Health officials in the US have spotted a worrying cluster of syphilis cases: five women have allegedly had their eyes infected with the pathogen after encountering the same man, which given the infection's unusual nature suggests a new strain of the syphilis bacterium may have evolved.

The syphilis bacterium, Treponema pallidum, is known to be able to infect the eyes (as well as the ears and the central nervous system), when left untreated. But it happens very rarely, which is why the experts at the Centers for Disease Control and Prevention (CDC) are concerned about a mutation.

"A common heterosexual partner in an ocular syphilis cluster has not been previously documented and suggests that an unidentified strain of T. pallidum might have been associated with increased risk for systemic manifestations of syphilis," write CDC staff in their published report.

The women involved, aged between 40 and 60, came to see healthcare professionals between March and May 2022. They presented symptoms including headaches, blurred vision, eye floaters, and photophobia (eye discomfort in bright light).

They all named the same man as a recent sexual partner. The man in question took some tracking down, but when he was tested, he was found to have early latent syphilis – a stage where the disease is still present, but there are no symptoms. He hadn't developed ocular syphilis, however, and was treated with penicillin.

Eye syphilis is only found in a few percent of overall cases (though it's on the rise). It usually affects those who have late-stage syphilis, those over 65, those reporting intravenous drug use, and those who are HIV-positive.

None of the women involved in this outbreak match any of those categories; to find five cases together in quick succession strongly suggests we're dealing with something new.

What's more, it's the first reported cluster linked to heterosexual transmission.

Tests on the man at the center of the cluster weren't able to identify any new strain of T. pallidum however, primarily because there were no ulcers or lesions to swab (which are needed for genetic testing to be most likely to produce results).

The CDC is now spreading the word that a new bacterial strain, which makes it easier for the disease to move to the eye and other areas of the body, may have appeared. Effective treatment relies on catching infections early – and successfully tracing contacts.

"A high index of clinical suspicion and thorough sexual history are critical to diagnosing ocular syphilis, otosyphilis, and neurosyphilis," writes the CDC team.

You can read the CDC report on the case cluster here.