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Enzootic Bovine Leukosis Part 2 — Control

Part 1 of this article on enzootic bovine leukosis (EBL) was about the troubling news that bovine leukosis virus (BLV) is transmissible to humans, and the revelation that the presence of BLV in human breast tissue is potentially associated with an increased risk of breast cancer.

This article presents the basics about EBL control and its significance as a production disease.

Once before, we stood on this side of the Atlantic, hands in pockets, trying to convince the world that BSE wasn’t a big deal; that it hadn’t really jumped the species barrier. We must remember the world’s response to BSE when it finally appeared in Canada — as illogical as it turned out to be — a response that smothered the beef industry for nearly a decade and cost the industry billions.

Many people assume leukosis is a dairy issue, one eliminated by pasteurization. At the same time, they forget that 15 per cent of our meat supply comes from culled dairy cows and, unfortunately, not all milk is pasteurized. Also, up to 10 per cent of beef animals carry the virus.

We have an opportunity to be more proactive this time, even if further research proves the breast cancer-EBL virus link turns out not to be substantiated.

Statements like, “there is no conclusive evidence of transmission,” and, “it is now generally thought that BLV is not a hazard to humans,” lulls the dairy and beef industries into a state of complacency. We must be very careful how the issue of EBL and human disease is handled from this point forward. Recent studies (2013-14) reporting BLV proviral or DNA segments in human tissues make data less speculative. Giving the impression that controlling EBL (the disease) and BLV (causative agent) is not in the industry’s best interest could become an unwieldy albatross around our necks, especially if research strengthens the association between bovine and human disease and if consumer sentiment escalates.

Initially, the reasons for BLV control centred on reduction of carcass condemnations and trade with countries that successfully implemented EBL eradication programs.

There is renewed interest in controlling BLV as a production disease, especially in the United States.

Affordable tests, using serum, have very good sensitivity (98 per cent) and specificity (100 per cent). Tests, using milk, are also very good: sensitivity (95 per cent) and specificity (99 per cent).

In the view of Professor James Evermann, Washington State University, the question really should be, why not test for BLV? In the best interest of the dairy and beef industries it would be prudent to move, at a bare minimum, toward concerted and co-operative voluntary BLV eradication programs.

Four options to consider:

1. No action taken to test for or remove BLV test positive cattle, the primary approach taken on many farms in the U.S. and Canada.

2. Herd monitoring by blood- or milk-based antibody testing, and changes instituted to reduce viral spread through animal-age-dependent management changes.

There are three distinct infection points:

  • Three to 20 per cent of infections occur during fetal development.
  • The second infection peak occurs during calfhood, when up to 40 per cent of infections take place. This may be due to feeding colostrum from BLV-positive cattle or infection by blood-contaminated dehorners, ear-tagging pliers, tattoo instruments or common-use needles.
  • The third peak was noted at the heifer/mature cow ages which took infection levels up to 80 per cent.

It only takes a fraction of a drop (0.001 ml) of blood to transmit disease. Ear tagging; tattooing, subcutaneous, intradermal, or intramuscular injections, dehorning and castration are all high-risk procedures.

3. The third and fourth options are similar. Initially test all cattle and close the herd, adding only BLV test negative cattle. The major difference is that in option three, animals are segregated depending on their BLV infection status; in essence, maintaining two subpopulations, BLV test negative and BLV test positive. In time the BLVs are phased out.

4. Cull any BLV-positive cattle and follow strict biosecurity procedures on all incoming cattle.

Common-sense production practices include:

  • Use separate needles for each animal during vaccination or therapeutic procedures.
  • Clean/disinfect blood-contaminated equip­ment used for tattooing, ear tagging, dehorn­ing, supernumerary teat removal, implants, esophageal feeders and other surgical procedures between animals.
  • Use disposable equipment whenever possible.
  • Use electrical or gas-burning devices rather than gouging equipment for dehorning.
  • Use a new or cleaned rectal palpation sleeve for each cow.
  • Use AI exclusively for breeding, ensuring embryo recipients, and all sires and dams are BLV free.
  • Control stable flies and other biting flies.
  • Segregate BLV positive cattle from negative cattle.
  • Cull BLV test positive cattle, especially those with lymphocytosis, high lymphocyte counts.
  • Minimize contact between newborn calves and BLV test positive cattle.
  • Avoid feeding unpasteurized colostrum.
  • Avoid contact with blood, tissues, and fluids at parturition.
  • Train all personnel on BLV-prevention and biosecurity practices, including testing and isolating herd additions and managing age groups separately while minimizing contact between groups.
  • Control blood-feeding insects, particularly in densely populated farm areas such as milking areas, free stalls and barns.

The search for an effective vaccine against BLV and other members of the retrovirus family has been elusive going back as far as the 1970s. A dependable vaccine that could be used for leukosis control is desperately lacking. Industry support for vaccine research on retroviral vaccines needs to be front and centre if EBL control is to jump forward.

About the author


Dr. Ron Clarke

Dr. Ron Clarke prepares this column on behalf of the Western Canadian Association of Bovine Practitioners. Suggestions for future articles can be sent to Canadian Cattlemen ([email protected]) or WCABP ([email protected]).

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