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What causes enzootic bovine leukosis? | How is the EBL virus spread? | Clinical signs of EBL | Control or prevention of EBL
Enzootic bovine leukosis (EBL) is a viral disease of cattle, which is sometimes referred to as leukosis, leukemia, bovine viral leukosis or bovine lymphosar-coma. It is characterized by a fatal malignant cancer in a small percentage of infected animals. The tumors consist of collections of lymphocytes, a type of white blood cell, that originate in the bone marrow. The major economic losses associated with EBL are related to the loss of sale and export opportunities. Many countries require imported animals to test negative for EBL and to have originated from a herd free of infection.

Although there are four syndromes of lymphosarcoma recognized in cattle, only the adult form is caused by a virus and is called enzootic bovine leukosis. The other three syndromes occur rarely, and their causes have not been determined. Cutaneous lymphosar-coma is a type of skin cancer in cattle under two and a half years of age. It often regresses spontaneously. Thymic lymphosarcoma affects the thymus gland, which is located on the lower neck of cattle. Usually, only calves are affected but the condition has been observed in animals up to two and a half years of age. Calf lymphosarcoma is a result of tumors in the lymph nodes, bone marrow and other internal organs. Both of the latter forms of lymphosar-coma are always fatal.

What Causes Enzootic Bovine Leukosis?

EBL is caused by a virus, which has a worldwide distribution. This virus only resides inside lymphocytes. Because it is intracellular, the virus escapes the animals immune defences. Once infected, an animal is infected for life.

Natural infections occur only in cattle; however, sheep and goats can be infected experimentally. Although all breeds of cattle are susceptible, dairy cattle are more frequently infected presumably due to their close confinement. The disease appears to spread more easily during the summer in North America, possibly a result of transmission by biting insects.

How Is The EBL Virus Spread?

Less than 20 per cent of the calves born to EBL positive dams are themselves born infected with the virus. Those calves born infected are a result of the virus crossing the placental barrier and infecting the developing fetus. Research has indicated that neither the egg nor the sperm become infected. Consequently there is no risk of spreading EBL virus by embryo transfer techniques, provided the embryos are "washed" properly before the transfer. Although the disease has not been spread using artificial insemination, there are reports of females becoming infected after natural matings with infected bulls.

The spread of EBL virus to susceptible animals requires close contact with infected animals. Blood is the most common source of infection. Infected lymphocytes in blood can be transmitted by biting insects such as horseflies, which act as a mechanical vector. Blood contaminated needles, dehorners, and ear tagging and tattoo equipment also transmit the infection. As little as 1/1000 of a millilitre of blood can contain enough infected lymphocytes to transmit the virus.

Placing infected blood on plastic sleeves has been shown experimentally to transmit the virus to susceptible animals during rectal palpation.

Clinical Signs Of EBL

It is important to understand that livestock infected with the EBL virus are not necessarily diseased. Less than 5 per cent of the cattle infected with the virus develop tumors and die of lymphosarcoma. Clinical signs of the disease usually do not occur until a long time (up to 4-8 years) after infection with the EBL virus. Diseased animals only represent the tip of the iceberg of infected animals. Why only a few animals develop clinical disease is not understood; but genetic, environmental and immunological factors appear to be involved.

Economic losses associated with impaired milk production or reproductive failure of infected but non-diseased animals have not been adequately identified. If there are economic losses, they don't appear to be large.

When tumors do develop and result in disease, the clinical signs observed are related to the organ systems involved. For this reason, the condition may be difficult to diagnose clinically. Signs observed in decreasing order of frequency include:
weight loss
decreased milk production
enlarged external lymph nodes
(in front of the flanks and shoulders)
reduced appetite
enlarged internal lymph nodes
(felt on rectal palpation)
partial paralysis of the hind legs
abnormal breathing
bulging eyes
There is no treatment for affected animals. About 5 to 10 per cent of affected animals die suddenly without any signs of illness being observed. Animals infected with EBL virus will pass slaughter inspection because the infection is not transmissible to humans. Once clinical signs develop or tumors are detected in more than one internal organ, the carcass is likely to be condemned.

Diagnosis of the disease is based on the observation of clinical signs and is confirmed by an autopsy or laboratory examination of biopsy tissue. When an animal has been diagnosed as having clinical enzootic bovine leukosis, the animal should be slaughtered or euthanized for humane reasons.

Control Or Prevention Of EBL
Once an animal becomes infected with the EBL virus, it remains infected for life. Antibodies usually appear in the blood about two weeks after the animal becomes infected. These antibodies can be detected in blood samples by using the agar gel immunodiffusion (AGID) technique. The AGID test therefore is useful in detecting infected animals before clinical signs develop. Calves under six months of age may give a false positive test because of antibodies absorbed from colostrum of infected cows.

In some countries the prevalence EBL has been drastically reduced using blood tests to detect infected animals which are then culled for slaughter. At the present time, there is no national policy to eradicate EBL from Canada because the cost of such a program far outweighs the costs currently associated with the disease. Individual herd owners may wish to eradicate EBL from their own herds on a voluntary basis.

EBL can eventually be eradicated, but success depends on several factors. These include: the prevalence of EBL virus infection in the herd, the degree to which the farmer adheres to the recommended sanitary measures, and the willingness to cull animals testing positive for EBL. The following recommendations have been successful in herds with a low prevalence of infection.

1. Test all animals in the herd. Cull and slaughter animals that test positive. Infected cows in the last month of gestation or first month of lactation may give false negative results.

2. Calves that are less than six months of age can be included in the testing or isolated from all other animals in the herd and tested after they reach six months of age.

3. Testing should be repeated for the entire herd at three to six month intervals. Eradication is reportedly successful in 90 per cent of the herds after the second test and slaughter, but a herd should not be declared free of infection until all animals have had at least three negative tests, three to six months apart.

4. All herd additions should have at least two negative tests for EBL. The first test should be done not more than 30 days before purchase and the second should be done no later than 60 days after purchase. Herd additions should be isolated until the second test is found to be negative.

5. Use individual sterile syringes and needles for all vaccinations and injections. Use disinfected instruments for each surgical procedure, e.g., teat removal. Ensure that bleeding is prevented.

6. Use electric dehorners and avoid the use of gouge dehomers. Ensure that bleeding is prevented.

7. Use individual plastic sleeves for all rectal palpations or during artificial insemination.

8. Ensure that insect control is effective in order to minimize transfer of blood by blood sucking insects.

9. Wash and disinfect nose leads, ear taggers and other equipment contaminated with blood before using on another animal.

In herds that have a high prevalence of infection, culling all positive animals may be impractical or financially impossible. Adhering to the following recommendations will reduce the level of infection in the herd with eradication as the ultimate goal.
Test all animals in the herd and segregate the positive animals from the negative by a minimum of
200 metres. A separate farm site for negative animals is an ideal situation. Cull as many positive animals as possible.
Calves from negative cows should be raised separately from those from positive cows. All calves should be fed colostrum and milk from EBL negative cows only. The use of individual calf hutches is useful in preventing the spread of EBL virus among young calves.
Follow points 3 to 9 of eradicating EBL in herds with a low infection rate of the disease.

Prepared by:
G.W. Ollis, D.V.M.

Adapted from Alberta Agriculture Agdex 663-7

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