Chancroid is an uncommon infection in the United States and most other developed countries, but is a major cause of genital ulcer disease in many parts of the developing world . In sub-Saharan Africa, for example, chancroid is the predominant cause of genital ulcers . Definitive diagnosis of chancroid requires detection of the causative organism, Hemophilus ducreyi. Chancroid may be underdiagnosed in the United States since most sexually transmitted disease (STD) clinics do not have the capability of isolating H. ducreyi and many clinicians do not have clinical experience with the infection.
MICROBIOLOGY AND PATHOGENESIS
Hemophilus ducreyi is a highly infectious bacterium. Data from a human model of experimental infection demonstrates that inoculation of a single colony forming unit (cfu) results in papule formation in 50 percent of cases; inoculation of 100 cfu leads to papule formation in 90 percent of cases . Organisms likely gain access to tissues via abrasions in the skin acquired during sexual activity. The likelihood that inoculation will lead to papule formation appears to be mainly influenced by inoculum dose and host factors .
Hemophilus ducreyi is a small, fastidious, gram-negative rod that requires an enriched growth medium containing hemin and usually serum for successful cultivation . Cultures must be delivered expeditiously to the laboratory and incubated at 33º to 35ºC in high humidity with CO2 enrichment. Small, heterogeneous colonies appear on culture medium after 48 to 72 hours. The gray to tan translucent colonies slide intact across the agar plate when pushed.
When examined by Gram stain, organisms from culture often clump in long parallel strands, producing a so-called "school of fish" appearance. This morphology can occasionally be seen in gram-stained smears from clinical specimens, but it is not a consistent or reliable clinical finding.
The histologic and immunophenotypic characteristics of chancroid lesions were described in a report of 11 men, five of whom were coinfected with HIV . A superficial purulent exudate was typically present in the epidermis, and a perivascular and interstitial mononuclear cell infiltrate was seen in the dermis. Neutrophils were prominent in most patients who were not HIV-infected, but were less common in those who were HIV-positive. The mononuclear cell infiltrate contains many CD4+ lymphocytes, which may explain the increased risk of HIV transmission among persons with chancroid [7-9].