Bovine Viral Diarrhea and Mucosal Disease (BVD / MD)

Bovine Viral Diarrhea (BVD) is a highly contagious, economically significant viral disease of cattle caused by Bovine Viral Diarrhea Virus (BVDV), a Pestivirus within the family Flaviviridae. The disease manifests in two distinct forms: acute (transient) infection and mucosal disease (MD), a fatal condition arising in persistently infected (PI) cattle. BVDV infection can cause a spectrum of clinical presentations including subclinical infection, acute febrile illness with diarrhea, immunosuppression leading to secondary infections, reproductive failure (embryonic death, abortion, congenital defects), and the severe thrombocytopenia and hemorrhagic syndrome. Mucosal disease is a sporadic, invariably fatal condition characterized by severe erosive and ulcerative lesions in the gastrointestinal tract, occurring when a PI animal becomes superinfected with a cytopathic (CP) BVDV strain. The disease affects cattle of all ages, but clinical signs are most severe in young stock and immunologically naïve herds. Economically, BVD causes substantial losses through decreased milk production, reduced weight gain, increased mortality, reproductive inefficiency, and increased susceptibility to other pathogens. Control programs focus on biosecurity, vaccination, and identification and culling of PI animals.

The primary causative agent is Bovine Viral Diarrhea Virus (BVDV), a single-stranded, positive-sense RNA virus belonging to the genus Pestivirus in the family Flaviviridae. BVDV is classified into two biotypes based on cytopathic effect in cell culture: non-cytopathic (NCP) and cytopathic (CP). Both biotypes can cause acute infections, but only NCP strains can establish persistent infection when the fetus is exposed before approximately 125 days of gestation. CP strains arise from NCP strains via mutation or recombination and are responsible for the development of mucosal disease in PI animals. BVDV is further divided into two species, BVDV-1 and BVDV-2, with multiple subtypes (e.g., 1a, 1b, 2a, 2b). BVDV-2 strains are often associated with more severe clinical disease, including hemorrhagic syndrome and high mortality. The virus is enveloped and relatively labile in the environment, being inactivated by heat, detergents, and common disinfectants. Transmission occurs primarily via direct contact with bodily secretions (nasal discharge, saliva, feces, urine, milk, semen) from acutely or persistently infected cattle. PI animals shed virus continuously and at high titers throughout their lives, serving as the main reservoir for herd outbreaks. Fomites, contaminated equipment, and biological products (e.g., modified live vaccines) can also transmit the virus. The virus can cross the placenta and infect the fetus, leading to a range of outcomes depending on gestational age.

BVDV has a worldwide distribution, with seroprevalence rates varying from 40% to 90% in unvaccinated cattle populations. The disease affects both dairy and beef cattle, with higher morbidity in intensively managed herds. Age is a significant factor: acute infections are most common in cattle aged 6 months to 2 years, while PI animals are typically identified in calves and young stock. The prevalence of PI animals is generally low (0.5% to 2%) but can be higher in herds with poor biosecurity. Seasonality is not pronounced, but outbreaks may occur more frequently in winter when cattle are housed indoors, facilitating virus spread. Herd size and management practices influence transmission; large herds with high turnover and commingling of cattle from multiple sources are at increased risk. Morbidity in acute outbreaks can reach 30% to 50%, but mortality is usually low (<5%) except in severe BVDV-2 infections or mucosal disease, where case fatality is nearly 100%. Economic losses arise from decreased milk production (up to 20% reduction), reduced growth rates, increased culling, reproductive losses (abortions, stillbirths, congenital defects), and increased veterinary costs. The presence of PI animals is the most critical factor for endemic persistence within a herd.

BVDV primarily targets epithelial cells and cells of the immune system, including lymphocytes, macrophages, and dendritic cells. After entry via the respiratory or oral route, the virus replicates in the upper respiratory tract and tonsils, then spreads via the bloodstream to lymphoid tissues and various organs. Acute infection leads to viremia, which is typically cleared within 10-14 days. The virus induces immunosuppression by causing lymphopenia, particularly affecting T cells, and impairing neutrophil function, leading to increased susceptibility to secondary bacterial infections (e.g., pneumonia, diarrhea). In pregnant cattle, the virus crosses the placenta. If infection occurs before day 45 of gestation, embryonic death and resorption may occur; between days 45 and 125, the fetus may become immunotolerant to the virus, leading to the birth of a PI calf. PI calves lack a specific immune response to BVDV and shed the virus continuously. Mucosal disease develops when a PI animal becomes infected with a CP BVDV strain, which is antigenically homologous to the NCP strain. The CP virus replicates extensively in mucosal epithelial cells, causing severe erosive and ulcerative lesions in the oral cavity, esophagus, forestomachs, abomasum, and intestines. The exact mechanism of cell death involves apoptosis and necrosis, leading to severe diarrhea, dehydration, and death. In some acute infections, particularly with BVDV-2, thrombocytopenia occurs due to viral replication in megakaryocytes, resulting in hemorrhagic diathesis.

Intrinsic factors include age (young cattle more susceptible), immune status (naïve animals at higher risk), pregnancy (fetal infection leads to PI), and genetic susceptibility (some breeds may be more resistant). Stress factors such as weaning, transport, overcrowding, poor nutrition, and concurrent infections can exacerbate clinical disease. Extrinsic factors include lack of biosecurity, introduction of new animals without quarantine, commingling with PI animals, contaminated equipment (e.g., needles, ear taggers), and use of contaminated biological products. Poor herd management practices, such as inadequate vaccination programs, high stocking density, and poor hygiene, increase the risk of transmission. In dairy herds, the presence of PI animals in the milking herd can lead to widespread exposure. In beef herds, the use of bulls persistently infected with BVDV can result in venereal transmission and reproductive losses.

Clinical signs vary depending on the strain, immune status, and age of the animal. Acute BVDV infection may be subclinical or cause mild to moderate disease. Common signs include fever (104-107°F), depression, reduced feed intake, decreased milk production, nasal discharge, ocular discharge, oral erosions (often subtle), diarrhea (ranging from mild to severe), and increased respiratory rate. In severe cases, especially with BVDV-2, hemorrhagic syndrome may occur, characterized by petechiae and ecchymoses on mucous membranes, bloody diarrhea, epistaxis, and prolonged bleeding from injection sites. Immunosuppression can lead to secondary infections such as pneumonia and enteritis. In pregnant cattle, reproductive signs include embryonic death, abortion (usually between 4-6 months of gestation), stillbirth, and congenital defects (e.g., cerebellar hypoplasia, ocular defects, skeletal abnormalities). PI calves may appear normal at birth but often have poor growth, are more susceptible to infections, and may develop mucosal disease. Mucosal disease is characterized by severe depression, high fever, profuse watery diarrhea (often with blood and mucus), erosive lesions on the oral mucosa, tongue, dental pad, and interdigital spaces, and rapid dehydration. Affected animals typically die within 1-2 weeks. Herd-level signs include an increase in abortions, stillbirths, and weak calves, as well as an increase in respiratory and enteric disease in young stock.

Differential diagnoses for BVD include other viral and bacterial causes of diarrhea and mucosal lesions. Key differentials include: 1) Bovine Coronavirus Infection: Causes diarrhea in calves and winter dysentery in adults; distinguished by rapid onset, high morbidity, and absence of persistent infection. 2) Rotavirus Infection: Primarily affects neonatal calves; diagnosis via antigen detection in feces. 3) Coccidiosis: Causes bloody diarrhea in young cattle; fecal examination reveals oocysts. 4) Salmonellosis: Caused by Salmonella spp.; presents with fever, diarrhea, and septicemia; diagnosed by fecal culture. 5) Malignant Catarrhal Fever (MCF): Caused by ovine herpesvirus-2; characterized by high fever, erosive lesions, corneal opacity, and lymphadenopathy; history of sheep contact. 6) Rinderpest (now eradicated): Severe erosive lesions and high mortality; ruled out by absence of disease. 7) Foot-and-Mouth Disease (FMD): Vesicular lesions on the mouth, feet, and teats; distinguished by vesicle formation and rapid spread. 8) Bovine Papular Stomatitis: Causes papules and erosions on the muzzle and oral cavity, but is benign. 9) Bluetongue: Causes oral ulcers, coronitis, and fever; transmitted by midges. 10) Infectious Bovine Rhinotracheitis (IBR): Causes respiratory signs and abortions; may have oral lesions but less severe. Definitive diagnosis relies on virus isolation, antigen detection (ELISA, PCR), and serology.

The diagnostic approach for BVD involves a combination of herd history, clinical signs, and laboratory testing. The algorithm begins with a thorough herd history, including vaccination status, recent introductions, reproductive performance, and presence of clinical signs. Physical examination of affected animals may reveal fever, oral erosions, diarrhea, and other signs. For individual animals, blood samples should be collected for virus isolation, antigen detection (ELISA or PCR), and serology (paired samples for antibody titers). Ear notch samples are commonly used for antigen detection (IHC or PCR) to identify PI animals. In cases of abortion, fetal tissues (spleen, thymus, lung) should be submitted for virus detection. For herd screening, pooled samples (e.g., bulk milk, pooled ear notches) can be tested using PCR. If PI animals are suspected, a two-step testing protocol is recommended: initial antigen test (e.g., ear notch IHC or PCR) followed by a confirmatory test after 3-4 weeks to rule out transient infection. Necropsy of affected animals can reveal characteristic lesions, and histopathology can support the diagnosis. Differential diagnoses should be ruled out based on laboratory results and clinical presentation.

Hematology may reveal leukopenia, particularly lymphopenia and neutropenia, during acute infection. Thrombocytopenia may be present in severe cases. Serum biochemistry may show mild elevations in liver enzymes (AST, GGT) and bilirubin. In cases of diarrhea, electrolyte imbalances and metabolic acidosis may be present. Virus detection methods include virus isolation from blood, nasal swabs, or tissues; antigen-capture ELISA; and RT-PCR, which is highly sensitive and specific. Serology using virus neutralization (VN) or ELISA can detect antibodies, but paired samples are needed to confirm acute infection. For PI identification, ear notch samples are tested for BVDV antigen using immunohistochemistry (IHC) or PCR. Bulk milk testing using PCR is useful for herd surveillance. In aborted fetuses, virus can be detected in fetal fluids or tissues. Histopathology of affected tissues, especially the gastrointestinal tract, may show erosive and ulcerative lesions with necrosis of epithelial cells and lymphoid depletion.

Imaging modalities are not commonly used for the diagnosis of BVD, but they may be helpful in evaluating secondary complications. Thoracic radiography or ultrasonography may reveal evidence of pneumonia in immunosuppressed animals. Abdominal ultrasonography may show thickened intestinal walls or fluid-filled loops in cases of severe enteritis. In cases of congenital defects, such as cerebellar hypoplasia, advanced imaging (CT or MRI) of the brain may be performed, but this is rarely indicated in clinical practice. Ultrasonography can also be used to assess fetal viability and detect fetal abnormalities in pregnant cattle. However, the diagnosis of BVD is primarily based on laboratory testing rather than imaging.

Histopathological examination of tissues from affected animals reveals characteristic lesions. In acute BVD, lymphoid depletion and necrosis are observed in lymph nodes, spleen, and Peyer's patches. Erosive and ulcerative lesions in the oral cavity, esophagus, forestomachs, and intestines show epithelial necrosis, with infiltration of inflammatory cells. In mucosal disease, extensive ulceration and necrosis of the gastrointestinal mucosa are prominent, with intranuclear inclusion bodies occasionally seen. In the brain of fetuses with cerebellar hypoplasia, there is a reduction in the number of Purkinje cells and granule cells. Immunohistochemistry can detect BVDV antigen in formalin-fixed tissues, particularly in PI animals. Cytological examination of oral lesions may show necrotic debris and inflammatory cells, but is not specific. In cases of hemorrhagic syndrome, bone marrow examination may reveal megakaryocytic hypoplasia.

There is no specific antiviral treatment for BVD. Management focuses on supportive care and control of secondary infections. Affected animals should be isolated and provided with clean, dry bedding and easy access to feed and water. Fluid and electrolyte therapy is essential for animals with severe diarrhea to correct dehydration and acidosis. Oral rehydration solutions or intravenous fluids (e.g., lactated Ringer's solution) may be administered. Non-steroidal anti-inflammatory drugs (NSAIDs) such as flunixin meglumine (1.1-2.2 mg/kg IV) can reduce fever and inflammation. Antibiotics may be indicated to treat or prevent secondary bacterial infections, especially pneumonia and enteritis. For example, ceftiofur (2.2 mg/kg IM or SC q24h) or oxytetracycline (10-20 mg/kg IM or IV q24h) may be used. In cases of severe thrombocytopenia, blood transfusions may be considered, but prognosis is poor. For PI animals, there is no treatment; they should be culled to prevent further spread. Prevention is key, including vaccination and biosecurity measures.

The prognosis for acute BVD is generally good, with most animals recovering within 1-2 weeks. Mortality is low (<5%) except in severe cases with hemorrhagic syndrome or secondary infections. However, reproductive losses can have a significant economic impact. PI animals have a poor prognosis, as they are at high risk of developing mucosal disease, which is invariably fatal. Mucosal disease has a case fatality rate of nearly 100%, with death occurring within 1-2 weeks of onset. The prognosis for the herd depends on the prevalence of PI animals and the effectiveness of control measures. Herds with a high proportion of PI animals may experience ongoing losses and require aggressive intervention.

After an outbreak, it is crucial to identify and remove PI animals from the herd. This involves testing all animals, especially calves, using ear notch IHC or PCR. Repeat testing after 3-4 weeks is recommended to confirm PI status. Vaccination programs should be reviewed and implemented to protect susceptible animals. Biosecurity measures should be enhanced, including quarantine of new arrivals and testing before introduction. Monitoring for reproductive performance, such as abortion rates and calving intervals, can help assess the impact of BVD. Bulk milk testing can be used for ongoing surveillance in dairy herds. Regular herd health visits should include a review of vaccination protocols and biosecurity practices.

Pearls: 1) Ear notch testing is the gold standard for identifying PI animals; always confirm with a second test. 2) PI animals are the main source of virus; culling them is essential for control. 3) Vaccination with modified live vaccines can provide rapid immunity, but should not be used in pregnant cattle or in calves under 4 months of age due to potential immunosuppression. 4) In acute outbreaks, consider testing bulk milk for BVDV to assess herd status. 5) BVDV can cause immunosuppression, so look for concurrent infections. Pitfalls: 1) Do not rely solely on clinical signs for diagnosis, as many cases are subclinical. 2) Avoid using modified live vaccines in pregnant cattle, as they can cause fetal infection. 3) Do not forget to test new animals before introducing them to the herd. 4) Be aware that PI animals may appear healthy; testing is the only way to identify them. 5) In cases of abortion, always consider BVDV as a differential diagnosis.

There is no specific antiviral drug for BVD. Supportive care includes: 1) Fluid therapy: For dehydrated animals, administer isotonic fluids such as lactated Ringer's solution or normal saline at a rate of 40-60 mL/kg IV over 24 hours, adjusted based on hydration status. For severe acidosis, add sodium bicarbonate (1-2 mEq/kg IV) as needed. 2) NSAIDs: Flunixin meglumine (Banamine) at 1.1-2.2 mg/kg IV or IM once daily for up to 3 days. 3) Antibiotics: For secondary bacterial infections, use broad-spectrum antibiotics such as ceftiofur (Excede) at 2.2 mg/kg SC once, or oxytetracycline (Liquamycin) at 10-20 mg/kg IM or IV every 24 hours. 4) Vitamin B complex may be given to anorexic animals. 5) In cases of severe thrombocytopenia, consider blood transfusion (10-20 mL/kg IV). Withdrawal times must be observed: for flunixin, milk 36 hours, meat 4 days; for ceftiofur, milk 0 hours, meat 3 days; for oxytetracycline, milk 96 hours, meat 28 days. Always consult the label and a veterinarian for specific protocols.

Landmark studies have established the role of PI animals in BVDV transmission and the importance of their removal for control. Research by Houe (1999) demonstrated that PI cattle are the primary source of infection and that their identification and culling can lead to eradication. Studies on vaccination have shown that both modified live and inactivated vaccines can reduce clinical disease and reproductive losses, but modified live vaccines provide more rapid and robust immunity. A meta-analysis by Newcomer et al. (2015) concluded that vaccination with BVDV vaccines is effective in reducing the incidence of PI calves. The economic impact of BVD has been quantified in several studies, with losses estimated at $10 to $40 per cow per year in endemic herds. Control programs in European countries, such as the Scandinavian eradication programs, have demonstrated that systematic testing and removal of PI animals can lead to national eradication. Current recommendations from the AABP and ECBHM emphasize biosecurity, vaccination, and PI testing as the cornerstones of BVD control.