Feline Abscesses: Causes, Symptoms and Treatment
Background: Feline abscesses are common feline wounds encountered in small animal practice and occur when normal or opportunistic organisms are inoculated into the skin. An abscess is a localized accumulation of pus which is composed of inflammatory cells and the infectious organism(s) contained within a cavity. There are several factors that put cats in situations that increase the risk of acquiring an injury that can lead to an abscess. These risk factors include: intact male cats permitted to roam outdoors, multi-cat households, and maintained feral cat populations. In addition, certain patient characteristics may predispose a cat to abscess development. These include immunosuppression from medications, feline leukemia virus infection, feline immunodeficiency virus infection, diabetes mellitus, chronic renal failure, and hyperadrenocorticism.
Pathogenesis: Abscesses may form when a causative organism is inoculated into the skin and subcutis. This occurs from bite wounds, trauma, or hematogenous spread of a systemic infection to the subcutis. Bite wounds from cat to cat fights are most common, and oral flora is the source of bacteria recovered from fight wound abscesses. It is known that healthy cat mouths contain obligate and facultative anaerobic bacteria, and Pasturella multocida has been isolated from the nasopharynx of healthy cats. Traumatic injury from foreign object such as a stick or plant material may also inoculate an agent not normally found on feline skin into the subcutis.
Introduction of an organism into the subcutis disrupts the microenvironment. If the injury is from a cat fight, the puncture wound heals quickly often without the injury being recognized. Bacteria then have conditions in which to flourish. The host mounts a systemic inflammatory response against the foreign bacteria resulting in abscess formation and clinical signs. With inflammation, purulent exudate accumulates, and, if not resorbed, a fibrous capsule forms. Continued production of pus within this closed nonexpansile cavity results. Increased pressure within the contained space ultimately leads to a ruptured abscess. If the abscess does not rupture, granulation tissue develops within the cavity and the organism then has an environment in which to persist. A persistent organism explains the intermittent swelling and/or draining tracts one may see clinically.
Organisms involved in feline abscesses: As mentioned, abscesses often result from cat to cat bites. Such abscesses are often a polymicrobial infection. There appears to be synergy between oral flora bacteria under abscess conditions and this may result in the overgrowth of more than one pathogen. For example, P. multocida is often isolated with Porphyromonas spp. The anaerobic primary pathogen may be isolated with or without aerobic bacteria. Anaerobic bacteria may not be isolated from culture samples collected from a ruptured abscess, even if there is cytologic evidence to support an anaerobic infection. When an abscess ruptures, conditions for growth and survival change. The conditions that permitted growth of the anaerobe in a closed abscess may no longer be favorable for growth once ruptured. Therefore, cytology and culture results can differ.
Aerobic bacteria that have been isolated from feline abscesses include P. multocida (most common), Staphylococcus spp., Streptococcus spp., members of the Enterobacteriaceae family, and Pseudomonas spp. Anaerobic bacteria that have been isolated from feline abscesses include Fusobacterium spp., Bacteroides spp., Porphyromonas spp., Prevotella spp., Peptostreptococcus spp., Clostridium spp., and Actinomyces spp. Other organisms have been reported to cause feline abscesses and some of these agents include Nocardia spp., Mycobacteria spp., Stemhylium sp., Cladosporium sp., Exophiala spinifera (dermatiacious fungus), Yersinia pestis (plague), Mycoplasma, Mycoplasma-like bacteria (ie L-form bacteria), and Corynebacterium equi.
History and clinical signs: For cat bite abscesses, cats are commonly presented for a rapidly appearing painful swelling. Lesions often occur under the fur on the caudal ventral abdomen, face, ventral neck, tail, shoulder, trunk and limbs; sites commonly bitten during fighting. Lesions are characterized by swelling, pain, heat, purulent discharge if ruptured, malodor, erythema, and perhaps loss of function. The mass-like lesion may be fluctuant to firm with or without soft pockets. The abscess is often surrounded by moderately erythematous skin and may have fistulous tracts. Depending on the causative agent, nodules or furuncles may be present. Abscesses illicit a systemic inflammatory response, so signs including fever, lethargy, anorexia, hiding, aggression and limping may be seen. If the injury penetrated a body cavity, resulting in an intracavity abscess, signs of sepsis may be present. There may be clinical signs attributable to an underlying primary disease, so complete history and examination are warranted in all cases.
Diagnosis: History, clinical signs and a battery of diagnostic tests serve to evaluate the patient. A complete blood count, serum biochemistry, urinalysis and FeLV-FIV tests may be recommended.
Cytology should be the initial test performed upon presentation, as it provides valuable information and may serve to eliminate several differentials in a short period of time. Samples may be obtained for cytologic evaluation via fine needle aspiration or by swabbing exuded purulent discharge. Samples should be placed upon a clean glass slide and initially stained routinely with DiffQuik®. Examination using light microscopy under oil immersion (1000x) may reveal intra and extra-cellular organisms, neutrophils, macrophages, keratinocytes, or no organisms at all. Additional stains may be performed and could include acid fast staining for Nocardia spp., or periodic acid Schiff staining (PAS) for fungal elements. Sampling an open tract or old surface discharge may only yield secondary contaminants.
Because abscesses are a deep infection, cultures and susceptibility tests are indicated. I recommend aerobic and anaerobic, fungal culture, and atypical mycobacterial culture. For culture, material collected using aseptic technique via fine needle aspiration is appropriate as is tissue collected via punch or excisional biopsy for maceration tissue culture. Less desirable specimens may be obtained from under a crust. Exudate from an open draining tract may result in growth of contaminants and misquide therapy choices.
Biopsy for dermatohistopathology evaluation may be indicated. Standard punch-biopsy technique is adequate or excisional biopsy may be performed. Sedation or general anesthesia may be required. Results should give a morphologic description and an etiologic diagnosis. If no etiologic diagnosis is reached, dermatohistopathology is still useful to for identification of the disease pattern and to eliminate differentials. The dermatohistopathologist may perform special stains if the histologic pattern is consistent with a particular infection. Special stains may include PAS, acid fast or a silver stain.
Treatment Surgery: Minor surgery is typically indicated in the management of feline abscesses. The goal is to establish and maintain drainage, remove the nidus of infection, and to remove any foreign material if present. One must first consider the health status of the patient, the site of the abscess, and the temperament of the pet in developing the surgical plan. The hair coat surrounding the lesion should be clipped widely and the entire cat should be evaluated for puncture sites and other wounds. The surgical sites should be prepared for aseptic surgery and sterile drapes and sterilized instruments should be used. Once the site is aseptically prepared, the surgeon makes a stab incision into a dependent soft or fluctuant aspect of the abscess. The abscess is evacuated and copiously flushed using sterile saline. Debridement may be required depending on the integrity of the tissue. Extension of the stab incision may be needed to permit continued drainage. Larger abscesses may require more extensive surgery or placement of a drain to facilitate repetitive flushing procedures and/or drainage. Basic nursing care should be instituted following surgery. Bandaging, placement of an Elizabethan collar, and use of warm compresses ensure a favorable surgical outcome. The pet should be restricted from outdoor activity during recovery. If the abscess was determined to be from a cat fight, then permanent restriction from going outside decreases the risk of another occurrence.
Antimicrobial therapy: Selection of antibiotic therapy should be based upon standard principles of antimicrobial usage. The choice of antibiotic should be effective against the probable organism and be known to reach the site of infection. Because abscesses are deep infections, selection should ultimately be based upon culture and antibiotic susceptibility testing. The antibiotic should be bacteriocidal, broad spectrum, and effective against aerobic and anaerobic bacteria if known to be a cat bite abscess.
Pending assessment of antibiotic susceptibility testing results, there are several appropriate antibiotics suitable for first-line usage. Amoxicillin-clavulanic acid, some cephalosporins and clindamycin are good initial choices. Treatment adjustments may be needed based upon antibiotic susceptibility test results.