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Answer: Dermatophilus congolensis. This patient had pitted keratosis due to Dermatophilus congolensis, a Gram-positive bacillus first described in 1915 as a cause of contagious dermatitis in cattle in the Belgian Congo (1). This organism is associated with a range of skin diseases in cattle, sheep, deer, horses, and occasionally humans (2). Organisms resembling Dermatophilus congolensis have also been isolated in an elephant with streptotrichosis in Cambodia (3). Humans acquire infection through traumatic inoculations following contact with infected animals, although insect bites have also been proposed. Human skin infections are often mild and self-limiting, although recurrent infections can occur. These skin infections can present as pitted keratolysis, folliculitis, pustular eruptions, distal onycholysis, or subcutaneous nodules. More severe infection with necrotizing granulomatous lymphadenopathy has also been described to occur in an immunosuppressed child (4). Both human and animal cases have been described to occur in different continents, predominantly the tropical regions (5).
In this case, the presence of motile zoospores, agar pitting, beta-hemolysis, and better growth at 5% CO2 than under aerobic atmospheric conditions helped distinguish Dermatophilus congolensis from other aerobic actinomycetes, like Nocardia or Streptomyces. Motile zoospores are released from developed sporangia under optimal growth conditions (6). This can be induced in the laboratory by incubation in liquid medium for 3 to 5 days. 16S rRNA gene sequencing showed a 99.4% match (514/517 bp) with Dermatophilus congolensis strain NBRC 105199. The isolate has now been added to our laboratory's local MALDI Biotyper database, allowing rapid identification with a high confidence score using direct plating with formic acid.
There are currently no published clinical breakpoints for Dermatophilus congolensis. Disk susceptibility testing was performed on Mueller-Hinton sheep blood agar, which showed large zones for ampicillin, tetracycline, erythromycin, cefoxitin, trimethoprim-sulfamethoxazole, and vancomycin.
In this case, no antibiotic was given to the patient. The lesions healed spontaneously after a month. Treatment with antimicrobial agents is controversial, as spontaneous recovery is often achieved by keeping the infected area dry. Dermatophilus congolensis infects the stratum corneum of the skin, which is avascular and out of reach of topical and systemic antimicrobials. In reported human cases, intramuscular streptomycin, oral ampicillin, norfloxacin, and topical neomycin have been used with minimal success (7).