Focus on disease: Flystrike in rabbits
Dr Molly Varga BVetMed DZooMed MRCVS
Published September 2018
Flystrike (blowflystrike, myiasis) is a regular occurrence in pet rabbits during the 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 are feeding resulting 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 (eg urine scalding, abscesses, wounds). In the UK most cases of flystrike are due to Lucilia sericata. Because 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 either there is a disease process that is affecting grooming (eg head tilt secondary to inner ear disease or E.cuniculi infection, osteoarthritis, or spondylosis) or there is a disease process that is directly affecting the skin (urine scalding, cutaneous abscess, 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 3 days these moult to 2nd and then 3rd stage maggots, which are larger and will cause direct damage to the skin. Maggots will not develop if the environmental temperature is below 9 degrees centigrade, or is 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 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.
Initial treatment of flystrike must comprise analgesia, treatment of infection, addressing toxic shock if this is present as well as removing or killing and 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 (eg 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 flurorquinolones 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 ins 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 maybe 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, and 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.
An understanding of the underlying causes of flystrike is critical for its prevention in the future. Management of these 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- eg 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.
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