BVDV in calves can present in a number of ways. Immune suppression is probably the most costly manifestation. “All strains of BVDV have some type of immune suppression,” says Vic Cortese, DVM, PhD, Dipl. ABVP, Pfizer Animal Health. “Even the mild strains will cause immune suppression that can last approximately three weeks.”
Cortese notes that work has shown that when BVDV is circulating through a dairy, other diseases become more prominent. “You may be trying to fight these other diseases, but the underlying immune suppression caused by a BVDV infection is to blame.”
“If you have multiple PI calves along with inadequate management, you have the potential for a lot of clinical disease,” says Scott Smith, DVM, The Dairy Authority, LLC, Greeley, Colo. “Cattle are much more susceptible to other viral and bacterial infections. You may see a higher incidence of general scours and pneumonia and an increase in death loss. Salmonellosis, coccidiosis and other opportunistic diseases can often be found in populations with circulating BVDV.”
Clinical signs and PIs
Acute BVDV can also cause “bleeder syndrome” due to platelet destruction. Bleeder syndrome is only caused by the Type II BVDV strains, and not all of the Type IIs will cause this syndrome. Depletion and destruction of throm-bocytes causes a lack of clotting. “In young calves, you see hemorrhaging in the sclera and petecchial hemorrhages in the mouth,” says Cortese. “In adults, you’ll see nosebleeds.” Hematomas at injection sites, dripping blood from injections, severe blood loss from dehorning or castration and free blood in the thoracic cavity upon necropsy are also signs of bleeder syndrome.
One way to begin eliminating persistently infected (PI) BVDV animals from the dairy is to identify PI calves and then identify and test (and cull) any of their PI dams.
Cortese adds that BVDV is the only cause of severe platelet losses in North America, and a platelet count in a bleeder case can tip you off to a BVDV infection. Numbers will often drop below 100,000, and usually hemorrhaging will occur when below 50,000. Platelet numbers this low are a good indication of bleeder syndrome. Often, a lab may report too few platelets to count.
Persistently infected (PI) calves may or may not be visually evident. PI calves may be weak and runted and 10-15 pounds lighter than normal. However, on a dairy especially, PI calves often look normal though they are immunologically frail. “They tend to be the first to die in a respiratory outbreak,” says Cortese. “They can’t clear infections and are the primary source of BVDV spread on dairies.”
Smith adds that a calf with BVDV can present in a number of ways, especially if there is a secondary infection involved. “Respiratory symptoms are seen the most frequently, but you may also see diarrhea and lameness. Treatment is based mostly on supportive care and treatment of secondary bacterial infections.”
Cortese notes that 50 percent of PI calves will die within the first year of life. Another 40 percent will die before two years of age and only 10 percent make it to two years. PI animals tend to die near birth and later can die of mucosal disease, chronic BVDV and lameness.
The older the animal is, the less likely it is to be a PI animal because of this immunologic frailty of PI animals.
Creating a PI in utero
BVDV can affect the unborn calf in a variety of ways, mostly dependent upon the timing of exposure (see sidebar). If a nonimmune pregnant cow – this does not necessarily mean an unvaccinated cow – is exposed early in the first trimester, stillbirths, early embryonic death and mummification can occur. Later in the first trimester, between days 60-100, BVDV exposure can result in PI calves. In the second trimester, exposure to BVDV is likely to cause abortions and birth defects.