Abscesses, which are characterized by an accumulation of pus, can occur in almost any tissue (Table 50-1). Percutaneous abscesses are the most common bacterial infections of feline skin and probably the most common site of abscess formation in dogs and cats. The following discussion focuses on feline cutaneous abscesses, although the principles of diagnosis and treatment are similar in any location. See infections in the various system chapters (Chapters 84 to 92) for a discussion of abscesses in these locations. Abscesses develop more frequently in cats than they do in dogs, owing to the tough, elastic nature of feline skin, which readily seals over contaminated puncture wounds, causing the accumulation of subcutaneous exudates. Sharp teeth and fighting behavior, especially of adult males, are important predisposing factors to abscess formation. The size and degree of abscess formation also depend on many other factors, including the overlying skin tension, amount of dead space, and gravitation of exudate below the point of penetration. Pus-filled cavities that form above the puncture site drain easily. Cavities that gravitate below the puncture site become overdistended and may repeatedly drain through the puncture site without complete resolution.

Because feline abscesses usually result from bites and scratches, the most common organisms found within them are resident oral microflora (Table 50-2).* Although more difficult to cultivate, anaerobes are more frequently isolated than are aerobes. The supposition that Pasteurella multocida is the most common organism involved in cat abscesses is incorrect if one considers the concurrent anaerobic bacterial isolation.⁸⁷ Anaerobes can often be isolated in greater frequency. P. multocida is likely the most common facultative aerobic organism contaminating cat bite wounds (see Table 50-2). Cats that sustain bite injuries are more likely to become infected with feline immunodeficiency virus (FIV) (see Chapter 12) or feline foamy virus (see Chapter 15).

Clinical Findings

The clinical signs of abscess formation in cats reflect the site and severity of the infection. Abscesses are usually located around the cat’s legs, face, back, and base of the tail (Fig. 50-1). Some cats have a noticeable swelling with few other signs of illness, whereas more extensive infection is associated with fever (39.7° C to 40.6° C [103.6° F to 105.2° F]), anorexia, depression, and regional lymphadenomegaly. Pain is usually present at the site of infection, and obvious swelling or warmth may or may not occur. Mature abscesses that are ready to discharge are usually tender with a soft, fluctuant central area. Feline skin easily stretches over distended abscesses. Redness or discoloration is rarely apparent unless the blood vascular supply has been compromised. Drainage of white, creamy, purulent material occurs spontaneously or after surgical lancing. Foul-smelling, red-brown discharges occur, especially with tissue necrosis and virulent anaerobic bacterial infection. Systemic signs often abate once the abscess ruptures, and the only evidence of infection may be matted hair at the site of drainage. Careful examination in the area of swelling usually reveals a small puncture wound covered by a crust. Recurrent abscesses at the same site suggest underlying osteomyelitis or neoplasia, poor surgical drainage, immunodeficiency (e.g., feline leukemia virus [FeLV] or FIV infection), antimicrobial resistance, or the presence of a foreign body. Abscesses associated with foreign bodies, underlying osteomyelitis, or certain organisms such as Nocardia or Mycobacterium also tend to recur, persist, or spread in tissues (Fig. 50-2). Abscesses in dogs and cats are often associated with noticeable regional lymphadenomegaly. Sometimes the overlying skin may become necrotic, and exudate drains externally (Fig. 50-3).

Additional clinical manifestations reflect various sequelae that can occur. Lameness or paralysis will be apparent with myositis, osteomyelitis, septic arthritis, or diskospondylitis. Depression, stiffness, lethargy, nuchal rigidity, or seizures can be seen with meningitis. Osteomyelitis also causes chronic or recurrent draining fistulas that only temporarily respond to antibacterials. Respiratory distress, dyspnea, and stridor are noted with pyothorax, sinusitis, and rhinitis. Vestibular signs can be noted if otitis media or interna develops. Signs of bacteremia and systemic infection include malaise and fever or reflect those of another organ system of hematogenous localization.

Retrobulbar abscess or cellulitis is associated with proptosis, decreased globe mobility, oculonasal discharge, chemosis, conjunctival congestion, protrusion of the third eyelid, pain while opening the mouth, periocular swelling or discomfort, lethargy, weight loss, and fever. Corneal ulcers may develop from exposure keratitis or physical trauma caused by the discomfort. Swelling may be present behind the last upper molar.

Diagnosis

Determining the presence of an abscess is usually based on clinical history and examination. Abscesses should be expected in cats that develop an acute onset of unexplained fever, anorexia, or lameness, even in the absence of an obvious swelling, given that abscess formation may be delayed or hidden. The differential leukocyte count can help in determining the extent of infection and the animal’s ability to control it. A low count with an inappropriate shift is associated with diffuse infection or cellulitis. A mature neutrophilia is more characteristic of a walled-off or mature abscess. A nonregenerative (occasionally microcytic) anemia is present in accordance with chronic inflammation and iron sequestration.⁷⁹ Severe leukopenia (a cell count of under 4000/mm³), with or without an associated anemia, may be present in cats infected with FeLV or FIV that develop chronic or recurrent abscesses as a result of immunosuppression. Unlike those in FeLV- or FIV-infected cats, the leukograms usually improve in immunocompetent cats after the abscesses drain. Because a high proportion of outdoor cats with cutaneous wounds have FIV infection,³⁷ testing for this disease should be done at the time of initial examination, and at least 30 days thereafter, to determine if they became infected from the bite incident.

Ultrasonography can be used to view the extent of a swelling and accumulation of fluid or pus in soft-tissue planes.1,6 Grass awns characteristically appear as double or triple spindle-shaped echogenic shadows within soft tissues.^36a Minimally invasive ultrasound-guided retrieval of foreign material has also been helpful in avoiding standard surgical methods.⁸⁸ Radiographic contrast procedures may be helpful in determining the extent or depth of a draining fistulous tract. Injection of contrast via an indwelling Foley catheter can be followed by radiography, computerized tomography scans, or magnetic resonance imaging. Cerebrospinal fluid evaluation and radiographic imaging is needed to detect central nervous system abscesses within the cranial vault or spinal canal.⁷⁵

Causes of recurrent or nonhealing abscesses in cats include retroviral infections, as previously discussed, underlying osteomyelitis, neoplasia or foreign body, or infection with organisms such as Nocardia, Mycobacterium, fungi, or parasites such as Cuterebra. Chronic or recurrent draining abscesses should be evaluated by radiography for underlying osteomyelitis, by cytology and culture for possible fungal or unusual bacterial infection, or by surgical exploration for the presence of a foreign body or parasite. Culture of abscess wounds for bacteria is not generally indicated because surgical debridement and empiric antimicrobial therapy are often effective. Anaerobic bacteria are difficult to culture.

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