38 Fly strike in a rabbit (myiasis)
Myiasis, or fly strike, is unfortunately a common problem with domestic rabbits, often with severe consequences. When attracted to the rabbit by certain conditions, flies will lay their eggs on the rabbit, which subsequently hatch into maggots (larvae). These maggots feed on the living tissue and undergo several moults, each time increasing their digestive capabilities.
The disease is very distressing for the owner, and will often go unnoticed for several days. More importantly, fly strike is extremely serious and painful for the rabbit, which not uncommonly necessitates euthanasia in advanced cases.
A 7-year-old male, neutered English lop rabbit (Oryctolagus cuniculus), weighing 1.8 kg, was presented in mid-July as an emergency, with a 2- to 3-day history of lethargy, inappetance and generally not being himself. The owner had noticed an increased smell from his hutch and whilst cleaning him out saw he had a wet and dirty rear end. On closer examination live maggots were found.
With the majority of sick rabbits, complete and thorough history taking is vital. The owners may not think the information is relevant to the presenting problem, but around three-quarters of problems in rabbits and other exotic pets are related to poor husbandry and the owners’ lack of knowledge about proper diets, housing and healthcare requirements.
On presentation, the rabbit was quiet and depressed, but responsive to stimulus. Clinical examination revealed an elevated heart rate of 280 beats per minute and fast respiration (approx. 60 breaths per minute), both of which could have been related to stress at being examined or pain. There was a small amount of salivary staining around the mouth and on the forepaws, suggesting increased salivation. Abdominal palpation demonstrated mildly dilated, gas-filled loops of bowel with less ingesta than normal, consistent with the inappetance. Otoscopic examination of the oral cavity revealed subjectively elongated teeth, with several lingual spurs on the lower arcades.
Thorough examination of the perineal region revealed maggots and lesions consistent with fly strike. The skin around the perineum, inguinal area and tail base was grossly inflamed, moist and exudative (Fig. 38.1). There was faecal and urine contamination of the area, causing matting of the fur in places (Fig. 38.2). Stage L1–L3 maggots were seen, suggesting that initial egg laying occurred at least 40 hours ago (Table 38.1). Several skin wounds were found in the perineal and tail base area, with L2 and L3 stage maggots moving in and out of these wounds (Fig. 38.3).
Careful and systematic examination, particularly around the tail base, perineum and inguinal area, is required to avoid missing early cases of myiasis. A common site for maggots to feed is in the two deep folds of skin either side of the anal orifice. These are the inguinal glands and in the healthy rabbit are normally filled with a yellow–brown odiferous deposit. The L1 stage maggots are only a few millimetres in length and can be missed during a brief examination. These larvae only feed on skin debris and exudate, and so will cause mild inflammation, but not gross skin damage alone. Clumps of small eggs, looking like grains of rice, may be seen within the coat. In older cases of myiasis, larger second- and third-stage larvae will be present, which will be easier to see and are likely to be associated with more severe soft tissue damage.
The clinical signs encountered with a case of myiasis will depend on the duration of the disease. In the very early stages (up to 18 hours at 27ºC), fly eggs will be present, but the eggs will not have hatched. This means no fly strike damage has yet occurred, but the rabbit may have been presented due to signs of the underlying disease, such as reduced appetite or perineal soiling. If fly eggs have already hatched, they will have begun attacking the epidermal tissues (to varying degrees depending on life stage) and the number of maggots will dictate the extent of lesions present. Environmental conditions of at least 60% humidity and 9–11ºC are needed for larval development.
The perineum is a very sensitive area and conscious examination should be kept brief, simply to assess the degree of damage. For more detailed examination and treatment, sedation or general anaesthesia (Table 38.2) must be used.
|Fentanyl/fluanisone||0.2–0.3 ml/kg i.m.|
• General anaesthesia can subsequently be induced with midazolam (0.5–2 mg/kg i.v.) to effect, or isoflurane by mask
• Long recovery (3–6 h) but can be shortened by antagonizing with buprenorphine (0.01–0.05 mg/kg i.m.) or butorphanol (0.1–0.5 mg/kg i.m.)
|Midazolam||0.5–2.0 mg/kg i.v.|
|Medetomidine and ketamine|
|Medetomidine, ketamine and butorphanol|
|Fentanyl/fluanisone and isoflurane (Isoflo; Abbots)||(F/F) 0.3 ml/kg i.m., then mask induction with isoflurane|
|Fentanyl/fluanisone and midazolam||(F/F) 0.3 ml/kg i.m., then (M) 0.5–2 mg/kg i.v. 10–20 min later|
|Isoflurane or sevoflurane only||Either starting on 1% and increasing by 0.5% every few minutes, or start with 5% for isoflurane or 8% for sevoflurane|
• Used for anaesthesia of critically ill rabbits, who tend not to struggle or breath-hold with mask induction
|Propofol||10 mg/kg IV|
Full assessment of the damage caused by the maggots is required to give the client an accurate prognosis. This assessment should to be carried out early in the investigation, using sedation or anaesthesia as appropriate. Where the larvae have attacked deeper tissues the prognosis is poor, and if they have entered the body cavity it becomes grave. Conversely, if larvae have not progressed beyond L1, or the damaged caused by L2 larvae is not yet severe or deep, then treatment may be initiated with reasonable hopes of success.
There are no differential diagnoses for fly strike; however, determination of the underlying cause is necessary. Blowfly myiasis does not occur in the healthy rabbit. There is invariably a cause of perineal soiling or urine scalding that creates an appropriate environment for greenbottle flies to deposit their eggs.