A skin-ulcer outbreak in Sierra Leone's schoolchildren was first mistaken for anthrax

When clustered skin ulcers began appearing among schoolchildren in the Kayassie community of Safroko Limba Chiefdom, Bombali District, Sierra Leone, the first suspicion was anthrax [s1]. The real cause, identified by genomic sequencing and reported on 1 October in the CDC journal Emerging Infectious Diseases, was something less expected: Haemophilus ducreyi, a bacterium best known as the agent of the sexually transmitted infection chancroid [s1].

From an anthrax scare to sequencing

The outbreak was reported to Sierra Leone's National Public Health Agency after clustered cases of lower-limb cutaneous ulcers, predominantly in schoolchildren, began on 29 October 2025 [s1]. Because Bacillus anthracis was the initial concern, close student-to-student contact was restricted at the school while laboratory testing was launched [s1].

Anthrax was ruled out quickly. The first 7 skin swabs were tested with quantitative PCR targeting anthrax genes and came back negative [s1]. Metagenomic sequencing on those same samples then found H. ducreyi sequences in 4 of the 7, with no other relevant pathogen identified; confirmatory real-time PCR turned all 7 positive [s1].

As the outbreak grew, another 149 swabs were collected, bringing the total to 156 [s1]. Of those, 96 — 61.5% — tested positive for H. ducreyi [s1]. Critically, no DNA from Treponema pallidum, the cause of yaws, was detected in any of the 156 samples by quantitative PCR [s1], which rules out the disease that would normally top the list of suspects for childhood skin ulcers in the region.

A steadily spreading outbreak

This was not a brief flare. Case counts climbed through the winter: 266 cases by 24 December 2025, 386 by 13 January 2026, and 403 by 10 February 2026 [s1]. The roughly 403 affected were overwhelmingly schoolchildren [s1].

The lesions themselves followed a consistent pattern — progressive ulcers, sometimes with localized swelling, occurring mainly on the lower limbs, where minor skin injuries can easily develop [s1]. They typically showed an irregular border and a dry, crusted base without the pus of a bacterial abscess [s1]. Recovery progressed steadily once cases were treated and managed [s1]. The concentration in schoolchildren, and the setting of a single community and chiefdom, point to person-to-person spread through everyday contact and shared skin breaks rather than a sexual route — the transmission pattern now increasingly recognized for this organism outside the chancroid context in which it was first described.

Confirming the culprit

Establishing that H. ducreyi was genuinely the cause, rather than a bystander, took several lines of evidence. The team sequenced genomic DNA from 22 swabs — 12 positive and 10 negative for the bacterium — and again found no sign of the yaws pathogen; the most abundant bacteria overall were ordinary skin and wound colonizers, Staphylococcus aureus and Klebsiella pneumoniae [s1].

Growing the organism proved harder than detecting its DNA. Of 61 swabs inoculated in the field onto enriched blood agar and cultured under low-oxygen conditions at 33°C for 96 hours, H. ducreyi grew from 9 [s1]. The authors note that ambient transport temperatures and the lack of added carbon dioxide likely held the isolation rate down [s1]. From those cultures they sequenced 9 complete bacterial genomes [s1].

Why the identification matters

The finding fits a shift in how tropical skin ulcers are understood. For decades, yaws — caused by a subspecies of the same bacterium behind syphilis — was treated as the primary cause of such lesions in children across West and Central Africa, the South Pacific, and South Asia [s1]. More recent work has established H. ducreyi as a leading bacterial cause of non-genital cutaneous ulcers in yaws-endemic areas [s1], and the Sierra Leone outbreak is a sharp illustration of that reassignment playing out in real time.

The practical stakes are in the response. An anthrax scare, a yaws assumption, and a chancroid-associated bacterium each imply different containment and treatment paths, and only one of them was correct here. Rapid genomic and PCR work turned a misattributed outbreak into a specific diagnosis within weeks — the kind of laboratory capacity that lets public health authorities match the intervention to the actual pathogen rather than to the first guess. For a cluster of ulcers in schoolchildren that could easily have been chalked up to yaws, that distinction determined what was done.

Sources

• Haemophilus ducreyi Bacteria Causing Cutaneous Ulcer Outbreak in Children, Sierra Leone, December 2025 — Emerging Infectious Diseases (CDC), 1 October 2026