What is an abscess?
An abscess is a walled-off pocket of infection, which contains infective bacteria, in order to stop them spreading throughout the body – a potentially lethal situation. They are fairly common in pet rabbits and can occur anywhere in the body. Common sites include the skin (after injury, bite wound or surgery) or in the head and neck, frequently secondary to dental or nasolacrimal duct disease.
What abscesses do
To understand the process better, let’s take a look at how a typical abscess is formed. Imagine a rabbit with a small bite wound. The presence of bacteria multiplying in the wound sends signals to the body’s immune system. Cells of the immune system respond like the cavalry, approaching the infected area from all directions and surrounding it. The bacteria may continue to multiply and cause damage up to a point, but most of their food and oxygen supply will be cut off. The bravest of the immune cells invade the area in a kamikaze act and the majority of bugs will be killed off in the ensuing battle.
The point is eventually reached where the centre of the battlefield consists of dead bacteria, immune cells and neighboring tissue, which disintegrates to form the creamy liquid which we know and love as pus!
Meanwhile, the surrounding tissue and remaining immune cells band together to form a wall, or capsule – a highly effective blockade, which prevents further spread of any remaining infection. Unfortunately, a few bacteria usually manage to adapt and survive cocooned in the thick wall of the abscess where the immune system cannot get to them. These bugs are than able to lie dormant until an opportunity arises for them to reactivate.
Over time, fluid build-up inside the abscess cavity increases the pressure within until the abscess bursts, expelling pus and bacteria out of the body where it can do no harm. This is usually the best option for the rabbit: indeed “letting out the pus” forms the basis for successful treatment. Under certain circumstances, however, this can go disastrously wrong. For example, if an abscess in the gut wall bursts into the abdominal cavity, the consequences can be devastating: bacteria may escape and multiply causing overwhelming infection before the immune system has a chance to contain it.
Abscesses that don’t burst can remain in status quo for many months or years with no ill effects to the rabbit. Sometimes the remaining bacteria continue to multiply slowly with an increase in abscess size, but any deterioration in the rabbit’s health or nutritional status may allow large numbers of bacteria to build up and eventually overpower the local defence mechanisms and escape to cause widespread infection.
One particularly unpleasant scenario that sometimes occurs is when the bacteria that have escaped settle in different locations around the body and cause new abscesses to form. This is called “seeding” and the doses of antibiotics required to penetrate multiple abscesses that cannot be removed surgically would cause the rabbit major side effects without much hope of success. Sadly, the only humane option if this occurs is palliative care and euthanasia when deemed necessary.
It’s not just trauma (eg bites and scratches) that can cause abscesses to form. “Foreign bodies” can help bugs hitch a ride through the skin and initiate formation of an abscess. Foreign bodies include grass seeds, splinters or wood and sutures left in after surgery – which is why it is important to remove non-dissolving sutures once a wound has healed, and to check that the dissolving type break down and disappear correctly over time.
How are abscesses treated?
Treatment will vary depending on the site and underlying cause of the abscess, but there are certain basic principles which apply to them all.
If an abscess is anything other than small and superficial skin abscess, the rabbit will need sedation or general anaesthesia so the abscess can be thoroughly probed and cleaned out. The subsequent treatment may also cause significant discomfort, so the bunny will need prolonged and adequate analgesia.
The first step is to get rid of the pus. The abscess cavity must be opened and drained, and the abscess wall removed. Dead (necrotic) tissue often sticks to the base of the cavity and must be removed (debrided). Even scrupulous debridement is bound to leave some bugs behind, so the importance of good wound care cannot be stressed enough. If the skin over an abscess heals too soon, bacteria will become trapped inside and reactivate, and the whole cycle will begin again. Therefore the wound cavity is often kept open (some have to be “packed” to achieve this) until it heals from the inside. Daily or twice daily cleaning of the wound cavity is a key part of successful abscess treatment.
Many vets also give antibiotics. Systemic antibiotics – given by mouth or injection – destroy any bacteria that manage to escape into the body. Antibiotics can also be placed inside the abscess cavity to kill off any residual bugs from the abscess as well new ones trying to get in from the outside. Many antibiotics have difficulty gaining access to the bugs in the abscess cavity itself which is why the recurrence rate is high if the capsule isn’t completely removed.
Rabbit pus is semi-solid and can be lumpy (a bit like refined cottage cheese!) so it doesn’t drain as freely as in other animals. The most common bacteria involved in rabbit abscesses are Pasteurella multiocida and Staphylococcus aureus.
The only way to be certain what bacteria are causing an abscess is to take a swab and send it to a laboratory to perform a “culture & sensitivity” test (C&S). It’s a good idea to obtain a C&S when the abscess is drained. The sample should be taken from the abscess wall to try and get some live bacteria, since those in the pus are usually already dead. This also helps the vet to decide which antibiotics are most suitable to fight a particular bacteria.
Some rabbits don’t seem too bothered by their abscesses and carry on life as normal during treatment, but some need careful nursing and supportive care. One specific point is that clean, soft bedding is essential to reduce the potential for contamination of the wound. “Vet bed” is comfortable and is easy to launder. Hay must still always be available to eat but should be kept well away from any wounds.
Analgesia should always be given, since rabbits show very subtle signs of pain and discomfort, which can often be overlooked.
Abscess treatments – old and new
Over recent years, new treatments have been developed and old ones rediscovered in the fight against abscesses.
Ultimately the best treatment is complete surgical excision, but often, due to the location or size of an abscess, this may not be possible. In these cases one or a combination of the following treatments may be implemented.
Gels (eg Intrasite) help to stimulate normal tissue growth and repair at the edges and base of wounds. Some preparations also inhibit bacterial growth and some newer preparations actually contain antibiotic as a paste and can be used to pack the cavity and left for longer.
Antibiotic solutions can be soaked into gauze used to pack abscess cavities, but still need to be changed daily. This method – applying relatively high concentrations of antibiotic directly to the wound – allows the use of antibiotics which would be toxic if given systemically.
Care still has to be taken, though, because the drug may be absorbed into the body via the wound which can upset the normal gut function of the rabbit, which is dependent on friendly bugs which may be killed off by the antibiotics.
Dextrose – An ancient treatment coming back into fashion is very strong dextrose (sugar) solution. Normally, bacterial growth in the body increases as sugar concentrations rise, because bugs use sugar as a food source. This is why diabetic animals get more infections. However, a 50% dextrose solution is so concentrated that it not only inhibits the growth of most bacteria, but actually kills them by sucking the water out of them so that they die of dehydration. The dextrose can also help draw out fluid that would otherwise accumulate in the cavity. Dextrose solution is soaked onto gauze and packed into the wound, filling the cavity. The dressing must be changed at least once a day – otherwise fluid coming out of the wound will dilute the dextrose, and the dextrose may start to be absorbed into the body, negating its beneficial effects.
Manuka honey – In addition to being a strong sugar solution, as above, honey has antibacterial properties and has been used to treat infected wounds in humans for centuries. Honey promotes the formation of clean, healthy granulation tissue, and acidifies the wound, promoting healing.
Prior to commencing treatment with honey, the abscess will require surgical drainage and cleaning and removal of all the necrotic tissue and pus. Twice daily application (which owners can be shown to do), using a syringe to introduce the honey into the cavity is normally recommended. Treatment can be continued for weeks and it doesn’t matter if the rabbit licks at the honey.
Antibiotic beads: Some abscesses are too deep and painful to clean every day, or are inaccessible. Antibiotic impregnated polymethylmethacrylate (PMMA) beads can be packed into the abscess cavity which is then sewn shut. The choice of antibiotics is not simple, as some do not work well with the beads and some may cause antibiotic associated diarrhoea if ingested by the rabbit. The beads are left there for anything up to a few months and provide a slow but continuous release of antibiotic as well as filling in the hole left by the abscess and preventing it being filled by newly formed pus.
Calcium hydroxide has been used for many years treating facial abscesses in rabbits, with some success. The solution is syringed into the cleaned out abscess cavity and removed after a week or so. The main problem with calcium hydroxide is its effects on soft tissue. It is caustic, and causes tissue necrosis and nerves, blood vessels and muscles can be very seriously affected. For this reason many vets now steer clear of the treatment, but potentially it may still have its place in the treatment options of rabbit abscesses.
Hyaluronidase may help to break up pus and make it easier to remove.
N-acetylcysteine (Parvolex), also helps break up pus and mucus, as well as stimulating some immune functions and inhibiting certain aspects of bacterial activity. It can be very effective, but smells rather unpleasant!
Location and outcome
The site and size of the abscess often determine how difficult the abscess will be to eradicate. Abscesses on the foot or abdomen – subject to the pressure of being walked or laid upon – will take much longer to heal. Confining the rabbit to a cage or small, clean area of flooring might help.
Abscesses around the tail are at high risk of contamination, and other sites are subject to excessive licking, which can contaminate the wound and prevent it from healing. Abscesses developing close to the vital organs such as the windpipe or heart can cause serious disease by compressing these organs, and their position can make surgical treatment difficult if not impossible.
Dental abscesses are another particular problem. They may be the first obvious sign of teeth problems, and the severity of the underlying dental disease often dictates the eventual outcome. Any abscess in the head or neck area should ideally prompt a thorough search for dental disease including skull X-rays or CT scans to determine if the abscess has started to erode bone or cause osteomyelitis (bone infection). Osteomyelitis is associated with a much poorer prognosis and requires aggressive treatment to have any hope of long-term success; complete cure is not common but control of spread with treatment may be possible.
Having said this, not all head or neck abscesses are due to dental disease. Sharp grass seeds or strands of hay have been known to penetrate the skin and cause abscesses.
If things are not going well……
If an abscess that is being treated does not seem to be getting better, your vet will ask him/herself a few questions:
- Is there a residual foreign body preventing the wound from healing completely?
- Was a hidden pocket of infection left behind?
- Has the abscess spread to underlying tissues such as bone?
- Is there another problem going on which is helping the infection persist?
- Has the right antibiotic been used at the right dose for the right length of time?
- Is the rabbit getting what the vet has prescribed? Rabbits that spit out their antibiotic may be better treated with daily injections for a few days!
Despite all efforts, some rabbits do not survive their abscess problems. They may be unresponsive or impossible to treat successfully.
Treatment failure is quite common and often unavoidable due to the underlying cause; the size and site of the abscess; the infecting organism; and the general state of health of the rabbit.
The bottom line
Abscesses are common afflictions. Although they prevent the spread of infection, they may themselves cause considerable problems if not detected or dealt with promptly. Treatment options have improved and the combination of aggressive surgical intervention followed by fastidious wound care can lead to the resolution of even rapidly enlarging lesions.
Some abscesses require extensive and several treatment options, which can be drawn out and expensive. Others abscesses are not possible to cure and the rabbits’ quality of life will require constant reassessment.
This article first appeared in “Rabbiting On” in Winter 2000. It was updated by Dr Lucy Hansen in June 2002, ready for publication on this website. Updated and reviewed in December 2012 by Claire Speight RVN. Reviewed by Richard Saunders BSc BVSc CBiol MSB CertZooMed DZooMed (Mammalian) MRCVSin Feb 2013.
Copyright © Dr Lucy Hansen/RWF 2002