Medical practice

'New' sexually transmitted infection 'MG' may be widespread

"A sexually transmitted infection could have infected hundreds of thousands of people in the UK," The Guardian reports. 

The infection – mycoplasma genitalium (MG) – causes few, and often no, symptoms. It is unclear whether it could trigger complications such as infertility.

Many media sources describe MG as a new infection, but it was actually discovered in 1981, although at the time it was unclear if it was a sexually transmitted infection (STI).

New research suggests it could be. A large study of UK adults found 1 in 100 adults aged 16 to 44 were infected with MG, with the majority showing no symptoms. 

Black men and men from deprived areas were most likely to carry the bacteria, while infection risk increased for those with more sexual partners and those who did not practice safe sex.

MG infection was linked to a higher risk of post-sex vaginal bleeding – a possible sign of disease – but this was tentative, and the only sign the infection might be causing disease.

This study provides a prevalence estimate and insight into risk factors, but leaves the question of potential long-term harm unanswered. This question requires further investigation using different study types.

However, you can protect yourself from MG and other STIs by practicing safe sex. The humble condom offers the best protection against STIs and can be used during penetrative, oral and anal sex.

Where did the story come from?

The study was carried out by researchers from London-based Universities in England and was funded by the Medical Research Council, the Wellcome Trust, the Economic and Social Research Council, and the Department of Health, with support from an NIHR Academic Clinical Lectureship.

The study was published in the peer-reviewed International Journal of Epidemiology on an open-access basis, so it is free to read online.

Generally, the UK media reported the story accurately. Most UK coverage focused on the possibility that thousands of adults were infected without knowing it – a so-called "stealth STI", as most people don't experience any symptoms.

Some potential harms from MG infection – such as possible female infertility linked to pelvic inflammatory disease – were mentioned in the media, but do not come directly from the study text.

That said, the media coverage usually came with the caveat that the long-term effects of MG infection are largely unknown. 

What kind of research was this?

This was a cross-sectional study looking at whether MG infection was likely to be sexually transmitted, as well as its prevalence in Britain and the risk factors associated with infection.

MG is a bacterium, which evidence identified by the research team says might be linked to genital urinary diseases in men and women, such as post-coital bleeding and urethritis (inflammation of the urethra).

The researchers say there are currently no large population-based epidemiological studies of MG that include prevalence, risk factors, symptoms and co-infection in men and women across a broad age range. Hence, there is doubt about whether it is an STI, how common it is, and uncertainty about whether it causes sexually transmitted diseases (STDs).

Cross-sectional studies are one of the best ways of assessing the prevalence of an infection like MG. However, they are not able to prove cause and effect – that different sexual behaviours increase the risk of MG infection. That said, they can point to highly probable links that can be investigated more robustly in the future using different study designs.

What did the research involve?

Data for this research came from 8,047 respondents to a National Survey of Sexual Attitudes and Lifestyles (Natsal-3) who lived in England, Wales or Scotland from 2010 to 2012.

Participants were interviewed from 2010 to 2012 using computer-assisted face-to-face and self-completion (CASI) questionnaires, which included questions on participants’ sexual lifestyles, history of STIs and current STI symptoms.

Following the interview, a sample of participants was invited to provide a urine sample for testing. The researchers obtained 189 samples from 16 to 17-year-olds who had not been sexually active and 4,507 urine samples from the rest of the 16 to 17-year-olds. They also obtained the same from a sample of 18 to 44-year-olds who reported at least one sexual partner in their lives.

MG infection rates were calculated for 16 to 44-year-olds who reported at least one sexual partner in their lives. They were calculated separately for different age groups and for men and women. Factors linked to MG infection were analysed, such as ethnicity, education level, deprivation levels and sexual behaviours – such as number of sexual partners and unprotected sex in the last year.

What were the basic results?

Just over 1 in 100 men (1.2%, 95% confidence interval (CI) 0.7 to 1.8%) and women (1.3%, 95% CI 0.9 to 1.9%) aged 16 to 44 had an MG infection.

There were no positive MG tests in men aged 16 to 19, and prevalence peaked at 2.1% (1.2 to 3.7%) in men aged 25 to 34 years. By contrast, prevalence was highest in 16 to 19-year-old women, at 2.4% (1.2 to 4.8%), and decreased with age.

The strongest risk factors linked to MG infection were men of Black ethnicity (adjusted odds ratio (AOR) 12.1; 95% CI 3.7 to 39.4) and men living in the most deprived areas (AOR 3.66 95% CI 1.3 to 10.5).

For both men and women, MG was strongly associated with an increased number of total and new partners, and unsafe sex, in the past year. No infections were detected in those reporting no previous sexual experience.

More than 9 out of 10 men (94.4%) and over 5 in 10 women (56.2%) with MG did not report any STI symptoms in the past month.

Women with MG were much more likely to report vaginal bleeding after sex (AOR 5.8; 95% CI 1.4 to 23.3) than those without MG. This, the study authors say, may be a sign the infection is causing disease, but they admit they don't know with any certainty. For example, women with MG were no more likely to report other symptoms that are usually associated with pelvic inflammatory disease, such as pelvic pain, abnormal vaginal discharge or dyspareunia (pain during sexual intercourse).

How did the researchers interpret the results?

The researchers summed up their findings in three key messages:

  • "This study strengthens evidence that MG is an STI: there were strong associations with risky sexual behaviours, with behavioural risk factors similar to those in other known STIs, and no infections were detected in those reporting no previous sexual experience.
  • Given the uncertainty on the natural history and clinical implications of infection, especially in women, we report that although asymptomatic infection was common, we found a strong association with post-coital bleeding in women. Therefore, in addition to MG being an STI, it can also be an STD.
  • MG was identified in over 1% of the population aged 16-44, and among men was most prevalent in 25 to 34-year-olds, who would not be included in STI prevention measures aimed at young people."


This British population study found that around 1 in 100 men and women aged 16-44 living in England, Wales and Scotland are infected with MG, and that it is likely to be transmitted by sexual contact.

The STI doesn't lead to symptoms in the vast majority of men and around half of women. The study wasn't able to tell if the infection was causing disease, but there were tentative signs that it might. For example, more women with MG infection reported vaginal bleeding after sex than those without MG – a possible, but by no means strong, sign the infection may be causing disease.

The overall prevalence masked interesting variation by age, ethnicity and gender. For example, male prevalence of MG was highest in those aged 25 to 34, at 2.1%, whereas in women it peaked earlier in those aged 16 to 19 years, 2.4%.

There are a number of potential biases in this study – for example, non-participation bias to the survey, and bias from non-provision of the urine sample. In each case, the groups taking part might be different to those who chose not to – potentially influencing the results. While this remains a possibility, the authors were aware of the risk and took measures to minimise the influences. For example, the statistical analysis took account of some factors and the team compared the background of the participants taking part with those of the wider population. 

This showed that the group who participated in the study were similar to the British population at large, at least in terms of ethnicity, marital status and self-reported general health.

The study team suggest they may have underestimated MG prevalence in women, as the urine test they used is less effective than an alternative, using vaginal swabs.

To summarise, the study was based on a large number of people living in Britain – over 4,000 urine samples and interviews – so can be considered relatively reliable and applicable to the UK population.

We don't routinely screen for MG infection in adults in Britain, so this study might spark debate about whether we should. To better inform that debate, we need more information about the possible disease-causing effects of the infection: is it harmless, or does it do lasting damage that needs treatment to stop or prevent it? At the moment, we don't seem to have a clear idea.

Even if we don't know the long-term effects of MG infection, it is simple to minimise your personal risk. Ways to prevent MG infection are likely to be the same as for other STIs, such as using condoms during oral, anal and regular sex.

Read more about safe sex and reducing your risk of STIs

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