Facing an alarming surge of syphilis cases a group of US doctors recently deployed an experimental strategy: asking high-risk patients to take a single pill of a common antibiotic after unprotected sex to try to prevent infection.
San Francisco has become the first US public health authority to issue formal guidance recommending the strategy following the release of two studies showing doxycycline can slash transmission rates for the potentially life-threatening disease.
In July, a US study found that taking a single dose of doxycycline within 72 hours of having sex without using a condom reduced the risk of contracting syphilis, chlamydia and gonorrhoea by more than 60 per cent among people at high risk of contracting sexually transmitted infections.
This week a French study evaluating the use of doxycycline and the meningococcal B vaccine to prevent bacterial STIs, both in combination and separately, reported similar results. The trials have raised hopes antibiotics can become a powerful tool in the fight against the diseases.
“It is a major step forward in the fight against STIs,” said Yazdan Yazdanpanah, director of the French agency ANRS, which funded the study.
He said the positive trial results should lead to changes in global health recommendations for preventing syphilis, chlamydia and gonorrhoea.
Doctors working in sexual health said the rollout of doxycycline was needed because of soaring rates of infections transmitted during intercourse, which had prompted US authorities to warn of a sexual health “crisis”.
The number of syphilis cases in the US jumped more than a quarter last year to 171,000, the highest level in more than 70 years, according to the Centers for Disease Control and Prevention.
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But the strategy has raised concerns among some experts, who argue prescribing doxycycline to prevent bacterial STIs could boost resistance to an antibiotic that is also used to prevent malaria and treat other types of infections.
“Having more options to prevent bacterial STIs among high-risk groups is certainly needed and the study data for preventing syphilis and chlamydia is quite convincing,” said Professor Cindy Liu, chief medical officer at the Antibiotic Resistance Action Center at the George Washington University public health school.
“But we need to address the concern that this could trigger a rise in doxycycline-resistant MRSA and undermine the antibiotic’s usefulness,” she said, referring to the infection caused by staph bacteria. “I think we need more data on this, as well as the change in antibiotic-resistant bacterial STIs.”
Liu said San Francisco authorities’ guidance to doctors on doxycycline should have more emphasis on the need for patients to be regularly tested for STIs. The study showed doxycycline was less effective for preventing gonorrhoea, which is on the brink of becoming untreatable and can increase a patient’s risk of contracting HIV.
Gonorrhoea, which causes pain and inflammation, has built up resistance to seven antibiotics since treatments became widely available in the 1930s. One of the only treatment courses is an injectable antibiotic called ceftriaxone.
Some experts are also concerned using doxycycline to prevent STIs could undermine use of condoms, which most doctors agree should remain the main tool for controlling the spread of sexual diseases.
But others say condom use has fallen sharply among high-risk groups, such as men who have sex with men. This is mainly because of the rollout of PrEP, a once-a-day pill that protects people against HIV infection. This highlights that new tools are urgently needed to halt the spread of STIs, they say.
“People are already using doxycycline off label — it is not just being used in these studies, mainly because some patients really need this intervention,” said Troy Grennan, associate professor in the division of infectious diseases at the University of British Columbia.
He said it was important to continue studying antimicrobial resistance and send a message that doxycycline should be a targeted intervention for a small number of people. “This is not a universal STI prevention strategy for everyone,” said Grennan, who has prescribed doxycycline to some high-risk patients.
Researchers have been studying doxycycline’s effect on STIs for several years but it was preliminary results from a US study presented in July, involving just over 500 people in high-risks groups, that prompted San Francisco’s public health authorities to issue new guidance.
The treatment was 88 per cent effective at preventing infection with chlamydia in HIV-negative people and 87 per cent for syphilis, although its ability to stop gonorrhoea was a more modest 55 per cent, according to the study, which has not yet been peer reviewed.
The trial found a fifth of people diagnosed with gonorrhoea were resistant to doxycycline and this increased during the study. There was no evidence of resistance in syphilis or chlamydia.
Annie Luetkemeyer, director of medicine and infectious diseases at the University of California San Francisco and a principal investigator on the study, said more surveillance data from communities where doxycycline was being prescribed was required to determine if the strategy would increase antibiotic resistance.
“We are revaluating the impact . . . on bystander bacteria like Staph aureus, commensal Neisseria (found in the throat) and the bacteria in the gut,” she said.
The CDC said it was “excited” by the initial findings of the UCSF study, which was funded by the National Institutes for Health. But Leandro Mena, director of the CDC division of STD prevention, said the agency wanted to review the full data and consider the issue of anti-microbial resistance before issuing formal guidance to doctors.
Mena said the agency was also encouraged by a recent study which suggested a vaccine targeting meningitis had shown promise in reducing the risk of gonorrhoea infections by as much as 40 per cent. Even a partially effective vaccine could have a significant effect in reducing rates of transmission, he said.
“If we could have a vaccine that could effectively reduce the risk of getting these infections. You know that would be the Holy Grail,” Mena added.
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