Large Animal Oncology, Part 1
I do not encounter tumors as frequently as my small animal colleagues. The greatest reason for this is simply that many of my patients are raised for food and therefore do not live as long as companion animals. Cancer in sheep, swine, and steers raised for beef certainly would be at risk for developing cancers, but they just aren’t living long enough to find out. But what about dairy cows, some of whom stick around for years, and horses?
The most common cancers I’ve encountered in practice in dairy cows involve two types: ocular and lymphatic. Ocular cancer is usually seen in the form of squamous cell carcinoma and starts as a small growth, usually on the eyelid. However, over time, these tumors can grow large and invasive, sometimes affecting the entire eyeball itself. This condition is common enough that farmers refer to it as simply “cancer eye.” This cancer is also common in one particular beef breed: the Hereford. Cattle with white faces seem to be more prone.
Treatment for this type of cancer is removal of the tumor. If the tumor is small and does not involve the eyeball, we can easily surgically remove it. If the tumor has invaded the eye, then the eye and all affected surrounding tissue must be removed. We do enucleation procedures on the farm — a little sedation and a lot of local anesthetic is used while the cow remains standing. Cows recover extremely well after this procedure.
Lymphosarcoma is the other cancer commonly encountered in bovines. Interestingly, a cow can sporadically develop this cancer, or can contract it through infection with bovine leukemia virus, or BLV. Recent surveys of the cattle industry in the U.S. estimate about 40 percentof dairy cattle and 10percent of beef cattle are infected with this virus, which is transmitted via blood. Not all cattle with BLV will develop cancer.
Lymphosarcoma is one of the great imitators in bovine medicine. A cow with enlarged lymph nodes is a suspect, but so is the cow with chronic weight loss and high liver enzymes, or the cow with chronic diarrhea, or the cow that died suddenly. Lymph tissue is throughout the body, so lymph tumors can crop up almost anywhere, either internally or externally.
There is no cure for BLV. Likewise, a cow with lymphosarcoma doesn’t really have any treatment options. There are no chemotherapeutic treatments approved for use in cattle and even if there were, they would most likely be cost-prohibitive on many farms. Most cattle suspected to have lymphosarcoma are either shipped to slaughter before they get too sick, or are euthanized on the farm.
Next week, we’ll take a look at equine cancers.
Dr. Anna O’Brien