Dual-Acting Penicillin: Non-Surgical Treatment of Head Abscesses in Rabbits

(updated December 2013)

An abscess is a localized swelling in the tissues caused by an accumulation of pus, which is composed mostly of dead white blood cells and the bacteria those noble cells have killed. These can occur almost anywhere in a rabbit's body, and while the center of an abscess is filled mostly with dead cells, the outer margins are often seething with live bacteria being walled off in a fleshy capsule by the rabbit's defense mechanisms.

Rabbits may suffer abscesses in various locations in the head, but the most common occur in the maxilla, which houses the upper arcade of teeth, and the mandible, or lower jaw. These infections are usually associated with dental disease.

Why are Cranial Abscesses Such a Problem?

Because rabbits engage in cecotrophy (ingestion of cecotropes), cranial abscesses that originate at the teeth often contain bacteria normally found in the rabbit's mouth and gastrointestinal tract (GIT). This can make treatment problematic, since you don't want the antibiotics used to treat the infection to eliminate beneficial intestinal bacteria. The loss of normal bacterial flora can allow harmful bacterial populations to explode, causing life-threatening dysbiosis in the cecum and lower GIT.

Treatment of Cranial Abscesses in Rabbits: A Brief History

Years ago, the only option for treatment of an abscess in a rabbit's head was invasive surgery, debridement of the site, and implantation of antibiotic-impregnated beads to allow slow release of bacteriostatic or bactericidal drugs. While it is still a good idea to debulk a large abscess and remove as much purulent tissue as possible, some abscesses are difficult to surgically access. In such cases, medical treatment with antibiotics can be remarkably effective at reducing the size of the abscess, and in many cases even eliminating the infection.

In 2000, Marcy Rosenfield (Moore) published the anecdotal results of a small study on dual-acting penicillin's efficacy against cranial abscesses in rabbits, entitled Successful Eradication of Severe Abscesses in Rabbits with Long- Term Administration of Penicillin G Benzathine/Penicillin G Procaine. The announcement turned the rabbit lay community on its head, with many people unwilling to believe the reported results. But Rosenfield proved to be ahead of the curve. Administration of dual-acting Penicillin G is now a commonly accepted protocol for the treatment of cranial abscesses in rabbits, and can be remarkably effective.

In Periodontal Bacteria in Rabbit Mandibular and Maxillary Abscesses, Tyrell et al. (2001) identified bacteria commonly responsible for jaw abscesses/osteomyelitis in rabbits, as well as antibiotics effective against them. The investigators found that 96% of bacteria found in rabbit jaw abscesses were sensitive to penicillin, and that 100% were sensitive to clindamycin.

When administered orally to rabbits, the lincosamine antibiotics (e.g., clindamycin) and the penicillins can trigger fatal dysbiosis and enterotoxemia. Some drugs in these two classes are harmful even when injected (e.g., clindamycin, ampicillin, and others), as they are delivered to the intestinal tract via the bloodstream, and can still wreak havoc with the normal GIT microbial ecosystem.

Clindamycin can be effective if embedded in slow-release, resin beads implanted in an abscess site. However, because the drug could accidentally enter the mouth, some veteriarians prefer not to risk using it in this manner.

Dual-acting Penicillins: An Effective Medical Option

Benzylpenicillin, commonly known as penicillin G is effective against a wide range of gram-positive bacteria that commonly inhabit rabbit cranial abscesses. It is administered via injection, as it is degraded by the acidic environment of the stomach. When delivered by injection, the drug bypasses the GIT and does not trigger dysbiosis. Pen G is a fast-acting salt of penicillin: therapeutic levels of the drug can occur in the tissues within an hour.

Benzylpenicillin can be chemically modified with a stabilizer known as benzathine to form a salt called benzathine benzylpenicillin. The stabilizer increases the "hang time" of the benzylpenicillin in the tissues, allowing slow release of the drug, which, once it enters the bloodstream, is metabolized into the active form, our old friend benzylpenicillin.

Various manufacturers sell a combination of Penicillin G (usually coupled with Procaine, a local anesthetic) and Benzathine Benzylpenicillin as a "dual acting" penicillin. The Pen G/Procaine enters the tissues and bloodstream relatively quickly to fight bacteria, and the Benzathine salt is released more slowly, allowing therapeutic levels of the drug to remain in the tissues for approximately 48 hours. Commercial names for this combination include Bicillin, Duplocillin, Durocillin, Combipen, and others. But they are essentially the same thing: a combination of a fast acting (Benzylpenicillin, or Pen G) and slow-release (Benzathine Benzylpenicillin) penicillin.

Penicillins: Use With Caution

Because these formulations are usually quite viscous, it is essential that they be injected subcutaneously to avoid accidental injection into a blood vessel. Even when injecting subcutaneously, the practitioner will draw back on the syryinge plunger to be certain that a blood vessel has not been breached, as introduction of the thick penicillin suspension directly into the bloodstream can cause a debilitating or fatal stroke.

While penicillin itself has very low toxicity, it is unfortunately a powerful sensitizing agent. Many animals (including humans) can become sensitized to it and suffer dangerous anaphylactic (i.e., "allergic") reactions to it without warning after having tolerated it very well for long periods. No one is certain why certain individuals develop sensitivity to penicillins while others can take it for years without reaction. But it is wise to always be aware of the possibility of sensitization. Once a rabbit exhibits any type of reaction to a penicillin injection, the drug should never be used on that individual again, since anaphylactic reactions tend to become more severe with repeated exposure to a given allergen.

Bottom Line: It Might Be Worth a Try

Treatment of rabbit cranial abscesses with dual-acting penicillin is now a relatively common and generally accepted protocol used by many rabbit-savvy veterinarians wherever this drug combination is available. There are numerous anecdotal accounts of dual-acting Pen G/Benzathine being used to successfully treat retrobulbar (i.e., behind the eye) abscesses, recalcitrant inner ear infections (often--but not always--accompanied by torticollis), and especially infections originating around the teeth. Especially when surgery is not an option, treatment with dual-acting penicillin injections every 48 hours should be considered a viable and promising option.

Dana Krempels, Ph.D.
Department of Biology
University of Miami

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