General Characteristics and Epidemiology
Basic features of orthopoxviruses
- Largest and most complex viruses
- Family: Poxviridae
- Genus: Orthopoxvirus
- DNA virus:
- Nucleosome (instead of a capsid)
- Proteolipid envelope
- Oval or brick-shaped
Clinically relevant species
Notable orthopoxviruses that infect humans include:
- Variola virus (smallpox)
- Monkeypox virus
- Vaccinia virus (used in the smallpox vaccine)
- Cowpox virus
- Orthopoxvirus infections are rare events.
- The last case of naturally occurring smallpox was in the 1970s.
- Considered eradicated in 1980
- Mortality had been as high as 25%–30%.
- Sporadic epidemics have occurred in Africa
- Mortality can be as high as 17%.
- < 150 cases reported
- Most cases occur in individuals < 18 years of age
|Monkeypox||Unknown (maybe small rodents)|
|Vaccinia||Unknown||Skin-to-skin contact (often from those recently vaccinated)|
|Cowpox||Rodents||Direct contact with infected animals (often cows)|
Host risk factors
More severe disease tends to occur in:
- Immunosuppressed patients
- Certain skin conditions (e.g., eczema)
Viral replication cycle
- Unlike many other viruses:
- Viral replication is cytoplasmic.
- No cell receptor or specific protein is targeted by the virus to gain entry into the cell.
- Entry into a host cell is mediated by endocytosis → viral core is released into the cytoplasm
- Intermediate genes are expressed → DNA replication
- Progeny virions are assembled → mature virions are released from the cell upon cellular lysis
- Inhaled secretions and virus → entry into the respiratory tract
- Viral replication → spread to regional lymphatic tissue → replication → viremia
- Dissemination to lymphoid organs → secondary viremia → symptoms
- Virus localizes in dermal blood vessels → endothelial swelling
- Infection of epidermal cells → cutaneous manifestations
- Vaccinia: virus replicates at the site of inoculation → localized cutaneous manifestations
The following table summarizes notable clinical diseases caused by orthopoxviruses:
Diagnosis and Management
The diagnosis can be supported with the following testing:
- PCR for viral DNA from lesion specimens
- Serology (not specific for individual viruses)
- Viral culture
- Electron microscopy of lesion specimens
- There is no known treatment for these diseases.
- Management is supportive.
- Potential antiviral treatments:
- FDA-approved for the treatment of smallpox infection
- Effectiveness is unknown
- Has in vitro activity against orthopoxviruses
- No clinical data in humans
The smallpox vaccine is not only responsible for the eradication of smallpox but also appears to:
- Confer protection against some other orthopoxviruses, such as monkeypox
- Blunt the clinical course of smallpox if given after exposure
- Chickenpox: primary infection caused by the varicella–zoster virus. The typical clinical presentation includes prodromal symptoms, an oral enanthem, and a generalized, intensely pruritic vesicular rash. These lesions appear in crops and will be in varying stages of evolution. The diagnosis is primarily clinical. Management is supportive, though antiviral therapy can be used in certain patient populations.
- Molluscum contagiosum (MC): viral infection limited to the epidermis. Molluscum contagiosum is mainly seen in children < 5 years of age. Lesions are grouped, flesh-colored, dome-shaped papules with central umbilication. The disease is mild in immunocompetent patients and self-resolves within months. Diagnosis is usually clinical. This infection is highly transmissible, and patient education is key in management. Cryotherapy with liquid nitrogen is the 1st-line therapy in patients who require it.
- Hand, foot, and mouth disease: mild, self-limited disease caused by coxsackie group A virus. Patients may have fever, mouth pain, and pharyngitis. An oral enanthem may occur. Skin lesions can be macular, papular, or vesicular and often include the palms, soles, and buttocks. Diagnosis is usually clinical. Management is supportive.
- Bacillary angiomatosis: condition seen in HIV/AIDs patients that results in angiomatous skin lesions due to Bartonella infection. The skin lesions are usually protuberant, red papules or nodules that are friable. Painful osteolytic bone lesions can also occur. Diagnosis can be made with biopsy, cultures, and PCR. Management includes antibiotics and antiretroviral therapy for HIV.
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