Largest viruses, and comprise
enveloped complex capsids.
They have double-stranded linear DNA.
Unlike most DNA viruses, they replicate in the host cytoplasm; thus, they must encode enzymes necessary for replication, such DNA and RNA polymerases.
Morphology
- The core membrane that surrounds the viral DNA and proteins.
- Lateral bodies lie to the sides of the viral core, which is surrounded by an envelope.
Aka, variola virus.
Transmitted via aerosols, lesions, and fomites.
There are two key strains,
variola major and
variola minor; the major form is more deadly.
Severity of smallpox can range from mild to severe hemorrhagic disease.
Smallpox was associated with high morbidity and mortality until its eradication in the late 1970s.
Global vaccination programs lead to the eradication of Smallpox by 1980; since then, vaccination has been discontinued in the United States.
General pathogenesis of smallpox
Viral entry is typically via aerosols, followed by
replication in the upper respiratory tract.
The virus infects
macrophages and is disseminated to the lymph nodes, where replication continues.
Subsequent viremia disseminates the virus the spleen, liver, bone marrow, and other lymph nodes, where the virus continues to replicate;
this period is typically asymptomatic.
Secondary viremia is produces fever, diarrhea, vomiting, fatigue, and malaise;
As the virus accumulates in the small vessels of the dermis, the vessels begin to hemorrhage and lesions form.
Ultimately, the lesions crust or scab over, and scarring may occur.
An effective
T-cell response can limit the virus and lead to resolution; however, infection often leads to death.
Common, especially in children.
Transmitted via direct contact or fomites.
Can also be spread via
auto-inoculation, which occurs when a person spreads the virus from one part of his body to another.
This virus, unlike smallpox, is dermatropic, and remains in the epidermis; it does not disseminate in the circulation and cause systemic illness.
Typically, the lesions resolve on their own, though this can take several years; upon resolution, there is no latent infection.
In the image, we can see the "pearl-like" pinkish bumps with their characteristic central dimples.
Orf infection is typically acquired from sheep or goats; in animals, the virus typically produces oral lesions and is therefore called "sore mouth infection."
Monkeypox, as its name suggests, was first discovered in research monkeys; however, rodents may serve as reservoirs that aid in transmission.
Until an outbreak in the United States in 2003, monkeypox was confined to Africa.
Cowpox, as its name suggests, was traditionally associated with cows; this virus infects the cows' udders, and is transmitted during milking.
In less agricultural settings, rodent reservoirs play an important role in transmission, because they can spread the virus to humans or other animals, such as cats.
