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Monkeypox, a disease that rarely shows up outside a belt of countries across Central and West Africa, has exploded into the news recently, with cases reported in the United Kingdom, Spain, Portugal, the United States, Sweden, Italy, and likely Canada.

At this point, the cases are mainly being detected by clinics that treat sexually transmitted diseases and are being seen in men who have sex with men. But the World Health Organization and the Centers for Disease Control and Prevention have cautioned that to assume the virus is only circulating in a single subset of the population risks missing cases that may be occurring among other people.

Monkeypox causes a flu-like array of symptoms, but also comes with a distinctive rash; one telltale sign is the fact that lesions often appear on the palms of hands. So far it seems that the cases are being caused by viruses from the West African clade, which triggers milder disease than the other family of viruses, called the Congo Basin clade. All monkeypox viruses are cousins of the one that caused smallpox, the only human virus to have been eradicated.


STAT had many questions about monkeypox. Fortunately, Andrea McCollum, the poxvirus epidemiology team lead in the CDC’s division of high consequence pathogens and pathology, had many answers.

Excerpts from the conversation have been lightly edited for clarity and length.


Do we know how efficient monkeypox virus is at transmitting from person to person?

Monkeypox is transmissible really from the time when signs or symptoms appear, throughout the entire course of illness. And the definition of “course of illness” is until all lesions have healed, crusts have separated, and a fresh layer of skin has formed. That can be quite a long period of time. That can be several weeks.

How transmissible it is is a little bit of a trickier question. We don’t have really good contemporary estimates of R-naught. [R-naught is the figure that estimates how many people an infected person, on average, will infect.] We don’t really have any estimates of R-naught for the West African clade. Most of our estimates come from Congo Basin. And most of those estimates are less than 1. But I will remind you that you can have an R-naught of less than one and the agent can still be transmitted person to person.

I think we can take away a lot from what we know about monkeypox in Congo Basin and in West Africa. Even if human-to-human transmission is documented, it is generally documented among very close contacts. So family members, people taking care of ill patients. Or health care providers.

On the CDC’s website, it says transmission can occur via respiratory droplets. But is it really thought that most of the transmission is via skin-to-skin contact, effectively?

I don’t think we have a really good handle on whether most of it is respiratory or skin-to-skin. I think historical literature and certainly learning from smallpox, which was a closely related virus, we knew respiratory droplet transmission, particularly in the early stages of transmission, did contribute significantly. And we think a lot of that is due to lesions often developing within the oral cavity. Those lesions are chock-full of viruses and of course you can imagine with saliva it’s easily spread.

We do know that lesions themselves, including the ones that present on the surface of the skin, they’re loaded with virus. So they are infectious. If a patient’s been in bed, then the lesions have exudate and pus and that gets on the bed linens and the virus is in that material.

Are the lesions so distinctive that people will go see a doctor to get them looked at? Or might they think it was hives or something like that?

It depends on the person and it depends on the extent of the rash. I think certainly if people have a very defuse, disseminated rash across multiple parts of the body and it’s very visible and evident, that may prompt somebody to go to a medical provider. If it’s more contained to a single body site or a few body sites that can easily be covered up by clothing, then maybe they’re less likely.

Monkeypox patients that I’ve spoken with, they often talk about quite a protracted illness with kind of flu-like syndrome with respiratory involvement. They talk about a lot of malaise, achiness. They’re tired. And the lesions themselves often are described as being very painful, irrespective of where they occur on the body.

That’s what we usually hear from patients, that due to these sorts of signs or symptoms, they knew that they were really sick.

Do these lesions scar in the way that smallpox did? Or chickenpox, for that matter?

Yeah, they can lead to hypo- or hyper-pigmentation and scarring, yes. Darker-skinned individuals may present after healing with hypo-pigmentation, lighter areas where lesions were.

How many lesions do people typically get?

There’s a range of the number of lesions that people present with, from just a handful all the way up to several hundred. It can be quite severe.

Are they itchy like chickenpox?

No. One of the clinical signs we tell clinicians to work through is that generally up until the scabbing stage, orthopoxvirus lesions are generally painful and chickenpox are itchy. It’s only during that healing phase [with monkeypox] when there’s crusting and the skin is regenerating a bit, patients mention itching.

Is it one of those conditions where the older you are when you contract it, the worse the illness is?

I think that’s something we don’t really have a good handle on.

There’s a couple of aspects with age. One could certainly be concerned about underlying immunosuppression just due to the nature of age and other underlying health conditions. But individuals who have prior smallpox vaccination do have some degree of protection against monkeypox. So we would expect those individuals to potentially have maybe a milder form of disease. But again, this is something we really haven’t teased out in individuals who had vaccination 50 years prior, 60 years prior. [The United States stopped vaccinating against smallpox in 1971.]

A number of the cases are in men who have sex with men, which raises the question of sexual transmission. But is it really sexual transmission? It’s being transmitted because of skin-to-skin contact or exchange of saliva if there are lesions in the mouth. Is that correct?

We don’t have any data on virus in semen or vaginal fluids. But what we do know is that it requires close contact. And that’s what certainly occurs during intimate contact. So I think for us this is not entirely different from what we already know about monkeypox in terms of close contact.

Now there may be things we learn later on as we learn about more of these cases. And if there is a significant component about intimate contact that we can try to tease out, then this may present that opportunity. But at this point, everything we’re hearing and seeing is consistent with what we know about monkeypox, which is that it requires close contact.

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