Bovine Virus Diarrhea (BVD) is one of the most significant viral infections of cattle. BVD was first recognized as a disease syndrome in 1946. Today, 70 to 90 percent of the world's cattle population is seropositive for BVD. 1 There are at least two genotypes, type 1 and type 2; and two biotypes, cytopathic and noncytopathic. Both type 1 and type 2 genotypes have cytopathic and noncytopathic biotypes as members; and both type 1 and type 2 genotypes have many different strains, some of which are more deadly than others.2 Recently, the type 2 genotype has caused many of the most severe cases of BVD.3,1,4

Most BVD virus (BVDV) infections are subclinical, but the clinical disease syndromes can be grouped into three categories: acute BVD, in utero infections, and diseases in persistently infected (PI) animals.1

Acute BVD can vary greatly in presentation from fever, depression, and runny nose and eyes, to diarrhea to respiratory disease, and can end in complete recovery or death depending on several factors: including, immune status of the animal, strain they are infected with, and age of the animal.4,1,2 BVDV has a profound immunosuppressive effect on infected cattle.5 Infected cattle are more susceptible to many respiratory and intestinal pathogens.6 BVDV also is an important component of Bovine Respiratory Disease Complex (BRDC).7

In utero infections with BVDV can result in abortion, persistently infected animals, congenital defects, or normal, immuned calves8,9 depending on the stage of gestation the cow is in and her immune status when she is infected with the virus. The noncytopathic biotype is responsible for all in utero infections.1 If a cow is infected with BVD virus in the first trimester of pregnancy, the fetus will most likely die. The cow will reabsorb the fetus, abort, or give birth to a mummified fetus. The abortions usually are sporadic and at a low rate10; usually only 2 to 7 percent in an outbreak.1 If the cow is infected with BVDV between 60 and 120 days of gestation, the calf may be persistently infected (PI).11 These animals are lifelong carriers of BVDV and shed large quantities of virus in all secretions throughout their lives.12 The immune system in calves less than 120 days of gestation is not capable of responding properly to BVDV, therefore, the virus multiplies in the calf. When the immune system becomes competent the virus is recognized as "self," and the calf is "immune tolerant" to that strain of BVDV for life; it never develops an immune response to that strain. Infection with BVDV between 100 and 180 days in gestation may result in congenital defects such as cerebellar hypoplasia, hydrocephaly, cataracts, and other similar defects.13 Infection of the cow in the last trimester of gestation, when the calf's immune system is functional, will yield a normal, immunized calf.

Persistently infected animals can result from in utero infection as described above, or by birth from a PI dam. The prevalence of these cattle are low (0.5 to 3 percent); but their potential to shed large quantities of virus and infect other animals in the herd is tremendous.14 Persistently infected cows always give birth to PI calves, and seronegative cows (cows that have not mounted an immune response to BVDV) are much more likely to give birth to PI calves. However, some seropositive cows can give birth to PI calves if their circulating antibodies do not cross react with the virus they are exposed to. PI calves often are "poor doers", and are more susceptible to other calfhood diseases due to the immunosuppressive effects of BVDV. Sometimes, however, PI calves may appear normal and healthy.15 PI calves reportedly have death rates of 50 percent in the first 12 months of life.1 Some probably die from other calfhood diseases, but many die from BVD-Mucosal Disease (BVD-MD). BVD-MD occurs when persistently infected animals, harboring noncytopathic BVD, are exposed to a cytopathic variant of BVD. This exposure most likely is due to mutation of the noncytopathic strain to a cytopathic strain.16 BVD-MD is characterized by profuse diarrhea with severe erosions and ulcers on all mucosal surfaces. It most often occurs in cattle 6 to 24 months of age, and is nearly 100 percent fatal.1

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