Cutaneous Blowfly Myiasis – Rabbit

Introduction

Blowfly larvae are frequently found on the soft skin of the underside of the neck but more commonly at the vent of pet rabbits and are usually accompanied with some degree of internal malaise in the affected rabbit, sometimes precedent, sometimes subsequent to the infestation. In general the flies are attracted by putrefactive sulphur-rich volatile compounds produced by bacteria whereas the presence of ammonia encourages oviposition. The first stage larvae are non-pathogenic. It’s only the second and third stages that cause tissue damage.

Predisposing Causes: when considering accompanying internal illnesses, include uterine hyperplasia and coccidiosis.

Clinical Signs

• Depressed and dehydrated
• Necrotic smell – a characteristic odour
• Usually perineal or caudal fold lesions
• The presence of ages and larvae
• Underlying disease(s)

Assessment of myiasis

• Rafts of eggs are seen on the fur
• L1’s are only a few millilitres long and hard to see and are non-pathogenic
• L2’s and L3’s are associated with considerable soft tissue damage
• Tertiary damage (cf. burn injuries) is indicative of euthanasia

Development of Lucilia serricata in hours (Cousquer 2006)

°C	Eggs hrs	L1 hrs	L2 hrs	L3 hrs	Prepupa hrs	Pupa hrs	Total days
16	41	53	42	98	148	393	32
21	21	31	26	50	118	240	20
27	18	20	12	40	90	168	14

Treatment

The subject should therefore be approached as a very sick animal and fluids and other supportive remedies given as required to hasten recovery.

• Analgesics should not be omitted from the treatment regime
  • carprofen (Rimadyl®: Pfizer) IM
  • ketoprofen (Ketofen®; Merial Animal Health) IM
• Fluids should be given intraperitoneally or intravenously, I favour intraperitoneal - a bolus can be given to avoid too much handling of the patient
• antibiotics usually oxytetracycline (Engemycin® 5%; Intervet) SC

Then attend to the removal of the offending eggs and larvae and to prevent any further damage. It is often surprising how much damage the larvae can produce without causing the death of the rabbit. The animal is placed on a grid (a cake-cooling tray) over a cat litter tray. The area is sprayed with ivermectin (Panomec®; Merial Animal Health diluted 1:10 with water, not propylene glycol (due to its irritancy especially on broken skin), keeping within the dose rate although Okerman (1994) reports good results after a simple injection SC) and the maggots are picked off individually with a thumb forceps or washed through the grid to the tray underneath which may contain an insecticide solution. A nit comb is useful for removal of the eggs and dry heat (from a hair-dryer) reduces development and activates the larvae making therm more easy to see and remove.

• Manage the wound
• Treat the underlying disease

Management of Myiasis Wounds

• Cleansing
• Hydrogels inhibit larval development
• Intrasite
• Vapour permeable dressings
• Opsite
• Suturing dressings in place
• Granuflex or Duoderm

(See Cousquer 2006)

Prevention of Blowfly Myiasis

• Hygiene and the removal of the predisposing cause are obviously of prime importance in the prevention of this distressing condition.
  • Diet correction is nearly always necessary.
  • Entire does should be spayed.
• Fly repellent
  • Permethrin (Xenex Ultra)
    • Larvicide
    • Adulticide
    • Fly repellent
    • lasts two weeks
  • Cyromazine (Rear Guard)
    • prevents L1 – L2
    • does not repel flies
    • lasts 10 weeks

Note on permethrin (Xenex Ultra®)

The Kill Time50 of permethrin on adult flies is 4 – 4.5 minutes and it is used to repel mosquitoes and green bottle. Permethrin also kills maggots: In vitro 500 mcgm per cm2 kills 100% of all larval forms in 5 minutes. However, it is not recommended to apply Xenex Ultra to the skin unless it’s intact because it’s irritant - permethrin is a wax and requires propylene glycol as a solvent to allow the product to flow from the phial onto the rabbit’s skin. It also takes a while to build up at wound sirtes etc when administered percutaneously so is not recommended for use in acute cases of myiasais requiring treatment – it takes too long to be absorbed from the site of application and then to work through the discharges and matted fur around the site of infestation.

Emergency procedure

Give ivermectin, oxytetracycline, the NSAID and fluids, keep the animal warm and wait for an effect for an hour or two before you pick the maggots off. Mild antiseptics, 1% chlorhexidine (Vetasept®; Animalcare) or 10% povidone iodine (Tamodine®; Vetark Professional), are syringed into the various cavities made and/or enlarged by the larvae. I prefer to have the rabbit placed on a cake-cooling tray during the procedure (the wire mesh allows the maggots to fall through and keeps the fur of the rabbit reasonably dry – this is very important as wetting can lead to further fur loss and hypothermia), usually under hypnosis or deep sedation or anaesthesia (ketamine/xylazine).

Prevention

• Hygiene and the removal of the predisposing cause are obviously of prime importance in the prevention of this distressing condition.
• Diet correction is nearly always necessary.
• Entire does should be spayed.
• A fly repellent may be helpful but may be impractical.
  • Avermectins may be useful
  • Fipronil (Frontline®; Merial) is toxic to rabbits.

Cyromazine (Rearguard®; Novartis) is licensed for use in rabbits as a preventative – it prevents the moult from L1 to L2 and has a ten week duration of action.

References

• Cousquer, G. (2006) Veterinary care of rabbits with myiasis In Practice 28 342 – 349
• Okerman, L. (1994) Diseases of Domestic Rabbits. Blackwell Scientific Publications 2nd Edition