When the bovine fetus is exposed to the virus that causes BVD (bovine viral diarrhea) during the first 125 days of gestation, it can become persistently infected with the virus. During this early time of fetal life, the immune system is underdeveloped, and the calf essentially does not recognize this virus as "foreign." These animals remain infected for life; because of shedding, they remain a constant source of infection to others in the herd.

Dr. Cortese, of Pfizer Animal Health, estimates that 1.5% to 2% of U.S. calves less than one year of age are persistently infected; 10% to 50% of calves may be carriers in some herds. Persistently infected calves are loaded with virus. A test on serum from the calf has been used to detect the presence of the virus. Virus is also present in the skin and is particularly associated with hair follicles. A new test that uses immunohistochemistry can be performed on formalin-fixed sections of skin, such as an ear notch sample. Antibodies to the virus are used to attach to viral particles. Markers then attach to the complex to visualize the location of viral presence. Tests are under way to determine differences in sensitivity or specificity between the serum and skin tests. At this point, either test can be used to determine the presence of persistently infected BVD calves.