During the last few years, we have been testing more and more herds for Johne's disease. In most cases, we use a blood test to establish an initial infection rate. However, in several herds, we requested that a test for bovine leukosis be run simultaneously with the test for Johne's. The additional test was not because we had initiated a leukosis control program, but because it seemed like a prudent step since we were already drawing blood from the entire herd.
Much to our surprise, test results revealed about 90 percent of the adult animals in several herds tested positive for leukosis. And, these particular herds were not considered high-risk.
What it is
Bovine leukosis is a viral infection of cattle, sheep and goats. It invades the lymph system, and multiplies in the white blood cells called lymphocytes.
Most cows do not show any outward signs of infection. But about 5 percent of infections do progress to lymphocytosis - an increase in white blood cell concentrations - and, finally, to lymphosarcoma.
Lymphosarcoma is a true cancerous condition in which all or some of the lymph nodes become en-larged and tumors develop. Symptoms can vary, depending on where the tumors are located. Heart failure, paralysis, intestinal blockage, and kidney or liver failure may result. Cows with clinical lymphosarcoma are a complete loss.
In the other 95 percent of infected animals, indirect losses include increased culling rate and loss of livestock breeding sales, particularly for export markets. Research suggests there's very little difference in productive or reproductive performance between infected and uninfected animals, except in the 5 percent that develops lymphosarcoma.
The infection is spread by the transfer of infected lymphocytes, which can occur by:
- The use of common needles for injections.
- The use of common palpation sleeves.
- Feeding colostrum from infected cows to newborn calves.
- The use of tattooing or dehorning equipment without disinfecting between animals.
- Biting or blood sucking insects.
Direct contact between infected and uninfected animals is not likely to spread the infection.
Control the spread
You can control the spread of leukosis by:
- Using a new or disinfected needle for each injection.
- Using a new palpation sleeve for each animal.
- Pasteurizing colostrum, or only feeding colostrum from cows that test negative for bovine leukosis.
- Disinfecting tattoo and dehorning equipment after each calf.
- Implementing an effective fly, louse, and mange control program.
None of these control measures are particularly burdensome. But, most herds do not practice them.
In addition, several trends within the industry have probably helped spread the leukosis virus. The high prevalence rates observed among the herds in my practice - almost at random - may be quite typical.
It seems that we give more and more injections to cows - Pre-Synch, Ov-Synch, vaccinations, and BST. Over the course of one lactation, each cow can receive more than 30 injections. To its credit, Monsanto has packaged BST in single-use syringes. However, it is very likely that the other injections will be administered out of multiple-dose vials, with the same needle being used for several animals. It is not hard to see how a herd with a moderate rate of bovine leukosis infection could easily turn into a herd with a high rate of infection.
Without an effective leukosis control program, don't be surprised if clinical lymphosarcoma becomes a problem in your herd.
Brian Gerloff is a veterinarian and operates Seneca Bovine Service in Marengo, Ill.