Bovine leukemia virus
Encyclopedia
Bovine leukemia virus is a bovine virus closely related to HTLV-I, a human tumour virus.
BLV is a retrovirus
which integrates a DNA intermediate as a provirus
into the DNA of B-lymphocytes of blood and milk. It contains an oncogene
coding for a protein called Tax. Nevertheless in its natural host the cattle
leukemia is rare. Because the oncogenic properties of the virus were discovered early, a search for evidence of pathogenicity humans started soon after discovery. Mostly farm workers drinking raw milk were tested for disease, especially for leukemia. But neither leukemia nor other signs of infection could be detected. So many in many states it was not tried to get rid of this infection**. Testing strategies have recently changed since the virus was first detected in Cows; "Only very recently have currently available and highly sensitive assays such as Western blot and ELISA been employed in testing human sera. Buehring et al (2003) detected antibodies against BLV p24 capsid antigen in 74% of human sera tested using Western blot, while none of the samples that had given the most intense reaction was positive when tested with one of the earlier techniques."
In Europe attempts were made to eradicate the virus by culling infected animals. The first country considered to be free of infection was Denmark**. Soon the United Kingdom followed. Like the North American states, those of the Eastern block in Europe did not try to get rid of the virus. But the Eastern Europe states started to become leukosis free after the political changes at the end of the last century. A very disturbing quote from a USDA fact sheet, "The high individual animal prevalence of BLV reported in the Dairy 1996 study suggests that testing and culling seropositive animals may not be a cost effective method to control the disease. Instead, preventing disease transmission by implementing preventive practices would likely be more cost-effective."
Natural infection of animals other than cattle and buffalo
are rare, although many animals are susceptible to artificial infection. After artificial infection of sheep most animals succumb to leukemia. Rabbits get a fatal AIDS
like disease similar to rabbit-snuffles, different from the benign human snuffles
. But it is not known whether this naturally occurring rabbit disease is linked to BLV infection. "Although several species can be infected by inoculation of the virus, natural infection occurs only in cattle (Bos taurus and Bos indicus), water buffaloes, and capybaras. Sheep are very susceptible to experimental inoculation and develop tumours more often and at a younger age than cattle. A persistent antibody response can also be detected after experimental infection in deer, rabbits, rats, guinea-pigs, cats, dogs, sheep, rhesus monkeys, chimpanzees, antelopes, pigs, goats and buffaloes."
Some long term studies may be necessary, as there appears to be a correlation in instances of cancer among butchers and slaughterhouse workers. "Several studies have been carried out in an attempt to determine whether BLV causes disease in humans, especially through the consumption of milk from infected cows. There is, however, no conclusive evidence of transmission, and it is now generally thought that BLV is not a hazard to humans."
and HMS/TSP like neurological disorders.
are more likely to provide a source for infection.
In general BLV causes only a benign mononucleosis
-like disease in cattle
. Only some animals later develop a B-cell leukemia
called enzootic bovine leukosis. Under natural conditions the disease is transmitted mainly by milk
to the calf. Infected lymphocytes transmit the disease too. So for artificial infection infected cells are used or the more stable and even heat resistant DNA. Virus particles are difficult to detect and not used for transmission of infection. It is possible that a natural virus reservoir exists in the water buffalo.
s are to be found explains the many symptoms observed by veterinarian
s: enlargment of superficial lymph nodes, a digestive form, a cardiac form, a nervous form, a respiratory form, etc.
Lymph node enlargement is often an early clinical sign.
An unexpected clinical finding is protrusion of cunjunctival membrane, due to enlargement of retro-ocular lymph nodes.
Diagnosis relies on Agar Gel Immunodiffusion, ELISA and PCR. Post mortem findings are characteristic and include widespread white tumours in most organs.
Testing and removing positive animals from the herd is one method of control. In herds where the disease is widespread, it is important to limit spread by avoiding contact with blood between animals.
BLV is a 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...
which integrates a DNA intermediate as a provirus
Provirus
A provirus is a virus genome that is integrated into the DNA of a host cell.This state can be a stage of virus replication, or a state that persists over longer periods of time as either inactive viral infections or an endogenous retrovirus. In inactive viral infections the virus will not replicate...
into the DNA of B-lymphocytes of blood and milk. It contains an oncogene
Oncogene
An oncogene is a gene that has the potential to cause cancer. In tumor cells, they are often mutated or expressed at high levels.An oncogene is a gene found in the chromosomes of tumor cells whose activation is associated with the initial and continuing conversion of normal cells into cancer...
coding for a protein called Tax. Nevertheless in its natural host the cattle
Cattle
Cattle are the most common type of large domesticated ungulates. They are a prominent modern member of the subfamily Bovinae, are the most widespread species of the genus Bos, and are most commonly classified collectively as Bos primigenius...
leukemia is rare. Because the oncogenic properties of the virus were discovered early, a search for evidence of pathogenicity humans started soon after discovery. Mostly farm workers drinking raw milk were tested for disease, especially for leukemia. But neither leukemia nor other signs of infection could be detected. So many in many states it was not tried to get rid of this infection**. Testing strategies have recently changed since the virus was first detected in Cows; "Only very recently have currently available and highly sensitive assays such as Western blot and ELISA been employed in testing human sera. Buehring et al (2003) detected antibodies against BLV p24 capsid antigen in 74% of human sera tested using Western blot, while none of the samples that had given the most intense reaction was positive when tested with one of the earlier techniques."
History
High prevalence of virus was found from testing by USDA. "As part of the 2007 dairy study, bulk tank milk was collected from 534 operations with 30 or more dairy cows and tested with an Enzyme Linked-Immunosorbent Assay (ELISA) for the presence of antibodies against BLV. Results showed that 83.9 percent of U.S. dairy operations were positive for BLV (table 1)."In Europe attempts were made to eradicate the virus by culling infected animals. The first country considered to be free of infection was Denmark**. Soon the United Kingdom followed. Like the North American states, those of the Eastern block in Europe did not try to get rid of the virus. But the Eastern Europe states started to become leukosis free after the political changes at the end of the last century. A very disturbing quote from a USDA fact sheet, "The high individual animal prevalence of BLV reported in the Dairy 1996 study suggests that testing and culling seropositive animals may not be a cost effective method to control the disease. Instead, preventing disease transmission by implementing preventive practices would likely be more cost-effective."
Natural infection of animals other than cattle and buffalo
Bovinae
The biological subfamily Bovinae includes a diverse group of 10 genera of medium to large sized ungulates, including domestic cattle, the bison, African buffalo, the water buffalo, the yak, and the four-horned and spiral-horned antelopes...
are rare, although many animals are susceptible to artificial infection. After artificial infection of sheep most animals succumb to leukemia. Rabbits get a fatal AIDS
AIDS
Acquired immune deficiency syndrome or acquired immunodeficiency syndrome is a disease of the human immune system caused by the human immunodeficiency virus...
like disease similar to rabbit-snuffles, different from the benign human snuffles
Snuffles
Snuffles can refer to:* Not a common term for a runny nose - see Rhinitis.* A cartoon character - see Snuffles .* A nickname for Sirius Black in the Harry Potter books.* A type of teddy bear made by Gund - see Gund Snuffles...
. But it is not known whether this naturally occurring rabbit disease is linked to BLV infection. "Although several species can be infected by inoculation of the virus, natural infection occurs only in cattle (Bos taurus and Bos indicus), water buffaloes, and capybaras. Sheep are very susceptible to experimental inoculation and develop tumours more often and at a younger age than cattle. A persistent antibody response can also be detected after experimental infection in deer, rabbits, rats, guinea-pigs, cats, dogs, sheep, rhesus monkeys, chimpanzees, antelopes, pigs, goats and buffaloes."
Some long term studies may be necessary, as there appears to be a correlation in instances of cancer among butchers and slaughterhouse workers. "Several studies have been carried out in an attempt to determine whether BLV causes disease in humans, especially through the consumption of milk from infected cows. There is, however, no conclusive evidence of transmission, and it is now generally thought that BLV is not a hazard to humans."
Research
Because of the close relationship between BLV and HTLV-I the research on BLV is important. One can use the experience with BLV for understanding HTLV-I induced diseases like ATL the adult T-cell leukemiaAdult T-cell leukemia
Adult T-cell leukemia/lymphoma is a rare cancer of the immune system's own T-cells.Human T cell leukemia/lymphotropic virus type 1 is believed to be the cause of it, in addition to several other diseases.-Signs and symptoms:...
and HMS/TSP like neurological disorders.
Transmission
Many potential routes of BLV transmission exist. Transmission through procedures that transmit blood between animals such as gouge dehorning, vaccination and ear tagging with instruments or needles that are not changed or disinfected between animals is a significant means of BLV spread. Rectal palpation with common sleeves poses a risk that is increased by inexperience and increased frequency of palpation. Transmission via colostrum, milk, and in utero exposure is generally considered to account for a relatively small proportion of infections. Embryo transfer and artificial insemination also account for a small number of new infections as long as common equipment and/or palpation sleeves are not used. While transmission has been documented via blood feeding insects, the significance of this risk is unclear. The bottom line appears to be that transmission relies primarily on the transfer of infected lymphocytes from one animal to the next and that BLV positive animals with lymphocytosisLymphocytosis
Lymphocytosis is an increase in the number or proportion of lymphocytes in the blood, usually detected when a complete blood count is routinely obtained. Lymphocytes normally represent 20 to 40% of circulating white blood cells...
are more likely to provide a source for infection.
In general BLV causes only a benign mononucleosis
Infectious mononucleosis
Infectious mononucleosis is an infectious, widespread viral...
-like disease in cattle
Cattle
Cattle are the most common type of large domesticated ungulates. They are a prominent modern member of the subfamily Bovinae, are the most widespread species of the genus Bos, and are most commonly classified collectively as Bos primigenius...
. Only some animals later develop a B-cell leukemia
Leukemia
Leukemia or leukaemia is a type of cancer of the blood or bone marrow characterized by an abnormal increase of immature white blood cells called "blasts". Leukemia is a broad term covering a spectrum of diseases...
called enzootic bovine leukosis. Under natural conditions the disease is transmitted mainly by milk
Milk
Milk is a white liquid produced by the mammary glands of mammals. It is the primary source of nutrition for young mammals before they are able to digest other types of food. Early-lactation milk contains colostrum, which carries the mother's antibodies to the baby and can reduce the risk of many...
to the calf. Infected lymphocytes transmit the disease too. So for artificial infection infected cells are used or the more stable and even heat resistant DNA. Virus particles are difficult to detect and not used for transmission of infection. It is possible that a natural virus reservoir exists in the water buffalo.
Clinical signs of bovine leukosis and diagnosis
The variety of organs where white blood cellWhite 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 are to be found explains the many symptoms observed by veterinarian
Veterinarian
A veterinary physician, colloquially called a vet, shortened from veterinarian or veterinary surgeon , is a professional who treats disease, disorder and injury in animals....
s: enlargment of superficial lymph nodes, a digestive form, a cardiac form, a nervous form, a respiratory form, etc.
Lymph node enlargement is often an early clinical sign.
An unexpected clinical finding is protrusion of cunjunctival membrane, due to enlargement of retro-ocular lymph nodes.
Diagnosis relies on Agar Gel Immunodiffusion, ELISA and PCR. Post mortem findings are characteristic and include widespread white tumours in most organs.
Treatment and control
No treatment is available for the disease.Testing and removing positive animals from the herd is one method of control. In herds where the disease is widespread, it is important to limit spread by avoiding contact with blood between animals.