What is infective panniculitis?

Panniculitis is inflammation within the subcutaneous tissue. It has a variety of causes. An infective cause is more likely in those with immunodeficiency (a weakened immune system).

Immunodeficiency may be due to disease, such as infection with human immunodeficiency virus (HIV), or with the use of immune suppressive drugs (e.g. prescribed after organ transplantation). Immunodeficiency may lead to infection with unusual organisms and in unusual sites, including subcutaneous fat.

Staphylococcus

Atypical mycobacterium

Cause unknown

How does infective panniculitis present?

Infective panniculitis, in general, has no specific features. The clinical presentation depends on:

• The type of organism
• Its virulence (how harmful it is)
• The route of inoculation
• The immune state of the individual.

How is the diagnosis of infective panniculitis made?

Whilst history is important, the diagnosis usually requires microbiological and histological confirmation. On biopsy, most infectious panniculitis causes a lobular pattern of inflammation. A lobular pattern is nonspecific although there are sometimes clues that an organism is responsible. Cultures and special stains are required to distinguish these.

Bacterial causes of panniculitis

Common bacteria that cause panniculitis include:

• Streptococcus pyogenes
• Staphylococcus aureus
• Pseudomonas spp.
• Kleibsiella spp.
• Nocardia spp.
• Brucella spp.

Bacterial panniculitis can appear in immunocompetent as well as immunosuppressed individuals. It can develop as a result of direct inoculation or from seeding from systemic infection.

The classical histopathological appearance is a lobular or mixed lobular/septal suppurative panniculitis with heavy infiltrate of neutrophils.

The choice of antibiotic depends on the bacteria. If the organism is unknown, broad-spectrum antibiotic such as amoxicillin-clavulanic acid is often chosen.

Mycobacterial panniculitis

Mycobacteria have a wide range of skin manifestations and panniculitis is uncommon. Most mycobacterial panniculitis is due to non-tuberculous or atypical mycobacteria. They include:

• Mycobacterium chelonae
• Mycobacterium fortuium
• Mycobacterium abcessus
• Mycobacterium avium-intracellulare complex
• Mycobacterium marinum
• Mycobacterium leprae (leprosy, rare)
• Mycobacterium ulcerans

Mycobacterial panniculitis is more frequently seen in immunocompromised individuals. It is usually the result of spread through the bloodstream, which results in widespread lesions. Trauma is the usual means of inoculation in individuals with a normal immune system and results in a single lesion.

Histology demonstrates lobular panniculitis, sometimes with granuloma formation. Zeihl-Neelson, auramine-rhodaine or Fite-Faraco stains are specialised mycobacterial stains that help highlight the organism. Cultures are more sensitive and allow for accurate identification of the mycobacterial species but can take many weeks. DNA probes and DNA PCR are quick ways of species identification.

Treatment regimes are specific to the species of mycobacteria. Treatment is usually continued for 6–12 months or at least 6–8 weeks after clinical resolution.

Fungal panniculitis

Fungal panniculitis can be separated into disseminated disease or classical subcutaneous mycosis.

• A disseminated fungal infection affects very unwell immunocompromised individuals, who present with multiple inflamed subcutaneous nodules.
• Classical subcutaneous mycosis develops in healthy individuals as a single nodule on the lower leg and then becomes an ulcer or abscess that does not respond to antibiotics.

Biopsy, fungal culture and specific histology stains (silver-impregnated procedure and periodic acid-Schiff) are required to make the diagnosis.

Treatment of fungal panniculitis depends on the identification of the organism.

• Disseminated fungal infections carry high mortality and require a long duration of antifungal treatment unless the immune system is reconstituted.
• Classical subcutaneous mycosis is extremely difficult to cure and a long duration of antifungal treatment is required often preceded by complete surgical excision.

Viral panniculitis

The medical literature about viral panniculitis is limited to a few case reports. Cytomegalovirus (CMV) has been reported to cause panniculitis in an immunocompromised patient.