Bovine Viral Diarrhoea (BVD) - Persistently Infected Calves

Calves become persistently infected with bovine viral diarrhoea virus when they are infected in utero before day 120, when their immune system is not fully developed. They become immunotolerant to the infective virus strain and no immune response is elicited against the infective viral strain.

PI calves often go to full term and are delivered normally. They are typically small, and initially may develop normally. These calves are viraemic and shed BVDV most of the time. They do not have antibodies against BVD.

Only the ncp biotype can cause persistent infection of the bovine fetus. Calves persistently infected with an ncp BVD virus will develop mucosal disease if the virus mutates spontaneously into the cp biotype or the calf is infected with a cp virus closely related to its own ncp BVDV strain. Most PI calves die from mucosal disease before the age of two years.

A calf showing severe diarrhoea typical of mucosal disease

Severe erosions in the intestinal tract as a result of mucosal disease

Some PI animals grow to become breeding animals.When these animals are bred they will always give birth to a PI calf.

BVDV infections that occur later in gestation result in abortion, stillbirths or the birth of normal, healthy calves seropositive for BVDV.

Preventing fetal infection

Fetal protection refers to the ability of a vaccine to prevent the transmission of the BVDV from dam to the fetus during the first 120 days of gestation.

Vaccination with an inactivated BVD vaccine prevents the infection of the fetus. This has been demonstrated in severe challenge studies.