Breeding bulls may be source of bovine leukosis virus

Bovine leukosis virus is a widespread viral infection in cattle in North America.

Dairy cattle tend to have a higher level of infection with estimates of 14 percent of all dairy cows being infected with the virus. Older cows are often more likely to be infected.

The virus is usually spread from animal to animal through contact with contaminated blood of an infected animal. The most common methods of spread may be biting insects such as flies or through the use of tattooing instruments, contaminated needles or dehorning instruments and perhaps even by rectal palpation in some circumstances.

All of these methods allow blood from an infected animal to be introduced to a non-infected animal.

Occasionally transmission occurs from the cow to the calf through infected colostrum or by blood contamination at birth.

Some recent research has started to question if breeding bulls could be a potential source of BLV transmission.

Leukosis virus can cause disease, but it is important to understand that only a small percentage of infected animals will ever develop clinical signs.

Fifty to 60 percent of cattle that are persistently infected with the virus will remain asymptomatic and never show any outward signs of infection. Thirty to 40 percent of infected cows may show elevations in their white blood cell count and still remain outwardly healthy. A very small percentage (five to 10 percent) of infected cows will develop clinical lymphosarcoma, usually occurring between four to eight years of age.

Lymphosarcoma clinical signs include lymph node enlargement throughout the body. In some cases, internal organs may also be affected. These cows will lose weight, may bloat or even become paralyzed, depending on which lymphoid tissue is most severely affected.

It should be emphasized that most cows, even if they are infected, rarely show clinical signs, and lymphosarcoma only affects a small percentage of cows later in life. Because of this, the disease has relatively little economic significance to most commercial producers.

However, in some parts of the world, such as Europe, Australia and New Zealand, eradication programs have been implemented, and if a producer wants to export genetic stock, embryos or semen to these countries, then bovine leukosis virus infection levels may be important because some countries will have import restrictions.

Much of the BLV research has occurred in the dairy industry. Ron Erskine and colleagues at Michigan State University first demonstrated that the use of bulls for natural breeding in dairy herds has been associated with a higher BLV prevalence in those herds.

Natural breeding is of course much more common in the beef industry than in the dairy industry, and if bulls can pass the infection on to cows through breeding, this would be of some significance to the beef industry.

Another Michigan research study has estimated a high level of infection in beef bulls with 25 percent of 385 bulls that were examined for breeding soundness testing positive for BLV.

Several studies have failed to isolate the bovine leukosis virus from the semen of infected bulls, but Oscar Benitez-Rojas from Michigan State University recently reported on his research at the 2018 American Association of Bovine Practitioner’s conference.

Benitez-Rojas and his colleagues selected 121 beef bulls from 39 Michigan beef herds. They collected blood and semen and took a prepuce smegma sample from each bull, which was collected in a similar fashion to what a veterinarian would collect for trichomoniasis testing.

Forty-five percent of the bulls were positive for BLV antibodies in their blood, which meant they had been exposed to the virus, and 49 percent of the 39 herds had at least one bull test positive. This prevalence is high compared to recent estimates in beef cows in Western Canada.

They were unable to demonstrate the presence of the virus in semen of the positive bulls, but in 7.4 percent of the positive bulls they did demonstrate that the virus was present in the smegma sample. This would suggest bulls could potentially be a source of BLV transmission during breeding.

However, Benitez-Rojas also reported on a breeding trial where negative heifers were exposed to bulls with positive BLV smegma samples and was unable to demonstrate transmission of the virus during a breeding season.

This trial was small and had limited exposure to infected bulls, which means we can’t completely rule out bulls as a possible source of the virus.

The risk of BLV transmission through bulls could go up as the length of breeding exposure increases or as bulls show evidence of high white blood cell counts and higher viral loads. More research will probably be necessary to evaluate this potential source of transmission.

Researchers at the University of Saskatchewan have recently evaluated beef cows in Western Canada to determine the prevalence of BLV infections. This research was part of the Western Canadian Cow-Calf Surveillance project funded by the Beef Cattle Research Council.

In this study, only 14.6 percent of western Canadian cow-calf herds had at least one cow test positive for BLV. The prevalence of infected cows was quite low with only 2.3 percent of cows testing positive. These prevalence estimates are much lower than what was found in the recent studies in Michigan.

Exposure to BLV is not unusual in dairy herds and in some beef herds across Western Canada. Purebred producers who export genetics might want to consider a control program in consultation with their veterinarian. However, most commercial cow-calf producers would probably not see an economic benefit from attempting to control the virus due to the low prevalence rates and the limited impact of clinical disease.

John Campbell is a professor in the department of Large Animal Clinical Sciences at the University of Saskatchewan’s Western College of Veterinary Medicine.

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