Syphilis is one of the oldest sexually transmitted diseases (STDs), with an outbreak among French soldiers first recorded in 1495. Before the advent of penicillin, the disease was known to cause muscle weakness, dementia, blindness, and even death.

Today, of course, syphilis can be cured, but a cluster of cases on the West Coast in which syphilis has infected patients’ eyes and even led to blindness should serve to remind us that even curable STDs can cause serious complications.

Since December, San Francisco has seen seven cases of uveitis, a swelling of the middle layer of the eye, caused by syphilis. There have also been cases in Orange County, San Diego, San Mateo, Santa Barbara, Sacramento, and Los Angeles. In that same time, there have been six cases in Washington, four of them in King County, which is where Seattle is located.

At least two of the cases in Washington have led to blindness. Most of those infected on the West Coast have been men who have sex with men (MSM) and many, though not all, have been HIV-positive.

Syphilis, which is caused by a bacterium, usually first appears in the body as firm, round, painless sores. If left unnoticed and untreated, the sores will go away within three to six weeks. The bacterium, however, stays in the body and can progress to cause an all-over rash as well as fever, swollen glands, sore throat, and headache. It can also cause central nervous system issues, including meningitis, a swelling of the protective membranes covering the brain and uveitis.

Though neurological issue are often thought of as symptoms of late syphilis, in many of the current cases vision problems appear to be a very early symptom.

Gil Chavez, deputy director and state epidemiologist at the California Department of Public Health, told Rewire in an email, “Although information on the cases is not complete, we know that in many of the cases, ocular symptoms were the first signs of syphilis infection. We know that syphilis can present with eye symptoms alone, without any of the traditional signs or symptoms such as genital ulcers or rash.”

Public health experts are investigating and trying to determine why so many cases of ocular syphilis have been recently diagnosed. Susan Phillip, director of disease prevention and control at the San Francisco Department of Public Health, told Rewire that it has long been known that the organism that causes syphilis “has a predilection for the central nervous system.”

The new outbreak may be a result of sexual networks (the patients involved may know each other or have common sexual partners) or it may be a strain of the bacterium that is more likely to infect the eye.

“We don’t know exactly why the numbers have increased recently,” Chavez wrote. “While it is possible that these cases are caused by a new strain that favors ocular tissue, we don’t have laboratory data that support this hypothesis.”

The fact that the patients who have been diagnosed are mainly MSM who are HIV-positive may again be a function of sexual networks (two of the patients diagnosed in Washington were sexual partners) or it may be a result of the increased STD testing that MSM and those who are being treated for HIV usually receive.

Because these areas have such high rates of syphilis—Los Angeles had the highest number of syphilis cases in the country in 2013, San Francisco the third highest, and Kings County the tenth—providers are more familiar with the disease and may be more likely to recognize the symptoms of ocular syphilis than would providers elsewhere.

As they investigate the reasons behind the outbreak, public health experts in these areas are urging both providers and patients to be on the lookout for symptoms of ocular syphilis.

“We think of STDs as being an isolated issue of the genitals, but this issue underscores that STD infections can manifest in a host of different ways and healthcare providers of every ilk, as well as patients themselves, need to be fully informed about risk factors and symptoms of possible infection,” William Smith, executive director of the National Coalition of STD Directors, told Rewire.

The San Francisco Department of Public Health issued an advisory to clinicians urging them to test patients with visual complaints for syphilis; to ask patients with known or suspected syphilis if they’ve had headaches or any changes in their vision or hearing, in order to spot cases of neurosyphilis; to refer patients with visual complaints to an ophthalmologist; to perform a lumbar puncture; and to treat promptly.

Phillip explained that ocular syphilis is more difficult to treat than cases that present with genital sores or a rash. Those cases are typically treated with a one-time injection of antibiotics. Ocular syphilis, in contrast, requires patients to be admitted to the hospital in order both to monitor their vision and provide strong intravenous antibiotics. A ten-day course of IV antibiotics is the standard course of treatment.

“The concerning thing about this outbreak,” Phillip said, “is that some people have had continued visual defects after treatment. That’s a great clinical concern and it becomes a public health concern. We want to make sure no one experiences devastating complications such as blindness.”

She noted that her department is “not trying to scare people but to make people aware that while syphilis is a treatable disease, rates are increasing and there are risks of complications.”

“With the advent of PrEP and HIV treatment that can suppress viral loads—some people are deciding that they are going to use those methods for HIV prevention and not necessarily reduce their number of partners or use condoms consistently,” Phillip continued. “This outbreak is a reminder that there can be potentially serious complications for other STDs and people need to think holistically about their sexual health.”

Phillip also noted that during outbreaks, it is important that public health and community members work together.

“Public health is very actively involved in trying to stop the transmission of syphilis,” she said, adding that public health workers often track down partners of infected individuals and inform them of their risk and the need to get tested. This is all done anonymously—the name of the infected individual is never released.

Still, some are reluctant to discuss their sexual histories with public health officers. Phillip hopes that this will change: “We’re not the sex police; we just want to keep the community as healthy as possible.”