Parasitology - Myiasis-producing flies Flashcards
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:
- By the parasiitic behaviour of the maggots
or
- By the location of the parasitism on the host
What are the classifications?
- 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)
- 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.
See photo.
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?
- Downgrading of hides by warbles’ bore holes
- Production losses & injury due to gadding (running away, freaking out at sight of warble flies)
- Trimmed-meat losses - green-coloured butcher’s jelly around larvae due to eosinophils around oesophagus (H. linneatum) & warbles under skin
- ** Paraplegia due to larval death & toxin release** / pressure on spinal cord with H. bovis
- 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?
Sheep
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
↓
Production losses
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
(4-10 generations/year)
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,
wrinkly skin
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?
Prevention:
Prophylactic insecticide treatment - kill larvae with hand spray, plunge dip, spray race, jetting
Treatment:
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.
