The surveillance of antimicrobial susceptibility in H. ducreyi is complicated by the technical difficulties of performing sensitivity testing. Data are available from very few centres.

H. ducreyi has developed resistance to a number of different antibiotics but with the exception of two strains isolated in Singapore in the early 1980s, resistance to erythromycin has not been reported, and it therefore remains the recommended treatment. Ceftriaxone and ciprofloxacin are suitable alternatives, since in vitro resistance has not been reported to either, although frequent treatment failures were observed with ceftriaxone among both HIV-positive and HIV-negative patients in a study conducted in Nairobi in 1991. Single-dose azithromycin therapy appears to be another promising alternative, but further data are required.

Plasmid-mediated resistance has been found against ampicillin, sulphonamides, tetracycline, chloramphenicol, and streptomycin. All H. ducreyi strains now contain beta-lactamase coding plasmids, several of which have been described. Neither penicillin nor ampicillin is now effective against chancroid. Tetracycline resistance too is widespread. As with N. gonorrhoeae, H. ducreyi can also carry a large plasmid capable of mobilizing smaller, non-conjugative resistance plasmids. Trimethoprim and tetracycline resistance can occur in the absence of plasmids.

Resistance to sulphonamides is now widespread, and strains with reduced sensitivity to trimethoprim are becoming increasingly prevalent in South-East Asia, in parts of Africa and in North America. Where strains remain sensitive to trimethoprim, treatment with this agent alone or combined with a sulphonamide remains effective.

Plasmid-controlled aminoglycoside-inactivating enzymes have reduced the usefulness of these antibiotics in treating chancroid in South-East Asia. At present this is not the case in Africa or elsewhere.