Bovine viral diarrhoea (BVD) is a disease of cattle and occasionally other ruminants which is caused by infection with the bovine viral diarrhea virus (BVDV). There are different genogroups of the BVDV: BVDV type 1 and BVDV type 2. These are further broken down into subgenotypes represented as a, b, c, etc. BVDV type 1 contains 12 subgenotypes (BVDV-1a to BVDV-1l) and BVDV type 2 has two subgenotypes (BVDV-2a and BVDV-2b). Each type can be cytopathic or noncytopathic.
Both types of BVDV (type 1 and 2) are found in cattle worldwide, although the prevalence of each type varies depending on the region. All ages are susceptible to infection, but most cases involving clinical disease occur in young cattle between 6 months to 2 years of age.
The severity of the disease induced by most types of BVDV varies and can develop in acute or transient form. Clinical signs of the acute form tend to last 1-3 days and consist of fever over 104°F (40°C), increased respiratory rate, transient loss of appetite, excessive nasal secretions and eye discharge, diarrhea, depression and reduced production of milk. The chronic form of B
Some infections involving BVDV type 2 have been reported to cause more severe disease which lasts from 3-7 days. Clinical signs observed consist of ulcerations in the mouth, high fever (over 107°F [41°–42°C]), lesions along the coronary band and interdigital cleft, and diarrhea. Some cattle develop thrombocytopenia, which results in swollen lymph nodes, ulcerations and erosions in the gastrointestinal tract, and petechial hemorrhages around the eyes, mouth and vulva.
When pregnant cattle are infected, BVDV can cross the placental barrier and infect the fetus. The effects vary depending on the stage of fetal development and the strain of the virus. Infection during the first 4 months of the pregnancy may cause abortion, reduced growth and embryonic resorption. Congenital malformations and neurological disorders may occur when infected between 4 to 6 months of the pregnancy. Fetal infections can also cause premature birth, stillbirth, weak calves and fetal mummification.
Calves who survive the pregnancy tend to remain carriers for life. They act as a natural reservoir for the virus, shedding large amounts in their secretions and feces into the surrounding environment. Calves infected with the noncytopathic BVDV strain usually have a short life span and die before they reach 2 years of age.
A less common but often fatal form of BVD is mucoasal disease. This occurs in persistently infected cattle and is caused by infection with the cytopathic strain of BVDV. Infections can occur one of three ways.
- Persistently infected calves born with the noncytopathic BVDV strain. Over a period of time, this strain can mutate within in the calf internally.
- Cattle exposed to calves infected with the cytopathic strain.
- Exposure to the modified live virus vaccine.
Mucosal disease can present in acute or chronic form.
- Acute Form: Causes extensive damage to lymphoid tissue, resulting in necrosis of the tissue and leukopenia. Clinical signs include erosive lesions in the mouth and nares, dysenteric diarrhea, loss of appetite and dehydration. The duration of the disease is usually a few days and often ends in death, especially without medical intervention.
- Chronic Form: Clinical signs are less severe than the acute form and consist of lameness (caused by coronitis and eruptive lesions of the interdigital cleft), weight loss, and intermittent diarrhea.
Transmission
BVDV is transmitted to cattle through exposure to persistent carriers and spread by biting insects, biologic products, semen, fomites and other animals.
