Management programs and control strategies designed to eliminate the bovine viral diarrhea (BVD) virus infection in cattle herds must consider two ways the virus spreads within a herd.
· From one animal to another (horizontal transmission) as infected animals secrete the virus and the virus enters a susceptible animal through the mouth or respiratory tract.
· From an infected dam's bloodstream to her fetus during pregnancy (vertical transmission).
Horizontal transmission of BVD virus to calves or adult cattle results in a temporary (transient) infection that's usually mild, but can occasionally result in severe disease.
The main negative health effect of BVD is it can inhibit conception and/or cause abortion in susceptible females. It also suppresses the immune system, making infected animals more susceptible to other diseases. In addition to contributing to disease in infected cattle, horizontal infection from one animal to a pregnant dam can lead to vertical transmission of BVD virus to her fetus during pregnancy.
Fetal infection and the PI
The BVD virus is able to cross from the dam's bloodstream to the fetus with high efficiency. Fetal infection can lead to early embryonic death, abortion, birth defects and stunting.
The lesions associated with fetal infection of the BVD virus include brain malformations, spinal cord defects, cataracts and other eye abnormalities, sparse hair coats, a short lower jaw, growth retardation and lung immaturity.
The immune status of the dam, the stage of gestation, and the characteristics of the virus itself are important factors in determining the result of BVD virus infection of pregnant cows and heifers. If the fetus recognizes the virus as an infection, it can fight it off and result in the birth of a normal calf; or it can become a persistently infected (PI) calf, carrying and spreading the virus its entire life.
PI cattle are the result of fetal exposure to the non-cytopathic biotype of BVD virus prior to the development of a fairly mature immune system at about 125 days of gestation. Although a high percentage of PI calves die at or near birth, or at least by weaning, as many as 50% of PI calves may survive long enough to enter the breeding pool or a feedlot.
The spread of BVD
The primary source of BVD virus is PI cattle. These animals are a much more efficient transmitter of BVD virus than transiently (temporarily) infected animals because they secrete higher concentrations of virus for a longer period of time.
BVD-PI females of breeding age are a source of horizontal transfer of BVD virus. In addition, they will always produce a PI calf.
As a result, PI suckling calves are considered to be the primary source of BVD virus in breeding herds — depending on the timing of the introduction relative to the breeding season and the resulting immune status of the herd during early gestation.
Horizontal transmission of BVD virus to susceptible cattle has been shown to occur after only one hour of direct contact with a single PI animal. Horizontal transmission of the virus from either PI or transiently infected animals to susceptible cattle can occur directly via nose or mouth contact with virus-containing body fluids. In addition, air transmission over short distances seems likely.
Persistent infection has a clustered distribution, which means a few herds may contain several PI cattle. Clustering of multiple PI animals in a herd is primarily due to exposure of numerous susceptible dams to a PI or transiently infected source of BVD virus prior to day 125 of gestation.
Economic effects of PIs
Infection with BVD virus has been associated with respiratory disease outbreaks in feedlot situations. PI cattle also can spread the virus to susceptible cattle during marketing and trucking. PIs have been shown to have an impact on health performance of susceptible penmates and cattle in adjacent pens.
The cost of one PI animal in a cow-calf herd reportedly ranges from $14.85 to $24.84/cow/year. Cow-calf producers should work with their veterinarian to determine if their herd is likely to be experiencing production and economic losses due to BVD.
The economic value of whole-herd screening for PI animals in cow-calf herds is influenced by:
· The likelihood of finding at least one PI animal in the herd,
· The negative production effects when PI animals are present,
· The cost of screening and identifying PI animals, and
· The value of animals sold.
Because of the potential production and economic losses associated with BVD, all cow-calf producers should work with their veterinarians to establish biosecurity plans to reduce the risk of buying or creating a PI animal. Biosecurity plans for BVD in cow-calf herds include a sound vaccination program and use of diagnostic testing for PI status of any herd replacements or additions.
Biosecurity also involves application of a vaccination protocol to reduce the risk of fetal infection in the event of cow herd exposure to an animal shedding BVD virus.
Modified-live vaccines (MLV) have inherent properties that may enable them to stimulate more complete protection against transplacental infection. For that reason, one recommendation is to vaccinate unstressed, healthy heifers with a MLV.
Vaccine administration should be timed so a protective immune response coincides with the first four months of gestation.
Bottomline: While vaccination does provide some protection from fetal infection, BVD virus control is generally achieved by a combination of:
1) Diagnosis and removal of PI cattle,
2) Vaccination with an MLV, and
3) A biosecurity system that prevents the introduction of PI animals into the herd and minimizes contact with infected animals.
For more information, go to the “BVD Information Links” posted on the Internet by the Academy of Veterinary consultants: www.avc-beef.org.