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

Wohlfahrtia spp.

• Obligatory parasite

• Occurs in North America

• Parasite of mink and sometimes humans

Recognition

• Large 8-14mm long

• Pale grey

• Black stripes on thorax

• Black spots on abdomen

Life cycle

• Larvae deposited directly onto host

• No egg stage

• Larvae moult twice on host then fall to ground to pupate

Pathogenesis

• Larvae penetrate intact skin
  • Cause boil like swellings

Links

• Viruses

• Ectoparasiticides

• Blowfly strike