Myiasis Producing Flies

INSECTA PARASITES

Introduction

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

Oestridae

• The larvae of Oestridae spp. are obligatory parasites

• The three important species of veterinary interest are Hypoderma, Oestrus and Gasterophilus

Hypoderma spp.

• Also known as warble flies

• Important cattle parasites
  • Also parasitise horses, small ruminants and sometimes humans

• The two main species in cattle are H. bovis and H. lineatum

• H. diana parasitises deer

• Infestation is largely confined to the Northern Hemisphere

Recognition

• Large 13-15mm long

• Similar to bumble bees in appearance
  • Yellow abdomen
• Band of black hairs

• One pair of wings

Life Cycle

• Adult flies emerge on warm, sunny days between June and August

• White eggs are laid around the belly and legs of the animal
  • Eggs are attached to hairs by cement and a small terminal clasp
  • H. lineatum lays a whole row of eggs as it approaches the animal stealthily
  • H. bovis only deposits a single egg before the animal runs away ('gadding) as it makes a loud buzzing noise

• The adult lives for 3 weeks

• Females can lay several hundred eggs in their life

• A few days after eggs are laid, larvae emerge and crawl down the hairs into the hair follicles

• Larvae penetrate the skin through wounds made by biting flies

• Larvae migrate through the subcutaneous tissue towards the diaphragm
  • Enzymatic secretions and mouth hooks aids larval migration

• After 3 months larvae reach winter resting sites where they remain from November to Feburary/March whilst moulting to the L2 stage
  • Epidural fat in the spinal canal for H. bovis
  • Wall of the oesophagus for H. lineatum

• Larvae then migrate back to the subcutis along the midline of the back where they bore a breathing hole through the skin and moult to the L3 stage
  • Larvae are palpable as distinct swellings called warbles

• L3 larvae emerge after 4-10 weeks where they fall to the ground and pupate under loose vegetation

• Adult flies emerge from the pupa after one month where they copulate, lay eggs and die within two weeks

• H. lineatum are on the wing 6-8 weeks before H. bovis

• There is only one generation of warble flies per year

Pathogenesis

• Causes myositis

• Production losses
  • Condemnation and down-grading of hides
  • Reduced milk yield and reduced weight gain
  • Injury from stock panic
  • Trimmed meat losses from H. lineatum
    • 'Butcher's Jelly' around warbles which is green due to mass eosinophil attraction

• Paraplegia resulting from:
  • Toxin release
  • Pressure on the spinal cord (H. bovis)
  • Bloat from pressue on the oesophageal wall (H. lineatum)

Control

• Total eradication should be aimed for

• Old methods include popping out warbles
  • But could lead to anaphylactic shock

• Ectoparasiticides
  • Systemic organophosphorus insecticides in pour-on formula
  • Avermectins and milbemycins in pour-on and injectible formulations

• Timing is crucial for treatment
  • Larvae residing in winter resting sites, if killed, can lead to bloat and paraplegia
  • It is safe to treat in the autumn before larvae reach their winter resting sites and in the spring when the warbles have migrated to the midline of the back
  • Ivermectin can be given at any time without risking host infection as larval antigen is released much slower

Legislation in the UK

• 'Warble Fly Order 1978' requires all clinically affected animals to be treated

• Notifiable disease

• 'Warble Fly Infected Area Order 1983'

• For more information on the warble fly orders, see here

Oestrus ovis

• Also known as the sheep nasal bot fly

• Larvae parasitise the nasal chambers of sheep and goats

• Found in most sheep rearing areas of the world

Recognition

• 13-15mm long

• Grey colouring

• Black spots on abdomen

• Clear wings

• Larvae have distinct black bands on each body segment

Life Cycle

• Larvae are squirted into the nostils of sheep in a jet of liquid

• The larvae crawl caudally into the nasal cavity and feed on the nasal mucosa and mature before returning to the nostrils

• Larval development takes up to two months
  • Larvae can overwinter in the nasal cavity if deposited late in the summer

• Once the larvae have developed they are sneezed out and pupate on the ground

• The adult fly emerges one months later

• Adult flies only live for 2-3 weeks

Pathogenesis

• Adult flies can annoyance
  • Interrupts feeding
  • Leads to a decreased weight gain

• Larvae cause nasal irritation, nasal discharge and sneezing
  • Irritate the nasal mucosa with oral hooks and spines causing a viscous exudate to be produced from which they feed
  • Heavy infestations lead to erosion of the bones in the sinuses (turbinate bones)
  • Penetration of the brain leads to false gid (high stepping gait and incoordination)

• Apthology in the nasal cavity

Control

• Systemic insecticides can be used in heavy infestations

• In warmer countries, strategic prophylactic treatment can be used

Gasterophilus spp.

• Also known as the horse bot fly

Recognition

Life Cycle

Pathogenesis

Control

Dermatobia hominis

Recongition

Life Cycle

Pathogenesis

Control

Calliphoridae

Recongition

Life Cycle

Pathogenesis

Control

Screw Worm Myiasis

Recongition

Life Cycle

Pathogenesis

Control

Wohlfahrtia sp.

Recongition

Life Cycle

Pathogenesis

Control

Links

• Viruses

• Ectoparasiticides