INTRODUCTION

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.

CASE PRESENTING SIGNS

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.

CASE HISTORY

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.

In this case the relevant history was:

• The rabbit was acquired at 1 year old as a pet for the children and was kept alone. There has been no known contact with other rabbits. The family also had a pet dog.

• The rabbit was housed in a standard-sized rabbit hutch (approx. 4 ft × 1.5 ft × 2 ft), with a separate area for urination and defaecation.

• Handling had initially been every day, but more recently the patient had been handled less frequently. Access to the garden was allowed two or three times per week.

• Vaccinations against myxomatosis and viral haemorrhagic disease were not continued since the original vaccination course with the previous owner. The owner was unaware they required boosting every 6 and 12 months respectively.

• No other preventative healthcare, such as blowfly strike prevention, was used.

• The diet offered was made up of commercial muesli-type mixed rabbit food fed ad lib. Each morning the owner would discard the leftover portions and refill the bowl with fresh food. Hay was offered daily, with a small amount being consumed. In addition, a selection of fresh or leftover vegetables was given once daily.

• The owner reported that the rabbit occasionally got a ‘mucky bottom’, which the owner cleaned as required. There had been a slight reduction in appetite over the preceding few weeks, which the owner thought was due to the hot weather.

• The owner became concerned when her rabbit had become lethargic, with a reduced appetite, and on closer examination she noticed the maggots within the fur of the rear end. She did not attempt removal of the maggots, but sought emergency veterinary attention.

CLINICAL EXAMINATION

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).

Figure 38.1 Extent of the damage caused by fly strike in this case. The fur has already been carefully clipped away in the anaesthetized rabbit.

Figure 38.2 Close-up of the wound in Fig. 38.1.

Table 38.1 Development rate of L. sericata (hours) at three different temperatures

Figure 38.3 L3 maggots in the wound. Careful examination and removal are required to ensure none are missed.

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.

Table 38.2 Anaesthetic protocols for fly-struck rabbits

Anaesthetic agent(s)	Dose	Notes
Fentanyl/fluanisone	0.2–0.3 ml/kg i.m.	• Provides sedation and good analgesia • 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.) • Schedule 2 drug so has to be recorded
Midazolam	0.5–2.0 mg/kg i.v.	• Can be used alone for sedation but only allows minor procedures
Medetomidine and ketamine	(M) 0.2 mg/kg i.m. followed by (K) 10–15 mg/kg i.v. to effect	• Allows approx. 20 min anaesthesia • Intubation can be performed
Medetomidine, ketamine and butorphanol	(M) 0.2 mg/kg (K) 10 mg/kg and (B) 0.5 mg/kg all given together s.c.	• Inhalational agent will be required for more painful or prolonged procedures • Medetomidine can be antagonized with atipamezole at 1–2 mg/kg i.m. • Allows easy i.v. catheter placement
Fentanyl/fluanisone and isoflurane (Isoflo; Abbots)	(F/F) 0.3 ml/kg i.m., then mask induction with isoflurane	• 10–20 min for fentanyl/fluanisone to be effective • Pre-oxygenation and slow incremental induction reduces the risks of breath holding
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	• 30–45 min surgical anaesthesia • Avoids the need for anaesthetic equipment, other than oxygen if required • Allows easy i.v. catheter placement • Fentanyl/fluanisone can be antagonized as above
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 • Many will already have received an analgesic premedicant • Pre-oxygenate for several minutes • Sevoflurane is less irritant for mask induction • Incremental induction may be beneficial, but this author prefers the speed of rapid induction on 8% sevoflurane
Propofol	10 mg/kg IV	• As with other species, apnoea is common