Diagnosis
Cellulitis is most often a clinical diagnosis, readily identified in many people by history and physical examination alone, with rapidly spreading areas of cutaneous swelling, redness, and heat, occasionally associated with inflammation of regional lymph nodes. While classically distinguished as a separate entity from erysipelas by spreading more deeply to involve the subcutaneous tissues, many clinicians may classify erysipelas as cellulitis. Both are often treated similarly, but cellulitis associated with furuncles, carbuncles, or abscesses is usually caused by S. aureus, which may affect treatment decisions, especially antibiotic selection. Skin aspiration of nonpurulent cellulitis, usually caused by streptococcal organisms, is rarely helpful for diagnosis, and blood cultures are positive in fewer than 5% of all cases.It is important to evaluate for co-existent abscess, as this finding usually requires surgical drainage as opposed to antibiotic therapy alone. Physicians clinical assessment for abscess may be limited, especially in cases with extensive overlying induration, but use of bedside ultrasonography performed by an experienced practitioner readily discriminates between abscess and cellulitis and may change management in up to 56% of cases. Use of ultrasound for abscess identification may also be indicated in cases of antibiotic failure. Cellulitis has a characteristic "cobblestoned" appearance indicative of subcutaneous edema without a defined hypoechoic, heterogeneous fluid collection that would indicate abscess. Differential diagnosis
Other conditions that may mimic cellulitis include deep vein thrombosis, which can be diagnosed with a compression leg ultrasound, and stasis dermatitis, which is inflammation of the skin from poor blood flow.
Other animals
Horses may acquire cellulitis, usually secondarily to a wound (which can be extremely small and superficial) or to a deep-tissue infection, such as an abscess or infected bone, tendon sheath or joint. Cellulitis from a superficial wound usually creates less lameness (grade 1–2 of 5) than that caused by septic arthritis (grade 4–5). The horse exhibits inflammatory edema, which is hot, painful swelling. This swelling differs from stocking up in that the horse does not display symmetrical swelling in two or four legs, but in only one leg. This swelling begins near the source of infection, but eventually continues down the leg. In some cases, the swelling also travels distally. Treatment includes cleaning the wound and caring for it properly, the administration of NSAIDs, such as phenylbutazone, cold hosing, applying a sweat wrap or a poultice, and mild exercise. See also
Haemophilus influenzae cellulitis
Helicobacter cellulitis
Tuberculous cellulitis

References
Further reading
Stevens, DL; Bisno, AL; Chambers, HF; Dellinger, EP; Goldstein, EJ; Gorbach, SL; Hirschmann, JV; Kaplan, SL; Montoya, JG; Wade, JC (15 July 2014). "Practice guidelines for the diagnosis and management of skin and soft tissue infections: 2014 update by the infectious diseases society of America". Clinical Infectious Diseases. 59 (2): 147–59. doi:10.1093/cid/ciu296. PMID 24947530. External links
"Cellulitis". MedlinePlus. U.S. National Library of Medicine.