The presence of lymphomas is a leading reason for condemnation of beef and dairy cows at slaughter.
They may have been infected with bovine leukosis virus.
Bovine leukosis was not considered an important disease 25 years ago, but prevalence studies in Canada and the United States show it is circulating in many herds at very low levels. Once considered a greater problem among dairy cattle, it has appeared in beef herds and could be a reason for poor productivity.
The scientists involved in the Western Canadian Beef Cattle Surveillance Network tested 2,000 samples and found that 2.3 percent of the cows were positive for bovine leukosis antibodies.
“That means they have been exposed to the virus at some point. A very small proportion of those will go on to have clinical disease,” said John Campbell of the University of Saskatchewan’s Western College of Veterinary Medicine.
“It was significantly higher in Manitoba than it was in Alberta and Saskatchewan.”
Incidence was quite low in Alberta, but nine percent of cows were positive in Manitoba.
“Across the three provinces there were 14.6 percent of the herds (that had) at least one positive cow,” he said.
These are approximate figures and do not mean the cows will sicken.
“Just because a cow tests positive for leukosis doesn’t mean it is going to get sick with any of the diseases that are associated with that virus,” Campbell said.
“Lots of animals live with it for their entire life, and a relatively small percentage get leukemia and cancers associated with bovine leukosis virus.”
An ongoing study at Michigan State University’s College of Veterinary Medicine has found low levels of the disease in five midwestern states.
“Bovine leukosis disease was not considered important 25 years ago. The tide is starting to turn on that,” Michigan State researcher Dan Grooms said during an animal health session at the recent National Cattlemen’s Beef Association convention in Phoenix.
The disease is a part of the retrovirus family and targets lymphatic cells and blood.
It suppresses the immune system and can lead to tumours, leukemia and other forms of cancer. Vaccine efficacy may be lessened.
“This is a chronic disease, so once infected always infected,” he said.
It has been detected more often in dairy cattle. Infected cows milk less and could be culled earlier.
“We now know cattle that are affected with BLD have production losses,” Grooms said.
“In an industry where cow longevity is already not that great, bovine leukosis disease reduces the survivability of that cow on a dairy operation.”
Researchers do not know the level of production losses in beef cattle, but surveys should indicate the prevalence and potential costs.
Grooms’ research followed 3,325 cattle from five midwestern states to estimate prevalence among cow-calf operations.
Information is just coming in, but the virus was found from zero to three-quarters of the cows in the herds studied.
The stages of infection start at the subclinical level.
The cows don’t have tumours or a lot of white cells, but they do have lymphocytes that are n
ot working properly. This could affect immunity and increase the risk of other diseases.
The next stage sees a proliferation of white blood cells, which could proceed to leukemia.
About five percent of infected cattle can progress to developing tumours anywhere in the body.
Grooms is also checking the prevalence among bulls and whether they could introduce the disease through semen or smegma, which is the mucus material in the prepuce of bulls.
His sampling has found that two-year-old bulls had a 20 percent prevalence, but this increased to about 50 percent as bulls matured.
A serological response could be found six weeks after infection once an animal becomes infected.
Dairy farms using live bulls for breeding have a significant risk factor for the disease.
Frozen semen from positive bulls did not transmit the disease.
“We know this virus is destroyed by freezing,” Grooms said.
“One of the management practices in the dairy industry to reduce the risk of transmitting it is freezing the colostrum.”
The virus lives in white blood cells and spreads mostly through blood.
It can also be transmitted in colostrum, snot, saliva, semen and smegma.
“Any way that blood or lymphocytes can be transmitted from one animal to another has the potential of transmitting this virus,” he said.
Diagnosis uses blood tests and milk tests for dairy.
There is no treatment, so control starts with stopping transmission.
Bovine leukosis can be spread via:
- common needles
- tattoo pliers
- fly control
- ear tagging
- rectal sleeves
- colostrum management (pasteurization of colostrum or milk will kill the virus)
- sharing bulls
Source: The Western Producer