An elderly (in her 60’s) woman went to the beach with her grandchildren, accompanying them as they dug for crabs and picked shells near a creek.
Three days later, she presented with high fever accompanied by severe right lower limb pain and redness. She has a background history of diabetes mellitus which was not well controlled. She was hypotensive at the Emergency Department, requiring fluid resuscitation, and had extensive erythema with bullae formation from the foot up to the knee. She denied having been bitten or stung by anything, and had not stepped on anything sharp at the beach. An urgent debridement was scheduled for what appeared to be necrotising fasciitis.
Question: What are the likely pathogens and what would be the choice of empirical antibiotic coverage?
[Updated 29th May 2015]
This lady is mildly immunocompromised (poorly controlled diabetes mellitus) and has necrotising fasciitis after having been to a beach. She was playing with her grandchildren at a creek, with possible exposure to stagnant water. The possible pathogens for her condition are many, but the most likely three are as follows:
- Group A Streptococcus (Streptococcus pyogenes)
- Vibrio vulnificus
- Aeromonas spp.
Other less common organisms include Staphylococcus aureus (CA-MRSA is uncommon in Singapore) and Klebsiella pneumoniae.
Group A streptococcus is the most common cause of necrotising fasciitis, and must be covered in all such cases. It is universally susceptible to penicillin, and in the specific instance of necrotising fasciitis, clindamycin (if the organism is susceptible) may improve clinical outcomes.
Vibrio vulnificus is a Gram-negative bacterium that is related to the organism causing cholera – Vibrio cholerae. It can be a cause of severe infections in immunocompromised patients, particularly those with liver disease or cancer. It is present in marine environments (which is its natural habitat), and infections with this organism have been reported from shellfish ingestion. Antibiotic treatment is with a third-generation cephalosporin, and the addition of doxycycline may improve clinical outcomes in patients with concomitant skin and soft tissue involvement.
There is an excellent if excessively detailed review of Aeromonas spp. here. This is another group of Gram-negative bacteria that is associated with aquatic (but not salt water) environments, causing disease in fish, other animals and humans. In humans, they are most commonly associated with gastroenteritis, although immunocompromised patients may severe more severe illnesses including septicaemia and severe skin and soft tissue infections. Aeromonas spp. are interesting in that they harbour a large range of inducible beta-lactamases, including carbapenemases, and there is a potential for resistance developing during therapy if beta-lactam antibiotics are used (in a way, this is similar to Pseudomonas aeruginosa). They are almost universally suceptible to the fluoroquinolones.
What would be an appropriate choice of empirical antibiotics for this patient? This is subjective, but the old and reliable combination would IV crystalline penicillin, IV clindamycin and IV ceftazidime until culture or at least Gram stain results from debrided infected tissue are available. In this particular case, the organism cultured from intra-operative specimens was Aeromonas hydrophila, and the patient was switched to fluoroquinolones, making a complete recovery after a lengthy hospitalisation with repeated debridement of the right lower limb.