Enzootic bovine leukosis is caused by the bovine leukemia virus. A lymphosarcoma, or cancer of the lymph system, is the end result in a small number of cows that are exposed to this virus.
In blood tests, some dairy herds have a 10 to 20 percent incidence of cows that have been exposed and are what we call seropositive. This means they are infected with the virus but are not clinical.
The good news is that only about 0.5 to one percent will develop clinical disease in their lifetime. It should be noted that the incidence and exposure rate in beef cows in Canada is very low.
The natural way this virus is spread is what we call horizontal transmission from cow to cow or mother to calf through anything containing lymphocytes, which are things like blood or milk. This could be blood at the time of calving or blood and lymphocytes in the colostrum when the calf nurses.
Lymphocytes are certain white blood cells that are a component of blood. Thus, transmission is very hard to prevent unless we know all positive cows and their calves are removed immediately and the calves are given colostrum from a non-positive dam.
It has been shown experimentally that 1/1000 of a millilitre of blood can transmit the disease when injected into a negative animal. This means that very little blood or fluid transmission is necessary.
Dehorning dairy calves surgically and not cleaning instruments or disinfecting dehorners has been incriminated. The use of paste dehorners or electric dehorners where no blood is created can minimize this avenue.
In the future, more polled bulls will start to make their way into the dairy industry, and this may help in other ways because dehorning is a stressful event for a young calf.
With sexed semen producing enough replacement heifer calves, some dairy producers are using beef semen on their older cows as a way to increase the value of the calf, so selecting polled (preferably homozygous polled) beef bulls will also help the dehorning issue.
The other surgical procedure done on dairy heifer calves is removal of supernummery teats, so again, good disinfection of instruments between calves is critical.
Other conditions where blood transmission needs to be looked at are rectal palpation, needling when processing, and blood-feeding insects.
When herds have high levels of positive cows, veterinarians may opt to either segregate the cows and palpate them last or change rectal sleeves in between cows. As you can well imagine, this is a labour intensive process, especially when using ultrasound.
Most dairies do a pretty good job controlling insects such as mosquitoes and horn flies.
Many herd health veterinarians have started using a new needle every time to minimize spread.
Dairy calves often get at least two ear tags, which is another possibility of blood transmission.
Tattooing would be another highly possible form of blood transmission. We probably don’t disinfect taggers and tattoo pliers often enough.
We always use new needles if bleeding cows for surveillance, but blood transmission is possible if the vacutainer holder is not cleaned regularly.
We need to think of all the ways blood transmission occurs on dairies. For example, bleeding can occur during hoof trimming if a sole abscess is pared out, so those hoof instruments are potential sources of infection.
It may be too late to do much for a calf that picked up BLV in the first few days, but taking precautions and having procedures in place to prevent blood transmission will reduce spread to others.
The blood test to detect BLV has been used for years and is very accurate.
Bulls going into artificial insemination studs are tested for BLV, and certain countries require them to be negative if exporting. It can then become a real trade barrier for valuable papered or bred dairy heifers destined for export.
I’m not sure why the dairy incidence is so much higher than the beef incidence in Canada. Polled bulls have eliminated the need for dehorning most beef cattle, but that has only occurred to a huge degree in the last 15 to 20 years. Most beef herds change needles every 10 head or so, but they definitely use new needles if intravenous injections are given.
The positive cases of BLV are not clinical until tumours start to develop. Lymph nodes become extremely enlarged and the cow’s body condition goes down.
A huge accumulation of fluid may occur if the cancer is in the abdomen.
The incubation of the clinical form is four to five years, which is normally just outside the average dairy cow’s productive life. We tend to see more cases as cows live longer.
It was thought that milk production isn’t affected in asymptomatic carriers, but that is a hard thing to prove. Milk production has been steadily going up because of genetics, husbandry and nutrition despite the incidence of BLV.
Some herds have seen younger animals clinically affected, which means there is a threshold where the clinical infection incidence will go up.
A grad student at the University of Calgary’s veterinary medicine school will attempt to answer many questions about BLV in our dairy herds.
Decreasing the incidence over time is probably the only economically feasible way to achieve a virus-free herd. Testing and eliminating viable healthy milking cows would be a huge financial hardship.
Has the incidence in the dairies just gradually become higher? Does it have something to do with dairy cattle being in closer proximity?
Also, the average age of beef cows is much higher than dairy cattle, so it is a puzzling scenario.
I only saw one to two clinical cases in beef cattle in all my years of practice. The veterinary community and provincial and federal veterinary bodies had talked years ago about putting more money into tackling production limiting diseases such as Johne’s disease, bovine viral diarrhea and BLV.
That was right about the time BSE hit in 2003, and we have since put large amounts of infrastructure dollars into surveillance and testing for that disease.
I think we have had good control for BSE for a very long time, so maybe we need to get help tackling these other diseases in cattle in the future.
There are a couple of very rare forms of BLV in calves, one where they can essentially be born with the disease and another that affects the thymus, which is lymphoid tissue in the chest. This is particularly prominent in newborns.
Talk to your herd health veterinarian about BLV prevention strategies or to see if testing should be implemented on your dairy.