Bovine Viral Diarrhea (BVD) and Mucosal Disease – NAVLE Study Guide

Overview and Clinical Importance

Bovine Viral Diarrhea (BVD) is one of the most economically significant infectious diseases affecting cattle worldwide. Caused by Bovine Viral Diarrhea Virus (BVDV), a member of the genus Pestivirus within the family Flaviviridae, this disease causes immunosuppression, reproductive losses, respiratory disease, and gastrointestinal disorders. Despite its name, BVD does not specifically affect the digestive tract but rather has immunosuppression as its hallmark feature.

Mucosal Disease (MD) is a distinct, invariably fatal syndrome that occurs only in persistently infected (PI) cattle when they become superinfected with a cytopathic biotype of BVDV. Understanding the relationship between BVD and MD is critical for NAVLE success.

Etiology and Classification

Virus Classification

BVDV is classified within the Pestivirus genus, which also includes Border Disease Virus (sheep) and Classical Swine Fever Virus (pigs). The virus is a small (40-60 nm), enveloped, single-stranded positive-sense RNA virus.

BVDV Genotypes and Biotypes

Pathogenesis

Transmission Routes

Horizontal transmission: Direct contact with infected cattle, nasal/oral secretions, feces, urine, milk, semen, and contaminated fomites. PI animals shed virus continuously at 1000x the level of acutely infected animals.

Vertical transmission: Transplacental infection is the key mechanism for creating PI animals. The outcome depends critically on gestational timing.

Critical Gestational Windows and Fetal Outcomes

Persistently Infected (PI) Animals

PI animals are the cornerstone of BVDV epidemiology and serve as the primary reservoir for viral transmission. Key characteristics include:

• Born immunotolerant to their infecting NCP BVDV strain - virus recognized as self
• Shed massive quantities of virus continuously through all body secretions
• Typically seronegative (no antibodies) unless vaccinated or superinfected with heterologous strain
• Often unthrifty, poor doers, but may appear clinically normal
• Only 20% survive to 2 years of age; high susceptibility to secondary infections
• PI dams ALWAYS produce PI offspring

Mucosal Disease Pathogenesis

Mucosal disease (MD) is a uniformly fatal condition that can ONLY develop in PI animals. It occurs when a PI animal becomes superinfected with a cytopathic (CP) biotype of BVDV that is antigenically similar to their resident NCP virus.

Sources of CP Virus

• Internal mutation (most common): The resident NCP virus undergoes spontaneous recombination or mutation to become CP
• External exposure: Contact with other cattle shedding CP virus
• MLV vaccination: Modified-live BVDV vaccines containing CP virus (rare)

Because the PI animal is immunotolerant to its resident NCP virus, it cannot mount an immune response against the antigenically similar CP virus. The CP virus spreads to GI epithelium, causing keratinocyte necrosis, erosion, and ulceration throughout the alimentary tract.

Clinical Presentations

Summary of BVD Clinical Syndromes

Mucosal Disease Clinical Findings

Classic Presentation

• Sudden onset depression, fever, and complete anorexia
• Hypersalivation (excessive salivation with drooling)
• Oral ulcerations: Erosions on muzzle, dental pad, tongue, hard palate, gums
• Nasal discharge: Serous to mucopurulent, with nasal erosions
• Profuse watery diarrhea: Contains blood, mucus, and mucosal shreds in terminal stages
• Lameness: Coronitis and interdigital skin erosions/ulceration
• Rapid weight loss and severe dehydration leading to death in 5-10 days

Congenital Defects (Fetal Infection 100-150 Days)

Pathological Findings

Gross Pathology of Mucosal Disease

• Oral cavity: Erosions and ulcerations on tongue (especially lateral borders), dental pad, hard palate, gums, and pharynx
• Esophagus: Characteristic LINEAR erosions and ulcerations along mucosal folds
• Rumen: Erosions on pillars and papillae
• Abomasum: Mucosal edema and rounded ulcerations
• Small intestine: Erosions over PEYER'S PATCHES (hallmark finding), hemorrhagic mucosa
• Large intestine: Ulcerations in cecum, proximal colon, and rectum
• Lymphoid tissue: Mesenteric lymph node enlargement, edema; thymic atrophy
• Skin: Coronitis, interdigital erosions, occasional alopecia

Histopathology

• Necrosis of keratinocytes in stratum spinosum of epithelium
• Intestinal crypt necrosis
• Severe lymphoid depletion in Peyer's patches and lymph nodes
• Viral antigen demonstrable in epithelial cells by IHC

Diagnosis

Diagnostic Testing Overview

PI Animal Detection Protocol

The Ear Notch Test is the most practical method for PI detection in the field:

• Collect dime-sized ear notch sample at tagging (can use ear tag tissue plug)
• Test by pooled RT-PCR (pools of 10-36) or individual ACE
• If positive pool, test individual samples by ACE or IHC
• CRITICAL: Retest positive animals at least 3 weeks later to confirm PI status
• Two positive results at greater than or equal to 3 weeks apart = CONFIRMED PI

Mucosal Disease Diagnosis

The diagnosis of MD is confirmed by:

• Clinical signs + characteristic gross pathology
• Isolation of BOTH NCP and CP BVDV biotypes from tissues
• The paired viruses are antigenically similar (homologous)
• Animal is typically seronegative (no antibodies) unless recently superinfected with heterologous strain

Differential Diagnosis

Mucosal disease must be differentiated from other erosive/ulcerative diseases:

Treatment

CRITICAL: There is NO specific treatment for BVD or mucosal disease. Management is strictly supportive:

Prevention and Control

Vaccination

Recommended Vaccination Protocol

• Heifers: Vaccinate 2-4 weeks BEFORE breeding with MLV vaccine
• Pregnant cows: Use only KILLED vaccines if vaccination during pregnancy is necessary
• Bulls: Test for PI status before purchase; vaccinate annually
• Calves: Initial vaccination at 6-8 months when maternal antibodies wane
• Boosters: MLV annually; killed vaccine every 4-6 months

Herd Control and Eradication Strategies

• Test and Remove: Identify and eliminate ALL PI animals - cornerstone of eradication
• Biosecurity: Test all incoming animals for PI status before introduction
• Closed herd: Maintain closed herd or source from BVD-free accredited herds only
• Vaccination: Vaccinate ALL breeding cattle to prevent fetal infection and PI calf production
• Surveillance: Test all calves at birth (ear notch); bulk tank milk monitoring in dairy herds
• Fence line separation: Double perimeter fencing to prevent contact with neighbor cattle