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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.
 
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.
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==Oestridae==
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'''[[Oestridae|''Oestridae'']]
[[Image:Warble flies.jpg|thumb|right|150px|Warble Fly - Geo. H. Carpenter, Cambridge University Press 1913 - Wikimedia Commons]]
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*The larvae of ''Oestridae spp.'' are '''obligatory''' parasites
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*The three important species of veterinary interest
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**''Hypoderma''
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**''Oestrus''
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**''Gasterophilus''
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===''Hypoderma spp.''===
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*Also known as warble flies
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*Important cattle parasites
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**Also parasitise horses, small ruminants and sometimes humans
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*The two main species in cattle are
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**''H. bovis''
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**''H. lineatum''
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*''H. diana'' parasitises deer
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*Infestation is largely confined to the Northern Hemisphere
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'''Recognition'''
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*Large 13-15mm long
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*Similar to bumble bees in appearance
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**Yellow abdomen
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**Band of black hairs
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*1 pair of wings
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'''Life cycle'''
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*Adult flies emerge on warm, sunny days between '''June and August'''
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*White eggs are laid around the belly and legs of the animal
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**Eggs are attached to hairs by cement and a small terminal clasp
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**''H. lineatum'' lays a whole row of eggs as it approaches the animal stealthily
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**''H. bovis'' only deposits a single egg before the animal runs away ('''gadding''') as it makes a loud buzzing noise
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*The adult lives for 3 weeks
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*Females can lay several hundred eggs in their life
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*A few days after eggs are laid, larvae emerge and crawl down the hairs into the hair follicles
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*Larvae penetrate the skin through wounds made by [[Biting Flies|biting flies]]
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*Larvae migrate through the subcutaneous tissue towards the diaphragm
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**Enzymatic secretions and mouth hooks aid larval migration
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*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
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**Epidural fat in the spinal canal for ''H. bovis''
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**Wall of the [[Oesophagus - Anatomy & Physiology|oesophagus]] for ''H. lineatum''
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*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
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**Larvae are palpable as distinct swellings called '''warbles'''
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*L3 larvae emerge after 4-10 weeks where they fall to the ground and pupate under loose vegetation
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*Adult flies emerge from the pupa after 1 month where they copulate, lay eggs and die within 2 weeks
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*''H. lineatum'' are on the wing 6-8 weeks before ''H. bovis''
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*There is only '''1 generation''' of warble flies per year
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'''Pathogenesis'''
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*Causes [[Muscles Inflammatory - Pathology#Arthropods|myositis]]
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*Production losses
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**Condemnation and down-grading of hides
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**Reduced milk yield and reduced weight gain
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**Injury from stock panic
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**Trimmed meat losses from ''H. lineatum''
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***'Butcher's Jelly' around warbles which is green due to mass [[Eosinophils - WikiBlood|eosinophil]] attraction
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*Paraplegia resulting from
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**Toxin release
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**Pressure on the spinal cord (''H. bovis'')
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**Bloat from pressure on the [[Oesophagus - Anatomy & Physiology|oesophageal wall]] (''H. lineatum'')
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'''Control'''
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*Total eradication should be aimed for
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*Old methods include popping out warbles
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**But this could lead to anaphylactic shock
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*[[Ectoparasiticides]]
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**Systemic organophosphorus [[Ectoparasiticides|insecticides]] in pour-on formula
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**Avermectins and milbemycins in pour-on and injectible formulations
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*Timing is crucial for treatment
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**Larvae residing in winter resting sites, if killed, can lead to bloat and paraplegia
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**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
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**Ivermectin can be given at any time without risking host infection as larval antigen is released much slower
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'''Legislation in the UK'''
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*'Warble Fly Order 1978' requires all clinically affected animals to be treated
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*Notifiable disease
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*'Warble Fly Infected Area Order 1983'
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*For more information on the warble fly orders, see [http://www.defra.gov.uk/animalh/diseases/notifiable/brucellosis/surveillance.htm| here]
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===''Oestrus ovis''===
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*Also known as the sheep nasal bot fly
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*Larvae parasitise the nasal chambers of sheep and goats
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*Found in most sheep rearing areas of the world
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'''Recognition'''
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*13-15mm long
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*Grey colouring
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*Black spots on abdomen
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*Clear wings
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*Larvae have distinct black bands on each body segment
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'''Life cycle'''
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*Larvae are squirted into the nostrils of sheep in a jet of liquid by the adult fly
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*The larvae crawl caudally into the nasal cavity and feed on the nasal mucosa and mature before returning to the nostrils
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*Larval development takes up to 2 months
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**Larvae can overwinter in the nasal cavity if deposited late in the summer
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*Once the larvae have developed they are sneezed out and pupate on the ground
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*The adult fly emerges 1 months later
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*Adult flies only live for 2-3 weeks
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'''Pathogenesis'''
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*Adult flies cause annoyance
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**Interrupts feeding
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**Leads to a decreased weight gain
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*Larvae cause nasal irritation, nasal discharge and sneezing
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**Irritate the nasal mucosa with oral hooks and spines causing a viscous exudate to be produced from which they feed
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**Heavy infestations lead to erosion of the bones in the sinuses (turbinate bones)
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**Penetration of the brain leads to '''false gid''' (high stepping gait and incoordination)
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*[[Respiratory Parasitic Infections - Pathology#Oestrus ovis|Pathology in the nasal cavity]]
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'''Control'''
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*Systemic [[Ectoparasiticides|insecticides]] can be used in heavy infestations
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*In warmer countries, strategic prophylactic treatment can be used
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===''Gasterophilus spp.''===
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[[Image:Horse Bot Fly.jpg|thumb|right|150px|Horse Bot Fly - Dennis Ray - Wikimedia Commons]]
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*Also known as the horse bot fly
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*Obligate parasites of equids
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*Spend most of life cycle in the equine stomach
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**Of little pathogenic significance
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*Three important species (in the UK)
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**''G. intestinalis'' which is the most common
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**''G. nasalis''
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**''G. haemorrhoidalis'' which is rare
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*Two other important veterinary species
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**''G. nigricornis''
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**''G. inermis''
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'''Recognition'''
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*Medium to large flies at 10-20mm long
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*Look similar to drone bumble bees
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**Body covered with dense yellow hair
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**Dark coloured hairs produce a banding pattern
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*Clear wings with brown patches
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'''Life cycle'''
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*Adults are most active in '''late summer'''
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*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
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**''G. intestinalis'' eggs are laid in the hair of the shoulders and fore legs
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**''G. nasalis'' eggs are laid in the intermandibular area
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**''G. haemorrhoidalis'' eggs are laid around the lips
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*Creamy-white eggs
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*1-2mm in length
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*Larvae crawl into the mouth and penetrate the tissues of the tongue and buccal mucosa
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*Larvae then emerge after a couple of weeks and are swallowed
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*Larvae pass into the [[Forestomach - Anatomy & Physiology|stomach]] and attach to the gastric mucosa
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**Larvae are now known as '''bots'''
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*Each species attaches to a specific part of the stomach
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**''G. intestinalis'' attaches to the cardiac region
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**''G. nasalis'' attaches to the pylorus
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*After 10-12 months in the stomach, the larvae detach and are passed out in the faeces
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**''G. haemorrhoidalis'' attaches to the rectal mucosa before being passed out
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*Larvae pupate on the ground
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*Adults hatch after 1-2 months and survive from a few days to 2 weeks
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**Adults have non-functional mouthparts so cannot feed
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*There is only '''1 generation per year''' in temperate regions of the world
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'''Pathogenesis'''
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*Adults cause annoyance when egg laying
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**Disturbance and panic can ensue
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*Larvae cause a marked inflammatory reaction when attached to the gastric mucosa
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**Ring like thickening around the base of each attached larvae
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*Large numbers of larvae may interfere with the passage of food and action of gastric sphincters
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*''G. haemorrhoidalis'' can cause mild irritation to the rectal wall
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*Host reaction to larvae in the mouth is minimal
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'''Control'''
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*Treatment of horses with [[Ectoparasiticides|insecticides]] over winter
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**Breaks the life cycle as all the population are present as bots in the [[Forestomach - Anatomy & Physiology|stomach]]
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*If eggs are present in late summer, the horse's coat can be sponged with an [[Ectoparasiticides|insecticide]]
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**Stimulates hatching
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**Kills larvae
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===''Dermatobia hominis''===
 
===''Dermatobia hominis''===
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