Haemophilus ducreyi

A painful open sore on the genitals usually has one of three causes: herpes, syphilis, or chancroid. They can look alike, and in studies doctors judging by appearance alone identified chancroid correctly only about half to three quarters of the time. This test looks for the bacterium behind chancroid, so you know which infection you're dealing with and which treatment fits.

H. ducreyi (Haemophilus ducreyi) is that bacterium. A positive result means it was found in your sample, which points to an active infection. There's no healthy level here, only present or absent, and the test is most useful when you have a sore or painful swollen glands in the groin.

How the Infection Unfolds

The bacterium gets in through tiny breaks in the skin or the lining of the genitals. It stays outside your cells, clinging to the tissue around them, and it needs heme to grow, which it pulls from the hemoglobin in your blood.

Experiments in which volunteers had the bacterium placed into small scratches on the arm show how quickly it moves. A raised bump appeared within a day at 95% of the sites. About two thirds of the sites (69%) went on to become painful pus-filled blisters, while about a quarter (27%) cleared up on their own. Those figures are averages across many volunteers. Blister rates vary from trial to trial, and one recent study reported about 53% to 60%.

The same experiments show why timing matters for a swab. Live bacteria could be recovered from the lesion surface only some of the time, in 13% to 41.2% of attempts during the first week. Blisters held far more bacteria than early bumps: 12 of 15 blister biopsies grew the organism, against 1 of 7 bumps. This was arm skin, so the genital timeline may differ, but it's the clearest picture available.

What this means for you: an early bump or a healing sore can test negative even when the infection is real. A well-developed sore, sampled at its base, gives the test its best chance.

Chancroid

The infection this bacterium causes is called chancroid. It starts as a tender red bump that breaks down into a deep, painful ulcer with ragged edges and a pus-covered base. Lymph nodes in the groin often swell and hurt, and in more severe cases they fill with pus, rupture, or keep draining through the skin.

In tropical regions the same bacterium also causes long-lasting skin ulcers on children's legs. Those infections spread without sex, from strains closely related to the ones that cause genital ulcers.

HIV Risk

An open chancroid sore makes HIV (the virus that causes AIDS) easier to catch and easier to pass on. The ulcer breaks the skin barrier and draws in the same immune cells that HIV infects.

The link runs the other way too. In a rural Ugandan population followed for about two years, women already living with HIV were about three and a half times as likely to pick up a new H. ducreyi infection as women without HIV. That study tracked infection with antibody blood tests rather than swabs.

What this means for you: a positive result is a reason to get an HIV test right away, and a reason to treat the sore promptly if you're already living with HIV.

Where Chancroid Still Circulates

Chancroid was once a leading cause of genital ulcers in much of the world. After clinics began treating ulcers with regimens that cover all the common causes at once, its share fell sharply, and many surveillance sites now find little or none.

It hasn't disappeared. A 2024 review pooling 42 studies of 9,780 people with genital ulcers found the bacterium in 11.8% of samples, and recent clinic surveys in Malawi found it in 18% to 23% of people with genital sores. In other places it's close to absent: a study of 434 ulcer cases in Manaus, Brazil, found none, and an evaluation across four US public health labs detected none.

So a positive result in a low-prevalence country is unusual, but it isn't a reason to dismiss it. Travel, partners from regions where chancroid persists, and local outbreaks all bring it back into play, and one traveler who returned to the Netherlands from an Indonesian island turned out to have a skin ulcer caused by it.

Telling It Apart From Herpes and Syphilis

Herpes is the most common cause of genital ulcers in most of the world, and two or three of these infections can turn up together in the same sore, which is part of why appearance alone misleads.

Blood tests for syphilis don't settle the question either. Early syphilis can still test negative on antibody tests. In places where yaws (a skin infection caused by a close relative of the syphilis bacterium) is common, children with ulcers caused only by H. ducreyi have tested positive on syphilis antibody tests.

How the bacterium is detected changes how often a real infection gets caught.

Sources: Trees and Morse; Suntoke et al.; Chen et al. (1997).

What this means for you: a negative result from genetic detection on a swab of the sore is fairly reassuring, while a culture or antibody result is weaker evidence either way. Check your report for the method, because results from different methods can't be compared head to head.

A Vaginal Swab With No Sore

Where the swab touches matters more for this test than for most genital infection tests. The highest yield comes from the base of an open ulcer, where the bacteria live. Swabs of general vaginal fluid away from any sore rarely find it, because carrying this bacterium in the genital tract without symptoms is uncommon. It has been documented in the vagina and cervix, though, so it isn't impossible.

Screening studies bear that out. Among 79 pregnant women in Kilifi, Kenya, whose vaginal swabs were run on a broad genetic panel, none tested positive for it. Cervical samples from 299 incarcerated women in São Paulo, Brazil, turned it up rarely compared with common infections like trichomoniasis.

A negative vaginal swab with no sore present tells you there's no sign of active chancroid at that moment, which is the expected answer. There's no evidence that routine screening without symptoms adds value, so if a sore appears later, test the sore itself.

When Results Can Be Misleading

• Sampling away from the sore: a swab of vaginal fluid that misses the ulcer base can come back negative even when chancroid is present.
• Recent antibiotics: antibiotics taken before the swab, even for something else, can suppress the bacterium and leave a negative result while the sore is still healing.
• Timing in the life of the sore: early bumps and healing ulcers shed few bacteria, so a negative at those stages is less convincing than one from a fully formed ulcer.
• Antibody tests confused with direct detection: in Dakar, Senegal, older antibody tests caught only 59% to 71% of infections confirmed by genetic testing and flagged positive in up to 43% of people who weren't currently infected, and antibodies have stayed detectable for up to 8 months after diagnosis.

The method behind this test isn't stated on its listing, and accuracy varies widely between genetic detection, culture, and antibody methods, so check your report for which one your lab used.

Testing Again

A single negative covers one moment. Having chancroid once doesn't protect you: in experimental reinfection studies, a first infection didn't prevent a second.

Treatment doesn't always finish the job. The bacterium comes in two main family lines, called class I and class II. In Papua New Guinea, skin ulcers caused by class II strains lasted longer. Infections that came back after azithromycin treatment were driven mainly by one class I strain, likely because class I spreads more easily.

The practical cadence is symptom-driven. Test any new genital ulcer or painful swollen groin node as soon as it appears. US treatment guidelines call for a recheck 3 to 7 days after starting treatment, and a sore that isn't clearly improving by then should be retested from the ulcer. Test any later sore from scratch. If you have ongoing exposure where chancroid persists, keep it on your sexual health checklist and treat a new sore as the trigger to test, since scheduled swabs without symptoms have little evidence behind them.

Acting on a Positive or Unclear Result

A positive result calls for prescription antibiotics, so this is one of the times a clinician needs to be involved. Test for syphilis and herpes at the same time, because coinfections are common, and get an HIV test. Partners you had sex with in the 10 days before your symptoms started should be examined and treated even if they feel fine. Groin nodes that are swollen and full of pus may need to be drained.

A negative result with a sore still present calls for a closer look. Retest with genetic detection from the base of the ulcer, add a herpes swab and syphilis testing, and repeat syphilis antibody testing a few weeks later if the first one is negative, since early syphilis can be missed. Mention any travel or partners from regions where chancroid still circulates.

If a sore keeps going despite treatment for its presumed cause, or tests disagree with each other, see a sexual health or infectious disease specialist. A mixed infection, a missed chancroid, or a treatment-resistant strain are the usual explanations, and each needs a different fix.