Cellulitis

A short-term bacterial infection that affects the skin and under the skin tissue (subcutaneous) which causes painful, warm, red, swollen skin. The bacteria involved in this condition are most likely either streptococcus pyogenes or staphylococcus aureus.

Epidemiology

Affects over 14 million cases in the US annually [1]

0.2 to 24.6 per 1000 person-years (types of measurement that take the number of people in the study and the amount of time each person spends in the study) [2]
The incidence of cellulitis is approx 200 cases per 100,000 individuals globally [3]
Cellulitis commonly affects middle-aged and older people [4]
Cellulitis affects approximately 1 in 40 people per year [5]

Source: Waikato District Health Board; DermNetNZ

Cellulitis
Cellulitis affecting the leg with redness and swelling

Source: DermNetNZ.org

Cellulitis
Leg swelling with redness

Pathophysiology [6]

Medical Student
- Entry of bacteria; most likely strep pyogens or staph A
- Can be reached through breakages in the skin barrier (due to cracked skin, surgery, trauma etc)
- Organisms enter the wound site (likelihood of organism overwhelming the immune response depends on risk factors)
- Organisms can either spread to the lymph nodes or penetrate the blood vessels
- The presence of the organism leads to the spread of it to the endocardium, bone; causing systemic inflammation
- This inflammation produces the symptoms of fever, malaise and chills.
- Cellulitis also triggers the inflammatory proteins, cytokines, that activate an immune response.
- This triggers local inflammation (causing warmth and pain to the site on injury), an accumulation of pus (bacteria and dead skin cells) and systemic inflammation (presented in fever, chills and malaise).
Patients
- Entry of bacteria can be reached through breakages in the skin barrier (due to cracked skin, surgery, trauma etc)
- Bacteria enter the wound site
- This can either spread to the lymph nodes (areas inside the body that filter foreign substances) or penetrate the blood vessels
- The presence of the bacteria leads to the spread of it to the heart, bone; causing inflammation throughout the body
- This inflammation produces the symptoms of fever, sickness and chills.
- Cellulitis also triggers the inflammatory substances, that activate a response from the body to the infection.
- This triggers inflammation, causing warmth and pain to the site on injury; an accumulation of pus (bacteria and dead skin cells) and systemic inflammation (presented in fever, chills and malaise).

Causes

The bacteria that causes cellulitis exists on the skin usually but when it enters the layers under the skin- it can be harmful and cause an infection.
Infection sites can be created by:
Cuts/grazes
Burn
Animal/human/insect bites
Puncture wound
Leg ulcers
Dry and cracked skin eg eczema, athlete’s foot etc.
Surgical wounds [7,8]

Risk factors [2]
- Breakage in the skin
- Leg oedema
- Venous insufficiency/ surgery
- Obesity
- Pregnancy
- Conditions that increase the risk of infections (eg. diabetes Mellitus, immunocompromised conditions, liver or renal disease)
- Chickenpox
- Alcohol abuse
- Increased age
- Lymphadenectomy (lymph nodes are removed)

Source: NHS

Cellulitis
Cellulitis affects the eye causing the whites of the eye to become red- this doesn’t always happen

Source: Mind The Gap

Cellulitis
Ulcers and cellulitis in patients with deep vein thrombosis (people with blood clots are at a higher risk of skin infections)

Presentations
- Tenderness of the skin
- Warmth
- Swelling (oedema)
- Peau d’orange (resembles the skin of an orange on the skin)
- Patches of skin (petechiae)
- Fever
- Chills
- Fast heartbeat (tachycardia)
- Headache
- Low blood pressure (hypotension)
- Delirium in severe infections (confusion caused by infections)
- Pain
- Nausea and vomiting in severe cases [7,9]

Investigations

Assessment
- Eron classification system (categorised from Class 1- no signs or co-morbidities to Class 4- sepsis or life-threatening infections)
- Take a history of symptoms, recent trauma, underlying conditions and risk factors for cellulitis.
- Examining the person for obvious skin breakages, and presentation of the skin (redness, warmth, edges etc).
- Excluding differential diagnosis
- Investigations such as blood tests, skin biopsies and swabs are not always necessary for diagnosis.
- Tissue culture should be strongly considered for the identification of the causative organism because of the increased risk of fungal infection. [2,7]

Differential diagnosis [10]

Erysipelas (skin infection affecting the skin)
Necrotising fasciitis or gas gangrene (severe wound infection)
Varicella zoster (eg. chickenpox)
Septic arthritis
Bursitis (inflammation of fluid-filled sacs between tissues of the body)
Gout (inflammation of the joint in the big toe)
Insect bites eg Lyme disease

Management [7]

Medical Students
- Lifestyle
  - Immobilisation and elevation of the area in the body affected to reduce oedema.
  - Cool, wet dressing will reduce warmth, pain and swelling
- Uncomplicated, Eron stage 1-2 cellulitis
  - Oral medication is good for mild infections
    - Dicloxacillin
    - Cephalexin
  - For penicillin allergies
    - Clindamycin
  - Patients with cellulitis after an animal bite
    - Amoxicillin. clavulanic acid
    - For penicillin allergies
    - Clindamycin plus oral fluoroquinolone or double-strength sulfamethoxazole/trimethoprim
  - Patients with cellulitis exposed to fresh or brackish water (more salt than freshwater)
    - Cephalexin
    - Cefazolin with fluoroquinolone
  - Patients with cellulitis exposed to salt or brackish water (more salt than freshwater)
    - Doxycycline
  - Patients with 3-4 episodes of cellulitis despite treatment
    - Benzathine penicillin
    - Erythromycin
    - Penicillin V
- Complicated, Eron stages 3-4 cellulitis
  - Severe cellulitis that is likely to be associated with the case of cellulitis in cases of Staph aureus infections. Treatment must cover MRSA (methicillin-resistant staphylococcus aureus) too.
  - For signs of MRSA infection with suggested complicated cellulitis:
    - Double strength sulfamethoxazole/trimethoprim
    - Doxycycline
    - Linezolid
    - Clindamycin
  - Patients who have severe infections/ symptoms or when oral medication has failed- leading to secondary care:
    - Vancomycin
    - Linezolid- for highly resistant MRSA
    - Daptomycin
    - Teicoplanin
  - The use of alternative medications for severe infections with S. aureus (including MRSA) is based on:
    - Availability
    - Ease of administration
    - Adverse effect profile
    - Cost
  - Alternative medications include:
    - Linezolid or tedizolid
    - Delafloxacin
    - Omadacycline
    - Ceftaroline and ceftobiprole (only available in Canada and Europe)
    - Dalbavancin, oritavancin, and telavancin
Patients
- Lifestyle
  - Reduce the mobility and elevation of the area in the body affected to reduce swelling.
  - Cool, wet dressing will reduce warmth, pain and swelling
- Uncomplicated, mild-moderate cellulitis
  - Oral medication is good for mild infections with antibiotics
  - Antibiotics such as:
    - Dicloxacillin
    - Cephalexin
      - For penicillin allergies
        Clindamycin
  - Patients with cellulitis after an animal bite
  - Antibiotics such as:
    - Amoxicillin and clavulanic acid
      - For penicillin allergies
        Clindamycin plus oral fluoroquinolone or double strength sulfamethoxazole/trimethoprim
  - Patients with cellulitis exposed to fresh or brackish water (more salt than freshwater)
  - Antibiotics such as:
    - Cephalexin
    - Cefazolin with fluoroquinolone
  - Patients with cellulitis exposed to salt or brackish water (more salt than freshwater)
  - antibiotics such as:
    - Doxycycline
  - Patients with 3-4 episodes of cellulitis despite treatment
  - Antibiotics such as:
    - Benzathine penicillin
    - Erythromycin
    - Penicillin V
- Complicated, severe cellulitis
  - Severe cellulitis that is likely to be associated with the case of cellulitis in cases of Staph aureus infections. Treatment must cover MRSA (methicillin-resistant staphylococcus aureus) too.
  - For signs of MRSA infection with suggested complicated cellulitis:
  - Antibiotics such as:
    - Double strength sulfamethoxazole/trimethoprim
    - Doxycycline
    - Linezolid
    - Clindamycin
  - Patients who have severe infections/ symptoms or when oral medication has failed- leading to secondary care:
  - Antibiotics such as:
    - Vancomycin
    - Linezolid- for highly resistant MRSA
    - Daptomycin
    - Teicoplanin
  - The use of alternative medications for severe infections with S. aureus (including MRSA) is based on:
    - Availability
    - Ease of administration
    - Side effect profile
    - Cost
  - Alternative medications include:
  - Antibiotics such as:
    - Linezolid or tedizolid
    - Delafloxacin
    - Omadacycline
    - Ceftaroline and ceftobiprole (only available in Canada and Europe)
    - Dalbavancin, oritavancin, and telavancin

Source: NHS

Cellulitis
Lower legs affected by cellulitis

Source: Mind The Gap

Cellulitis
Scaling, plaque formation and bleeding irregular borders seen in cellulitis

Complications

Uncommon but serious
Blood infections are known as bacteremia
Suppurative arthritis (infection in the joint)
Osteomyelitis (bone infection)
Endocarditis (inflammation of the layer of the heart)
Tissue death (gangrene) [8,11]

Myths behind Cellulitis
- Skin that is red and swollen is definitely cellulitis
- All skin and soft tissue infections require antibiotics
- Cellulitis in hospitals is always associated with MRSA
- All patients with insect bites and redness have cellulitis
- If redness extends the original borders of cellulitis, it always means it’s getting worse
- Repeat infections will not happen when someone is taking antibiotic medication [12,13]

Questions you may want to ask your doctor
- How might I have gotten this infection?
- How long does treatment need to start working?
- How do I manage treatment if I’m already taking medications?
- Are there any alternatives to current medications?
- How can I prevent repeat infections?
Support
- American Academy of Dermatology
- Alberta
- Centres for Disease Control and Prevention