17 October 2017

Why we need to worry about M. genitalium

Clinical Communicable Disease

New data on Mycoplasma genitalium transmission have revealed just how contagious this emerging STI can be.

A Melbourne-based study, which included around 400 people, investigated the likelihood of M. genitalium transmission in a large Australian cohort.

It found that around half of women who had sexual contact with an infected partner tested positive for M. genitalium.

“If a woman comes into the clinic and says, ‘My boyfriend has just phoned me and says he’s been diagnosed with M. genitalium’, you know that there is a 48% chance that she’s actually got the infection, which is quite high,” Associate Professor Catriona Bradshaw, the lead author, said.

If the patient reporting exposure was a heterosexual male, then the probability of infection was lower, at around 30%.

This likely reflected the lower susceptibility of the urethra compared with the vagina and cervix in women, said Professor Bradshaw, who is a sexual health physician.

Gay men had a similar likelihood of infection to women, but only when tested at both the urethral and rectal sites.

“So, this gives the clinician some data to base their discussions about their need to presumptively treat that person,” said Professor Bradshaw.

With the recent approval of two commercial assays by the TGA, it was expected M. genitalium testing will become increasingly common in general practice,  Professor Bradshaw said.

The prevalence of M. genitalium infection is around 1% to 3% of the population, according to international studies.

Recommended treatment for symptomatic patients with proven infection and susceptibility is with azithromycin. However, treatment of sexual contacts is not as straightforward.

In Australia, the recommendation is that every recent sexual contact receive presumptive treatment.

This lowers the risk of one partner re-infecting the other, and ensures treatment in cases where the person is a no-show to the second appointment. But it also means that for every two to three people given presumptive treatment, only one would actually carry the infection.

In the US and the UK, however, only people who have a diagnosis of M. genitalium are treated. This approach cuts down the unnecessary use of antibiotics.

And, because of antibiotic resistance, there is also an issue with the effectiveness of presumptive treatment.

Resistance to the first-line antibiotic, azithromycin, was now more than 50%, Professor Bradshaw said.

Giving this drug presumptively will not be an effective strategy in individuals who have resistant disease.

“So in this situation a much more nuanced discussion really needs to occur,” said Professor Bradshaw.

If the patient was unlikely to come back for a follow-up appointment, they might prefer to be treated on the spot, with the knowledge that the antibiotic might not be effective if they had resistant disease, she said.

Other patients may prefer to wait for the test results, which can determine antibiotic effectiveness.

The Melbourne study could be used to inform guidelines in the future, sexual health physician Dr Terri Foran said.

“But guidelines do change in both directions,” she said.

Evidence around transmission rates could lead to more relaxed guidelines around presumptive treatment in Australia.

“Or it may be that this sort of evidence from Australia makes people in places in the US and the UK think twice about saying that it is not so important to treat partners,” she said.

Emerging Infectious Diseases 2017, 11 October

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