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Monkeypox has caught much of the world by surprise. But to Anne Rimoin, the warning signs have been obvious for years.

“What we’re seeing makes perfect sense,” says Dr. Rimoin, a professor of epidemiology at the University of California, Los Angeles’s school of public health.

Over decades, a confluence of factors – including declining population immunity, environmental degradation and growing international travel – has set the stage for an opportunity for monkeypox to leap beyond the African countries where it is usually found, she says.

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All it took was for the virus to be imported to the right place at the right time to spark the outbreak now occurring in parts of the globe that rarely or never saw the disease before, she says. “It seems to just have been an unfortunate roll of the dice.”

As of June 15, there have been 2,103 confirmed cases of monkeypox reported this year to the World Health Organization, many in regions where the disease is usually unheard of, including North America and Europe. In Canada, there have been 210 confirmed cases, 171 of which were in Quebec. The unprecedented spread of the disease prompted the WHO to hold an emergency committee meeting on Thursday to determine whether it is an international public health emergency. The committee is expected to issue a statement in the coming days.

But as Canada and other countries mobilize to extinguish the disease on their own soil, scientists such as Dr. Rimoin point to a long-neglected need to tackle monkeypox at its source. And unless the world learns from this outbreak, it will continue to be caught off guard by the emergence of rare diseases, they warn.

“An infection anywhere is potentially an infection everywhere,” Dr. Rimoin says. “Viruses or other pathogens do not require passports or visas.”

Although some reports have suggested the new cases may be traced back to party events in the Canary Islands and Berlin, the WHO says undetected transmission may have been occurring for some time in places that haven’t historically had monkeypox. To understand why this outbreak is now occurring requires going back to 1980. That’s when the World Health Assembly declared smallpox was eradicated, owing to mass vaccination. Since monkeypox is a close relative, vaccines for smallpox offered protection against both.

But while the natural reservoir for the smallpox virus is humans, with monkeypox, it’s believed to be rodents. (The name originates from its discovery in Denmark among imported macaques in 1958. The WHO is expected to officially give it a new name, amid concerns it is inaccurate and discriminatory.)

The end of routine vaccination against smallpox has left large swaths of the global population unprotected against other viruses in the smallpox family. And over the years, as climate change, population mobility, wars and floods forced an increase in human and wildlife interactions, monkeypox has re-emerged, says Sameer Elsayed, an infectious diseases physician and medical microbiologist at Western University.

By 2007, Dr. Rimoin and her colleagues found a massive increase of monkeypox in humans in the Democratic Republic of the Congo from the 1980s. Meanwhile, a large outbreak of human monkeypox occurred in Nigeria in 2017 and is still continuing.

All cases identified outside Africa have been linked to the virus circulating in West Africa, Dr. Elsayed says.

In a way, this is fortunate. The type, or clade, of monkeypox found in Nigeria, known as the West African clade, is less harmful than the Congo Basin clade, Dr. Elsayed says. Since the start of this year, WHO has so far reported one death worldwide in Nigeria.

One of the likely reasons why the West African clade has taken off outside Africa, but not the Congo Basin clade, is because there is more international travel to and from oil-rich Nigeria than the Democratic Republic of the Congo, he says. No cases of the Congo Basin clade has been seen outside of Africa – at least not yet, he says. ”It’s certainly possible.”

The new outbreak isn’t just unusual because of its geographic spread. At York University, postdoctoral fellow Nicola Bragazzi and his colleagues studied data from six clusters from continental Europe, Britain and Australia.

Unlike typical monkeypox, which often occurs in children and is equally seen in males and females, Dr. Bragazzi says he and his team found most of the new cases were in men, and more than half were among individuals in their 30s. The symptoms were also unusual.

The most prevalent was fever, followed by swollen lymph nodes in the groin and skin rashes. More than 30 per cent of cases had genital and anal lesions, whereas, typically, lesions are concentrated on the face and extremities.

Cautioning against stigmatization, Dr. Bragazzi says one of the risk factors he and his team identified is being a young man who has sex with men. He suggests the differences they found compared with previous outbreaks point to the potential of a new route of transmission: through sex.

So far, there isn’t convincing evidence the virus itself has changed to account for the various unusual aspects of this new outbreak, though that is still being investigated, says professor Scott Weese of the Ontario Veterinary College at the University of Guelph.

Compared with the coronavirus that causes COVID-19, orthopoxviruses, the genus to which monkeypox belongs, are relatively stable, says David Evans, a medical microbiology and immunology professor at the University of Alberta. The fundamental difference is that coronaviruses are RNA viruses, while orthopoxviruses are DNA viruses, which “don’t mutate at anywhere near the same rate,” he says.

The possibility monkeypox could be sexually transmitted isn’t something people considered previously, and it may help public health authorities track cases and prevent further spread, says Dr. Evans, who is a former member of the WHO’s smallpox advisory committee. In hindsight, it’s reasonable to think it could be transmitted sexually, not just between men, but men and women, he says.

Dr. Theresa Tam, Canada’s Chief Public Health Officer, has emphasized monkeypox can be spread to anyone through close contact, as well as through contact with contaminated objects and surfaces.

Efforts to contain monkeypox shouldn’t be limited to human-to-human transmission, Dr. Weese says. They should include paying attention to the risk of viral spillback into animals, he says. Even though that risk may be small in Canada, he notes this type of spillover occurred with the virus that causes COVID-19, which has since been found in deer domestically.

“Ultimately, if it stays a human problem in Canada, it’s controllable,” he says. “What we don’t want to do is have it living in our wildlife because then, we’ve got that reservoir” to allow sporadic cases to continually occur.

What applies to monkeypox applies to all kinds of other emerging infectious diseases, Dr. Weese says, noting viruses such as HIV and Ebola have all come from animals. A huge number of factors drive our risk of exposure to new diseases, including climate change, population growth in developing countries, urban expansion, agriculture and other factors that increase human incursion into wild areas, he says.

And though there are no easy solutions, the threat of emerging diseases is one more reason to reduce the stress on the planet, he says. “We’re stressing our ecosystem and it’s coming back to bite us.”

While the odds of a new virus emerging in Canada are slim, new infectious diseases are more likely to come from ecologically rich areas, such as parts of Africa, Asia, and South America, he says. And wealthy nations ignore the disease risk from these parts of the world at their peril.

“You could have argued two years ago, ‘Why would Canada dump money into monkeypox research in West Africa?’” he says. “Well, the better we can control it there, the lower risk it is for us here.”

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