Flystrike in rabbits

Dr Molly Varga examines flystrike in rabbits, a preventable condition that refers to the damage done to skin when fly eggs are laid on its surface.

Flystrike (blowflystrike, myiasis) is a regular occurrence in pet rabbits during warmer months. It is a preventable condition that compromises the welfare of the affected animal(s) and can be fatal.

Flystrike refers to damage done to skin when fly eggs are laid on its surface. When these eggs hatch and become maggots, further damage to compromised skin occurs as the maggots feeding results in infected wounds.

It is important to remember that normal dry, clean skin that is free of caecotrophs will not attract flies. Therefore, by definition there is likely to be an underlying disease process affecting the rabbit’s willingness or ability to eat caecotrophs or the skin integrity itself (for example, urine scalding, abscesses, wounds).

In the UK, most cases of flystrike are due to Lucilia sericata. Since flystrike is multifactorial in nature, simply removing maggots and successfully treating wounds does not produce resolution of the underlying problem and certainly won’t prevent recurrence.

Pathogenesis and clinical signs

Understanding the pathogenesis of flystrike relies on the concept that clean, dry skin will not be affected. If the skin is not clean and dry, there is either:

• A disease process that is affecting grooming (for example, head tilt secondary to inner ear disease, or Encephalitozoon cuniculi infection, osteoarthritis, or spondylosis)
• A disease process that is directly affecting the skin (for example, urine scalding, cutaneous abscess, or bite wounds).

Once the fly is attracted to the area of skin, it lays eggs on the surface. Depending on the weather conditions, eggs can hatch as rapidly as within 12 hours, to form 1st stage maggots. At this stage, the maggots are tiny and not harmful, however within three days they will moult to 2nd and then 3rd stage maggots. These are larger and will cause direct damage to the skin. Maggots will not develop if the environmental temperature is below 9 degrees Celsius, or if the humidity in the local area of the skin is below 90%.

Most rabbits do not appear to experience pain from the wounds caused by maggots, however they often seem off colour and may well be lethargic and inappetant. Once an owner looks more closely at their animal, wounds and often the maggots are generally visible.

Diagnosis

Diagnosis of flystrike may be made on clinical signs alone - finding maggots on or in skin wounds is sufficient. However, it must be noted that it is not sufficient to diagnose and treat flystrike without also addressing the underlying causes. A thorough clinical examination, with additional diagnostics such as radiography, urinalysis and bloodwork are often warranted. Definitive diagnosis of the inciting cause will lead to better ongoing management and ultimately prevention of recurrence.

Treatment

Initial treatment of flystrike must comprise analgesia, treatment of infection, addressing toxic shock if this is present, as well as removing or killing any live maggots remaining. Secondary treatment involves clipping and cleaning the affected area of skin, wound debridement and irrigation, and topical treatment to encourage granulation and healing.

Analgesia in these cases can comprise both non-steroidal anti-inflammatory drugs, for example meloxicam, as well as an opiate (buprenorphine) or tramadol. It is thought that maggots may excrete some form of local anaesthetic causing wounds not to be painful whilst they are active inside, however these wounds will start to hurt when the maggots are removed.

Antibiotics are indicated in cases of flystrike, both to address the immediate damage caused by the maggots, but often also for some of the potential underlying causes, such as abscesses. It is preferable, but often impractical in these cases, to prescribe on the basis of culture and sensitivity results, so using a licensed medication initially is sensible. While fluoroquinolones are not first line antibiotics, many of these rabbits are unwell enough with significant enough infection to justify at least consideration of their use.

Should the rabbit present in a state of shock, then fluid therapy, active warming and possibly even corticosteroids, in severe cases, are indicated. Crystalloids can be given intravenously at 2-4ml/kg/hour as long as the rabbit is carefully monitored. Once the rabbit is stable, then the wound area can be clipped, cleaned and any visible maggots manually removed. This may be painful and the use of sedation or local anaesthesia may facilitate this procedure.

Avermectin preparations are commonly administered to kill any remaining maggots, for example injectable ivermectin, or authorised topical preparations (ivermectin or selamectin – off license).

Topical wound care is a vital part of the healing process for the damaged skin. After initial irrigation, where use of disinfectants may be warranted, care should be taken to preserve the granulation tissue during cleaning, restricting irrigation fluids to saline after the gross contamination has been removed. Wound gels (such as Intra Site) can be used to keep the granulation bed moist, and silver sulfadiazine creams can be used if infection appears deep seated.

Sometimes despite aggressive care, infection and toxic shock can be insurmountable for these rabbits, and euthanasia should be discussed at the initiation of care rather than further down the line. Owners need to be aware, that getting past the immediate problem is not enough, and an evaluation of what has caused it will still need to take place.

Prevention

An understanding of the underlying causes of flystrike is critical for its prevention in the future. Management of these underlying causes, in combination with owners checking their rabbits regularly, is the ideal option.

During the warmer months, owners are advised to check their rabbits twice daily, particularly those areas that could get soiled, for example the perineum or under the dewlap. Use of fly screens or flystrips can also be helpful. There are topical preparations that are authorised to prevent flystrike - cyromazine (Rearguard) or sprays that are insecticidal that can be applied (F10 with insecticide spray). Using these products is certainly helpful but not a replacement for careful monitoring or treating the underlying causes.

References

• Jenkins JR (2001) Skin disorders of the rabbit. Veterinary Clinics of North America-Exotic Animal Practice 4(2), 543-563
• Paterson, S, Varga M (2017) Skin diseases of rabbits. In: Ferrets, Rabbits and Rodents. Clinical Medicine and Surgery 4th Edition WB Saunders, Philadelphia
• White SD Bourdeau PJ, Meredith A (2003) Dermatologic problems of rabbits. Compend. Continuing Education for Veterinarians 25, 90-101.