Ask the Vet

BVD Is Confusing, Costly Disease

If a cow is infected with bovine virus diarrhea between approximately 40 to 120 days into gestation and does not abort, the calf may be born with a unique condition known as persistent infection (BVD PI). (DTN/Progressive Farmer photo by Jennifer Carrico)


The more I read about BVD, the more confused I get. What is the difference between BVD and BVD PI? What kind of disease does it cause? How important is it?


Bovine virus diarrhea is a common, complex, and confusing disease of cattle. Some feel it may be the costliest disease in cattle. The name is certainly confusing since many if not most cases do not have diarrhea. Many cases are subclinical, meaning they do not appear to be sick or display mild signs. The virus suppresses the immune system, opening the door for other diseases, most commonly causing reproductive and respiratory issues. Classic BVDV presents with fever, discharge from the nose and eyes, erosions of the muzzle and in the mouth, and in some cases, severe diarrhea.

BVDV's most significant effect occurs on the developing fetus. Abortions can occur at any stage of gestation from inapparent early embryo death to stillbirths at term. Even subclinically infected cows can abort and may occur months after infection. If a cow is infected between approximately 40 to 120 days and does not abort, the calf may be born with a unique condition known as persistent infection (BVD PI). The calf's developing immune system does not recognize the virus as dangerous, accepts it as "normal" and will never mount an immune response to clear the virus from its body. It will be a permeant carrier and continuously sheds large amounts of the virus in tears, nasal discharge, saliva, urine and feces. These calves become a major source of infection within the herd, at sale barns, in stocker operations and in the feedlot. BVD PI calves can shed a thousand times more viral particles than transiently infected non-PI animals. If a PI is infected with the cytopathogenic strain of BVD, a severe and often fatal bloody diarrhea known as mucosal disease can occur.

Producers and the herd veterinarian need to create a comprehensive vaccination program to prevent BVD. Many vaccines are labeled to provide fetal protection, which means if the cow is transiently infected with BVDV, the developing calf will be protected. A good biosecurity program is always essential to prevent exposure of the herd to disease. Purchased cattle should be from a well-managed herd and be quarantined for three to four weeks before introducing them into the herd. Testing can be very helpful in finding and removing any PIs from the herd. Since a PI cow will always have a PI calf, testing all calves, bulls and any cows that did not calve is a very effective herd screening method. A tissue sample in a special solution is required and can be obtained by ear notching or a special gum that collects the sample into a small prenumbered tube.



For Christmas this year, my wife gave me a notebook of every column I have written for DTN/Progressive Farmer over the last 22 years. It is an amazing and treasured present and a whole lot of questions and answers. I appreciate all the readers who wrote or emailed questions and to all the readers of the magazine. I hope this column has been helpful.

As I look back over the years, I owe huge thanks to several people who helped with this column. Jack Odle, the editor, and Joe Link, the managing editor, gave me this opportunity. For the past several years, Vicki Myers has been in charge of making me get questions and answers in on time and editing my responses to make them much better and shorter than what I sent her. Vicki retired from ag journalism at the end of 2023 to pursue a new career. Thank you, Vicki, for all you did to help me and the magazine over the years.

I would like to welcome Jennifer Carrico to DTN/Progressive Farmer as the new Senior Livestock Editor and look forward to working with her for many years.


Editor's Note: Please contact your veterinarian with questions about the health of your herd or other animals. Every operation is unique, and the information in this column does not pertain to all situations. This is not intended as medical advice but is purely for informational purposes.

Write Dr. Ken McMillan at Ask the Vet, 2204 Lakeshore Dr., Suite 415, Birmingham, AL 35209, or email