We know a lot about bovine viral diarrhea virus (BVDV) in cattle, but we still have a lot to learn about persistent BVDV infection. Cattle persistently infected (PI) with BVDV are lifelong carriers and shedders of BVDV. A PI animal is defined as one that was infected at 40-150 days of gestation. The developing immune system of the fetus actually recognizes the virus as part of itself, which is referred to as immunotolerance.
“Because the virus is recognized as part of the fetus, it is not cleared and therefore is allowed to replicate as long as the fetus is alive or, more importantly, as long as the animal is alive once it is born,” explains Dan Grooms, DVM, PhD, Michigan State University.
Both Type 1 and Type 2 BVDV can cause PI animals, and only non-cytopathic (NCP) strains will cause persistent infection. NCP strains of BVDV can also cause death and abortion.
In the general population of cattle, about one out of every 1,000 cattle are likely PIs; however, on individual farms this prevalence can be much higher. “Bunching” of PI cases is related to BVDV actively circulating on a farm and among a group of pregnant females that are at the correct stage of gestation for PIs to be created, says Grooms.
The exact mechanism of transference of infection from dam to calf is not known. “Some evidence suggests a vasculitis occurs with BVDV infection, which may lead to a breakdown of the maternal-fetal blood barrier, allowing the virus to cross to the fetus, and other studies suggest that trophoblasts may become infected and lead to direct transmission across the placenta,” says Grooms.
Does a PI dam always have a PI calf? Most likely. “Since a PI cow is viremic, virus crosses the placenta via the bloodstream, so there is virtually constant exposure of the fetus to BVDV virus in a PI cow,” says Bruce Brodersen, DVM, PhD, University of Nebraska Veterinary Diagnostic Center.
PI is a PI is a PI?
But, are all PIs the same? “Aside from being persistently infected, it is not known if PI calves are significantly different from one another,” says Broder-sen. Chris Chase, DVM, PhD,
PI animals are approximately 10 times more likely to become chronically ill and realized than non-PI animals, and they are approximately 10 times more likely to die.
Some PI animals are poor performers, and some are essentially normal, says Grooms. “Although we don’t know for sure, the differences are likely due to strain of virus and timing of fetal infection.”
Once a PI always a PI? Yes, based on current knowledge. There may be different levels of virus shedding and clinical manifestations, but virus can always be found somewhere in the PI and they are at high risk of transmitting the disease to susceptible cattle. However, some PIs can mount an immune response against heterologous BVDV that may cross react against the persistent virus and partially clear it, notes Grooms. “Typically, in these animals, we cannot find virus in serum, but we can find it in white blood cells and nasal swabs.”
“There is evidence that suggests animals can clear themselves of persistent infection,” says Brodersen. “If it actually occurs, it is extremely rare.” However, says Grooms, “The bottom line is that people should not think that PIs can potentially get better. They should be destroyed, plain and simple.”
Persistent infection can also lead to mucosal disease, a fatal manifestation that, by definition, only occurs in PIs. Brodersen says the classical definition of mucosal disease is when a PI animal is superinfected with a strain of cytopathic BVDV that is antigenically similar to the persistently infecting virus, mucosal disease can develop. Grooms adds that the most common way that mucosal disease occurs is when the persistent virus that is replicating in the PI animal acquires a specific mutation that turns the virus from an NCP to a cytopathic biotype. This then starts a cascade of events that leads to the animal’s death.
Dan Grooms, DVM, PhD, says some PI animals are poor performers and some are essentially normal.