Poxvirus infection

Last reviewed: 4 Jul 2021

Last updated: 20 Jul 2021

16 Jul 2021

US FDA approves new drug to treat smallpox

The US Food and Drug Administration (FDA) has approved the anti-viral drug brincidofovir for the treatment of smallpox disease in children and adults.

Although naturally occurring smallpox was eradicated in 1980, there have been long standing concerns that the variola virus could be used as a bioweapon.

The effectiveness of brincidofovir was studied in animals infected with viruses that are closely related to the variola virus as human trials are unethical. Effectiveness was determined by measuring animals’ survival at the end of the studies. More animals treated with brincidofovir survived compared to the animals treated with placebo. The approval was based on the FDA’s animal rule which allows findings from adequate and well-controlled animal efficacy studies to serve as the basis of an approval when it is not feasible or ethical to conduct efficacy trials in humans.

The safety of brincidofovir was based on clinical trials in humans for a non-smallpox indication, primarily patients who received haematopoietic stem cell transplants. The most common adverse effects were diarrhoea, nausea, vomiting, and abdominal pain.

Brincidofovir is only approved for the treatment of smallpox.

See Management: approach

See Management: treatment algorithm

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Human smallpox has been eradicated since 1977, but monkeypox is endemic in West and Central Africa. Cases of monkeypox have also been reported in the UK, the US, Israel, and Singapore.

If smallpox is suspected, the patient should be isolated immediately and the regional infectious diseases unit notified. All suspected cases should be managed by experts, including public health officials, to prevent a potential emergency situation. Monkeypox is also a notifiable condition.

Many other conditions present in a similar way, so it is important to keep in mind possible differentials as, since the eradication of smallpox, these are far more likely to be the cause.

Management is centred around isolation of the patient, infection control measures, and supportive care. Antiviral therapies are also now available.

Smallpox is prevented by vaccination, and is only recommended for laboratory staff working with orthopox viruses, and public health and health care response team members.

Definition

Viruses belonging to the Poxvirus family (genus Orthopoxvirus) that can naturally infect humans and mammals include variola virus (smallpox), monkeypox virus, vaccinia virus, cowpox virus, buffalopox virus, camelpox virus, and cantagalo virus. This topic focuses on human disease associated with smallpox and monkeypox infection.

Smallpox was a serious, contagious, and often fatal infectious disease caused by the variola virus. It was declared eradicated in 1980 following a global immunisation campaign led by the World Health Organization, with the last known natural case reported in Somalia in 1977. Smallpox infection generally resulted in acute onset fever and malaise, followed by a distinctive skin rash (i.e., raised firm lesions with umbilication) mainly occurring on the face, arms, and legs.[1] It was mainly transmitted between humans through the respiratory route (i.e., via saliva droplets); however, transmission from direct contact with infected bodily fluids was also reported. The incubation period of variola virus was 7 to 17 days with rash typically appearing 4 to 7 days later; the most infectious period was during the first week of illness.[1]

Monkeypox is a rare disease that is clinically nearly identical to smallpox, although usually less serious. There is one distinctive feature of monkeypox, which is marked lymphadenopathy.[2] The monkeypox virus can be transmitted to humans from different wild animals, such as non-human primates and rodents, although its natural host reservoir is unknown. Similar to smallpox, it can spread from human to human by the respiratory route or by direct contact with infected bodily fluids. The incubation period is usually 7 to 14 days, and disease typically lasts for 2 to 4 weeks.[3]

BMJ talk medicine podcast: recognise, refer and report Poxvirus infection, with Dr Tom Blanchard external link opens in a new window