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Understanding Monkeypox and How Outbreaks Spread

Monkeypox virions, left, obtained from a sample of human skin. Photographer: Smith Collection/Gado/Getty Images
Monkeypox virions, left, obtained from a sample of human skin. Photographer: Smith Collection/Gado/Getty Images (CDC)
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The global eradication of smallpox more than 40 years ago was one of the greatest achievements in public-health history, vanquishing a cause of death, blindness and disfigurement that had plagued humanity for at least 3,000 years. But, on the downside, it also led to the end of a global vaccination program that provided protection against other pox viruses. That includes monkeypox, which has been spilling over from its animal hosts to infect humans in Africa with increasing frequency since the 1970s. Monkeypox now represents a serious, evolving threat after sparking outbreaks in dozens of countries this year, mostly in Europe, demonstrating again how readily an infectious agent that emerges in one country can quickly become an international concern.

1. What’s monkeypox?

Monkeypox is a misnomer resulting from the fact that it was first discovered at the Statens Serum Institut in Copenhagen in 1958, when outbreaks of a pox-like disease occurred in monkeys kept for research. While monkeys are susceptible to it, just like humans are, they aren’t the source. The virus belongs to the Orthopoxvirus genus, which includes the variola virus, the cause of smallpox; the vaccinia virus, which is used in the smallpox vaccine; and cowpox virus. Monkeypox is less contagious than smallpox and the symptoms are milder. About 30% of smallpox patients died, while the fatality rate for monkeypox in recent times is around 3% to 6%, according to the World Health Organization.

2. What does monkeypox do?

After an incubation period of usually one to two weeks, the disease typically starts with fever, muscle aches, fatigue and other flu-like symptoms. Unlike smallpox, monkeypox also causes swelling of the lymph nodes. Within a few days of fever onset, patients develop a rash, often beginning on the face then spreading to other parts of the body. The lesions grow into fluid-containing pustules that form a scab. If a lesion forms on the eye, it can cause blindness. The illness typically lasts two to four weeks, according to the WHO. The person is infectious from the time symptoms start until the scabs fall off and the sores heal. Mortality is higher among children and young adults, while people whose immune system is compromised are especially at risk of severe disease. 

3. How is it normally transmitted?

Monkeypox doesn’t usually spread easily between people. Contact with the virus from an animal, human or contaminated object is the main pathway. The virus enters the body through broken skin, the respiratory tract or the mucous membranes in the eyes, nose or mouth. Transmission from one person to another is thought to occur through respiratory particles during direct and prolonged face-to-face contact. But it can also happen through contact with body fluids or lesion material, or indirectly through contact with contaminated clothing or linens. Common household disinfectants can kill it.

4. What’s unusual this time?

There have been multiple chains of human-to-human transmission occurring, including in sexual networks, in countries in which monkeypox isn’t normally present. 

• Cases don’t involve recent travel to places in West and Central Africa, where the disease is endemic.

• Although anyone can get monkeypox, most cases occur in men. In endemic areas of Africa, it was thought that was related to hunting practices, whereas in the current outbreak, most individuals are men who have sex with men, people with multiple sexual partners, or people who practice condomless sex.

• Flu-like symptoms haven’t always preceded the rash, and some patients first sought medical care for lesions in the genital and perianal region.

• In some cases, the lesions are mostly located at these sites, making them hard to distinguish from syphilis, herpes simplex virus, shingles and other more common infections, according to the US Centers for Disease Control and Prevention.

• Close skin-to-skin contact during sex is the primary mode of transmission among men who have sex with men.

• Semen from four patients in Italy collected around the time their symptoms appeared was positive for monkeypox DNA in three of the cases. It’s not yet known whether the fluid alone can transmit the infection.

5. Has monkeypox virus mutated?

Analysis of the genetic sequence of the virus collected from patients in Europe indicates that the current outbreak in non-endemic countries is caused by a strain that likely diverged from the monkeypox virus that sparked a 2018-19 Nigerian outbreak, according to a June 24 study in Nature Medicine. The authors, from Portugal’s National Institute of Health in Lisbon, identified some 50 genetic changes or differences compared with the original strain, including several mutations that made the virus more transmissible. That’s roughly 6-to-12 times more than scientists would expect based on the observed evolution of orthopoxviruses, they said. “Current data points for a scenario of more than one introduction from a single origin, with superspreader event(s) (e.g., saunas used for sexual encounters) and travel abroad likely triggering the rapid worldwide dissemination,” the authors said. The strain belongs to the West African clade, or branch on the evolutionary tree. Earlier research found that it was most closely related to viruses found in cases exported from Nigeria to the UK, Israel and Singapore in recent years. Monkeypox virus strains from this clade usually have a case-fatality rate of less than 1%. (That compares with 10% for a second clade called Congo Basin, which appears on the US government’s bioterrorism agent list as having the potential to pose a severe threat.)

6. How fast is it spreading?

From just a handful of cases in Europe in early May, more than 4,100 cases, mostly in men, were reported across the region, as well as in the Americas, the Middle East, Asia and Australia by late June. One death was reported in an immunocompromised person. Experts told a WHO meeting that monkeypox had been circulating undetected in Europe since at least April. Preliminary research estimates that among cases who identify as men who have sex with men, the virus has a reproduction number greater than 1, which means more than one new infection is estimated to stem from a single case. Overall, the reproduction number is estimated to be 0.8. Understanding the dynamics of spread is proving difficult. A UK study found anonymous sex has proved to be a barrier to effective contact tracing, with only 28% of men able to provide the names of recent sexual contacts. This may challenge efforts to stem transmission ahead of LGBTQ pride celebrations occurring in major cities around the world. Data from outbreaks in Canada, Spain, Portugal, and the UK suggest venues where men have sex with multiple male partners are helping to drive spread.

7. Where does monkeypox come from?

The reservoir host or main carrier of monkeypox disease hasn’t yet been identified, although rodents are suspected of playing a part in transmission. It was first diagnosed in humans in 1970 in the Democratic Republic of the Congo in a 9-year-old boy. Since then, most cases in humans have occurred in rain forest areas of West and Central Africa. In 2003, the first outbreak outside of Africa occurred in the US and was linked to animals imported from Ghana to Texas, which then infected pet prairie dogs. Dozens of cases were recorded in that outbreak.  

8. Is monkeypox a pandemic threat?

A meeting of the World Health Organization’s Emergency Committee on June 23 determined that, at present, the event doesn’t constitute a public health emergency of international concern. The committee acknowledged “the emergency nature of the event and that controlling the further spread of outbreak requires intense response efforts” and recommended reviewing the situation after a few weeks, once more information becomes available. Modeling by researchers at RTI International, a nonprofit research institute, predicts that if public health measures to curb ongoing outbreaks aren’t taken, the introduction of three cases in a non-endemic country could cause 18 secondary cases, 30 could cause 118 secondary cases, and 300 cases could cause 402 secondary cases. The findings, published June 23 in the Lancet Microbe journal, align with the WHO’s assessment that the overall public health risk at a global level is currently “moderate,” the authors said. “Observed outbreaks in non-endemic countries should be contained quite quickly, particularly when adequate mitigation measures are implemented,” they said.

9. How is it treated and prevented?

The illness is usually mild and most patients will recover within a few weeks; treatment is mainly aimed at relieving symptoms. For the purposes of controlling a monkeypox outbreak, the CDC says smallpox vaccine, antivirals, and vaccinia immune globulin can be used. Vaccination against smallpox can be used for both pre- and post-exposure and is as much as 85% effective in preventing monkeypox, according to the UK Health Security Agency, which is offering the Imvanex smallpox vaccine to close contacts of a person diagnosed with monkeypox. It lists cidofovir and tecovirimat as antiviral drugs that can be used to control outbreaks. Tecovirimat was approved by the European Medical Association for monkeypox in 2022 based on data in animal and human studies but isn’t yet widely available, according to the WHO. Newer vaccines based on non-replicating versions of the vaccinia virus have been developed, of which one has been approved for prevention of monkeypox. Limited supplies of two vaccines are available in the US for adults, the journal JAMA reported in late May:

• There are 100 million doses of ACAM2000, which was approved in 2007 for immunization against smallpox. Although it can be administered to people exposed to monkeypox if used under an expanded access investigational new drug protocol, it contains a live vaccinia virus linked to some serious potential adverse effects.

• There are 1,000 doses of JYNNEOS, a live, non-replicating vaccinia virus vaccine manufactured by Bavarian Nordic A/S and approved in 2019 by the US Food and Drug Administration for the prevention of smallpox and monkeypox.

The main way to prevent infection is by isolating patients suspected of having monkeypox in a negative pressure room and ensuring health staff wear appropriate personal protective equipment.

10. Is there a test for monkeypox?

Yes. Monkeypox is diagnosed using polymerase chain reaction (PCR) tests to detect viral DNA in specimens collected from the scabs or swabs of a patient’s lesions. In the US, these are available at state health departments and the CDC. 

(Adds section 5 on genetic sequence research; updates section 6 on how fast it’s spreading, and pandemic threat assessment in section 8.)

More stories like this are available on bloomberg.com

©2022 Bloomberg L.P.

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