Brachygnathic calf: Because of the shortened mandible, the tongue appears abnormally long.
Photo credit: Bruce Brodersen, DVM, PhD
Paul Walz, DVM, PhD, Auburn University, says transplacental infection is a common event in pregnant cattle exposed to BVDV. Most acute, postnatal infections are subclinical, yet infection in pregnant cattle may result in significant disease. The outcome of fetal infection with BVDV is dependent upon time of infection (i.e. gestational age of the fetus), organ system involved in the infection and properties of the virus (i.e. biotype, virulence and target cell range).
Days 0-45 of gestation
Infection of cattle prior to insemination results in impaired conception rates due in part to ovarian infection and dysfunction as a result of BVDV viremia. Viral antigen and ovaritis have been described in acutely infected cattle with BVDV. Conception and pregnancy rates are lower if the animals are viremic at the time of insemination. Retrospective determination of serologic status has indicated that cows that seroconvert during the early gestation period have significantly lower conceptions rates than cows that were immune. Additional work has demonstrated that overall conception rates were lower for herds defined by having a persistently infected (PI) carrier present.
Days 45-125 of gestation
The development of the fetal immune system occurs during this period of fetal development. The most outstanding feature or outcome of infection during this period is the development of persistent infection. By definition, cattle that are persistently infected with BVDV are immunotolerant to BVDV. Biotype is important during this period – infection with either biotype is capable of causing fetal death, however, only the noncytopathic (NCP) biotype is capable of causing persistent infection. Persistent infection occurs from an in utero exposure to NCP BVDV. The PI calf cannot recognize the infecting BVDV as foreign and never mounts an immune response to it.
Congenital malformations may be produced by BVDV infection during this period as well. The period of organogenesis occurs approximately between days 100-150 of gestation. Central nervous system malformation and defects in myelination have been reproduced in experimental infections prior to 125 days.
Days 125-175 of gestation
Abortion may be observed at any time during gestation as a result of BVDV infection, but most abortions attributed to BVDV seem to be reported during this time. Part of this may be bias as fetuses of this gestational age may be more likely to be submitted for a diagnostic workup. Following BVDV infection, expulsion of the fetus may occur shortly or may be delayed for several months after infection. Following experimental infection at around days 100-120 of gestation, abortion was observed 30-50 days after infection.
Congenital malformations may also be observed during this period as well, including cerebellar hypoplasia, hypomyelinogenesis, hydranencephaly, alopecia, cataracts, optic neuritis, brachygnathism, hydrocephalus, microencephaly, thymic aplasia, hypothrichosis, pulmonary hypoplasia and growth retardation. However, it has not been determined if congenital malformations are the direct result of viral infection on developing cells, or the host immune response in destroying virally infected cells.
Days 175 to term
Beyond day 175 of gestation, the fetus is determined immunocompetent. Infection during this period usually results in the birth of a calf that is clinically normal and seropositive on a pre-colostral blood sample. Detrimental effects of BVDV infection in late gestation seem to be less common, however, weak calves and abortions have been reported. In addition, recent data suggest that calves exposed to BVDV in utero that appear normal at birth may be predisposed to postnatal disease such as scours and pneumonia.
This information excerpted from the proceedings of Kansas State University’s Bovine Conference on Investigating Pregnancy Wastage in Cattle Herds, May 2003.