● @ Tuesday, December 16, 2008 ●
Lecture 6!
Poxviridae
Types
Chordopoxvirinae (poxviruses of vertebrates)
- A subfamily of the family POXVIRIDAE, containing eight genera comprising all the vertebrate poxviruses.
Eight Genera:
- Orthopoxvirus (e.g. Smallpox,Vaccinia)
- Avipoxvirus (eg. pox virus which affects birds only)
- Capripoxvirus (eg. poxviruses infecting sheep, goats, and cattle)
- Leporipoxvirus (eg. Myxoma virus(MYXV), Hare fibroma virus (FIBV) and Squirrel fibroma virus(SQFV))
- Molluscipoxvirus (eg. Molluscum contagiosum virus (MCV))
- Parapoxvirus (eg. Pseudocowpox virus, Squirrel parapoxvirus)
- Suipoxvirus (eg. swinepox virus)
- Yatapoxvirus (eg. Ya ba monkey tumour virus (YMTV) and Ta napox virus (TANV))
Entomopoxvirinae(poxviruses of insects)
- A subfamily of POXVIRIDAE comprising poxviruses infecting insects including members of COLEOPTERA; DIPTERA; LEPIDOPTERA; and ORTHOPTERA.
- Alphaentomopoxvirus (eg. Melolontha melolontha entomopoxvirus (MMEV) )
- Betaentomopoxvirus (eg. Amsacta moorei entomopoxvirus ‘L’ (AMEV) )
- Gammaentomopoxvirus (eg. Chironomus luridus entomopoxvirus (CLEV))
Special Features
•Largest family of viruses
•Can just be seen under light microscope?
•Unique Oval-shaped
•Antigenically, very complex
•Remain stable for hours in air
Genome
•Linear dsDNA, covalently cross-linked at ends
•Terminal repeated sequences
•Inverted repeats at both ends
Small pox virus
Smallpox is a serious, contagious, and sometimes fatal infectious disease. There is no specific treatment for smallpox disease, and the only prevention is vaccination.
Clinical Features
•Human only
•Respiratory Secretions
•Always associated with skin lesions
•At least 9 poxviruses cause disease in human
•Variola and Vaccinia
•12-7 days incubation
•Initially influenza-like symptoms
•Characteristic pustules
•Scarring of skins
•Neurological damage
•Blindness
•Death
You may mistake a severe chickenpox rash for a smallpox rash at first. But different viruses cause these illnesses.
Variola
Variola Major
The severe and most common form of smallpox, with a more extensive rash and higher fever.
Four types of variola major smallpox:
- ordinary (the most frequent type, accounting for 90% or more of cases);
- modified (mild and occurring in previously vaccinated persons);
- flat;
- hemorrhagic (both rare and very severe).
Historically, variola major has an overall fatality rate of about 30%;
however, flat and hemorrhagic smallpox usually are fatal.
Variola Minor
Less common presentation of smallpox, and a much less severe disease.
Death rates historically of 1% or less.
Vaccinia
Vaccinia virus is a big mystery in virology. It is not known whether vaccinia virus is the product of genetic recombination, or if it is a species derived from cowpox virus or variola virus by prolonged serial passage, or if it is the living representative of a now extinct virus.
Vaccinia virus was used for smallpox vaccination via inoculation into the superficial layers of the skin of the upper arm. However, with the eradication of smallpox, routine vaccination with vaccinia virus has ceased. Recent interest in vaccinia has focused on its possible usage as a vector for immunization against other viruses.
Vaccination
–used cowpox from milkmaids
–vaccinate an 8 year old James Phipps
–then challenged with small pox!
Primary response to vaccination
4 to 5days following vaccination with vaccinia virus, a papule appeared at the site of vaccination. 2 or 3 days later the papular lesion became vesicular, growing until it reached its maximum diameter on the 9th or 10th day. During this time, the draining lymph nodes of the axial were enlarged and tender. Many patients also presented a mild fever. The lesion dried from the center outward, and the brown scab fell off after about three weeks, leaving a scar- a mark by which previous vaccinees could be recognized.
Complications of Vaccination
Progressive vaccinia (vaccinia necrosum)- It is a severe, potentially fatal illness characterized by progressive necrosis at the site of vaccination. This occurred only in immunocompromised individuals with deficiencies in their cell-mediated immune system. There were only about 1.6 cases of progressive vaccinia per million vaccinations reported. See picture below for clinical manifestation of progressive vaccinia.
Where Smallpox Comes From
Smallpox is caused by the variola virus that emerged in human populations thousands of years ago. Except for laboratory stockpiles, the variola virus has been eliminated. However, in the aftermath of the events of September and October, 2001, there is heightened concern that the variola virus might be used as an agent of bioterrorism. For this reason, the U.S. government is taking precautions for dealing with a smallpox outbreak.
Transmission
Generally, direct and fairly prolonged face-to-face contact is required to spread smallpox from one person to another. Smallpox also can be spread through direct contact with infected bodily fluids or contaminated objects such as bedding or clothing. Rarely, smallpox has been spread by virus carried in the air in enclosed settings such as buildings, buses, and trains. Humans are the only natural hosts of variola. Smallpox is not known to be transmitted by insects or animals.
A person with smallpox is sometimes contagious with onset of fever (prodrome phase), but the person becomes most contagious with the onset of rash. At this stage the infected person is usually very sick and not able to move around in the community. The infected person is contagious until the last smallpox scab falls off.
Smallpox Disease
Incubation Period(Duration: 7 to 17 days)Not contagious
Exposure to the virus is followed by an incubation period during which people do not have any symptoms and may feel fine. This incubation period averages about 12 to 14 days but can range from 7 to 17 days. During this time, people are not contagious.
Initial Symptoms (Prodrome)(Duration: 2 to 4 days)Sometimes contagious*
The first symptoms of smallpox include fever, malaise, head and body aches, and sometimes vomiting. The fever is usually high, in the range of 101 to 104 degrees Fahrenheit. At this time, people are usually too sick to carry on their normal activities. This is called the prodrome phase and may last for 2 to 4 days.
Early Rash(Duration: about 4 days)Most contagious
A rash emerges first as small red spots on the tongue and in the mouth.
These spots develop into sores that break open and spread large amounts of the virus into the mouth and throat. At this time, the person becomes most contagious.
Around the time the sores in the mouth break down, a rash appears on the skin, starting on the face and spreading to the arms and legs and then to the hands and feet. Usually the rash spreads to all parts of the body within 24 hours. As the rash appears, the fever usually falls and the person may start to feel better.
By the third day of the rash, the rash becomes raised bumps.
By the fourth day, the bumps fill with a thick, opaque fluid and often have a depression in the center that looks like a bellybutton. (This is a major distinguishing characteristic of smallpox.)
Fever often will rise again at this time and remain high until scabs form over the bumps.
Pustular Rash(Duration: about 5 days)Contagious
The bumps become pustules—sharply raised, usually round and firm to the touch as if there’s a small round object under the skin. People often say the bumps feel like BB pellets embedded in the skin.
Pustules and Scabs(Duration: about 5 days)Contagious
The pustules begin to form a crust and then scab.
By the end of the second week after the rash appears, most of the sores have scabbed over.
Resolving Scabs(Duration: about 6 days)Contagious
The scabs begin to fall off, leaving marks on the skin that eventually become pitted scars. Most scabs will have fallen off three weeks after the rash appears.
The person is contagious to others until all of the scabs have fallen off.
Scabs resolvedNot contagious
Scabs have fallen off. Person is no longer contagious.
* Smallpox may be contagious during the prodrome phase, but is most infectious during the first 7 to 10 days following rash onset.
Eradication of smallpox
•Possible because
–no reservoir for VV except man
–VV causes only acute infection from which the patients dies, or
–obtains lifelong immunity
–VV is an effective immunogen
–certified by WHO in 1980 to be defeated!
Bioterrorism Today
No one can be infected by small pox virus anymore as it had been completely removed from the Earth.
However, Smallpox virus is known to exist in labs at the Centers for Disease Control and Prevention (CDC) in the U.S. and at the Institute of Virus Preparations in Siberia, Russia. But it may also be in other labs. Some people worry that terrorists could release the virus and spread smallpox to many people
References
http://www.springerlink.com/content/w333u18x11613x30/
http://microbewiki.kenyon.edu/index.php/Poxviridae
http://www.stanford.edu/group/virus/pox/2000/vaccinia_virus.html
http://www.bt.cdc.gov/agent/smallpox/vaccination/faq.asp
http://beamicrobkidnow.wordpress.com/2008/11/
