Some PI BVDV calves will be smaller and unthrifty at birth, but some won't.
Some PI BVDV calves will be smaller and unthrifty at birth, but some won't.

The effects of bovine viral diarrhea virus (BVDV) are wide-ranging in terms of reproductive issues, adult and calf health, and production. Much has been written about BVDV over the years and much has been and is still being discovered, but in spite of that misinformation “myths” about BVDV still remain.

Dan Givens, DVM, PhD, Auburn University, discusses the plausibility of the most persistent myths about BVDV. Givens categorizes these myths by:

Confirmed Research evidence confirms the truth of this myth.
Implausible Worthy of disbelief based on available research evidence.
Busted Research evidence confirms the fallacy of this myth.

    Here are three myths in a list of 15 about BVDV that Givens says are “busted”:

    Myth 4: A cow or heifer must already carry the BVDV virus before pregnancy in order to produce a PI calf.

    Busted. A cow or heifer only has to sustain an infection with BVDV before approximately four to five months of gestation to produce a PI calf. At that point in gestation, the calf’s immune system develops the competence to recognize BVDV as a viral intruder and prevent persistent infection. Just like there is variability in the age at which a calf will reach 65% of its mature body weight, there is variability in the gestational age at which immunocompetence is achieved.

    One study describes a set of twin calves in which one was persistently infected with BVDV and the other was not. Another study describes a PI calf that resulted when a pregnant cow was exposed to BVDV at a gestational age of 182 days. However, the vast majority of calves will not be susceptible to persistent infection after 150 days of gestation.

    At the other end of the window of susceptibility, PI dams will always produce PI calves by natural gestation. Insemination of non-viremic, seronegative heifers with BVDV-contaminated semen can result in a low percentage of PI calves. Controlled research does demonstrate that calves are less susceptible to persistent infection at 18 days of gestation compared to 30 days of gestation.

    Myth 5: Unless it’s PI, BVDV infection in utero doesn’t have that much effect on the calf.

    Busted. Limiting the in utero impact of BVDV to persistent infections is a fallacy. Without resulting in PI calves, BVDV can cause cerebellar hypoplasia, microencephalopathy, hydrocephalus, hydranencephaly, porencephaly, hypomyelination, cataracts, microopthalmia, optic neuritis, retinal degeneration, thymic hypoplasia, deranged osteogenesis, and growth retardation. While these calves will not be persistently infected and will be negative for virus if tested at birth, the etiology of congenital defects is a transient BVDV infection that disrupted organogenesis. Diagnostic, pre-colostral serum samples will contain anti-BVDV antibodies as the immune system won the war against this pestivirus but the combination of direct cellular damage by virus and the inflammatory response to BVDV causes multiple battle sites within the calf to be destroyed.

    Myth 6: PI calves will always look normal at birth.

    Busted. While calves persistently infected with BVDV can present as normal calves, many times they are small and unthrifty or can exhibit a congenital defect such as mandibular brachygnathism. In one study, all neonatal calves presenting with mandibular brachygnathism from a herd outbreak involving abortions and premature births tested positive for BVDV. Natural and experimental reports suggest that BVDV infection between 79 and 90 days of gestation can result in brachygnathism and persistent infection. In another study, the median live calf birth weights were 37 pounds for persistently infected calves in comparison to 65 pounds for uninfected calves (Givens et al., yet to be published).

    Read the full article and all 15 BVDV myths, including references, in “Busting BVDV myths” in Bovine Veterinarian here.