smallpox virus

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Related to smallpox virus: smallpox vaccine, influenza virus, Cowpox virus, measles virus


any member of a unique class of infectious agents, which were originally distinguished by their smallness (hence, they were described as “filtrable” because of their ability to pass through fine ceramic filters that blocked all cells, including bacteria) and their inability to replicate outside of and without assistance of a living host cell. Because these properties are shared by certain bacteria (rickettsiae, chlamydiae), viruses are now characterized by their simple organization and their unique mode of replication. A virus consists of genetic material, which may be either DNA or RNA, and is surrounded by a protein coat and, in some viruses, by a membranous envelope.

Unlike cellular organisms, viruses do not contain all the biochemical mechanisms for their own replication; they replicate by using the biochemical mechanisms of a host cell to synthesize and assemble their separate components. (Some do contain or produce essential enzymes when there is no cellular enzyme that will serve.) When a complete virus particle (virion) comes in contact with a host cell, only the viral nucleic acid and, in some viruses, a few enzymes are injected into the host cell.

Within the host cell the genetic material of a DNA virus is replicated and transcribed into messenger RNA by host cell enzymes, and proteins coded for by viral genes are synthesized by host cell ribosomes. These are the proteins that form the capsid (protein coat); there may also be a few enzymes or regulatory proteins involved in assembling the capsid around newly synthesized viral nucleic acid, in controlling the biochemical mechanisms of the host cell, and in lysing the host cell when new virions have been assembled. Some of these may already have been present within the initial virus, and others may be coded for by the viral genome for production within the host cell.

Because host cells do not have the ability to replicate “viral RNA” but are able to transcribe messenger RNA, RNA viruses must contain enzymes to produce genetic material for new virions. For certain viruses the RNA is replicated by a viral enzyme (transcriptase) contained in the virion, or produced by the host cell using the viral RNA as a messenger. In other viruses a reverse transcriptase contained in the virion transcribes the genetic message on the viral RNA into DNA, which is then replicated by the host cell. Reverse transcriptase is actually a combination of two enzymes: a polymerase that assembles the new DNA copy and an RNase that degrades the source RNA.

In viruses that have membranes, membrane-bound viral proteins are synthesized by the host cell and move, like host cell membrane proteins, to the cell surface. When these proteins assemble to form the capsid, part of the host cell membrane is pinched off to form the envelope of the virion.

Some viruses have only a few genes coding for capsid proteins. Other more complex ones may have a few hundred genes. But no virus has the thousands of genes required by even the simplest cells. Although in general viruses “steal” their lipid envelope from the host cell, virtually all of them produce “envelope proteins” that penetrate the envelope and serve as receptors. Some envelope proteins facilitate viral entry into the cell, and others have directly pathogenic effects.

Some viruses do not produce rapid lysis of host cells, but rather remain latent for long periods in the host before the appearance of clinical symptoms. This carrier state can take any of several different forms. The term latency is used to denote the interval from infection to clinical manifestations. In the lentiviruses, it was formerly mistakenly believed that virus was inactive during this period. The true situation is that lentiviruses are rapidly replicating and spawning dozens of quasi-species until a particularly effective one overruns the ability of the host's immune system to defeat it. Other viruses, however, such as the herpesviruses, actually enter a time known as “viral latency,” when little or no replication is taking place until further replication is initiated by a specific trigger. For many years all forms of latency were thought to be identical, but now it has been discovered that there are different types with basic and important distinctions.

In viral latency, most of the host cells may be protected from infection by immune mechanisms involving antibodies to the viral particles or interferon. Cell-mediated immunity is essential, especially in dealing with infected host cells. Cytotoxic lymphocytes may also act as antigen-presenting cells to better coordinate the immune response. Containment of virus in mucosal tissues is far more complex, involving follicular dendritic cells and Langerhans cells.

Some enveloped RNA viruses can be produced in infected cells that continue growing and dividing without being killed. This probably involves some sort of intracellular regulation of viral growth. It is also possible for the DNA of some viruses to be incorporated into the host cell DNA, producing a carrier state. These are almost always retroviruses, which are called proviruses before and after integration of viral DNA into the host genome.

Few viruses produce toxins, although viral infections of bacteria can cause previously innocuous bacteria to become much more pathogenic and toxic. Other viral proteins, such as some of the human immunodeficiency virus, appear to be actively toxic, but those are the exception, not the rule.

However, viruses are highly antigenic. Mechanisms of pathologic injury to cells include cell lysis; induction of cell proliferation (as in certain warts and molluscum contagiosum); formation of giant cells, syncytia, or intracellular inclusion bodies caused by the virus; and perhaps most importantly, symptoms caused by the host's immune response, such as inflammation or the deposition of antigen-antibody complexes in tissues.

Because viral reproduction is almost completely carried out by host cell mechanisms, there are few points in the process where stopping viral reproduction will not also kill host cells. For this reason there are no chemotherapeutic agents for most viral diseases. acyclovir is an antiviral that requires viral proteins to become active. Some viral infections can be prevented by vaccination (active immunization), and others can be treated by passive immunization with immune globulin, although this has been shown to be effective against only a few dozen viruses.
Comparison of shapes and sizes of viruses.
attenuated virus one whose pathogenicity has been reduced by serial animal passage or other means.
B19 virus a species belonging to the genus Erythrovirus that binds to the erythrocyte P blood group antigen and is the cause of erythema infectiosum. In patients with hemolytic anemia or sickle cell disease it causes aplastic crisis; it can also cause acute arthritis. Fetal infection can cause hydrops fetalis and spontaneous abortion or death in utero. Persistent infection in immunocompromised patients can lead to chronic bone marrow failure. Called also human parvovirus B19.
bacterial virus one that is capable of producing transmissible lysis of bacteria; see also bacteriophage.
coryza virus rhinovirus.
Coxsackie virus coxsackievirus.
defective virus one that cannot be completely replicated or cannot form a protein coat; in some cases replication can proceed if missing gene functions are supplied by other viruses; see also helper virus.
dengue virus a flavivirus, existing as four antigenically related but distinct types (designated 1, 2, 3, and 4), that causes both the classic and hemorrhagic forms of dengue.
DNA virus a virus whose genome consists of DNA.
Ebola virus an RNA virus almost identical to the Marburg virus but serologically distinct; it causes a disease similar to that caused by the Marburg virus.
encephalomyocarditis virus an enterovirus that causes mild aseptic meningitis and encephalomyocarditis.
enteric v's enterovirus.
enteric orphan v's orphan viruses isolated from the intestinal tract of humans and other animals.
Epstein-Barr virus (EBV) a herpeslike virus that causes infectious mononucleosis and is associated with Burkitt's lymphoma and nasopharyngeal carcinoma; see also epstein-barr virus.
fixed virus a virus whose virulence and incubation period have been stabilized by serial passage and have remained fixed during further transmission, as opposed to a street virus.
helper virus one that aids in the development of a defective virus by supplying or restoring the activity of the viral gene or enabling it to form a protein coat.
hepatitis A virus (HAV) any virus of the genus Hepatovirus that causes hepatitis a. This has the most rapid onset of the hepatitis viruses affecting humans; transmission is easier than for the hepatitis B and C viruses, but infection generally does not persist. While infection with this virus alone is usually not life-threatening, coincident infection with hepatitis C virus is generally rapidly fatal.
hepatitis B virus (HBV) a species of genus Orthohepadnavirus that causes hepatitis b.
hepatitis C virus a species of genus Hepacivirus that causes hepatitis c; its latency period may last 30 years or more.
hepatitis D virus (HDV) (hepatitis delta virus) an unclassified defective RNA virus, thought of as a parasite of the hepatitis B virus and transmitted in the same manner; it requires enzymes and other assistance from HBV to replicate. This virus magnifies the pathogenicity of hepatitis B virus many times and is the etiologic agent of hepatitis d.
hepatitis E virus an enterically transmitted calicivirus that causes hepatitis e.
hepatitis G virus (HGV) a parenterally transmitted flavivirus originally isolated from a patient with chronic hepatitis; most infections are benign, and it is uncertain what role, if any, HGV plays in the etiology of liver disease.
hepatotropic virus a virus that primarily affects the liver, such as the hepatitis viruses.
herpes virus herpesvirus.
herpes simplex virus former name for any virus that causes herpes simplex, now called human herpesviruses; see herpesvirus.
human immunodeficiency virus (HIV) either of two species of lentiviruses that cause acquired immunodeficiency syndrome (AIDS). HIV-1 is found around the world and HIV-2 is found primarily in West Africa. Progression of HIV-2 infection to AIDS is generally slower and less extreme than that of HIV-1. The virus is believed to induce permanent infection and has a propensity toward a subset of T lymphocytes called the CD4 cells. The infected cells become dysfunctional and eventually the host's immune system is overwhelmed or exhausted; death ensues, usually as a result of infection. The virus is not transmitted through casual contact; the most common routes of transmission are through sexual intercourse, direct exposure to contaminated blood, and transplacental transmission from mother to fetus.
Human immunodeficiency virus: retrovirus particle. From Copstead, 1995.
human T-cell leukemia virus (human T-cell lymphotropic virus) former names for human T-lymphotropic virus.
human T-lymphotropic virus (HTLV) either of two related species of retroviruses that have an affinity for the helper cell type of T lymphocytes. HTLV-1 causes chronic infection and is associated with adult T-cell leukemia and a type of myelopathy. HTLV-2 has been isolated from an atypical variant of hairy cell leukemia and from patients with other hematological disorders, but no clear association with disease has been established.
influenza virus any of a group of orthomyxoviruses that cause influenza; there are at least three serotypes or species (A, B, and C). Serotype A viruses are subject to major antigenic changes (antigenic shifts) as well as minor gradual antigenic changes (antigenic drift) and cause widespread epidemics and pandemics. Serotypes B and C are chiefly associated with sporadic epidemics.
influenza A virus (influenza B virus) (influenza C virus) species in the genera Influenzavirus A, Influenzavirus B, and Influenzavirus C; see influenza virus.
La Crosse virus a virus of the California serogroup of the genus Bunyavirus, the etiologic agent of La Crosse encephalitis.
latent virus one that ordinarily occurs in a noninfective state and is demonstrable by indirect methods that activate it.
lytic virus one that is replicated in the host cell and causes death and lysis of the cell.
maedi/visna virus a lentivirus that is the etiologic agent of a type of pneumonia in sheep.
Marburg virus an RNA virus occurring in Africa, transmitted by insect bites, and causing marburg virus disease.
masked virus latent virus.
measles virus a paramyxovirus that is the cause of measles.
mumps virus a paramyxovirus that causes mumps and sometimes tenderness and swelling of the testes, pancreas, ovaries, or other organs.
Norwalk virus a calicivirus that is common cause of epidemics of acute gastroenteritis, with diarrhea and vomiting that last 24 to 48 hours.
oncogenic v's an epidemiologic class of viruses that are acquired by close contact or injection and cause usually persistent infection; they may induce cell transformation and malignancy.
orphan v's viruses isolated in tissue culture, but not found specifically associated with any disease.
parainfluenza virus any of various paramyxoviruses that cause upper respiratory tract disease of varying severity.
poliomyelitis virus see poliovirus.
pox virus poxvirus.
rabies virus an RNA virus of the rhabdovirus group that causes rabies.
respiratory syncytial virus (RSV) any of a genus of single-stranded paramyxoviruses; the name is derived from the type of disease produced (respiratory infection) and the microscopic appearance of the viruses in cell cultures. RSV can cause a wide variety of respiratory disorders ranging from a mild cold to serious or even fatal disease of the lung in the very young and very old. It regularly produces an outbreak of infection each winter and virtually disappears in the summer months. The most severe infections in children are in the very young, especially those who are preterm, immunologically compromised, or suffering from a congenital heart defect or preexisting lung disorder. Adults at risk for infection include parents and others who are repeatedly exposed to young children, for example, pediatric nurses and day care attendants. The course of infection tends to be milder in adults than in children and about 15 per cent of affected adults have no symptoms. In the very elderly these infections may have the same degree of seriousness and clinical manifestations as in the very young.
RNA virus a virus whose genome consists of RNA.
rubella virus a togavirus that is the etiologic agent of rubella.
satellite virus a strain of virus unable to replicate except in the presence of helper virus; considered to be deficient in coding for capsid formation.
simian-human immunodeficiency virus a chimeric, engineered virus with the envelope of human immunodeficiency virus and the cytoplasm and nucleus of simian immunodeficiency virus; it is used in animal models because it is a better mimic of HIV than SIV is.
simian immunodeficiency virus (SIV) a lentivirus closely related to human immunodeficiency virus that causes inapparent infection in African green monkeys and a disease resembling acquired immunodeficiency syndrome in macaques and chimpanzees.
slow virus any virus that remains latent for long periods in the infected host before the appearance of clinical symptoms.
smallpox virus variola virus.
street virus virus from a naturally infected animal, as opposed to a laboratory-adapted strain of the virus.
vaccinia virus a species of orthopoxvirus that does not occur in nature and has been propagated for many years only in the laboratory for use as an active vaccine against smallpox. The present virus is derived from the original one used by Jenner, obtained from the lesions of cowpox, but the origin of the original virus remains unclear.
varicella-zoster virus former name for human herpesvirus 3; see herpesvirus.
variola virus the virtually extinct orthopoxvirus that is the etiologic agent of smallpox. No natural infection has occurred since 1977, and no reservoir of the virus now exists.
West Nile virus a virus of the genus Flavivirus, the cause of West Nile encephalitis; it is transmitted by Culex mosquitoes, with wild birds serving as the reservoir. It was originally endemic in Africa, Asia, and Europe, but recently spread to North America.
wild-type virus street virus.

va·ri·o·la vi·rus

a poxvirus of the genus Orthopoxvirus, the pathogen of smallpox in humans.
Synonym(s): smallpox virus

smallpox virus

variola virus.

va·ri·o·la vi·rus

(var-ī'ō-lă vī'rŭs)
A poxvirus of the genus Orthopoxvirus, the pathogen of smallpox in humans.
Synonym(s): smallpox virus.
References in periodicals archive ?
Henderson discusses at length his opposition to research with the live smallpox virus for the development of medical countermeasures such as antiviral drugs, which began in 1999 at the two WHO-authorized repositories and has continued since then.
Firstly, the tests used to determine the infectivity of smallpox virus, on which this presumption is based, were carried out at a temperature of 35[degrees] centigrade over a period of weeks.
Since data on natural exposure to smallpox virus are not available for contemporary populations, analysis of historical data is likely to provide the most convincing evidence (3).
The two-day meeting, which focuses on measures to counteract the deliberate use of the smallpox virus, will take place on October 21-22.
Weiner that produces a protein that is an essential component of the smallpox virus.
In the late 1940s, direct touch preparations from skin lesions were prepared in North Africa and sent to Toronto, Canada, where they were examined successfully for smallpox virus for up to 4 months after collection (11).
Weaponized smallpox virus may have an incubation period as short as 7 days, allowing very little time for vaccine administration.
the threat of smallpox virus being used as a bioterrorist weapon.
For example, smallpox virus would not be considered a useful biological weapon against a population universally immunized with vaccinia.
Monkeypox virus is closely related to human smallpox virus and causes a similar disease in both monkeys and humans.
The smallpox virus could potentially be one of the most dangerous biological weapons in the world, yet there are an estimated 40 million Americans who cannot safely be vaccinated," said George Painter, president and CEO of Chimerix.
Medical countermeasures against the re-emergence of smallpox virus.