Systematic Veterinary Virology

INTRA-UTERINE INFECTION

• BVDV can cross the placenta and infect foetuses of all ages. The outcome of these infections are largely dependent on the stage of gestation when infection occurs. Fetal calves develop immunocompetency at about 180 days of gestation
• If the cow is infected in the first month of pregnancy, the pregnancy is terminated either by abortion or resorption of the foetus by the dam.
• If the foetus is infected in the second to the sixth month of pregnancy, a variety of different syndromes are observed. The foetus may still be aborted  or the foetus survives full term with the resultant offspring born malformed, weak, "dwarfed", stillborn, or clinically healthy but "persistently infected" with pestivirus. Persistently infected calves may grow well, but generally are unthrifty compared to others the same age and they often suffer from chronic scours or pneumonia.
• If infection occurs around three to five months of pregnancy, the virus affects the developing nervous system of the foetus. Calves may not be able to stand or suck or may develop convulsions after birth because they are missing parts of their brain. These calves may also have eye abnormalities such as blindness and cataracts or bent up front legs. Some calves will survive but have a wobbly gait and may have a permanent head tremor.
• If exposure to a noncytopathic virus occurs between 42 and 125 days gestation, the fetus accepts the invading virus, becomes immunotolerant to that particular virus and will never mount an immune response to eliminate it. These calves are born persistently infected (PI).