Every year I get numerous calls about Johne's disease and bovine leukosis. Occasionally veterinarians or producers want to discuss the two of them at the same time. As much as I try to keep things separated, inevitably I have trouble keeping the listener from getting them confused. I thought it might be interesting to discuss them together in one session in order to contrast the differences. I think most practitioners have a pretty good understanding of the basics surrounding the diseases, particularly the clinical syndromes. Therefore I've decided to try to answer some of the most frequently asked questions and highlight some of the most frequently misunderstood issues concerning these diseases

How are these diseases spread in a herd, vertically and horizontally?

The classical means of transmission of M. avium paratuberculosis (MAP) is fecal-oral. Susceptibly is age-related. There is no magic age after which a calf becomes resistant to the organism; however, the number of organisms required to produce infection rises as age increases. Therefore, it is increasingly difficult for an animal to become infected as it ages. Studies have shown that about 10% of infected cows shed MAP in their milk and about 22% of infected cows shed MAP in their colostrum. Approximately 8% of infected cows without clinical signs and 25% of infected cows with clinical signs of paratuberculosis have infected fetuses.

The spread of BLV is primarily by direct contact. In fact, a separation of 2 to 3 m appears to be adequate to prevent transmission between cattle. The virus exists primarily in lymphocytes, therefore exchange of lymphocytes is typically required. The exchange of blood is the most efficient method of transmission. White blood cells are present in the body secretions so this may be the most frequent natural means of horizontal transmission. There are several proven and theoretical iatrogenic means of transmission. Tattooing, dehorning and even rectal examination with the same sleeve could spread the disease. Dehorning with a Barnes-type dehorner has been shown to be associated with increased seroconversion among a group of calves compared to no dehorning. Interestingly, in that study, calves that were dehorned with a Barnes, and then had the surgical wound cauterized, had no transmission of BLV. Rectal palpation appears to be a means of transmission of the virus both experimentally and in the field. However, there is evidence that routine rectal palpation with a single sleeve is less likely to cause transmission than intensive repeated palpation by inexperienced personnel. It seems reasonable to change sleeves between cows when a substantial amount of blood is visible on the sleeve. When no blood is visible, the risk is probably low. The use of individual needles is usually recommended as a control measure, but there is not a lot of research to support the transmission of BLV by needles when visible blood is not present. Perhaps clients' perception should play some role in our decision to use individual needles. Transmission by insects has been studied extensively. Only tabanids are considered to be a likely vector. This is not been proven in a field situation.

It is clear that BLV is shed in milk and colostrum. However it is not clear whether calves are likely to become infected by consuming BLV infected milk. It is clear, however, that colostrum and colostral antibody against BLV is protective. Despite the fact that transmission by consumption of milk is not considered a primary route of transmission, several studies have been performed to demonstrate that pasteurization inactivates BLV.