Visna virus
Encyclopedia
Visna virus from the genus
lentivirinae and subfamily Orthoretrovirinae, is a "prototype" retrovirus
that causes encephalitis
and chronic pneumonitis
in sheep. It is known as visna when found in the brain, and maedi when infecting the lungs. Life-long, persistent infections in sheep occur in the lungs, lymph node
s, spleen
, joint
s, central nervous system
, and mammary gland
s; The condition is sometimes known as "ovine progressive pneumonia
", particularly in the United States
, or "Montana Sheep Disease". White blood cell
s of the monocyte
/macrophage
lineage are the main target of Visna virus.
, Maedi-Visna virus was the first lentivirus to be isolated and characterized, accomplished in 1957 by Sigurdsson. "Maedi", Icelandic
for dyspnoea, and "visna", Icelandic for "wasting" or "shrinking" of the spinal cord
, refer to endemic
sheep herd conditions that were only found to be related after Sigurdsson's work.
Visna infection may progress to total paralysis leading to death via inanition
; however, if helped to obtain water and food, animals may survive for long periods of time, sometimes greater than ten years. Viral replication is almost exclusively associated with macrophage
s in infected tissues, however replication is restricted in these cells—that is, the majority of cells containing viral RNA do not produce infectious virus.
The disease was introduced to Iceland following an import of Karakul sheep from Germany
in 1933. The susceptibility of Maedi-Visna infection varies across sheep breeds, with coarse-wool breeds apparently more susceptible than fine-wool sheep. Attempts at vaccination against Maedi-Visna virus failed to induce immunity, occasionally causing increased viremia
and more severe disease. Eradication programs have been established in countries worldwide.
, Visna shares many characteristics with human immunodeficiency virus, including the establishment of persistent infection with chronic active lymphoproliferation; however, Visna virus does not infect T-lymphocytes. The relationship of Visna and HIV as lentivirus
es was first published in 1985 by Visna researcher Janice E. Clements
and colleagues in the HIV field. It has been postulated that the effects of Maedi-Visna infection in sheep are the "equivalent" of central nervous system disease and wasting syndrome found in human AIDS patients. Despite limited sequence homology with HIV, the genomic organization of Visna is very similar, allowing Visna infection to be used as an in vivo
and in vitro
model system
for HIV infection.
Research using Visna was important in the identification and characterization of HIV. Nucleotide sequence analysis demonstrated that the AIDS virus was a retrovirus related to Visna and provided early clues as to the mechanism of HIV infection.
Visna has also been a target for HIV denial hypotheses and conspiracy theories. One such conspiracy theory, promoted by Jakob Segal
among others, purports that HIV is a man-made hybrid of Human T-lymphotropic virus
maedi-visna.
Many countries have some sort of national programme to prevent and control a situation where the virus spreads.
Genus
In biology, a genus is a low-level taxonomic rank used in the biological classification of living and fossil organisms, which is an example of definition by genus and differentia...
lentivirinae and subfamily Orthoretrovirinae, is a "prototype" retrovirus
Retrovirus
A retrovirus is an RNA virus that is duplicated in a host cell using the reverse transcriptase enzyme to produce DNA from its RNA genome. The DNA is then incorporated into the host's genome by an integrase enzyme. The virus thereafter replicates as part of the host cell's DNA...
that causes encephalitis
Encephalitis
Encephalitis is an acute inflammation of the brain. Encephalitis with meningitis is known as meningoencephalitis. Symptoms include headache, fever, confusion, drowsiness, and fatigue...
and chronic pneumonitis
Pneumonitis
Pneumonitis or pulmonitis is a general term that refers to inflammation of lung tissue.Pneumonia is pneumonitis combined with consolidation and exudation...
in sheep. It is known as visna when found in the brain, and maedi when infecting the lungs. Life-long, persistent infections in sheep occur in the lungs, lymph node
Lymph node
A lymph node is a small ball or an oval-shaped organ of the immune system, distributed widely throughout the body including the armpit and stomach/gut and linked by lymphatic vessels. Lymph nodes are garrisons of B, T, and other immune cells. Lymph nodes are found all through the body, and act as...
s, spleen
Spleen
The spleen is an organ found in virtually all vertebrate animals with important roles in regard to red blood cells and the immune system. In humans, it is located in the left upper quadrant of the abdomen. It removes old red blood cells and holds a reserve of blood in case of hemorrhagic shock...
, joint
Joint
A joint is the location at which two or more bones make contact. They are constructed to allow movement and provide mechanical support, and are classified structurally and functionally.-Classification:...
s, central nervous system
Central nervous system
The central nervous system is the part of the nervous system that integrates the information that it receives from, and coordinates the activity of, all parts of the bodies of bilaterian animals—that is, all multicellular animals except sponges and radially symmetric animals such as jellyfish...
, and mammary gland
Mammary gland
A mammary gland is an organ in mammals that produces milk to feed young offspring. Mammals get their name from the word "mammary". In ruminants such as cows, goats, and deer, the mammary glands are contained in their udders...
s; The condition is sometimes known as "ovine progressive pneumonia
Pneumonia
Pneumonia is an inflammatory condition of the lung—especially affecting the microscopic air sacs —associated with fever, chest symptoms, and a lack of air space on a chest X-ray. Pneumonia is typically caused by an infection but there are a number of other causes...
", particularly in the United States
United States
The United States of America is a federal constitutional republic comprising fifty states and a federal district...
, or "Montana Sheep Disease". White blood cell
White blood cell
White blood cells, or leukocytes , are cells of the immune system involved in defending the body against both infectious disease and foreign materials. Five different and diverse types of leukocytes exist, but they are all produced and derived from a multipotent cell in the bone marrow known as a...
s of the monocyte
Monocyte
Monocytes are a type of white blood cell and are part of the innate immune system of vertebrates including all mammals , birds, reptiles, and fish. Monocytes play multiple roles in immune function...
/macrophage
Macrophage
Macrophages are cells produced by the differentiation of monocytes in tissues. Human macrophages are about in diameter. Monocytes and macrophages are phagocytes. Macrophages function in both non-specific defense as well as help initiate specific defense mechanisms of vertebrate animals...
lineage are the main target of Visna virus.
Viral infection
First described in 1954 by Bjorn Sigurdsson in IcelandIceland
Iceland , described as the Republic of Iceland, is a Nordic and European island country in the North Atlantic Ocean, on the Mid-Atlantic Ridge. Iceland also refers to the main island of the country, which contains almost all the population and almost all the land area. The country has a population...
, Maedi-Visna virus was the first lentivirus to be isolated and characterized, accomplished in 1957 by Sigurdsson. "Maedi", Icelandic
Icelandic language
Icelandic is a North Germanic language, the main language of Iceland. Its closest relative is Faroese.Icelandic is an Indo-European language belonging to the North Germanic or Nordic branch of the Germanic languages. Historically, it was the westernmost of the Indo-European languages prior to the...
for dyspnoea, and "visna", Icelandic for "wasting" or "shrinking" of the spinal cord
Spinal cord
The spinal cord is a long, thin, tubular bundle of nervous tissue and support cells that extends from the brain . The brain and spinal cord together make up the central nervous system...
, refer to endemic
Endemic (epidemiology)
In epidemiology, an infection is said to be endemic in a population when that infection is maintained in the population without the need for external inputs. For example, chickenpox is endemic in the UK, but malaria is not...
sheep herd conditions that were only found to be related after Sigurdsson's work.
Visna infection may progress to total paralysis leading to death via inanition
Starvation
Starvation is a severe deficiency in caloric energy, nutrient and vitamin intake. It is the most extreme form of malnutrition. In humans, prolonged starvation can cause permanent organ damage and eventually, death...
; however, if helped to obtain water and food, animals may survive for long periods of time, sometimes greater than ten years. Viral replication is almost exclusively associated with macrophage
Macrophage
Macrophages are cells produced by the differentiation of monocytes in tissues. Human macrophages are about in diameter. Monocytes and macrophages are phagocytes. Macrophages function in both non-specific defense as well as help initiate specific defense mechanisms of vertebrate animals...
s in infected tissues, however replication is restricted in these cells—that is, the majority of cells containing viral RNA do not produce infectious virus.
The disease was introduced to Iceland following an import of Karakul sheep from Germany
Germany
Germany , officially the Federal Republic of Germany , is a federal parliamentary republic in Europe. The country consists of 16 states while the capital and largest city is Berlin. Germany covers an area of 357,021 km2 and has a largely temperate seasonal climate...
in 1933. The susceptibility of Maedi-Visna infection varies across sheep breeds, with coarse-wool breeds apparently more susceptible than fine-wool sheep. Attempts at vaccination against Maedi-Visna virus failed to induce immunity, occasionally causing increased viremia
Viremia
Viremia is a medical condition where viruses enter the bloodstream and hence have access to the rest of the body. It is similar to bacteremia, a condition where bacteria enter the bloodstream.- Primary versus Secondary :...
and more severe disease. Eradication programs have been established in countries worldwide.
Model system for HIV infection
Though it does not produce severe immunodeficiencyImmunodeficiency
Immunodeficiency is a state in which the immune system's ability to fight infectious disease is compromised or entirely absent. Immunodeficiency may also decrease cancer immunosurveillance. Most cases of immunodeficiency are acquired but some people are born with defects in their immune system,...
, Visna shares many characteristics with human immunodeficiency virus, including the establishment of persistent infection with chronic active lymphoproliferation; however, Visna virus does not infect T-lymphocytes. The relationship of Visna and HIV as lentivirus
Lentivirus
Lentivirus is a genus of slow viruses of the Retroviridae family, characterized by a long incubation period...
es was first published in 1985 by Visna researcher Janice E. Clements
Janice E. Clements
Janice Ellen Clements is Vice Dean for Faculty at the Johns Hopkins School of Medicine and the Mary Wallace Stanton Professor of Faculty Affairs. She is a professor in the departments of Molecular and Comparative Pathobiology, Neurology, and Pathology, and has a joint appointment in Molecular...
and colleagues in the HIV field. It has been postulated that the effects of Maedi-Visna infection in sheep are the "equivalent" of central nervous system disease and wasting syndrome found in human AIDS patients. Despite limited sequence homology with HIV, the genomic organization of Visna is very similar, allowing Visna infection to be used as an in vivo
In vivo
In vivo is experimentation using a whole, living organism as opposed to a partial or dead organism, or an in vitro controlled environment. Animal testing and clinical trials are two forms of in vivo research...
and in vitro
In vitro
In vitro refers to studies in experimental biology that are conducted using components of an organism that have been isolated from their usual biological context in order to permit a more detailed or more convenient analysis than can be done with whole organisms. Colloquially, these experiments...
model system
Scientific modelling
Scientific modelling is the process of generating abstract, conceptual, graphical and/or mathematical models. Science offers a growing collection of methods, techniques and theory about all kinds of specialized scientific modelling...
for HIV infection.
Research using Visna was important in the identification and characterization of HIV. Nucleotide sequence analysis demonstrated that the AIDS virus was a retrovirus related to Visna and provided early clues as to the mechanism of HIV infection.
Visna has also been a target for HIV denial hypotheses and conspiracy theories. One such conspiracy theory, promoted by Jakob Segal
Jakob Segal
Jakob Segal was a biology professor at Humboldt University in the former East Germany...
among others, purports that HIV is a man-made hybrid of Human T-lymphotropic virus
Human T-lymphotropic virus
Human T-cell Lymphotropic Virus Type 1 , also called the Adult T-cell lymphoma virus type 1, a virus that has been seriously implicated in several kinds of diseases including HTLV-I-associated myelopathy, Strongyloides stercoralis hyper-infection, and a virus cancer link for leukemia...
maedi-visna.
Control programmes
External links
- Visna/maedi virus ICTVdB - The Universal Virus Database, version 4
- Ovine Progressive Pneumonia in Sheep, University of MinnesotaUniversity of MinnesotaThe University of Minnesota, Twin Cities is a public research university located in Minneapolis and St. Paul, Minnesota, United States. It is the oldest and largest part of the University of Minnesota system and has the fourth-largest main campus student body in the United States, with 52,557...
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