Seal bite
Seal bite infection, usually presenting as "seal finger," is an acute skin and soft-tissue infection passed from marine mammals to humans. It is caused mainly by bacteria of the genus Mycoplasma (particularly Mycoplasma phocacerebrale and Mycoplasma phocidale), which live naturally in the mouths and secretions of seals and sea lions. Biologically, Mycoplasma has no peptidoglycan cell wall, so beta-lactam antibiotics such as penicillins and cephalosporins have no effect on it. The hallmark is a tense, shiny, intensely painful swollen finger that can quickly progress to tenosynovitis and arthritis, with a risk of permanent finger deformity unless it is treated with a systemic antibiotic that blocks bacterial protein synthesis.
At Haute-Derma, our approach to seal finger focuses on distinguishing it early from ordinary bacterial infection, starting targeted antibiotics against Mycoplasma, and restoring hand function while improving the appearance of any scarring.
Pathophysiology
Source of Infection and Route of Entry
- Natural hosts: seals, sea lions and other pinnipeds.
- Transmission: the bacteria enter directly through a bite or tooth scratch, or when scraped skin on the hand touches seal skin, meat or blubber while handling or caring for seals or working at sea.
A Wall-Less Bacterium and the Structural Damage It Causes
- Intrinsic beta-lactam resistance: lacking a peptidoglycan cell wall, Mycoplasma phocacerebrale is fully resistant to penicillins and cephalosporins, the antibiotics most often used for routine bite infections.
- Tendon sheath invasion and fibrosing inflammation: the bacteria penetrate the dermis and subcutis and soon reach the flexor and extensor tendon sheaths of the finger. The intense, exudative inflammatory reaction compresses the microcirculation, causing local ischemia, severe interstitial swelling and destruction of the finger's joint cartilage.
Clinical Forms and Skin Findings
Finger Cellulitis (Classic Seal Finger)
- Incubation period: 3–21 days (typically 5–10 days) after the bite or exposure.
- Local skin signs: the affected finger swells to two to three times its normal size, with tight, shiny skin that is deep red or purple. The original bite may already have closed, yet the whole finger is extremely painful, with a throbbing pain that flares sharply at the slightest knock.
- Little or no pus: unlike staphylococcal infection (S. aureus), seal finger usually produces firm swelling and fibrotic soft-tissue edema without a localized abscess.
Progressive Tenosynovitis and Arthritis
- Spread along the tendon sheath: the infection tracks from the finger into the palm along the course of the tendon sheath.
- Joint stiffness and loss of function: the proximal and distal interphalangeal joints stiffen and the patient cannot bend or straighten the finger. If treatment is delayed, the bacteria destroy the joint cartilage, leaving a permanent fused, deformed joint (ankylosis).
Assessment and Diagnosis at Haute-Derma
Exposure History and Clinical Examination
- We ask about sea travel, contact with seals, marine animal training or handling of seal products within the past month.
- We check finger joint movement, examine the tense, shiny epidermis with dermoscopy and look for signs of compartment pressure in the finger.
Microbiology and Soft-Tissue Doppler Ultrasound
- High-frequency Doppler ultrasound of the finger: measures tendon sheath thickness, fluid within the sheath and the degree of inflammatory hypervascularity around the joint.
- PCR for Mycoplasma phocacerebrale / Mycoplasma phocidale DNA on tissue aspirate or finger joint fluid, since Mycoplasma is very difficult to grow on standard culture media.
Medical Treatment and Aesthetic Recovery at Haute-Derma
Targeted Antibiotic Regimens Against Mycoplasma
- First-line oral therapy:
- Doxycycline 100 mg by mouth twice daily for 14–21 days.
- Or tetracycline 500 mg by mouth four times daily for 14–21 days.
- Alternatives (for patients who cannot take tetracyclines):
- Clarithromycin 500 mg by mouth twice daily for 14 days.
- Or moxifloxacin 400 mg by mouth once daily for 10–14 days.
- Clinical warning: never rely on penicillin, ampicillin, cephalexin or Augmentin alone, because the organism has no cell wall for these drugs to act on.
Restoring Finger Function and Skin Appearance
- Splinting in the acute phase: the finger is splinted in a functional position for the first 3–5 days to ease pain and limit spread of the tenosynovitis.
- Low-level laser therapy (LLLT) and physiotherapy: LLLT helps reduce tissue swelling and stimulates microcirculation, and gentle finger exercises begin as soon as the acute pain settles to prevent joint stiffening.
- Scar repair: fractional CO2 laser combined with a serum containing epidermal growth factor (EGF) and madecassoside smooths the edges of bite scars and restores elasticity to the skin of the finger.
