Question
Haemophilus ducreyi
Answer
Haemophilus ducreyi is a small, Gram-negative coccobacillus, the causative agent of chancroid, a sexually transmitted genital ulcer disease, prevalent particularly in some tropical/developing regions and an important cofactor facilitating HIV transmission (genital ulcers increase both susceptibility to and transmissibility of HIV).
Clinical features: after a short incubation period (typically 4–7 days), a tender papule develops at the site of inoculation, which rapidly evolves into a painful, soft, ragged-edged genital ulcer (“soft chancre” — in contrast to the painless, indurated chancre of primary syphilis, an important clinical distinguishing feature) with a friable, purulent base; frequently accompanied by tender, often unilateral, suppurative inguinal lymphadenopathy (bubo), which can rupture spontaneously.
Laboratory diagnosis: Gram stain of ulcer exudate may show Gram-negative coccobacilli in a characteristic “school of fish” or “railroad track” parallel-chain arrangement, though sensitivity is limited; culture is technically demanding, requiring special enriched selective media and extended incubation, with relatively low sensitivity even under optimal conditions; PCR (nucleic acid amplification test) is now the most sensitive and preferred diagnostic method where available. Diagnosis is often made clinically in resource-limited settings, sometimes after excluding syphilis and herpes simplex virus as alternative causes of genital ulceration.
Treatment: a single dose of azithromycin or ceftriaxone (or a course of erythromycin/ciprofloxacin); treatment of sexual partners; the buboes may require aspiration if fluctuant, but incision/drainage is generally avoided to reduce the risk of chronic sinus formation.