What is myiasis?
Parasitism of living animals by the larvae of dipteran insects (flies). Of interest is the parasitic larval stage -- also called a maggot -- instead of the adult fly.
Myiasis can be classified in a couple of different ways:
1. By the parasiitic behaviour of the maggots
2. By the location of the parasitism on the host
What are the classifications?
1. Facultative or Obligatory - based on whether the larvae must become parasitic to complete its life cycle (obligatory) or whether the parasitic stage is optional (facultative)
2. Cutaneous, Nasal or Somatic - based on location of larval parasitism on the live animal; note that myiasis-causing flies are NOT all parasites of the skin. Some are respiratory and GIT parasites.
What are the two main FAMILIES of Diptera (order) that have myiasis-producing flies?
Oestridae & Calliphoridae
Which family of flies is facultative myiasis-producing, and which is obligatory myiasis-producing?
Calliphoridae flies are faculative: eg., greenbottles, bluebottles
Oestridae flies are obligatory: eg. warble flies, bot flies
Oestridae are truly grosser of the two.
Genera of the Oestridae family are obligatory myiasis-producing flies. What are these genera, and which is the one that affects skin?
Warble flies (species Hypoderma) - this is the one that leaves big holes in the skin
Horse bot fly (Gasterophilus) - targets GIT
Sheep nasal bot fly (Oestrus) - targets URT (nose)
What are some of the species of warble fly?
Hypoderma bovis, H. linneatum - cattle (eradicated in the UK)
H. diana - deer, sheep & horses (rare in UK)
How would you recognise a warble fly?
It is LARGE (13-15 mm).
Looks like a bumble-bee but with only ONE PAIR OF WINGS.
What is the life cycle of the typical warble fly that affects cattle?
ONE GENERATION PER YEAR:
Eggs laid in summer around host belly & legs, attached to hair by clasp & cement
few days later larvae hatch,
enter skin via insect-bite wounds
migrate SQ to diaphragm via mouth hooks & enzymes
3 months later reach
WINTER RESTING SITES:
epidural fat in spinal canal for H. bovis & oesophagus for H. linneatum
moult to L2 stage during winter (Nov.-March)
Spring: migrate to midline of back, bore breathing hole, moult to L3 “warbles” (see below)
4-10 weeks later, larvae emerge, fall to ground, pupate under veg
1 month later, adults emerge, copulate, lay eggs & die w/i one-two weeks (vestigial mouths, live off fat reserves)
What are the main differences between the life cycles of the two Oestridae warble-fly species, Hypoderma bovis & H. linneatum?
1. Winter resting sites:
H. bovis rests in epidural fat of the spinal canal - think of the damage to the spinal cord
H. lineatum rests in the wall of the oesophagus - think of bloat
2. H. lineatus adults emerge about six-eight weeks before H. bovis
What are the main veterinary &/or economic issues behind cattle infection by warble flies?
1. Downgrading of hides by warbles' bore holes
2. Production losses & injury due to gadding (running away, freaking out at sight of warble flies)
3. Trimmed-meat losses - green-coloured butcher’s jelly around larvae due to eosinophils around oesophagus (H. linneatum) & warbles under skin
4. Paraplegia due to larval death & toxin release / pressure on spinal cord with H. bovis
5. Bloat due to luminal blockage in oesophagus from dying larvae with H. linneatum
Warble fly (H. bovis) has been eradicated since 1978 legislation made it a notifiable disease, the introduction of Ivermectin and cooler summers.
What is used to treat it now (even though it's eradicated in UK)?
Systemic organophosphorous (OP) insecticides: transdermally injected; NB OP can only be used while larvae migrating from diaphragm (before Dec. 1) or when they’ve left winter resting sites (in spring) or it can cause anaphylaxis
Avermectins & Milbemycins - antimicrobials injectible & pour-on
Before, people used to just "pop out" the warbles under the hides, but there was a danger of rupture that could cause bronchospasm/anaphylactic shock
While Oestridae (family) are the disgusting obligatory myiasis-producing flies (warble flies, horse & sheep bot flies), the family Calliphoridae includes facultative myiasis-producing flies that cause "blowfly myiasis".
What are the main blowfly species in Europe?
Greenbottles - Luciata sericata
Blackbottles - Phormia terra-novae
Bluebottles - Calliphora erthyrocephala & C. vomitoria
What veterinary species is most affected by Calliphora blow-fly species in the UK?
What does it mean to say a sheep is blown?
What does it mean to say a sheep is struck?
Calliphora species such as greenbottles, blackbottles or bluebottles have laid eggs in the fleece.
A struck sheep means the eggs have moulted to larvae deep within the fleece.
What is the danger to sheep that have been struck by a blowfly?
Severe skin damage
Skin lesions extended & deepened by secondary blowflies
Secondary bacterial infection
What is the life cycle of a Calliphora blow fly?
Life cycle from egg to adult is about two weeks minimum.
Yellow-egg clusters laid in wounds, soiled fleece or carrion
24 hrs later, larvae hatch, crawl to skin, grow & moult 2x - this is when they cause lots of damage
1-2 weeks later, fully mature maggots ↓
fall to ground, pupate
1 week later (summer), adult emerges (pupae can also overwinter)
female reaches sexual maturity when she has protein meal, lays eggs
adults live ~ 1 month
How do the lengths of life cycle differ between obligatory myiasis-producing flies and facultative?
Obligatory (Oestridae - warble flies & bots): one generation a year
Facultative (Calliphorae - blowflies): 4-10 generations a year
Define the terms primary, secondary, tertiary blowfly in the context of facultative cutaneous myiasis.
Primary, secondary & tertiary refer to the blowfly’s ability to initiate a strike in a host & thus create a wound. The three types can work together to worsen a strike.
Primary blowfly: Larvae can penetrate intact skin, initiating a strike. Eg. Lucilia spp & Phormia spp.
Secondary: Can’t initiate, can only attack area already struck & extend area of damage. Eg. Most Calliphora spp & Chrysomya spp
Tertiary: Attack struck, dry carcass. Eg. Musca spp, Sarcophaga spp.
What are environmental and hygiene factors that pre-dispose sheep to a primary blowfly strike?
- Temperature: Warmth of spring & early
summer determine when pupae hatch
- Rainfall: Wet fleece attractive to adult
female laying eggs
- Host susceptibility: urine, faeces on
fleece, shearing & other wounds,
What is pizzle rot?
Blow-fly strike in sheep penile sheath.
How can blow-fly strike be prevented?
How can a blown sheep be treated?
Prophylactic insecticide treatment - kill larvae with hand spray, plunge dip, spray race, jetting
Insect Growth Regulators - hormone analogues that prevent further devt of larval stages. Eg. Cyromazine (pour- on, up to 2 months’ protection), Dicyclanil (pour-on, up to 4 months’ protection)
What do Lucilia seratica (greenbottles) blow flies look like?
Note especially colour of thorax and abdomen & wings.
As adults, they're medium sized (less than 10 mm long) with green metallic sheen to thorax & abdomen. They are very common.
They have clear wings.
What do the maggots (larvae) of the blow flies look like? How are they identified?
They are smooth.
About 10-15 mm long.
They have spiracles and stigmatic plates on the tail, which are used for identification of species.