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• The Latest News—Bovine leukemia virus, the cause of enzootic bovine leukosis, a malignant tumor disease in cattle



• Visit Us—At trade shows in the United States, Germany, Korea, Bolivia, Greece and France



• Technical Tip of the Month—Verification Plate Significance



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Learn more about bovine leukemia virus, the cause of enzootic bovine leukosis, a malignant tumor disease in cattle

Disease
Enzootic bovine leukosis is a malignant lymphoma disease of cattle, zebu and water buffalo.

There are four forms of leukosis in cattle. Three of them (calf, thymic, skin leukosis) are sporadic forms of leukosis of unknown cause. These forms are not contagious. The fourth, enzootic bovine leukosis (EBL), is a lymphoma disease caused by infection with bovine leukemia virus (BLV).¹

Bovine Leukemia Virus
BLV is a retrovirus of 80–100 nm in size that resides in blood lymphocytes where circulating antibodies are unable to neutralize it. Proviral DNA derived from a viral RNA template is permanently integrated into the host's cellular genome. Therefore, once an animal becomes infected with BLV, it is infected for life. Infection can occur at any age, but clinical onset of disease usually starts from three years of age. Up to 70% of infected cattle show chronic increase of circulating B lymphocytes, and 0.1–10% of infected cattle develop a B-cell lymphosarcoma.³

BLV is transmitted when biting insects transfer lymphocytes between cattle, when common bleeding needles or contaminated surgery equipment are used in veterinary treatments, or via colostrums from dams to calves. The virus rarely is present in nasal secretions, saliva, urine or semen. Diaplacental transmission is possible, but occurs in less than 10% of the of the infected dams.¹

Epidemiology
Enzootic bovine leukosis occurs worldwide, mainly in America, Australia, eastern Europe and Asia. BLV-eradication campaigns have been implemented in most European countries, the United States, Australia and other countries. European Community states such as Belgium, Ireland, Norway, Austria, Germany, Sweden and the Netherlands are free of BLV.

Enzootic bovine leukosis is economically significant because of premature culling or death of cattle as a result of lymphosarcoma. Other costly results of BLV infection can be the condemnation of carcasses at slaughter and losses from export restrictions. Countries that have implemented EBL-control programs may require BLV-free certification prior to shipping cattle into their regions.

Diagnosis
BLV infections are mostly asymptomatic. Only about 70% of infected cows develop persistent lymphocytosis, which can be detected using hematological techniques. Up to 10% of infected cows develop lymphosarcoma, which might be detected by clinical examination. Direct detection of BLV from blood lymphocytes is often performed using virus isolation or polymerase chain reaction (PCR), but this process is laborious and costly.

Most commonly, diagnosis is through indirect detection of EBL using serological testing for antibodies to BLV, such as agar gel immunodiffusion (AGID) and enzyme-linked immunosorbent assay (ELISA).

HerdChek* Bovine Leukemia Virus Antibody Test Kit

• Detects antibodies to gp51, an immunodominant response noted in early seroconversion
• Screening and verification format available
• No agar preparation
• Standardized reagents
• Objective measurement and interpretation of results
• Results in short time
• Easy handling—ready-to-use reagents
• Compatible with IDEXX xChek* ELISA software for data management

Sensitivity and Specifity
In order to evaluate the sensitivity of serologic tests, the OIE E4 BLV-positive standard has been produced. IDEXX Laboratories produced a standard equivalent to E4, which is tested in every batch of the HerdCHek Anti-BLV ELISA (Figure 1).

The HerdChek Anti-BLV ELISA, detects E4 positive in a 1:100 dilution, which meets the OIE requirements for testing pools of 10 serum samples.³

Figure 1:
Detectability of E4 equivalent standard diluted in negative serum in 90 minutes and overnight sample incubation protocols

In an independent study performed in Canada on 1,200 serum samples, the HerdChek Anti-BLV ELISA showed a high correlation with AGID (kappa value of 0.998), and a total negative and positive agreement of 99.86% and 100% respectively. In this study, the HerdChek Anti-BLV ELISA detected no false-negative serum samples.⁶

Please note
Antibodies to BLV are indicative of, but not protective against, infection. Seroconversion generally occurs 4–12 weeks after viral exposure. Calves from serologically positive dams can be serologically tested without interference of colostral antibodies if they are older than 6–9 months of age.²

Disease Control
To date there are no vaccines available commercially to prevent BLV infection, and no therapeutic means to cure the disease.

There have been three programs suggested for BLV eradication:

1. Test and implement corrective management methodology.
   In this program, seropositive cattle do not have to be slaughtered or segregated. Surveillance testing and measures to eliminate disease transmission are implemented. ⁷ Such measures include instrument disinfection, strictly using new needles for each animal, using new disposable sleeves for palpation and artificial insemination, and implementation of an insect control program. This program requires a long-term commitment to corrective management practices, and it still may take years to eliminate infection.



2. Test and segregate positive animals.
   This method is used frequently in North America. Positive tested cattle are kept in a separate facility and new cattle are tested to ensure they are free of BLV infection. Success using this method depends on the physical distance between infected and noninfected cattle.



3. Test and slaughter.
   This strategy is used extensively in Europe and Australia. Herds are tested in 6–24-month intervals. The higher the prevalence of BLV infection, the shorter the testing intervals should be. Cattle testing positive should be removed from the herd as soon as possible. ^4,5 In countries of the Europe Union, BLV was very successfully eradicated using this method.

References

1. Aiello SE, Mays A eds. Bovine leukosis. In: The Merck Veterinary Manual. Whitehouser Station, N.J.: Merck & Co., Inc.; 1998:521–522.

2. Burridge MJ, Thurmond MC, Miller JM, Schmerr MJ, Van Der Maaten Duration of colostral antibodies to bovine leukemia virus by two serologic tests. Can J Comp Med. 1982;46(3):270–271.

3. OIE. Enzootic bovine leukosis. In: OIE Manual of Standards for Diagnostic Tests and Vaccines. Paris, France; 2000:371–380.

4. Molloy JB, Dimmock CK, Eaves FW, Bruyeres AG, Cowley JA, Ward WH. Control of bovine leukemia virus transmission by selective culling of infected cattle on the basis of viral antigen expression in lymphocyte cultures. Vet Microbiol. 1994;39(3-4):323–33.

5. Pittler H, Lorenz RJ. Federal control of enzootic bovine leukosis in West Germany in the light of new European Economic Community considerations. Dtsch Tierarztl Wochenschr. 1988;95(7):260–263.

6. Simard C, Richardson S, Dixon P, Belanger C, Maxwell P. Enzyme-linked immunosorbent assay for the diagnosis of bovine leukosis: comparison with agar gel immunodiffusion test approved by the Canadian Food Inspection Agency. Ca J Vet Res. 2000;64:101–106.

7. Sprecher DJ, Pelzer KD, Lessard P. Possible effect of altered management practices on seroprevalence of bovine leukemia virus in heifers of a dairy herd with history of high prevalence of infection. J Am Vet Med Assoc.1991:199(5):584–588.

VISIT US AT THE FOLLOWING EVENTS

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St. Paul, Minnesota, United States—September 13–16, 2003
Allen D. Leman Swine Conference, RiverCentre, Touchstone Energy Place.

Kloster Banz, Germany—September 17–19, 2003
AVID Tagung

Seoul, South Korea—September 21–23, 2003
Asian Pig Veterinary Society Congress

Heraklion, Crete, Greece—October 1–4, 2003
5th International Symposium on the Epidemiology of Foodborne Pathogens in Pork

Santa Cruz, Bolivia—October 7–10, 2003
XVIII Congreso Latino-Americano de Avicultura, Hotel Los Tajibos, Booth 312

San Diego, California, United States—October 9–16, 2003
United States Animal Health Association and Annual Conference of American Association of Veterinary Laboratory Diagnosticians (USAHA/AAVLD) 2003 Annual Conference

Clermont-Ferrand, France—October 15—17, 2003
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TECHNICAL TIP OF THE MONTH

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This month's tip: Verification Plate Significance

What makes BLVv and other verification assays (PRVv, IBRv) different from most other assays is the presence of the normal host cell (NHC) well.

Verification plates require that two wells be run for each sample. One well is coated with virus antigen and the other well is coated with NHC antigens (i.e., a noninfected cell culture extract). The NHC antigens are used to assess whether immunoglobulins directed against culture components are contributing to the positive test results. Animals vaccinated with products made in mammalian cell culture may react as being seropositive on the screening plates in this kit, even in the absence of specific antibody (BLV, PRV or IBR).

The confirmation or verification of samples reactive for specific antibody is determined by calculation of a sample OD /NHC OD ratio. One then follows the specific insert interpretation to classify the samples as positive or negative.

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Studies conducted by IDEXX Laboratories Inc., Westbrook, Maine USA, 2003; data on file.

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