• Bovine viral diarrhea (BVD) and mucosal disease (MD) are clinically dissimilar disease syndromes and were originally described as separate diseases, but they are now known to have a common viral etiology. The virus can cause both acute disease, bovine viral diarrhea, and a protracted form of illness, mucosal disease, arising from persistent infection.
• BVD may occur at any age. MD is a persistent infection acquired in utero and characterized by high mortality and low morbidity.
• The BVD virus can be either non-cytopathogenic or cytopathogenic. Isolation and demonstration from both BVD and MD were serologically similar and gave the same experimental disease.
• The clinical evidence that abortions were constant findings of field outbreaks of BVD and also the demonstration of transplacental transfer of the virus to the fetus.
• Early fetal infection with the BVD virus may lead to persistent viremia and a failure to develop antibodies.
• Persistently viraemic animals could subsequently develop MD. Persistently viraemic animals are infected with the only non-cytopathogenic virus, whereas those developing MD are infected with both non-cytopathogenic and cytopathogenic forms.
• Mucosal disease is almost invariably a fatal disease with low morbidity. It occurs in cattle, which have a persistent BVD infection acquired as fetuses characterized by a specific immune tolerance to the infecting virus strains and consequent lack of antibody to it.
• In MD, if the fetus is infected at 110-120 days of gestation, i.e., before the age of immunocompetence, the virus becomes accepted as self and persists throughout the life of the animal. This explains the lack of antibodies to the persisting virus and the continued state of immunotolerance.
• Sometimes after birth, when the animal is about 6-18 months of age, superinfection of these persistently viraemic animals with the cytopathogenic bio-type may occur causing MD.
• Bovine viral diarrhea virus (BVDV) is an SS-RNA virus, genus Pestivirus in the family Flaviviridae, and is closely related to classical swine fever and Ovine Border disease viruses. This virus affects domestic and wild ruminants and pigs. There are two genotypes of the BVDV recognized.
• Both viral genotypes have been co-circulating in cattle throughout the world. This disease is reported from most regions of the world. Natural infections and diseases occur not only in cattle but also in sheep, pigs, goats, and a wide range of captive, free-living ruminants.
• BVDV also classified into two biotypes-Non cytopathogenic and cytopathogenic biotypes
• The most important sources of BVDV in nature are immuno-tolerant, persistently infected animals.
• High virus levels are usually present in their nasal secretions, saliva, tears, semen, milk, urine, and feces.
• Animals in contact with a persistently infected animal rapidly become infected. Vertical and horizontal transmission may occur. Disease transmits by direct or indirect contact.
• The virus may be isolated in several bovine monolayer cell cultures (e.g. kidney, lung, testis, or turbinate). The growth of both biotypes is usually satisfactory.
The clinical and pathologic manifestation of infection in individual cattle varies with age and pregnancy status. Three situations are considered,
• Postnatal infection in non-pregnant cattle
• Infection in pregnant cattle
• Persistent infection in calves and mucosal disease.
Postnatal infection of susceptible non-pregnant cattle
- Infection is most common in animals 8-24 months of age. Although calves may receive antibodies in colostrum, antibody levels decline by 3-8 months of age and animals can then become infected.
- Incubation period 5-7 days followed by pyrexia (40-41°C), leucopenia, viremia. In the majority, it is a subclinical infection.
- There may be diarrhea of explosive nature with very high morbidity but no mortality.
- Drop-in milk yield in dairy cows, oculo-nasal discharge, and mouth ulcers are referred to as BVD. Animals develop serum neutralizing antibodies which persist lifelong.
- The virus induces transient but profound immunosuppression.
- The virus suppresses interferon production, affects lymphocyte function, humoral antibody production, and phagocytosis which paves way for many respiratory and other diseases in calves.
- Semen from persistently infected bulls by AI or natural service infect cows which results in early embryonic mortality and repeat breeding.
- Infection of susceptible pregnant cattle
- The infection to the fetus causes abortion, weak calves, undersized calves, and congenitally deformed calves.
- The virus replicates in almost all the fetal tissues and the extent of damage is more in actively dividing cells.
- Regarding the nervous system, it causes cerebellar hypoplasia, dysplasia, cavitation of the cerebrum, and retinal displacement.
Immunological competence of the fetus
- Infection in early fetal life becomes a persistent viral infection in many tissues and organs. After birth, the calf remains infected for life.
- These calves excrete the virus in large quantity and transmit to all susceptible healthy animals. Therefore, a high probability of the development of MD is possible.
Persistently infected cattle
- In susceptible healthy herds in which the virus was recently introduced, the high proportion of calves in the next calving season become persistently infected.
- The mortality rate exceeds 50% in the preslaughter age i.e., 6-18 months ago, and the most classical clinical manifestations are that of MD. This disease is characterized by pyrexia, anorexia, profuse watery diarrhea, nasal discharge, buccal ulceration, and sometimes lameness. Death is within a few days to 3 weeks.
• Clinical diagnosis is a complex problem. Signs of the different syndromes may appear alone or in combination.
• Although many of the signs are similar to RP, the mortality in that disease is much higher.
• The oral lesions are very important, but their absence does not exclude BVD.
• This disease must be differentiated from conditions in cattle causing diarrhea, erosions/ulcerations of the gastrointestinal tract, reproductive failure, teratology, skin disease, laminitis, poor growth, and respiratory tract disease such as vesicular diseases, ingestion of caustic substances, mucosal disease complex: Rinderpest, bluetongue, papular stomatitis, malignant catarrhal fever.
• Samples to be collected- nasal, ocular discharges, blood, spleen, lymph nodes, lungs, and liver
• Identification of the agent- Persistently viraemic healthy animals resulting from congenital infection can be readily identified by isolation of the non-cytopathogenic virus in cell cultures from blood or serum.
• It is necessary to use an immune-labeling method to detect the growth of viruses in the cultures.
• Alternative methods based on direct detection of viral antigen or viral RNA in leukocytes are also available.
• Persistence of virus should be confirmed by resampling after at least 3 weeks.
• These animals will usually have no or low levels of antibodies to BVDV.
• Acute infection with BVDV is best confirmed by demonstrating seroconversion using sequential paired samples from several animals in the group.
• The testing of paired sera (acute and convalescent samples) should be done a minimum of 21days apart and samples should be tested side by side.
• The enzyme-linked immunosorbent assay (ELISA) for antibody and the virus neutralization test is the most widely used. Demonstration of the BVDV by CPE (but many BVDV strains do not produce CPE), FAT, PCR a much more productive method, is necessary for definitive diagnosis.
• Primary goal of a control strategy is the removal of persistently infected animals from a herd.
• All cattle should be screened for the virus including calves born in the herd for nine months after initiation of the program.
• Newly introduced animals should be tested for the disease before their introduction.
The quarantine period for newly purchased animals is 30 days.
• Strict managemental practices.
• Replacement animals especially breeding bulls.
• Cattle should not be allowed to mix with sheep and goats
• Modified Live BVD vaccine – Not recommended for pregnant cattle or in stressed populations
• Killed BVD vaccine, which is safer for administration even to pregnant cattle. A booster dose after 3 to 4 weeks of initial vaccination.
• Calves should be vaccinated at 4-6 months of age and again when they are 8-12 months old.
• Calves less than three months of age are generally not vaccinated because of potential vaccination failure associated with the presence of colostral immunity.
• Heifers and cows should be revaccinated 30-60 days before breeding and booster dose one month before calving or in the last trimester.