A CDC investigation into a cluster of rare, ocular syphilis cases that cropped up in southwestern Michigan between March and July 2022 found that the incidents may be linked by an unidentified strain of Treponema pallidum — the bacterium that causes syphilis.
The five cases of ocular syphilis occurred in women who had all been with one infected male partner. Researchers believe the male may have been infected with the unidentified strain of T. pallidum, which became "a risk factor for developing systemic manifestations of syphilis," according to the Nov. 28 report.
The shared male sexual partner of the five women had been diagnosed with and treated for early latent syphilis. He never developed ocular syphilis.
According to the CDC report, the specific, unidentified strain of T. pallidum he transmitted may have increased the likelihood that led his sexual partners' immune systems to manifest the bacteria in this particular way.
"This ocular syphilis cluster is the first documented with epidemiologic linkage among cases attributable to heterosexual transmission," the report states.
Further research is still needed to understand more about the specific strain, according to the report. However, surveillance and classification of different manifestations of syphilis have been ongoing in Michigan since 2020, when health officials debuted a "systemic manifestation checklist and algorithm" to help clinicians better classify cases of "ocular, otic and neurologic manifestations" of syphilis.