Editor’s note: this is the second in a two-part series.

Bovine Leukosis Virus, also commonly known as Bovine Leukemia Virus or BLV, has taken on increased significance within the U.S. dairy industry due to the recent link made by University of California-Berkeley medical researchers between the cattle disease and human breast cancer.

A national Extension webinar that featured Ray Sweeney, University of Pennsylvania veterinarian, addressed the widespread dairy and beef-cattle disease along with recent news that a Berkeley research team had detected Bovine Leukosis Virus in biopsied breast tissue taken from women. Sweeney said Bovine Leukosis has traditionally been thought not to infect people.

“The world has turned a little bit for us in the last month, believe it or not,” Sweeney said. “There’s now some new research which calls into question this conventional wisdom. And there is now evidence that the bovine leukemia virus can indeed infect people.”

The study, titled “BLV-Human Risk? Exposure to Bovine Leukemia Virus Is Associated with Breast Cancer: A Case-Control Study,” was reported Sept. 2 in the scientific journal “PLOS-One.” It reported that the bovine virus was found in human mammary tissue and more frequently found in samples from cancerous tissues than from non-cancerous control samples. Sweeney said he fears this cattle disease, widespread in the dairy industry and to a lesser extent in the beef industry, will make its way into the public spotlight.

Research has shown that almost 90 percent of Midwest dairy herds have at least one animal infected with Bovine Leukosis Virus. In the southern United States the estimate jumps to 99 percent. At the individual-animal level, it’s estimated that almost 40 percent of Midwest dairy cattle are infected. Again, the estimate is higher in the Southeast at just over 68 percent. Sweeney said it’s likely, on average, that 90 percent of all dairy herds in the United States harbor Bovine Leukosis Virus – and 50 percent of all cows have it.

It’s also in the U.S. beef herd, though the percentages are lower, probably due to the fact that beef cattle are raised differently than dairy cattle; they tend to be more spread out on pasture and range. Even so, it’s estimated that more than 36 percent of beef cattle in the northcentral portion of the country have Bovine Leukosis Virus and almost 8 percent of individual cows in the region do, too.

He said Bovine Leukosis Virus is a cancer-causing retrovirus that eventually manifests in cattle as Enzootic Bovine Lymphosarcoma. It’s related to Avian Leukosis Virus in birds, Feline Leukemia Virus in cats and Human T-Cell Lymphotrophic Virus. Distant cousins are Caprine Arthritis Encephalitis Virus in goats and Human Immunodeficiency Virus or HIV. Infection with these viruses is for life. The virus will always be present in white blood cells, so once a calf or cow is infected, she’ll remain infected for the rest of her life and will be a potential source of infection to herd-mates.

Bovine Leukosis Virus has a long incubation period before the onset of cancer; many, if not most, infected cows never become diseased – though Sweeney said there’s enough virus in one drop of blood to infect another cow. He said there have been no reports of cows contracting this virus or its close relatives from other species.

The virus is spread on a dairy in a number of ways.

  • Biting flies that take blood meals;
  • Multiple-use needles;
  • Dehorning and certain castration equipment that’s not properly sanitized between calves;
  • Rectal exams for pregnancy diagnosis if veterinary sleeves aren’t changed;
  • Colostrum or milk;
  • In-utero;
  • Direct contact such as nasal secretion and urine that have white blood cells in them; and
  • Semen. Sweeney said artificial insemination isn’t a concern because bull studs test for this virus, but it can be a concern with natural service.

Sweeney said there are tests for Bovine Leukosis Virus antibodies – both blood serum and milk tests. Cattle develop antibodies in their blood within six weeks of infection. Bulk-tank milk can also be tested for whole-herd detection. If one cow out of 100 has antibodies to this disease, it should show up on the bulk test. He said serum-testing isn’t recommended for calves under 6 months that may still be protected by their mothers’ antibodies.

Sweeney said the dairy industry has generally not paid too much attention to this disease, because the majority of cows never become sick. Only 2 percent to 3 percent of infected cows will develop cancer, usually after two to four years. Cows aren’t staying in dairy herds as long as they once did, so farmers don’t often see cancerous cows.

When lymphosarcoma – cancer – does manifest, it shows up in the abomasums, causing wall ulcers, “black tar” manure, and a cow that is pale and off feed. The heart is another common site, resulting in congestive heart failure, which manifests as brisket edema, bottle jaw and with fluid around the heart. The spinal cord can be affected, showing as weakness in the backend of a cow and eventual paralysis of the hind legs. It also appears in the eye socket behind the eye, so that a diseased cow’s eye will bulge. The uterus is another site for this cancer.

There is no practical treatment for the virus, Sweeney said. The jury is still out on if Bovine Leukosis Virus affects a cow’s immune function and general health, creates susceptibility to, for instance, mastitis, and causes reduced response to vaccines. The same goes for whether or not her milk production is impacted. Some studies have shown higher culling rates in Bovine Leukosis-infected herds and no differences in others. Sweeney said the virus might affect cow health even if cows don’t develop cancer.

Producers exporting cattle are potentially most economically impacted by Bovine Leukosis Virus because most countries require Leukosis-status. Screening of springing heifers for export typically results in a 10 percent to 20 percent refusal rate due to this virus being present. Farmers looking to have bulls put into the Artificial Insemination industry are also economically impacted. But most dairy farms are not losing much money to Bovine Leukosis Virus.

But if the human link is verified, Sweeney is concerned about the possibility of eventual mandatory control of these diseases in dairy cattle along with public perception and an impact on the consumption of dairy products. He encourages dairy producers to be proactive and talk to their veterinarians about testing for Bovine Leukosis Virus in their herd so they know their herd’s status. If the herd is virus-free, they might want to maintain a closed herd. A producer buying cattle has a 50/50 chance that each purchased animal is infected. Purchased cattle should be tested before being brought into the farm.

If it turns out a herd is infected, strategic options are:

  • Test, segregate and control, which is only reasonable if the prevalence of Bovine Leukosis Virus in the herd is low.
  • Live with it and do nothing, which Sweeney said might backfire if this erupts in a milk-image scare even though pasteurization of milk takes care of this virus.
  • Delay or prevent infection in young stock to reduce the prevalence and make the disease easier to eradicate in the future. Bovine-Leukosis-Virus-free heifers can be sold at a possible premium.

Sweeney said colostrum can be treated to kill the virus, either frozen or heat-treated to 145 degrees for 30 minutes. Disinfect dehorning and tattooing instruments. Require individual rectal sleeves and hypodermic needles.

While the economic impact of Bovine Leukosis Virus is probably less than other diseases dairy producers contend with, Sweeney said it might behoove them to take steps now to reduce prevalence in case the human link is confirmed. He stressed to be careful in interpreting the Berkeley results in terms of association and causation; this research doesn’t prove that this bovine virus causes breast cancer in people. It could be there because tissue was compromised by cancer in the first place, he suggested, and said more research is needed.

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