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Infected skin with cellulitis
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Cellulitis is a localized or diffuse inflammation of connective tissue with severe inflammation of dermal and subcutaneous layers of the skin. Cellulitis can be caused by normal skin flora or by exogenous bacteria, and often occurs where the skin has previously been broken: cracks in the skin, cuts, blisters, burns, insect bites, surgical wounds, intravenous drug injection or sites of intravenous catheter insertion. Skin on the face or lower legs is most commonly affected by this infection, though cellulitis can occur on any part of the body. The mainstay of therapy remains treatment with appropriate antibiotics, and recovery periods last from 48 hours to six months.
Erysipelas is the term used for a more superficial infection of the dermis and upper subcutaneous layer that presents clinically with a well-defined edge. Erysipelas and cellulitis often coexist, so it is often difficult to make a distinction between the two.
Signs and symptoms 
The typical symptoms of cellulitis is an area which is red, hot, and tender. The photos shown here of cellulitis are of mild cases, and are not representative of earlier stages of the condition.
Cellulitis is caused by a type of bacteria entering the skin, usually by way of a cut, abrasion, or break in the skin. This break does not need to be visible. Group A Streptococcus and Staphylococcus are the most common of these bacteria, which are part of the normal flora of the skin, but normally cause no actual infection while on the skin's outer surface.
Dental infections account for approximately eighty percent of cases of Ludwig's angina, or cellulitis of the submandibular space. Mixed infections, due to both aerobes and anaerobes, are commonly associated with the cellulitis of Ludwig's angina. Typically this includes alpha-hemolytic streptococci, staphylococci and bacteroides groups.
Predisposing conditions for cellulitis include insect or spider bite, blistering, animal bite, tattoos, pruritic (itchy) skin rash, recent surgery, athlete's foot, dry skin, eczema, injecting drugs (especially subcutaneous or intramuscular injection or where an attempted intravenous injection "misses" or blows the vein), pregnancy, diabetes and obesity, which can affect circulation, as well as burns and boils, though there is debate as to whether minor foot lesions contribute.
Occurrences of cellulitis may also be associated with the rare condition hidradenitis suppurativa.
The appearance of the skin will assist a doctor in determining a diagnosis. A doctor may also suggest blood tests, a wound culture or other tests to help rule out a blood clot deep in the veins of the legs. Cellulitis in the lower leg is characterized by signs and symptoms similar to those of a deep vein thrombosis, such as warmth, pain and swelling (inflammation).
This reddened skin or rash may signal a deeper, more serious infection of the inner layers of skin. Once below the skin, the bacteria can spread rapidly, entering the lymph nodes and the bloodstream and spreading throughout the body. This can result in influenza-like symptoms with a high temperature and sweating or feeling very cold with shaking, as the sufferer cannot get warm.
In rare cases, the infection can spread to the deep layer of tissue called the fascial lining. Necrotizing fasciitis, also called by the media "flesh-eating bacteria", is an example of a deep-layer infection. It is a medical emergency.
Risk factors 
The elderly and those with immunodeficiency (a weakened immune system) are especially vulnerable to contracting cellulitis. Diabetics are more susceptible to cellulitis than the general population because of impairment of the immune system; they are especially prone to cellulitis in the feet, because the disease causes impairment of blood circulation in the legs, leading to diabetic foot/foot ulcers. Poor control of blood glucose levels allows bacteria to grow more rapidly in the affected tissue, and facilitates rapid progression if the infection enters the bloodstream. Neural degeneration in diabetes means these ulcers may not be painful and thus often become infected. Those who have suffered poliomyelitis are also prone because of circulatory problems, especially in the legs.
Immunosuppressive drugs, and other illnesses or infections that weaken the immune system, are also factors that make infection more likely. Chickenpox and shingles often result in blisters that break open, providing a gap in the skin through which bacteria can enter. Lymphedema, which causes swelling on the arms and/or legs, can also put an individual at risk.
Cellulitis is also extremely prevalent among dense populations sharing hygiene facilities and common living quarters, such as military installations, college dormitories, nursing homes, oil platforms and homeless shelters. It is advised if a cabin is shared with a sufferer, urgent medical treatment should be given.
Cellulitis is most often a clinical diagnosis, and local cultures do not always identify the causative organism. Blood cultures usually are positive only if the patient develops generalized sepsis. Conditions that may resemble 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. Associated musculoskeletal findings are sometimes reported. When it occurs with acne conglobata, hidradenitis suppurativa, and pilonidal cysts, the syndrome is referred to as the follicular occlusion triad or tetrad.
Lyme disease can be misdiagnosed as staphylococcal- or streptococcal-induced cellulitis. Because the characteristic bullseye rash does not always appear in patients infected with Lyme disease, the similar set of symptoms may be misdiagnosed as cellulitis. Standard treatments for cellulitis are not sufficient for curing Lyme disease. The only way to rule out Lyme disease is with a blood test, which is recommended during warm months in areas where the disease is endemic.
Treatment consists of resting the affected area, cutting away dead tissue, and antibiotics (either oral or intravenous). Flucloxacillin or dicloxacillin monotherapy (to cover staphylococcal infection) is often sufficient in mild cellulitis, but in more moderate cases, or where streptococcal infection is suspected, then this course is usually combined with oral phenoxymethylpenicillin or intravenous benzylpenicillin, or ampicillin/amoxicillin. Pain relief is also often prescribed, but excessive pain should always be investigated as it is a symptom of necrotizing fasciitis. Elevation of the affected area is also important. As in other maladies characterized by wounds or tissue destruction, hyperbaric oxygen treatment can be a valuable adjunctive therapy, but is not widely available.
Other animals 
Horses may acquire cellulitis, usually secondary 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 will usually create less lameness (grade 1–2 out of 5) than that caused by septic arthritis (grade 4–5 lameness). The horse will exhibit inflammatory edema, which is marked by hot, painful swelling. This swelling differs from stocking up in that the horse will not display symmetrical swelling in two or four legs, but in only one leg. This swelling begins near the source of infection, but will eventually continue down the leg. In some cases, the swelling will also travel upward. 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. Veterinarians may also prescribe antibiotics. Cellulitis is also seen in staphylococcus and corynebacterium mixed infections in bulls.
See also 
- Dhingra, PL; Dhingra, Shruti (2010) . In Nasim, Shabina. Diseases of Ear, Nose and Throat. Dhingra, Deeksha (5th ed.). New Delhi: Elsevier. pp. 277–8. ISBN 978-81-312-2364-2.
- Scheinfeld NS (February 2003). "A case of dissecting cellulitis and a review of the literature". Dermatol. Online J. 9 (1): 8. PMID 12639466.
- Nowakowski J, McKenna D, Nadelman RB, et al. (June 2000). "Failure of treatment with cephalexin for Lyme disease". Arch Fam Med 9 (6): 563–7. PMID 10862221.
- Vinh DC, Embil JM (September 2007). "Severe skin and soft tissue infections and associated critical illness". Curr Infect Dis Rep 9 (5): 415–21. doi:10.1007/s11908-007-0064-6. PMID 17880853.
- Douso ML (2009). "Hyperbaric oxygen therapy as adjunctive treatment for postoperative cellulitis involving intrapelvic mesh". J Minim Invasive Gynecol 16 (2): 222–3. doi:10.1016/j.jmig.2008.12.007. PMID 19249714.
- Escobar SJ, Slade JB, Hunt TK, Cianci P (2005). "Adjuvant hyperbaric oxygen therapy (HBO2) for treatment of necrotizing fasciitis reduces mortality and amputation rate". Undersea Hyperb Med 32 (6): 437–43. PMID 16509286.
- Adam EN, Southwood LL (August 2006). "Surgical and traumatic wound infections, cellulitis, and myositis in horses". Vet. Clin. North Am. Equine Pract. 22 (2): 335–61, viii. doi:10.1016/j.cveq.2006.04.003. PMID 16882479.
- Fjordbakk CT, Arroyo LG, Hewson J (February 2008). "Retrospective study of the clinical features of limb cellulitis in 63 horses". Vet. Rec. 162 (8): 233–6. doi:10.1136/vr.162.8.233. PMID 18296664.
- Pathan MM, Khan MA, Bhonsle AV, Bhikane AU, Moregaonkar SD, Kulkarni MB (2012). "Cellulitis in a Red Kandhari Bull : A Case Report". Vetworld: 183–4.