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

Warble Fly - Geo. H. Carpenter, Cambridge University Press 1913 - Wikimedia Commons

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)

Pathology in the nasal cavity

Control

Systemic insecticides can be used in heavy infestations

In warmer countries, strategic prophylactic treatment can be used

Gasterophilus spp.

Horse Bot Fly - Dennis Ray - Wikimedia Commons

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

Human Bot Fly - Captain R. Goodman, United States Air Force

Human Bot Fly Larvae - Captain R. Goodman, United States Air Force

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

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