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The Centers for Disease Control and Prevention (CDC), the World Health Organization (WHO), and public health experts are all on the same page: Monkeypox can infect anyone. The virus, which usually causes a fever and bumpy, painful rash, can spread through skin-to-skin contact, but also through dirty surfaces, unwashed sheets, and even, over short distances, airborne spit.

At the same time, public health efforts have focused on men who have sex with men. In the US, where nearly 1,500 cases have been identified so far, the CDC is conducting outreach campaigns to the gay community, and although anyone, regardless of gender, who has had “high risk contact” with an infected person is eligible for a vaccine, many cities have also proactively made vaccines available to sexually active gay and bisexual men.

[Related: More than a million monkeypox vaccines are on their way in the US]

To plenty of people, that’s seemed like a contradiction: If monkeypox isn’t an exclusively sexually transmitted infection (STI), why have public health officials treated it that way? And does that emphasis risk stigmatizing men who have sex with men? 

The link between sex and monkeypox is genuinely confusing. Monkeypox does appear to be spreading through sexual interactions between men, but it’s not a STI. That means that while immediate strategies for controlling the virus have focused on men who have sex with men, in the long-term, ignoring other risk factors would be a mistake.

Sexually transmitted, or transmitted during sex?

Trying to categorize monkeypox as a sexual or non-sexual disease is an oversimplification, says Anne Rimoin, an epidemiologist at the University of California Los Angeles, and a longtime monkeypox researcher. “Sexual contact is a very effective way for the virus to spread,” she says, but that doesn’t make it an STI. 

STIs, like syphilis or gonorrhea, require sexual contact for transmission, often through bodily fluids. When it comes to spreading monkeypox, sex is incidental. “You could have no penetrative sex whatsoever,” says Gregg Gonsalves, a longtime HIV activist and scholar of epidemiology and health policy at Yale’s School of Public Health. “No swapping of bodily fluids. And just because [you] were close together in bed, you get monkeypox.”

Gonsalves compares monkeypox to a 2008 MRSA outbreak that first spread among men who had sex with men. The bacteria that causes MRSA is “often transmitted very similarly [to monkeypox], through close physical contact, often in sexual situations,” he says. “But guess where it showed up next: In health clubs, because contaminated towels were able to transmit the virus.”

So far, public health agencies have prioritized sex in their education and contact tracing to quelch the immediate outbreak: The virus seems to have spread globally through sexual contact networks of men who have sex with men.

According to a July 8 technical report from the UK Health Security Agency (UKHSA), which releases the most comprehensive data on monkeypox epidemiology, 97 percent of cases who had filled out a questionnaire reported having sex with other men, and 31 percent had had at least 10 sexual partners in the past three months. Test positivity rates among women and children are much lower than among men.

The CDC did not respond to Popular Science’s request for data on the role of sexual contact in US monkeypox cases.

Still, it’s possible that monkeypox is spreading undetected outside sexual contexts. The outbreak’s earliest cases were diagnosed at sexual health clinics, suggesting that sexually linked cases might have been more readily identified. And it’s been difficult for people who do report monkeypox symptoms to receive a test. “The diagnostics situation is very 2020-ish, sadly,” says Susan Butler-Wu, an expert in clinical testing at the University of Southern California, in an email, referring to the testing bottleneck early in the COVID pandemic.

At a press conference on July 15, Rochelle Walensky, the CDC director, said that the agency had expanded test capacity through partnerships with commercial labs, and could currently handle 70,000 tests a week. She anticipated that to expand next week. Jennifer McQuiston, deputy director of the CDC’s Division of High Consequence Pathogens and Pathology, said during the press conference that “we’ve got just a couple days of commercial testing started, but what I’ve seen is that they are testing outside [the gay community] and we’re not really seeing many positives.”

The UKHSA says that it hasn’t seen many cases among women or children, which would suggest the virus had moved into new transmission networks, but notes there is a “considerable data lag.”

Sexual transmission is sufficient to explain the UK’s outbreak, according to a model released last month by researchers at the London School of Hygiene and Tropical Medicine. That research, which hasn’t been formally reviewed by independent researchers, showed that monkeypox outbreaks would usually fizzle out. But if a few people with a high number of sexual partners were infected, it could spread rapidly to those partners’ next partners, and outwards. In simulated outbreaks based on self-reported sexual behaviors of young British people, monkeypox spread widely among men who have sex with men.

In other words, although other factors could be at play, sex alone appears to be enough to explain how a virus that had rarely spread from human to human could suddenly flare into a major outbreak.

“It just happens to find its immediate new ecological niche among networks of men who have sex with men with multiple partners,” says Gonsalves. “That’s the immediate situation analysis, and it has nothing to do with anybody’s biology—it has everything to do with the denseness of contacts.”

Atypical symptoms

The extra wrinkle with the current outbreak is that public health officials have described unusual symptoms of monkeypox. 

The WHO’s monkeypox reports have highlighted that the symptoms of the virus in 2022 have been “atypical,” at least compared to limited previous outbreaks. One of those atypical symptoms has included isolated sores around the genitals and anus. That lines up with investigations of an outbreak in Nigeria in 2017. There, researchers found that 68 percent of patients had genital rashes, which they noted was much higher than in previous outbreaks.

“The clinical outcomes in the 2022 outbreak are in some ways similar to what we observed in the 2017 outbreak,” writes Dimie Ogoina, a professor and physician of infectious diseases at Niger Delta University who led several of the 2017 investigations, in an email to Popular Science. In 2017, his team observed patients who “presented atypically,” with symptoms that began as a genital rash and no fever. But he says there are differences to the ongoing outbreak. “Only two patients to the best of my memory had isolated genital ulcers/rash in our cohort and the other cases who had genital ulcers also had associated skin lesions in other parts of their bodies. We did not report any cases of anal lesions, but anal lesions seem to be common in the 2022 outbreak.”

Those observations led the researchers to call for a closer examination of monkeypox and sex. “Sexual transmission is plausible in some of these patients through close skin to skin contact,” Ogoina’s team wrote at the time. “The role of genital secretions in transmission of human monkeypox, however, deserves further studies.”

It’s particularly important to study the possibility of non-lesion transmission, Ogoina says, because “it is unusual for a person with a painful genital rash to engage in sexual activity while the lesion is active.”

A recent study from Spain also suggested that monkeypox could spread directly through sex. The team isolated monkeypox DNA from saliva, semen, and rectal swabs, a first step in finding a new source of transmission. However, the study didn’t grow live virus from those samples, so it’s not clear if they could have actually been sources of transmission.

But UCLA’s Rimoin says to not put too much emphasis on these early reports. “Just because it is spreading through sexual networks doesn’t necessarily mean the virus has changed.”

There are other plausible explanations for the abundance of genital lesions. Typically, the virus’s initial rash appears at the site where a person was infected. And if it’s spreading during sex—which typically involves plenty of skin-to-genital contact—you might see a higher frequency of genital lesions without anything having changed about the virus.

The long term

Epidemiologists studying other outbreaks have found that in most cases, it’s fairly hard for monkeypox to spread from person to person. Clinicians studying the 2017 outbreak in Nigeria found that only a few cases were associated with community transmission—the rest had all come from an unknown animal host.

The rapid movement of this outbreak appears to be due, in part, to the fact that sex is an easy setting for the virus to spread. “It’s just a lot of skin-skin contact,” Rimoin says. “That’s the key.”

But with more than a thousand cases globally, more cases will pop up outside male-male sexual networks “almost certainly,” says Butler-Wu.

Tracking those cases could be one way of understanding how much monkeypox relies on transmission during sex to fuel an outbreak. The modeling team at the London School of Hygiene and Tropical Medicine noted in their paper that by comparing the number of cases linked to sexual contact against those in other contexts, researchers could provide an estimate on how easily the virus spreads outside of sexual contact.

[Related: The fight to stop the next pandemic starts in the jungles of Borneo]

So in the short term, Gonsalves says that by being explicit about sex and monkeypox, the gay community can develop harm reduction strategies, like asking sexual partners to check themselves for symptoms or get vaccinated. “We should be talking about trying to negotiate physical contact in a way that minimizes our risk, and we should be banging on the table to get access to vaccines,” he says.

But acknowledging the role of sex in the outbreak doesn’t have to mean ignoring other types of transmission. As long as monkeypox cases continue to persist, it’s not confined to sex. “By putting a point on sexual transmission,” says Rimoin, “anyone who’s doing things that they don’t consider to be sex will think they’re safe, and aren’t going to get it, and that is not the case.”

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