14.1 History
- Smallpox killed over 500 million people in the 20th century!
- Compared to 320 million deaths caused by wars, the Spanish flu and AIDS combined.
- Smallpox left people scarred by pockmarks and sometimes blind from corneal ulcerations.
- First evidence of smallpox
- Egyptian pharaoh Ramses V mummified remains (Chapter 1)
- Written descriptions: China (340 A.D.) and Southwestern Asia (910 A. D.)
- Smallpox carried by explorers and traders from the Old World to the New World where the population had no immunity to smallpox.
14.2 Clinical Features of Human Poxviruses
Smallpox (Variola)
- Smallpox is caused by two strains of the same virus:
- Variola major - more common, causes a severe form of the disease
- Variola minor - causes a mild form of disease 1-2% mortality rate
- WHO Smallpox Eradication Program - used smallpox recognition cards to search for cases in remote areas of India and Africa where the last pockets of smallpox existed.
- Also used monetary rewards to prompt reporting of cases.
4 Types of Variola major Smallpox
- Ordinary - accounts for 89%+ of cases, 30% mortality rate
- Vaccine modified - mild, occurred in 2.1% of previously vaccinated person, not lethal
- Flat or malignant pox - rare, very severe, represents 6.7% of cases, 90% mortality rate
- Hemorrhagic - rare, very severe, 2.4% of cases, 96% mortality rate
Variola minor
- Less common, mild disease, 1% or less mortality rate.
- The last known person to have natural smallpox of any kind, 1977, Merka, Somalia.
Clinical Presentation of Smallpox
- Average incubation period is 12-14 days.
- Infected individual is not contagious during the incubation period.
- First symptoms:
- Fever of 101 to 104 oF
- Splitting headache
- Severe backache
- Vomiting (50% of individuals infected)
- Diarrhea (10% of individuals infected)
- Delirium and convulsions (7-15% of individuals infected, usually children)
Smallpox - Prodromal Phase
- 2-4 day period
- May be contagious during this phase
- Infected individuals too sick to carry on normal daily activities.
- As the fever declines (2nd or 3rd day), a macular rash appears on the tongue and in the mouth.
- Rash becomes papular (raised) and vesicular (blistery).
- Contains large numbers of virus particles
- As vesicles rupture, large numbers of virus particles are liberated into the saliva. Person is MOST contagious at this stage!
- Lesions line the respiratory tract.
- Some infected individuals may experience a sore throat.
- When the vesicles in the mouth start to break, a skin rash appears: first on the face, spreading to the arms, legs, hands and feet within 24 hours.
- Rash is described as being centrifugal.
- Important in the diagnosis of smallpox.
- Skin vesicles mature into pustules by the 7th day.
- The 2nd week, the rash crusts over.
- Scabs fall off by days 22-27.
- In fatal cases, death occurs between the 10th and 16th day of illness.
- The infected individual remains contagious from the onset of the rash, until all of the scabs come off.
Monkeypox
- Rare infection.
- Before 1970, recognized as a disease of animals in the rainforests of central and western Africa.
- Between 1970-1986, first human cases reported from Western Africa and Congo basin of Africa as smallpox disappeared.
- The smallpox vaccine protects individuals from contracting monkeypox.
- 1996-1997 13 villages in Zaire: first human to human transmission of monkeypox
- Lack of smallpox vaccination and an epidemic in animals allowed the virus to jump the species barrier into humans
- June, 2003, U.S. multistate monkeypox outbreak -exotic pet swap meet
- Case fatality rate is 1-14%
- Signs and symptoms similar to ordinary or modified smallpox but milder.
- Infected individuals experience a fever, vesicular rash.
- Monkeypox victims suffer from lymphadenopathy during the early stages of disease.
Molluscum Contagiosum (MCV)
- Represents 1% of all skin infections
- Transmitted by direct contact, including sexual contact or more commonly through indirect contact - fomites such as sharing towels from swimming pools.
- Rapidly transmitted among children at daycares and kindergartens.
- Becoming a significant opportunistic infection of AIDS patients.
- Incubation period - 2-8 weeks.
- Can serve as a marker for severe immune deficiency.
- Causes pin, pearl-like lesions (1-5 mm in diameter) on the face (especially eyelids), arms, and legs.
- Lesion has a dimple in the center.
- Infection is usually self-limiting in individuals with a competent immune system.
- Secondary bacterial infections are a complication of molluscum contagiosum.
- The smallpox vaccine does not prevent or protect persons from MCV infection.
Vaccinia Virus
- Used to vaccinate individuals against smallpox.
- Also protects against monkeypox infection.
- The exact origin of vaccinia virus is unknown.
- Genetically it is most similar to buffalopox.
- Prototype of poxviruses - can be grown easily in the laboratory.
- Causes a localized skin infection.
- May cause a severe and systemic disease in persons who are immune compromised or have pre-existing conditions such as eczema or atopic dermatitis.
Laboratory Diagnosis of Poxvirus Infections
- 1949, last natural case of smallpox in the U.S.
- Very few physicians in the U.S. have seen an actual case of smallpox.
- Smallpox is a potential biological weapon.
- CDC has prepared information and instructions for physicians in case of a smallpox emergency.
Case Definition of Smallpox
- Illness with acute onset of fever greater than or equal to 101 oF.
- Followed by rash of firm, deep seeded pustules.
- Centrifugal pattern of lesions compared to chickenpox.
Preliminary Tests for Smallpox
- Electron microscopy and virus isolation on scrapings of pustules and scabs done by state health laboratories.
- ELISA assays
- PCR-RFLP analysis is the only way to acccurately distinguish between Variola, monkeypox and vaccinia infections.
14.3 Cellular Pathogenesis
- Very little known about the pathogenesis and virulence of variola virus.
- The knowledge we have, predates modern advances in molecular virology and immunology.
- Variola virus is stored safely in 2 international repositories:
- CDC, Atlanta, Georgia, U.S.
- State Research Center of Virology and Biotechnology (Vector), near Novosibirsk, Russia
- Must be worked with in a BSL-4 maximum containment facility
14.5 Naming and Structure of Poxviruses
- Variola derived from Latin varius (spotted) or varus (pimple)
- Poc or pocca - a bag or pouch, describes an exanthematous disease
- Late 15th century writers used smallpox to distinguish from the great pox, syphilus
Poxvirus Characteristics
- Largest of all animal viruses.
- Particles are brick-shaped
- Clinical specimens have two forms
- M or Mulberry form found in vesicular fluid
- C or Capsule form associated with dried scabs
- 350-270 nm on average.
- Internally, poxvirus particles have a nucleoid and two lateral bodies surround be an outer membrane (visualized by electron microscopy)
- dsDNA genomes with closed ends that are 130-230 kilobases in length
- Ends of the genomes have LTR sequences
Broad Host Range of Poxviruses
- Most poxviruses named after the animal which they were originally isolated (e.g. cowpox, camelpox).
Human Poxvirus Pathogens and Their Natural Hosts
- With the exception of variola and MCV, all poxvirus infections of humans today are zoonoses.
14.6 Vaccinia Virus Replication
- Vaccinia is the prototype of poxviruses.
- Grows in a wide host range of cells in culture (e.g. monkey, rabbit, human, hamster, mouse etc.)
- Within 12-24 hours, it can be grown in high titers.
- 200 bp genome, linear, dsDNA
- 200 genes
- Ends contain LTRs
- Ends of the genome contain unessential genes
- More than 40 different poxvirus genomes have been sequenced.
- 2 forms of virus particles in cell culture
- Extracellular Enveloped Virion (EEV), contains two membranes (golgi and plasma membrane)
- Intracellular Mature Virion (IMV) contains one membrane (derived from golgi)
- IMVs are released upon cell lysis.
- Each infectious form is thought to enter cells by different mechanisms.
Vaccinia Virus Entry and Uncoating
- No specific host-cell receptors have been identified.
- Some studies have shown that the A27L virion protein of vaccinia interacts with cell surface glycosaminoglycans (GAGs).
- GAGs are ubiquitously expressed on many different cell surfaces.
- During uncoating, the outer membrane of EEV is removed and the particle enters the cell, where it loses the 2nd membrane and the viral core passes into the cytoplasm.
Vaccinia Virus Replicates Solely in the Cytoplasm of the Cell
- Unique hallmark of vaccinia/poxviruses:
- Replicate genome in the cytoplasm.
- All other dsDNA viruses replicate genomes in the nucleus.
- Gene expression (transcription) is sequential
- 3 classes of mRNAs
- Vaccinia virus particle contains transcriptional machinery capable of synthesizing viral mRNAs that are recognized by the cell’s translational machinery.
- DNA dependent RNA polymerase
- Transcription factor (VETF)
- Capping enzymes
- Methylating enzymes
- Poly(A) polymerase
- D10 decapping enzyme involved in mRNA turnover
Vaccinia Gene Expression is Sequential and Temporally Regulated
- Early mRNAs encode enzymes and factors need for transcription of the intermediate class of mRNAs.
- Intermediate mRNAs encode enzymes and factors required for late gene expression.
- Late mRNAs expression occurs after DNA replication.
- Encodes factors and enzymes packaged into the virion.
- Structural proteins
Vaccinia Assembly, Maturation and Release
- After the late gene products are synthesized, infectious particles are assembled.
- Assembly involves interactions with the cytoskeleton (e.g. actin-binding proteins and microtubules).
- The particles are wrapped with a Golgi-derived membrane and transported to the periphery of the cell.
- EEV, picks up an additional, plasma membrane as it is released outside of the cell.
14.7 Poxviruses and Immune Evasion
- Poxviruses produce viroceptors and virokines.
- Viroceptors-altered cellular receptors that have lost their transmembrane anchor sequences.
- Secreted from infected cells.
- Sequester ligands onto the receptor portion of the protein.
- Virokines resemble host cytokines.
Other Types of Poxvirus Immune System Inhibitors
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