A rare manifestation of syphilis can affect the eyes, mimicking a range of ocular disorders. If left untreated, it can lead to blindness - and, according to new research, this problem appears to be on the rise.

Researchers from the University of Sao Paulo in Brazil and Flinders University in Australia analysed records from four medical centres in Brazil over a period of two and a half years.

They found 127 patients afflicted with ocular syphilis - 87 of whom had ocular inflammation, or uveitis, in both eyes. Half the patients were so badly affected, they were unable to drive.

The study is the largest series of ocular syphilis cases collated to date, constituting a valuable databank of the range of presentations it can take.

This could be of use to other countries around the world, where studies have found that ocular syphilis is on the rise - particularly the US, Europe, Asia and Australia, sometimes coinciding with HIV infection, possibly due to the effects of anti-HIV medication on the immune system.

"The 1990s and 2000s indicated that ocular syphilis was a rare diagnosis, accounting for less than 2 percent of all cases of uveitis," said ophthalmologist João Marcello Furtado of the University of Sao Paolo.

"More recent reports describe cohorts of up to 85 patients with ocular syphilis in the Americas, countries in Europe, and parts of Australasia, which shows it's not only a problem in Brazil."

The full history of syphilis is unknown and disputed, but the first written records of the disease in Europe date back to the end of the 15th century CE. For at least a few hundred years it was - quite literally - a pox on society.

It wasn't until the latter half of the 20th century that it was wrestled under some sort of control, due to the discovery and increasingly widespread use of antibiotics

Syphilis is caused by the bacterium Treponema pallidum, and it occurs in four stages. The first stage usually presents as skin lesions, which aren't always painful and can be easy to miss. The second stage can manifest a wide variety of symptoms, and therefore be very easy to misdiagnose.

The third stage is called the latent stage, wherein the disease doesn't show physical symptoms, but can be detected in body fluids with a test; and the fourth stage, known as the tertiary stage, can cause a type of inflammation called a granuloma, or affect the central nervous system or the cardiovascular system.

Weirdly enough, ocular syphilis can develop at any of these stages, and can involve almost any structure of the eye. The patient is most likely to experience changes to vision, such as blurred vision, according to a CDC advisory released in 2015.

The potential effect on permanent vision is the bad news. The good news is that the infection is relatively easy to treat with a dose of penicillin or other antibiotic.

If, that is, the condition is caught quickly.

"Our most important observation is the role of testing in making a timely diagnosis of ocular syphilis, which should limit the risk of vision loss," said ophthalmologist Justine Smith of Flinders University.

"Patients didn't present to clinics for treatment until they had a problem for some months, but it is not completely the fault of the patient.

"Doctors are no longer accustomed to seeing syphilis these days, so it may not be picked up for an extended period of time, during which patients may develop eye complications."

The paper has been published in Scientific Reports.