A previously healthy 27 year old man sustained a high spinal cord injury whilst diving into shallow water in a river in the Western Cape in late summer. He was found floating on the surface of the water after an unspecified time period, and taken to hospital. He required intubation and ventilation for respiratory support. Three days later he was transferred to a nearby tertiary hospital for specialist spinal surgery.
On arrival the patient appeared stable. However, about 24 hours later he deteriorated suddenly, becoming hypotensive, with respiratory distress, hypoxia and decreased level of consciousness. A pulmonary embolus was suspected. Despite resuscitation, his condition deteriorated and he died 24 hours later.
Laboratory results available about 12 hours after his sudden deterioration showed that his white cell count had risen sharply, from 4.1 x 109 /L the previous day, to 24 x 109/L. A blood culture taken at the time grew Gram-negative bacilli after 6 hours and the patient was started on ertapenem. Chest x-ray reported new, extensive bilateral changes.
Following his demise, the blood culture isolate was identified as Aeromonas hydrophilia, resistant to ampicillin and co-amoxiclav, but susceptible to cephalosporins, carbapenems, ciprofloxacin and aminoglycosides.
A tracheal aspirate taken 8 hours prior to his rapid deterioration showed 3+ WBCs on microscopy and cultured a scanty growth of Klebsiella pneumoniae, resistant to ampicillin only, as well as a scanty growth of a fully sensitive Haemophilus influenzae.
A second blood culture taken 20 hrs after starting ertapenem was negative. A forensic post-mortem confirmed the presence of bilateral pneumonia with associated pleuritis and small pleural effusions.
Question 1: What organisms are associated with infection in near drowning incidents?
Answer to Q1
As near drowning incidents occur mostly in young healthy males, one would expect a good recovery rate for associated pneumonia. However, case fatality rates of up to 60% have been reported, although the attributable mortality is unknown.
A variety of organisms have been associated with pneumonia following near drowning incidents, including:
Whilst 'exotic' organisms predominate, it is possible that the association of other more common organisms such as Pseudomonas aeruginosa remains unrecognised due to under-reporting of such infections in contrast to the rarer microorganisms.
Vibrio species, while commonly found in aquatic environments, typically result in soft tissue infections or bacteremia, rather than pneumonia.
While a number of opportunistic fungi are present in water, Pseudallescheria boydii and Aspergillus species are most commonly described as causing pneumonia in immunocompetent hosts. P. boydii may cause a delayed presentation of disseminated disease, typically involving the central nervous system.
Question 2: What is your interpretation of the pathogens grown on tracheal aspirate? Would you give empiric antimicrobials to patients with near drowning?
Answer to Q2
The two organisms isolated in small amounts from the tracheal aspirate could represent normal respiratory flora, and/or colonisation. The diagnosis of pneumonia following near drowning can be extremely difficult, and is based on clinical evidence of pneumonia and microbiological evidence of infection. Isolates obtained from blood culture are particularly informative.
A low threshold for starting empiric antibiotics is advised, to avoid potential serious consequences. Suggested antibiotic options include:
Prophylactic antibiotics are not generally recommended in near drowning incidents, although they may be considered in the setting of severe aspiration or exposure to contaminated water.
Question 3: What are the microbiological characteristics of Aeromonas hydrophilia?
Answer to Q3
Aeromonas are facultatively anaerobic, non-spore forming Gram-negative bacilli. Previously included in the Vibrionaceae family they are now classified as a separate family, with up to 24 species. However, not all species are fully accepted, and various controversies complicate the detailed taxonomy of Aeromonas. Most human infections are due to 3 species, namely A. hydrophilia sensu stricto, A. caviae and A. veronii bv. sobria.
Aeromonas species are ubiquitous in the environment, particularly associated with water, e.g. rivers, dams, ponds, and may be found in low concentrations in food.
They are also associated with outbreaks of disease in fish and amphibians, for example, red leg disease in frogs, and cause infections in insects and vertebrates. Human infections most commonly result from environmental or zoonotic exposure, for example, leeches used for medicinal purposes and snake bites may transmit Aeromonas spp.
In the laboratory:
Although the use of different methods for determining MIC values, including Etest® appears to be acceptable for most antibiotics, CLSI guidelines have been recently introduced for antimicrobial susceptibility testing which will facilitate disk diffusion testing for Aeromonas. One caution is that current knowledge of susceptibility patterns is based mainly on the 3 predominant human pathogens mentioned previously.
Question 4: Discuss the clinical manifestations and treatment of Aeromonas hydrophilia infection
Answer to Q4
Aeromonas species cause a wide spectrum of clinical disease, commonest being gastroenteritis, bloodstream and soft tissue infections.
Aeromonas, spp. are almost universally susceptible to fluoroquinolones which are probably the antibiotic of choice. Choice of beta-lactam antibiotics is complicated by the fact that most Aeromonas spp. possess chromosomal ampC type beta-lactamases (resulting in resistance to coamoxiclav and first to third generation cephalosporins), while a chromosomal metallo-betalactamase, CphA (conferring resistance to carbapenems) is also widely distributed.
While prophylactic antibiotics are not generally advised, near-drowning patients can experience very rapid onset of severe respiratory symptoms due to pneumonia. Diagnosis of pneumonia can be difficult and positive blood cultures can be extremely helpful. Aeromonas species are classic pathogens associated with pneumonia following near drowning.
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