Aquaculture Submissions

Prevention and Control of BVD Virus Infection

Introduction

 Bovine viral diarrhea (BVD) is caused by an RNA virus. Two distinct types of BVDV, Type 1 and Type 2, can be differentiated in the laboratory, based on their RNA makeup, the structure of their protein capsules and the antibodies that are made in response to their infection. Within each BVDV type, several different strains have also been identified. Furthermore, a specific virus may be either cytopathic or non-cytopathic, indicating its ability to cause visible damage to experimentally infected cells in the laboratory. BVD virus has a worldwide distribution, with serum antibody prevalence in cattle ranging from 50-90 percent. However, the incidence of acute clinical disease in the general cattle population is less than 5 percent. Disease problems associated with BVD virus include bovine viral diarrhea, immunosuppression, repeat breeding, abortion and mummification, congenital defects, and persistent infection.

 Persistent Infection:

Persistent infection (PI) with BVDV can develop when a fetus is exposed to a non-cytopathic strain of the virus before day 125 of gestation. PI animals are immunotolerant to the homologous strain of the virus. However, they can still make antibodies to different (heterologous) strains of BVDV. Animals under 3 months of age should have whole blood (buffy coat cells) submitted for testing because maternal colostral antibodies can neutralize the virus in their serum and lead to a false negative test. Fetal infection in the first trimester of gestation can result in abortion or the development of a mummified fetus. Congenital defects can result when the fetus is infected with BVDV in late first, second and early third trimester of gestation. The stage of fetal development determines the type of defect that occurs. The most common defect is cerebellar hypoplasia. Infection late in the pregnancy usually results in the birth of clinically normal calves.

Bovine viral diarrhea refers to a mild disease caused by a non-cytopathic BVD virus infection in immunocompetent cattle. In general, animals develop acute BVD 10-12 days post-infection with viremia starting approximately 4 days after exposure. At the time animals develop clinical disease they are usually starting to make neutralizing antibodies. These neutralizing antibodies lead to a false negative serum test. Since BVDV is also leukocyte associated, whole blood (buffy coat) is the sample of choice for isolation of BVDV from clinically ill animals.

Mucosal disease is the classic disease syndrome that people often associate with BVDV. The prevailing hypothesis is that mucosal disease occurs when immunotolerent, PI cattle are subsequently infected with a homologous strain of cytopathoic BVDV. Superinfection with cytopathic BVDV may result in acute mucosal disease or may persist as chronic mucosal disease. The source of the superinfecting cytopathic virus is from another animal or from a mutation of the persistently infecting noncytopathic virus. Animals with mucosal disease usually die in a few days of severe diarrhea and dehydration. Chronic form is usually manifested as intermittent diarrhea, oronasal and interdigital ulcerations.

 Acute Infection:

 Virus isolation must be done in the first 3-10 days post-infection. Some animals may be virus isolation positive for only 2-3 days post-infection. A whole blood sample is the best sample for BVDV isolation from acutely infected animals. In addition, swabs from mucosal and nasal surfaces can be collected and submitted for virus isolation. Paired acute and convalescent samples collected 30 days apart are required to identify four fold increase in serum antibody titers following convalescence. Many BVDV-associated abortions are virus isolation negative. The detection of antibodies in the fetus will confirm intrauterine infection. If the dam on the other hand, is antibody negative, BVDV can be ruled out as a cause of abortion. The diagnosis of BVDV-induced congenital defects in calves should include both virus isolation and serology to detect BVDV-specific antibody prior to uptake of colostrum.

 Persistent Infection:

 The identification of persistently infected animals is routinely made by virus isolation. In most cases serum is adequate for virus isolation. Due to colostral antibody, in young calves less than 3 months of age the best sample is whole blood in which the mononuclear cells are separated for virus isolation. Persistent infections should only be determined by identification of BVDV by virus isolation in sequential samples collected 30 days apart. By testing the animal 30 days apart it is possible at the same time to test for a four-fold increase in antibody titer should the first virus isolation have been due to acute infection.

 Herd Screening:

 In herds with BVD problems, blood should be collected from all breeding animals more than six months old, and samples should be submitted for virus isolation. Virus isolation using a microtiter immunoperoxidase detection is the most commonly used method for testing such large numbers of samples. In addition, any calves born for the next 9 months must be tested to ensure that no additional persistently infected animals are born that were in utero at the time of testing. All animals from which BVD virus is isolated should be culled from the herd. Animals that are kept for their genetic value should be re-tested in 30 days and should be culled if the second test result is still positive. Young stock should be kept isolated from the breeding herd until they are old enough to be tested or sold. Once vaccination programs are introduced, virus isolation becomes the only reliable method of identifying vaccinates that are persistently infected with BVD virus.

 - by Zuhair Bani Ismail, DVM

- edited by Luvan Anothayanontha, DVM