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Newsletter of Penn Dutch Cow Care October 2006

Hi Folks,

This month I'd like to tackle the topic of bovine leukemia virus (BLV), which is the most common reason that cows will develop leucosis/ cancer/ lymphosarcoma. There has been evidence that other cancers are linked to, or caused by, viruses. One such cancer is cervical cancer in women, which has been linked to a papilloma virus. There is also a canine virus transmitted during breeding. Bovine leukemia virus was originally identified in the mid-part of the 1900's and much research was done on it at veterinary schools. It is a retrovirus and as such it repetitively duplicates itself in the cow. Therefore antibody production (by B cells) against the virus is continual and lifelong. Leukemia is a disease of the B-cells whose growth has gotten out of control. Normal B cells are critical for a healthy immune system. Whether or not an exposed animal develops leukemia/ cancer is due to its genetic make-up. Less than 5% of animals exposed to the virus will develop cancer. That means that 95% of BLV infected cows show no symptoms, have normal immune systems and are productive

Clinical BLV infection is presented as a cow that is lumpy and bumpy, either observed externally at the lymph node areas or by rectal examination by detecting enlarged internal lymph nodes. The lymph nodes are affected because this is where B cell production normally occurs. Other presentations may include an eye that bulges out since there is a lymph node behind it. Or, there may be a tumor of the third eyelid which slowly progresses to invade the entire area. These can be snipped off (with anesthesia) if caught early. A slowly progressing "pinkeye" may actually be this condition. Also the lymph nodes at the top of the udder may become swollen despite no mastitis. The stomach and digestive tract can be affected as well - I have visually seen small tumors on the abomasum of some cows during surgery for a twisted stomach. The tumors can also be within the spinal cord, causing compression i.e. a pinched nerve. This will result in a weak cow that needs help rising for a few days and progresses to complete paralysis. Usually one tumor will predominate to give the presenting signs.

Unfortunately, BLV is a potentially contagious condition, mainly due to the B cells constantly circulating in the blood stream. Therefore, any blood contact between animals can result in a herd mate becoming infected. This can happen when using the same needles to give injections without proper sterilization (you should always use a new needle for each different animal). Using a single sleeve for reproduction exams can also pass it on - if there is blood on the sleeve (even un-seen blood). Mechanical dehorners and tattoo instruments can also pass it on if a proper antiseptic solution is used (chlorhexidine is effective). Biting insects, such as lice, mosquitoes and ticks may pass the infection on whereas stable flies and horn flies are less likely to. Bulls used in AI are very unlikely to transmit infection and natural breeding bulls may if they have a reproductive tract infection on top of being BLV positive.

Vertical transmission, or a calf being infected while in the uterus, may occur 10% of the time in a BLV positive cow. Herds which have high rates of BLV will have higher rates of in-utero transmission. Colostrum or milk of BLV infected cows may contain BLV. But, at least with colostrum, antibodies to the BLV will also be present and may provide against infection. Calves fed milk from infected cows up to weaning can become infected with BLV, but the frequency of occurrence of this is unknown. Using soured milk that reaches a pH of 4.4 and kept at 65° F will kill BLV, as will pasteurization. If considering pasteurizing milk to be fed to calves, also consider adding in yogurt to re-enhance the probiotic content.

Estimates of BLV infection in the US dairy herd range anywhere from 25% of cattle to 80% of cattle - depending on geography, climate and housing. Animals in herds confined with positive herd mates indoors will have higher rates of BLV infection. The diagnosis is either made by clinical signs or by the AGID laboratory method using whole blood drawn from individual cows. If an animal is positive on the AGID, she is truly positive. Some herds I have tested locally will have anywhere from 30-70% of animals infected - but remember only 5% will come down with clinical cancer.

Is it transmissible to humans? The jury is still out; however, one Berkeley, California study showed that healthy human volunteers had antibodies to BLV. This is probably due to the fragments of the virus that remain after pasteurization, rather than having the functional virus intact in raw milk, as pasteurization does kill it (but doesn't necessarily disintegrate it).

Control consists of strict adherence to hygienic methods of dehorning (do not use choppers or gouging instruments), single needle and syringe use, single rectal sleeve use (especially after a cow known to be positive with BLV), strict insect control and separation of positive animals from negative animals. I will gladly use a new sleeve on each cow during pregnancy checking if the farmer is committed to using single needles and syringes, etc.

NOTE: Axel Linde will have space to raise conventional heifers over the winter. You can call him at 717-529-6963. His farm is along Rt. 472 in Kirkwood.

For Bovinity Health, information on functional alternatives to antibiotics see:

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