Bovine Leukemia Virus Is Even More of a Threat Than Previously Considered

There are many areas to keep on top of when dealing with the health of your dairy herd.  Some are immediate and obvious (lameness, injury).  Others are recurring (mastitis, contagious diseases, assistance with calving).  And some, although undetectable, are continually attacking the health and productivity of your cattle. This last area is one that needs more consideration.

“Out of sight out of mind” only works until the negative effects mount up.  And, with so many of the more visible problems heading your “to do” list, you might prefer to “let sleeping dogs (or diseases) lie.”  After all, just keep your fingers crossed and hope the unseen and active problems remain that way. However, with dwindling margins, no dairy herd manager can afford to wait for a health crisis to make itself known — especially if it could have been prevented or, at least, downgraded to something less devastating than the death of a productive animal.

Bovine Leukemia virus is a case in point for a disease that is not being dealt with effectively.

Prevalence of BLV

When it comes to statistics, we are often comfortable with 10 to 15% or even 20%.  In the case of Bovine Leukemia Virus the stats are much higher. In the USA, recent surveys estimate that 44% of dairy cattle have BLV. This is very high compared to many places in the world. European countries, Australia and New Zealand have eradication programs in place that have led to negligible rates of BLV infection. In Canada there is the CHAH program but very few use it. In the USA there are voluntary control programs in place but, combined with the increasing incidence that occurs with increasing herd size and increasing age of cattle,  it is obvious that BLV is a continuing problem.

BLV Has Been Beneath the Radar

Bovine Leukosis Virus (BLV) is a retroviral infection that causes leukemia in cattle by targeting white blood cells and causing them to grow uncontrollably. The virus is transferred from cow to cow by BLV-laden white blood cells found in blood, saliva, semen and milk. The vast majority of BLV-infected cattle do not present with outward clinical signs. Only 5% will develop malignant tumors or lymphosarcomas. While approximately 30% of infected animals will have abnormally high white blood cell counts, only the most severe cases of BLV will exhibit enlarged lymph nodes, weight loss, loss of appetite, infertility and decreased milk production. That is why BLV stays off the priority radar of dairy managers.

Does BLV Alter the Immune System Response to Other Diseases?

It’s bad enough to have cattle affected by a virus but the idea that having that virus could also compromise their entire immune system is a problematic scenario. If that is indeed a possibility, then it is dangerous to be accepting of BLV in the belief that the small (5%) group does not represent a major health issue. Instead there is the  rising spectre of infectious diseases and mastitis pathogens which could be gaining a foothold in herds.

Operational Procedures Spread BLV

There are many health issues which are beyond the control and, to some extent, the management of dairy operators.  However, in the case of Bovine Leukemia Virus, there are definite causes and controls that can be handled by farm personnel.  The most obvious areas to consider are any equipment or procedure that expose animals to contaminated blood.

• used needles
• unsterilized equipment
• multiuse rectal sleeves
• biting insects
• tattooing
• rectal palpations
• dehorning

These seven are all contributors to the tumor growing, lymph node swelling that comes with Bovine Leukemia Virus (BLV). There is also evidence to suggest that BLV-infected dams may transmit the virus to offspring via infected blood, colostrum or placental transfer.

Management and Control of BLV

To ignore or underestimate the problem of BLV is to risk your dairy herd. Dr. Lew Strickland DVM, MS, DACT Bovine at the University of Tennessee suggests a management list that includes the following:

• Test all cattle entering the herd for BLV, and isolate them for 30 to 60 days. Test again at the end of the isolation period.
• Implement annual testing.
• If your herd is infected with BLV, maintain 2 herds; clean and infected.
• Perform all veterinary procedures on BLV-positive cows last.
• Use individual sterile needles for transdermal injection or blood collection.
• Visit http://www.safeneedledisposal.org to learn how to properly dispose of used sharps
• Disinfect all equipment between animals.
• Wash and rinse instruments in warm water, then submerge in an appropriate disinfectant.
• Use electric dehorners, or disinfect dehorning equipment between animals.
• Replace examination gloves and sleeves between animals.
• Use milk replacer to feed preweaned calves.
• Heat-treat or pasteurize colostrum.
• 140º for 30 minutes will kill BLV without damaging IgG antibodies.
• Use BLV-seronegative recipients for embryo transfer.
• Reduce numbers of biting insects.

What Damage Results from BLV – Transmission?

BLV presents multiple implications for your dairy herd.

• Tumors of the abomasum may lead to signs of cranial abdominal pain, melena (digested blood in feces), or abomasal outflow obstruction.
• Pelvic limb paresis progressing to paralysis can occur in animals with spinal lesions.
• Tumors in the eye socket cause protrusion of the globe, resulting in exposure keratitis,
• Tumors of the heart may be mild and undetectable clinically, or may cause arrhythmias, murmurs, or heart failure.
• Uterine lesions may lead to reproductive failure or abortion.

Once infected with BLV, cattle become lifetime carriers, since there are no vaccines or treatments that can eliminate the infection.

Numerous Negative Effects of BLV

The appearance of lymphomas in BLV-infected dairy cows has a direct economic impact on the industry, due to increased replacement costs, loss of income from condemned carcasses of cull cows and the inability to export cattle, semen and em­bryos to countries that maintain BLV control programs. Other losses may include reduced re­productive efficiency and decreased milk production. Beyond these obvious impacts there is the effect (not fully known) on the immune system of infected cattle. Because of BLV they have reduced ability to fend off other infectious diseases.

BLV Negatively Affects Immune Function and Vaccine Efficacy

One epidemiologist has reported a significant association between BLV infection and the occurrence of both clinical and subclinical mastitis. BLV-infected herds have also been found to have a higher risk of hoof problems, gastroenteritis, pneumonia, and culling when compared with BLV-free herds

One area of investigation that has received very little attention is the potential impact of BLV status and altered immune function on vaccine efficacy. Vaccine trials have been performed in attempts to vaccinate against BLV with some promising results. However, there are limited published reports on how cows may respond to non-BLV related immunization protocols based on BLV status. Unfortunately, the impact of BLV status on other vaccine programs has not been investigated.

The Bullvine Bottom Line

Whereas the observational studies do not necessarily prove that BLV causes other illnesses, the potential impact on the ability of cows to resist the development of health disorders is worth a closer examination. It is time to speed up the research and data collection.  Control and eliminate the BLV problem before the battle for dairy health is lost without a fight.

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