Medical Microbiology & Immunology
Lecture 22 Virology Poxviruses
1) To know the poxviruses genera that infect humans.
Orthopoxviruses
Parapoxviruses
Unclassified
Variola major: smallpox 3-40% fatality
Variola minor: (Alastrim) smallpox 1% fatality
Cowpox: mild lesions on hands
Vaccinia: used for vaccination
Monkeypox: similar to smallpox (less severe)
Orf: infection in sheep and goats. Human contact = single lesion, spontaneous regression
Pseudocowpox: Milker's nodules (similar to orf)
Molluscum contagiosum: contact (also sexual) "Pearl like" skin lesions, spontaneous
regression
2) To describe the viral structure and replication.
Structure: the most complex animal virus and largest (200-300 nm). Brick or oval
shaped, dsDNA genome surrounded by 2 lateral bodies and enclosed in a membrane.
Replication: rapidly adheres to receptors and is endocytized (cytoplasmic vacuole),
first uncoating removes outer membrane, second uncoating partially removes core
membranes and proteins, viral DNA-dependent RNA polymerase is activated,
transcripts are translated into virus-coated enzymes which further degrade core
proteins and release naked viral DNA. Early transcription leads to synthesis of viral
DNA polymerase, late functions begin with DNA replication, host protein synthesis is
inhibited by viral protein, assembled in the cytoplasm, and acquires an envelope
membrane (probably from the Golgi complex).
3) To explain the clinical features of smallpox. Virus is inhaled and replicates in the
upper respiratory tract epithelial cells before spreading to regional lymph nodes.
Viremia: target cells or skin, mucous membranes and internal organs. Fever and
malaise precede lesions, which are vesicles that appear on the face, arms, and lower
extremities simultaneously. Outcome varies from mild to severe fulminant
(genetics?). Convocations include: keratitis, encephalitis, pneumonia, and bacterial
superinfections.
4) To recognize the pros and cons of smallpox vaccination. Vaccination is vaccinia
virus given intradermally after one year of age.
Pros: guards against infection and prevents the spread of the virus. Smallpox
eradicated by who vaccination program, which began in 1967. Last known case
occurred in 1977 in Somalia.
Cons: protection is not lifelong, revaccination at 10-year intervals. Adverse reactions
to the vaccine include: generalized vaccinia (vesicles over entire body), encephalitis
(40% fatality), progressive vaccinia (immunosuppressed patients, lesions enlarge)
50% mortality, and death of fetus in pregnancy.
5) To describe the diagnosis and treatment of smallpox.
Diagnosis: clinical picture, serology.
Treatment: Methisazone (Marboran), reduces severity, must begin in early after
infection.
6) To identify other diseases associated with poxviruses. See question 1.
7) To discuss its potential use as germ warfare or in bioterrorism. Because smallpox
is one of the most stable enveloped viruses, it can be preserved for a long-time, if
freeze dried. It is unaffected with environmental conditions, in fact the scab form is
stable for one year at room temperature in as much as 13 years in a lab. It is in
activated at 65°C for 30 minutes and with bleach, ethanol (70%). Using modern
genetic recombination techniques, it may also be engineered to be more virulent.