What is the cause of monkeypox? Monkeypox is a member of the Poxviridae family of viruses called orthopoxvirus and is a double-stranded DNA virus. This subset includes the smallpox (variola), vaccinia, and cowpox viruses. A nice review of monkeypox virus is found in Scientific American.
How Is This Virus Transmitted and Where Is It Usually Found?
The name "monkeypox" originated in 1958 after the first cases were documented in monkeys being used for research. However, monkeys are not major carriers of the disease. Monkeypox is usually transmitted to humans by the bite of an infected animal, or by touching an infected animal's blood, body fluids, or fur. It's also possible to catch the disease by eating undercooked meat from an infected animal.
Carriers include rodents such as rats, mice, and squirrels.
The disease is usually limited to central and West Africa or to travelers to these regions.
Prior to the current outbreak (see below), the last two cases were in travelers who returned from Nigeria in 2021 (to Maryland and Texas). The case in Texas was in a US resident who had traveled on two commercial flights to and from Nigeria. Contact tracing revealed 200 contacts and none developed symptoms.
The last outbreak in the United States prior to that case was in 2003. A shipment of animals from Ghana imported 800 small mammals representing nine different species, including six types of rodents, to Texas. Some of the infected animals were housed near prairie dogs in an Illinois facility. Forty-seven people from Illinois, Indiana, Kansas, Missouri, Ohio, and Wisconsin became ill with monkeypox after having contact with infected pet prairie dogs.
Since 2016, confirmed cases of monkeypox have increased in countries where the virus is considered "endemic," including Central African Republic, Democratic Republic of the Congo, Liberia, Nigeria, Republic of the Congo, and Sierra Leone. There was an outbreak of over 80 cases in Nigeria in 2017.
Human-to-human transmission rates generally have been limited.
For instance, in an outbreak of monkeypox in the Democratic Republic of Congo (reported in 2016), attack rates were quite high in households; the waning immunity in the region from smallpox is very much a risk factor in the DRC.
Transmission via droplet respiratory particles usually requires prolonged face-to-face contact, which puts household members and other close contacts of active cases at greater risk.
What Does Monkeypox Have to Do With Smallpox?
Smallpox was eradicated in 1979 and routine smallpox vaccinations for US populations were stopped in 1972.
Remember, smallpox is the only virus to have ever been eradicated in humans due to four distinguishing features that many human pathogens do not have: (1) no animal reservoirs; (2) its symptoms are very characteristic (specifically, the "pox" rash), making it easy to isolate patients; (3) short infectious period; (4) a highly effective sterilizing vaccine.
Since the global eradication of smallpox, monkeypox has emerged as the most prevalent orthopoxvirus infection in humans. Because monkeypox virus is closely related to the variola virus, the smallpox vaccine can protect people from getting monkeypox. In fact, past data from Africa suggest that the smallpox vaccine is at least 85% effective in preventing monkeypox.
What Are the Symptoms of Monkeypox?
The presentation is similar to smallpox in humans, but with a very distinguishing symptom of lymphadenopathy (which may be generalized or localized to the axillary regions and the neck).
After infection, there is an incubation period that lasts, on average, 7-14 days.
After an initial febrile prodrome, a rash develops which usually progresses over four stages over a period of 2-3 weeks — macular, papular, vesicular, to pustular — before scabbing over and resolving. Sequelae can include secondary bacterial infections, pneumonias, gastrointestinal involvement, dehydration, and eye infections.
What Are the Treatments?
Patients are generally managed with supportive care and symptomatic treatment. Smallpox vaccination during or after a monkeypox exposure will help prevent the disease or make it less severe. Various antiviral treatments that can be considered if the patient is very ill (based on in vitro or animal studies) include cidofovir, brincidofovir, and tecovirimat.
What Is the Mortality Rate?
Monkeypox is usually a self-limited disease with the symptoms lasting from 2 to 4 weeks.
There are two distinct genetic clades of the monkeypox virus: the Central African (Congo Basin) clade and the West African clade. The Congo Basin clade has historically caused more severe disease and was thought to be more transmissible. Of note, the current outbreak (described below) seems to be caused by the variant with the lower mortality rate (the West Africa Clade, eg, 1% mortality rate).
How to Diagnose Monkeypox?
Monkeypox can be tentatively diagnosed if the characteristic skin lesions are present, or if other symptoms consistent with the disease are seen (eg, look for the more characteristic symptom of lymphadenopathy with monkeypox).
If monkeypox is suspected, health workers should collect a sample and have it tested through polymerase chain reaction (PCR) blood test, given its accuracy and sensitivity. Optimal diagnostic samples are from skin lesions — the roof of or fluid from vesicles and pustules, and dry crusts. Where feasible, biopsy is an option.
How Far-reaching Is the Current Outbreak?
Since May 13, 2022, there have been more than 100 cases of monkeypox reported to the World Health Organization in a currently occurring outbreak from nonendemic regions. Most cases across the world have been in Spain, Portugal, the United Kingdom (where the first European case was reported), Belgium, France, Germany, Italy, Sweden, Australia, and Canada.
As of this writing, there are seven confirmed or suspected cases in the United States: one in Massachusetts, one in New York, one in Florida, two in Utah, one in Washington State, and one in California.
Many of the cases are in men ages 30-55 who have sex with men, possibly linked to two large raves held in Spain and Belgium. Of note, sexual transmission of monkeypox has never been described and transmission is still thought to be from respiratory droplets during close contact, although lesions have been found in the genital and anal regions in some of the cases.
Can Monkeypox Outbreaks Be Contained Readily?
Yes, they usually can. Contact tracing is highly effective for an infection that is not spread that efficiently, that has very characteristic features like smallpox or monkeypox (unlike COVID-19), and when the affected community and medical doctors have been put on high alert as is occurring now. I am an HIV doctor in San Francisco and many large events are coming up, so we are alerting our patients to let us know if they have any unusual rashes or hear of any so that we can contact-trace, contain, and treat if necessary.
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Cite this: Monica Gandhi. Monkeypox: How We Got Here, How Smallpox Vaccines Can Help, and How to Contain It - Medscape - May 24, 2022.