Syphilis can affect the eyes. Wikimedia Commons

A sexually transmitted infection that affects the eye and can even result in blindness is on the rise, according to experts.

More than 200 cases of ocular syphilis across 20 US states were documented by the Center for Disease Control (CDC) between 2014 and 2016, prompting it to issue an advisory notice to clinicians.

Ocular syphilis is an inflammatory eye condition that can manifest itself after a syphilis infection. Symptoms include redness in the eyes, blurry vision, and, in the worst cases, vision loss. Syphilis only affects the eye in a small number of cases, however, the occurrence of ocular syphilis is rising in tandem with increasing incidence rates of the infection in the US and UK. Most of the cases involve HIV-positive men who have sex with other men.

The CDC recommends that those most at risk should be tested for syphilis, while doctors are urged to be more aware of eyesight complaints from these people. They also suggest that those who do test positive should seek treatment urgently.

Ocular syphilis is actually a manifestation of the syphilis bacterium infecting the brain or spinal cord – a condition known as neurosyphilis.

The syphilis infection itself, which is caused by a bacterium known as Treponema pallidum, often goes unnoticed because many of its symptoms such as sore throat, headache and skin rash resemble those of other common illnesses.

Primary syphilis occurs between 10 and 90 days after being infected with Treponema, usually resulting in a painless sore in the mouth, genitals or anus. The secondary stage occurs as the bacteria spreads, causing flu-like symptoms or a rash. Neurosyphilis is the potentially life-threatening tertiary stage that can manifest itself if the infection is left untreated for years.