What Is Mpox? How Do You Catch It? Is There a Vaccine?

July 1, 2022

Reeti Khare, Ph.D., discusses pressing questions about the symptoms, transmission dynamics, prevention and treatment strategies and basic virology of mpox.

Reeti Khare, Ph.D., discusses pressing questions about the symptoms, transmission dynamics, prevention and treatment strategies and basic virology of monkeypox.

Reeti Khare, Ph.D., Clinical Microbiologist, National Jewish Health and author of the Guide to Clinical and Diagnostic Virology, joined the ASM Studio at ASM Microbe 2022 to discuss pressing questions about the symptoms, transmission dynamics, prevention and treatment strategies and basic virology of mpox (formerly called monkeypox).

What is Mpox?

Mpox is an infection caused by mpox virus. It's a cousin to variola virus, which is the pathogen that causes smallpox. 

How Do Mpox and Smallpox Compare?

Clinically, smallpox and mpox can look very similar. Both can cause fever, chills, headaches, muscle aches and fatigue. But there are a couple of differences between the 2 viruses that are really important to remember.

Smallpox Has Higher Fatality Rate than Mpox

The biggest difference is the fatality rate. Smallpox has a fatality rate of up to 50%, whereas mpox comes nowhere close. There are 2 strains of mpox. The strain that’s involved in the 2022 outbreak is the West African Strain, which has a fatality rate of approximately 1%. The other strain, the Congo Basin Strain, has a higher of a fatality rate of up to 10%.

Mpox and Smallpox Produce Similar Rashes

One symptom that the 2 viruses share is a characteristic rash that appears like raised bumps over the skin. Over the course of 4-5 days, the bumps fill with fluid and puss and become pustules. Mpox and smallpox pustules are both characteristically umbilicated, meaning they have a little divot in the center. Over the course of 2-3 weeks, pustules will ulcerate, scab and fall off.

Another feature of these pox virus pustules is that they all tend to occur in the same stage. In other words, all mpox and smallpox lesions generally appear together, umbilicate, ulcerate and fall off together. This can be especially useful when trying to differentiate between other pox viruses. For example, chickenpox, which is caused by varicella zoster virus produces lesions that progress at different stages throughout the disease. New lesions may be forming at the same time that others are already ulcerating.

A lot of viruses can cause rashes, so another way to differentiate them is to look at their distribution pattern over the body. Mpox and smallpox tend to occur more centrifugally. That means the lesions tend to occur mostly on the extremities and face. Sometimes, lesions even occur on the palms of the hands and soles of the feet, and they can spread all over the body.

Chicken pox is essentially the opposite. It has more of a centripetal distribution, where most of the lesions tend to occur on the trunk and face, and fewer lesions occur on the extremities. Measles and rubella have a pour down distribution, where most of the lesions appear on the face and then decrease in concentration the further down the body that one goes.

Mpox Infects Both Humans and Animals

Although mpox can be acquired from other humans, the disease is generally acquired from animals. The virus can infect monkeys, rodents and other small animals, and this animal reservoir is really significant because it makes the virus difficult to eradicate.

Smallpox only has one host—humans, and thanks to significant global public health efforts to vaccinate and reduce person-to-person transmission, we were able to eradicate smallpox in 1980. Poliovirus is another good example of that. 

How Is Mpox Diagnosed?

Mpox lesions are actually the best way to test for the virus. Lesion fluid or tissue samples are collected with a swab or directly into viral transport media, then tested via PCR. Whole blood does not work as well for diagnosis.

While some U.S. labs are capable of conducting PCR analysis, using a general orthopox virus assay, samples that are positive get sent to the U.S. Centers for Disease Control and Prevention (CDC) for confirmatory testing.

Where Does Mpox Occur?

Until recently, mpox was primarily limited to some West African and Central African countries. That does not mean it wasn’t seen anywhere else. In fact, in 2003, a large outbreak occurred in the U.S., primarily in the upper midwestern states. Animals had been brought in from Africa for sale as exotic pets, and mpox hitched a ride. The virus was then transmitted from the exotic animals to prairie dogs that were housed in the same facility, and when those prairie dogs were sold as pets, the virus was transmitted to humans through bites, handling and/or cleaning the bedding and cages of infected animals.

How is Mpox Transmitted?

Notably, none of the cases in the 2003 outbreak were attributed to human-to-human transmission, but on May 7, 2022, the World Health Organization (WHO) began reporting the spread of  mpox in non-endemic countries. As of the date of this talk, June 10, 2022, more than 1,000 cases had been reported in almost 30 countries. Many infected individuals in this outbreak reported no travel or animal exposure, suggesting a significant human-to-human transmission element.

Human-to-human transmission of mpox, primarily occurs through direct contact with mpox pustules. However, the virus can be transmitted through other body fluids, for example respiratory droplets and other aerosols, as well as fomites that have been contaminated with that infected lesion fluid or body fluid.

Oral and anogenital lesions have been reported more frequently with this outbreak, and we’ve seen a higher frequency of infection in men who have sex with men. Still, we don’t actually know if this virus is being transmitted through close intimate contact, or if it really is being transmitted through sexual body fluids, and this is a subject that remains under investigation.

Is Mpox Going to Be the Next Pandemic? 

Hopefully not. Mpox is a DNA virus, meaning that it replicates a lot slower than RNA viruses, like the flu or SARS-CoV-2. This slow evolution, along with viable treatment/prevention options will likely limit the spread of disease.

How is Mpox Prevented and Treated?

The story of the smallpox vaccine is legend. Edward Jenner noticed that dairy maids that had been exposed to cowpox did not tend to get the more deadly smallpox, and that led to the first vaccine. Fortunately, the smallpox vaccine is also cross-protective against mpox—about 85%.

A couple of antivirals have also been tested against mpox. Tecovirimat is a pox-specific antiviral that prevents the pox viruses from exiting the cell, and cidofovir has been tested in animals and shown be effective. Having all of these things in place, before an outbreak starts to get crazy, definitely puts us in a better position than we were with the emergence of COVID-19.

Author: Ashley Hagen, M.S.

Ashley Hagen, M.S.
Ashley Hagen, M.S. is the Scientific and Digital Editor for the American Society for Microbiology and host of ASM's Microbial Minutes.