal
- Lays eggs in batches of 100-200

Adult flies survive for 1 month

Four generations can develop between May and September
- In warmer climates, up to 9 or 10 generations can develop per year

Flies can survive the winter as pupae and emerge the next spring

Epidemiology
Blowflies are divided into categories depending on their ability to initiate strike

Primary flies are capable of initiating a strike on living sheep. Larvae can penetrate intact skin
- Lucilia
- Phormia
- Calliphora spp.

Secondary flies cannot initiate a strike. Larvae attack an area already struck or damaged, extending it
- Calliphora spp.
- Chrysomya spp. (in warmer climates)

Tertiary flies attack lesions on carcasses which have become dry
- Musca
- Sarcophaga spp.

Predisposing factors to flystrike

Temperature
- Temperature in the spring will determine when the overwintering larvae hatch
- High temperature and humidity will create a microclimate in the fleece, attracting adult flies to lay eggs

Rainfall
- Persistent rainfall will make the fleece microclimate attractive to adult flies. Females lay eggs after the rain ceases
- Breeds with long, fine wool are the most susceptible

Host susceptibility
- This is increased when putrefactive odours develop on the fleece due to bacterial decomposition of organic matter following soiling with urine or faeces
- Merino sheep have a narrow breech area with excessive wrinkling making them more susceptible to soiling
- A narrow opening of the penile sheath in rams and wethers may result in accumulation of urine and increase blowfly strike in this area
- Cuts during shearing, fighting and barbed wire will also increase the incidence of blowfly strike

Blowfly season

Temperate regions in June to September
- Mostly in unshorn sheep in June
- Lambs from July to September

Warmer regions have a more prolonged season due to greater number of blowfly generations

Pathogenesis

Severe skin damage
- Larvae lacerate skin with oral hooks and liquefy host tissue by secreting proteolytic enzymes

Skin lesions are extended and deepened
- Secondary blowfly attack
- Flies attracted to odour of decomposing tissue

Secondary bacterial infection

Production losses
- Irritation and distress associated with skin lesions
- Poor weight gain (often the first clinical sign)

Risk of strike is highest in warm, moist weather

Clinical signs

Anorexia
Listlessness
Animals standing apart from flock

Fleece may appear darker, be damp and have a foul smell

Control

Prophylactic insecticide treatment
- Must kill larvae and remain in fleece to prevent flies from laying eggs
- Applied by spraying, dipping, spray race or jetting

Insect growth regulators
- Pour-on
- 2-4 month protection depending upon the product used

Effective worm control
- To minimise diarrhoea and therefore soiled fleece

Crutching to prevent soiling

'Mule's operation'
- Surgical removal of breech skin in Merino breeds

Tail docking of lambs

Proper carcass disposal
- Eliminates fly breeding sites

Vaccination
- Experimental use against Luculia cuprina in Australia

Screw Worm Myiasis

Screw worm larvae - John Kucharski - Wikimedia Commons

C. bezziana cause myiasis in both animals and humans

Located mainly in tropical regions

Larvae are obligate parasites

Recognition

Similar to Calliphora spp.
- Iridescent
- Clear wings
- Blue abdomen

Longitudinal stripes on thorax

Larvae have bands of spines
- Look like screws

Life cycle

Eggs laid in wounds or body cavities

Larvae feed as colonies

Larvae drop to the ground to pupate

Pathogenesis

Spiracles are exposed as larvae feed which expands the wound
- Creates a foul smelling lesion

Cause irritation and pyrexia

Control

In the USA
- Mass eradication through the release of sterile males
- Currently only persists where flies have migrated across the Mexican border

In Africa
- Introduced into Libya through the importation of infested livestock
- Sterile males released
- Eradication occurred in 1991

Maggot Debridement Therapy

Maggot therapy on a wound - Wikimedia Commons

Human medicine
- Sterile Lucilia sericata maggots used to treat infected and necrotic wounds
- Larvae secrete proteolytic enzymes and antimicrobial agents into the wound
- Larvae do not burrow under the skin or attack healthy tissue

Veterinary medicine
- Published reports rare
- Recently used successfully to treat a suppurative wound in a donkey that did not respond to conventional medical methods and surgery