The bovine viral diarrhea virus (BVDV) is a widespread problem for beef and dairy herds. BVDV can readily cross into other species, such as sheep, goats, deer, antelope and bison. However, I will confine my remarks to BVDV in cattle. I will further shorten the discussion by referring to the virus and the disease(s) it causes as bovine viral diarrhea (BVD).
Bovine viral diarrhea is a complicated disease to discuss. When BVD was first diagnosed and reported, textbook authors only addressed the most severe form of the disease. In reality, perhaps only 5% of the animals that become infected with the BVD virus develop clinical signs actually attributed to the virus. Furthermore, only about 8% of those clinical cases would fit the original textbook description of BVD, leaving 92% of the clinical BVD cases undescribed. In addition to the 92% clinical cases that were not described, the 95% of BVD-infected cattle that show no signs actually attributed to the virus were also not discussed. By no means are these statements intended to detract from the importance of BVD as a disease. BVD can be one of the most devastating diseases cattle encounter and one of the hardest to get rid of when it attacks a herd. The problem is, whenever producers look up the classic symptoms of BVD, they read about the most severe form of the disease and decide that they do not have the problem in their herd. The severe form of BVD is not the typical form of BVD that most producers encounter.
The BVD virus is an RNA virus that has the ability to replicate into many different variants. If the virus finds itself in a stressful environment or situation, another variant can readily be formed that can dominate the BVD virus population. These variants can result in changes in the virus's disease-causing capabilities.
The family of BVD viruses has recently been grouped into two genotypes, Type 1 and Type 2, and each of the genotypes has been divided into two biotypes, cytopathic (CP) and non-cytopathic (NCP). The distinction between the two biotypes, CP and NCP, is determined solely by how they behave in cell cultures in the laboratory; the CP-BVD virus will damage cell cultures (cytopathic) and the NCP-BVD virus will not (non-cytopathic). The basic difference between the Type 1 and Type 2 genotype BVD viruses is how severe a disease each causes in cattle. The Type 2 BVD virus has been responsible for the more severe outbreaks of the disease within the last few years. Both genotypes (Type 1 and Type 2) have been around for a long time. New diagnostic tests have enabled the laboratories to differentiate between the two. The disease syndromes caused by two different genotypes are basically the same, but the severity of the diseases caused by each is different. The various disease syndromes noted in cattle infected with BVD virus are mainly attributed to the age of the animal when it became infected and the virus biotype (CP or NCP).
The BVD virus manifests in several different ways in the bodies of cattle, which can complicate the disease and make diagnosis difficult. The NCP-BVD virus can infect an animal and remain in the animal as long as the animal lives. It does not stimulate a detectable antibody titer in the animal and can easily be passed to the fetus during pregnancy. In turn, the infected fetus is born, remains infected with the NCP-BVD virus, and, as an adult, transmits the virus to its own fetus. This is generally referred to as being persistently infected with NCP-BVD virus. Another complication is that, because of the lack of protective antibodies when persistently infected with NCP-BVD virus, the animal can become infected with CP-BVD virus. In other words, one BVD biotype infection is superimposed upon another BVD biotype infection. Finally, both NCP-BVD and CP-BVD viruses have the ability to suppress an animal's immune system: the viruses lower the animal's resistance to all diseases. With these properties of the BVD virus in mind, we will discuss the different forms of BVD infections.
Source: E. J. Richey, University of Florida Extension