Bovine Viral Diarrhea and Mucosal Disease Complex
Disease induced by BVDV varies in severity, duration, and organ systems involved. Acute disease results from infection of susceptible cattle with either noncytopathic or cytopathic BVDV. Acute BVD, also termed transient BVD, often is an inapparent to mild disease of high morbidity and low mortality. Biphasic fever (~104F [40C]), depression, decreased milk production, transient inappetence, rapid respiration, excessive nasal secretion, excessive lacrimation, and diarrhea are typical signs of acute BVD. Clinical signs of disease usually are seen 6-12 days after infection and last 1-3 days. Transient leukopenia may be seen with onset of signs of disease. Recovery is rapid and coincides with production of viral neutralizing antibody. Gross lesions seldom are seen in cases of mild disease. Lymphoid tissue is a primary target for replication of BVDV, which may lead to immunosuppression and enhanced severity of intercurrent infections.
BVD. Congestion and erosions in the ruminal mucosa
BVD. Inflammation of the abomasum (abomasitis, gastritis).
Some isolates of BVDV induce clinically severe disease that manifests as high fever (~107F [41-42C]), oral ulcerations, eruptive lesions of the coronary band and interdigital cleft, diarrhea, dehydration, leukopenia, and thrombocytopenia. In thrombocytopenic cattle, petechial hemorrhages may be seen in the conjunctiva, sclera, nictitating membrane of the eyes; and on mucosal surfaces of the mouth and vulva. Prolonged bleeding from injection sites also occurs. Swollen lymph nodes, erosions and ulcerations of the GI tract, petechial and ecchymotic hemorrhages on the serosal surfaces of the viscera, and extensive lymphoid depletion are associated with severe forms of acute BVD. The duration of overt disease may be 3-7 days. High morbidity with moderate mortality is common. Severity of acute BVD is related to the virulence of the viral strain infecting the animal and does not depend on viral biotype or genotype. In pregnant cattle, BVDV may cross the placental barrier and infect the fetus. The consequences of fetal infection usually are seen several weeks to months after infection of the dam and depend on the stage of fetal development and on the strain of BVDV. Infection of the dam near the time of fertilization may result in reduced conception rates. Infection during the first 4 mo of fetal development may lead to embryonic resorption, abortion, growth retardation, or persistent infection. Congenital malformations of the eye and CNS result from fetal infections that occur between months 4-6 of development. Fetal mummification, premature birth, stillbirth, and birth of weak calves also are seen after fetal infection. Persistent infection is an important sequela of fetal infection with noncytopathic BVDV. Persistently infected calves may appear healthy and normal in size, or they may show stunted growth and be prone to respiratory or enteric ailments. They often have a short lifespan, and death before 2 yr of age is common. Persistently infected cows give birth to persistently infected calves, but most calves sired by a persistently infected bull will not be infected with virus in utero. Lesions attributable to BVDV often are not seen in persistently infected cattle at necropsy. Antibody against BVD seldom is detected in persistently infected cattle in the absence of vaccination or superinfection with an antigenically heterologous BVDV. Persistently infected cattle exposed to BVDV that is antigenically different from their resident noncytopathic virus can produce antiviral antibody. Therefore, screening for persistent infection using the viral neutralization test to identify animals that lack antiviral antibody may not detect some persistently infected cattle.