Poxviruses (Poxviridae) are a distinct family, notable for their substantial size and intricate architecture. These pathogens shaped human history, notably through smallpox, and remain relevant in public health. Understanding their biology provides insight into their unique disease-causing mechanisms.
Understanding Poxviruses
Poxviruses are large DNA viruses that replicate entirely within the cytoplasm of infected cells, unlike most others. They are typically brick-shaped or ovoid, measuring 200–400 nm in length and 200–300 nm in diameter. Their size allows some to be visualized with a light microscope.
The complex virion includes a dumbbell-shaped core, two lateral bodies, and an outer membrane. This core contains a linear, double-stranded DNA genome (130-300 kilobase pairs) and replication enzymes, including DNA-dependent RNA polymerase. Poxviruses infect a broad spectrum of human and animal hosts.
Poxvirus-Related Diseases
Smallpox is the most historically significant poxvirus disease. Caused by the variola virus, it was a severe infectious disease with an estimated 30% fatality rate, though a milder form (variola minor) had a rate under 1%. It caused an estimated 300 million deaths in the 20th century before global eradication. The World Health Organization declared smallpox eradicated in 1980, a monumental public health achievement.
Molluscum contagiosum (MCV) is another common human poxvirus infection. It typically manifests as small, firm, painless bumps on the skin, often with a central dimple. While most common in children, it can also affect adults, particularly those with weakened immune systems. Lesions usually resolve within 6 months to 2 years.
Mpox (formerly monkeypox) is caused by the monkeypox virus. It presents with flu-like symptoms like fever, headache, muscle aches, and swollen lymph nodes, followed by a distinctive rash. The rash progresses from flat spots to pus-filled blisters, then scabs over and falls off, appearing on the face, hands, feet, or genital area. While generally milder than smallpox, mpox can cause serious illness and, rarely, be fatal, particularly in vulnerable populations.
Vaccinia virus, an orthopoxvirus related to smallpox, is notable for its role in the smallpox vaccine. Though the vaccine does not contain the smallpox virus, vaccinia infection can cause mild symptoms similar to smallpox, including rash, fever, headache, and body aches. Severe reactions are rare but can occur, particularly in individuals with compromised immune systems or pre-existing skin conditions like eczema.
How Poxviruses Spread and Are Prevented
Poxviruses primarily spread through direct contact with infected lesions, bodily fluids, or respiratory droplets. Smallpox, for instance, was mainly transmitted through prolonged face-to-face contact. Contaminated objects (fomites) like bedding or clothing can also facilitate transmission, though rare for smallpox. Mpox can spread through skin-to-skin contact, mouth-to-mouth contact, close proximity to an infected person, or via contaminated materials.
Prevention involves good hygiene, such as frequent hand washing, and isolating infected individuals to contain outbreaks. For molluscum contagiosum, covering lesions and avoiding sharing personal items helps prevent spread.
Vaccination is a highly effective prevention strategy. The smallpox vaccine, utilizing vaccinia virus, led to global eradication. Routine smallpox vaccination is no longer administered to the general public due to its eradication. However, JYNNEOS and ACAM2000 vaccines are available for mpox and recommended for individuals at high risk of orthopoxvirus exposure.
Managing Poxvirus Infections
Diagnosis typically involves clinical assessment and laboratory tests. A healthcare provider may visually inspect characteristic skin lesions. Laboratory confirmation often relies on molecular methods, such as polymerase chain reaction (PCR) testing, which detects viral DNA from lesion samples or body fluids. Electron microscopy can also visualize virus particles, though it is less sensitive than PCR and cannot differentiate between specific poxvirus species.
Treatment varies depending on the specific virus and illness severity. For many common infections like molluscum contagiosum, treatment may not be necessary as lesions often resolve without scarring. For persistent or extensive cases, physical removal methods such as cryotherapy (freezing), curettage (scraping), or laser therapy may be employed. Topical medications, including cantharidin, salicylic acid, or potassium hydroxide, are also used.
For severe poxvirus infections like mpox, supportive care manages symptoms like pain and fever. In certain cases, particularly for severe mpox or complications, specific antiviral medications may be considered. Tecovirimat is an antiviral approved for smallpox treatment and used for mpox due to its activity against orthopoxviruses. Other antiviral options include brincidofovir and cidofovir, which inhibit viral DNA replication and can be used with tecovirimat for severe cases.