Myiasis Producing Flies
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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
- The larvae of Oestridae spp. are obligatory parasites
- The three important species of veterinary interest
- Hypoderma
- Oestrus
- 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
- 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
- 1 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 aid larval migration
- After 3 months larvae reach winter resting sites where they remain from November to February/March whilst moulting to the L2 stage. These sites are different between the 2 species
- 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 1 month where they copulate, lay eggs and die within 2 weeks
- H. lineatum are on the wing 6-8 weeks before H. bovis
- There is only 1 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 pressure on the oesophageal wall (H. lineatum)
Control
- Total eradication should be aimed for
- Old methods include popping out warbles
- But this 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 nostrils of sheep in a jet of liquid by the adult fly
- 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 2 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 1 months later
- Adult flies only live for 2-3 weeks
Pathogenesis
- Adult flies cause 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)
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
- Obligate parasites of equids
- Spend most of life cycle in the equine stomach
- Of little pathogenic significance
- Three important species (in the UK)
- G. intestinalis which is the most common
- G. nasalis
- G. haemorrhoidalis which is rare
- Two other important veterinary species
- G. nigricornis
- G. inermis
Recognition
- Medium to large flies at 10-20mm long
- Look similar to drone bumble bees
- Body covered with dense yellow hair
- Dark coloured hairs produce a banding pattern
- Clear wings with brown patches
Life cycle
- Adults are most active in late summer
- Eggs are laid on the body and hatch spontaneously or are stimulated to hatch through an increase in warmth and moisture from the animal self-grooming. They are laid in different areas according to species
- G. intestinalis eggs are laid in the hair of the shoulders and fore legs
- G. nasalis eggs are laid in the intermandibular area
- G. haemorrhoidalis eggs are laid around the lips
- Creamy-white eggs
- 1-2mm in length
- Larvae crawl into the mouth and penetrate the tissues of the tongue and buccal mucosa
- Larvae then emerge after a couple of weeks and are swallowed
- Larvae pass into the stomach and attach to the gastric mucosa
- Larvae are now known as bots
- Each species attaches to a specific part of the stomach
- G. intestinalis attaches to the cardiac region
- G. nasalis attaches to the pylorus
- After 10-12 months in the stomach, the larvae detach and are passed out in the faeces
- G. haemorrhoidalis attaches to the rectal mucosa before being passed out
- Larvae pupate on the ground
- Adults hatch after 1-2 months and survive from a few days to 2 weeks
- Adults have non-functional mouthparts so cannot feed
- There is only 1 generation per year in temperate regions of the world
Pathogenesis
- Adults cause annoyance when egg laying
- Disturbance and panic can ensue
- Larvae cause a marked inflammatory reaction when attached to the gastric mucosa
- Ring like thickening around the base of each attached larvae
- Large numbers of larvae may interfere with the passage of food and action of gastric sphincters
- G. haemorrhoidalis can cause mild irritation to the rectal wall
- Host reaction to larvae in the mouth is minimal
Control
- Treatment of horses with insecticides over winter
- Breaks the life cycle as all the population are present as bots in the stomach
- If eggs are present in late summer, the horse's coat can be sponged with an insecticide
- Stimulates hatching
- Kills larvae
Dermatobia hominis
- Also called the human bot fly
- Larvae are important parasites of both humans and animals
- Specifically found in South America
Recognition
- Adult can grow up to 25mm in length
- Similar to Calliphora in appearance
- Blue-black
- Yellow-orange head and legs
- Larvae are distinctive as they taper towards the posterior end
Life cycle
- Eggs laid on blood sucking flies such as mosquitoes
- These hatch when the mosquito next lands on a warm blooded animal
- Larvae penetrate skin causing painful swellings
- Larvae emerge after 35-42 days and fall to ground to pupate
- 4 month life cycle
Pathogenesis
- In humans, the larvae are most often found in swellings on the head and limbs
- Larvae cause painful swellings and distress to cattle
- Larvae cause production losses
- Wounds caused by exiting larvae can increase the prevalence of attack by other myiasis flies
Calliphoridae
- 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
- Callipohora 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
- 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
- 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