Mad cow and kosher
Kosher laws do not prohibit feeding cows other cows, so kosher cattle are not immune from mad cow disease. However, "stunning" cattle, which is used to disable the cow in non-kosher slaughter, does push brain matter into other parts of the animal. Kosher slaughter, which involves slicing the throat, does not push brain matter into other animal meat. Also, because kosher slaughter also involves removing all of the blood, the meat is less likely to contain mad cow. Because of the difficulty in removing blood near the spinal column, much of that meat is not used. So, although the chances of contacting mad cow disease through kosher meat is reduced, it is not impossible. Additionally, a "downed" or obviously unhealthy cow would not be slaughtered at all in a kosher plant, but regular slaughter plants do slaughter downed and unhealthy animals.
Also, the brain of the cow is not ground into hamburger, as is sometimes done in non-kosher plants.
I hope this helps,