Leukosis showed up unexpectedly in this herd
At herd check Harry had me check an eye with a discharge. I did it very casually, as it appeared to be a cloudy cornea, a keratitis, which I treated topically not really examining the rest of the heifer. This was a 2-year-old. She was fresh three weeks, cleaned properly, and was milking about 70 pounds a day. The heifer was out of an excellent cow with 32,000 pounds of milk as a 4-year-old.
Three days later, Harry called to have some heifers examined, and he had my partner check that heifer's eye again. She was not eating as well as before. Her temperature was 103 degrees, there was slow rumen motility, she was a little humped up, and was slightly tender under the pyphoid cartilage. The eye was not better but was not showing light sensitivity as some pinkeye cases will do. My partner made a diagnosis of traumatic reticulitis (hardware), gave her a magnet, and put her on ampicillin daily.
Four days later, Harry called again saying that the heifer was really not much better. She was checked again, and we performed a laparotomy. The reticulum had a very thick wall adhered to the floor of the abdomen, but it did not feel like fibrin from a peritonitis, although there were some lumps that felt like abscesses.
Deciding not to proceed any further, Harry felt they would ship the heifer when she healed properly from the surgery. There was no hardware. Four hours later, she died with blood coming from the nose.
We all wanted a postmortem. Findings revealed a lymphosarcoma (confirmed by histopathology) involving the floor of the reticulum, the diaphragm, and the right atrium of the heart. This was not the immediate cause of death, but it would have been in the weeks ahead had the heifer lived longer.
Rare in young cows . . .
I had never seen adult lymphosarcoma in a heifer 25 months old. It is commonly seen in cows 5, 6, and 7 years old.
Bovine lymphosarcoma, or bovine leukosis, is a highly fatal malignant neoplasm. It is caused by a retrovirus which lives within the lymphocytes of the blood stream and the reticulo-endothelial system in the body (lymph nodes, liver, spleen).
The virus is spread by contact with an infected animal or with lymphocytes which are carrying the virus. Most animals infected do not develop tumors.
Harry was concerned because of his being a registered herd with a strong possibility of selling breeding stock, some overseas.
A negative herd for lymphosarcoma was definitely a must for him.
In a commercial herd, economic effect is not as great because the incidence of clinical cases is generally low. However, we do not know the effect on production and the immune system that a cow carrying a positive blood reaction and the virus, could exhibit.
Harry started by asking for an AGID blood test on all adults. Depending on the percentage of infection, a test and slaughter procedure would be considered.
If eradication (culling all positive cows) is not practical, then within-the-herd spread must be reduced by using individual needles, rectal sleeves, and proper cleaning of all instruments (dehorning, teat work, castrating) in surgery. Calves should be fed from negative cows. At present, there is no effective vaccine.
Harry is awaiting blood test results. He then will have to decide what to do about the leukosis in his herd.
Written by L. C. Allenstein, D.V.M.
Reprinted with permission from Hoard's Dairyman, October 10, 1998