Syphilis could become impossible to treat with key antibiotics, doctors warn amid concerns over a new drug-resistant strain.

Once a death sentence, the vast majority of syphilis cases today are curable with penicillin injections.

However, a new study reveals an aggressive new strain of the infection is more widespread than previously thought - and there are limited ways to control it.

There are two common types of syphilis: Nichols and Street Strain 14 (SS14).

In an analysis of syphilis samples, researchers by the University of Zurich found the most common to be SS14-Ω, a sub-set of SS14.

Alarmingly, 90 per cent of the SS14-Ω samples they analysed were resistant to antibiotics.

A new study reveals an aggressive new strain of syphilis is more widespread than previously thought - and there are limited means to control it

Although scientists have yet to detect any penicillin-resistant strains, the discovery is an ominous sign that it is adapting to modern medicine.

Cases of the chronic bacterial infection have shot up by 71 per cent in England since 2011, according to the latest figures.

Syphilis cases are also increasingly more common in the US. Infections rose 15 percent between 2013 and 14; and another 19 percent from 2014 to 2015, according to CDC data.

Scientists are unsure just how many of these cases can be put down to the relatively new strain that is growing resistant to key antibiotics.

SYPHILIS: WHAT IS IT, WHAT ARE THE SYMPTOMS, AND HOW IS IT TREATED? What is it? A chronic bacterial disease, syphilis can be contracted by other means but is typically a sexually-transmitted disease. In very rare cases, it can be spread through prolonged kissing, as well as the more common routes of transmission: vaginal, anal and oral sex. It comes from the bacteria Treponema pallidum. What are the symptoms? Sufferers develop sores, though these can often go ignored. The infection develops in stages. Stage one: Small, painless sores (like ulcers) on genitals or in the mouth

Appear within 10-90 days after exposure

They disappear within six weeks, and do not leave a scar, before developing to stage two Stage two: Rosy rash on the palms of the hand and soles of the feet

Moist warts in the groin

White patches inside the mouth

Swollen glands

Fever

Weight loss

This all fades away without treatment before developing into stage three Latent syphilis: Dormant, no symptoms Stage three: Without treatment it can progress to more severe issues with the heart, brain and nerves

Paralysis

Blindness

Dementia

Deafness

Impotence

Death How is it treated? In the early stages, patients can receive an injection of Benzathine penicillin G. This will not undo the internal damage but will eliminate the infection. For those with latent syphilis - and are unsure how long they had it - doctors recommend having three doses of the penicillin injection, seven days apart from each other. Advertisement

But they say the emergence of a drug-resistant forms could make syphilis, a potentially fatal STI, even harder to treat.

A study of syphilis infections in 13 countries in 2012 to 2013 shows how a drug resistant strain of the disease is far more widespread than previously thought.

SS14-Ω is particularly resistant to the class of antibiotics called macrolides, the researchers from the University of Zurich say in the journal Nature Microbiology.

The possibility that the second-line medication for the disease - used when penicillin injections are not appropriate - might lose their effectiveness has alarmed experts.

Lola Stamm, an epidemiologist at the University of North Carolina, said the findings should serve as a warning to doctors not to oversubscribe antibiotics.

'Physicians need to be extremely wary about using macrolide drugs to treat syphilis,' she said.

Dr Stamm said syphilis wasn’t yet resistant to multiple types of antibiotics - but she said that scenario could not be discounted in the future.

Doctors say that complacency and the difficulty in treating a disease that can remain symptomless for years has led to a rise in cases around the world.

Estimates suggest that there were 10.6 million cases worldwide in 2008, with many carrying the disease unaware they are actually infected.

Dr Yvonne Doyle, from Public Health England, said the return of syphilis was particularly worrying in London - which has increased by 163 per cent since 2010.

'While we've seen increases in many STIs in recent years, syphilis is fast emerging as a serious public health concern in the capital,' she said.

Cases of syphilis have been reported among both heterosexuals and men who have sex with men - but the latter group has been more affected.

Commonly, someone who is infected with syphilis may develop a painless ulcer or rash on the genitals, rectum or inside the mouth within a month.

Secondary syphilis will then follow. This is characterised by general symptoms such as a fever, headaches and night sweats.

If this is not treated with antibiotics it can then progress into late stage syphilis.

At this point the disease causes much more serious health complications that can include cardiovascular problems, central nervous disease and paralysis -- and can prove be fatal.

Doctors say regular sexual health check ups in sexually active people are the key to preventing the disease.

This comes after research commissioned by then Prime Minister David Cameron found antibiotic resistance will have caused 300 million premature deaths.

Globally, at least 700,000 die each year of drug resistance in illnesses such as bacterial infections, malaria, HIV/Aids or tuberculosis.