Bovine Virus Diarrhoea

VIRUSES FLAVIVIRIDAE

Antigenicity

• RNA virus closely related to Classical Swine Fever and Border Disease Virus
• 2 Serological Types
  • BVDV-1 is traditional, existing as two biotypes
    • BVDV-1nc: noncytopathogenic
    • BVDV-1c: cytopathogenic
  • BVDV-2 is an emerging hemorrhagic virus

Hosts

• Cattle

Pathogenesis

Small erosions of MDV/BVDV - vesicles are microscopic (Courtesy of Alun Williams (RVC))

Coalescing lesions of BVDV (Courtesy of Alun Williams (RVC))

BVDV-1c

• Infects cattle regardless of age
• Usually mild: diarrhoea with recovery in 10 dyas
• Immunosuppression can lead to secondary infection

BVDV-2nc

• Transient thrombocytopenia and leukopenia over 2 weeks
• Hemorrhages
• Secondary infection
• Death

BVDV-1nc

• Transplacental infection of naive heifers
• Outcome depends on age of fetus at contraction
  • 0-110 days: abortion or persistently infected (PI) calves born
  • 110-220 days: congenital damage with noticeable CNS and musculoskeletal lesions
  • 220 days to term: active immunity developed

Mucosal Disease

• Mucosal disease is caused by a superinfection of PI animals with a second homologous cytopathic biotype (eg BVDV-1nc followed by BVDV-1c)
• Infection typically occurs between 6-18 months of age but is variable
• Superinfection will quickly spread horizontally among PI animals
• Invariable fatal
• Characterized by oral and enteric erosions, particularly overlying Peyer's patches, and ulceration of the feet
• Animals can show anorexia, depression and/or diarrhoea for 2-5 days before death
• Vaccination can lead to iatrogenic infection in undiagnosed PI calves

Epidemiology

• A major concern is that it can be confused with FMD (especially as it often occurs with clinical signs of salivation and depression)
• Virus is widespread: 60-70% exposure by 4 years of age
  • Often may sweep through a whole colony of young stock causing profuse diarrhoea (perhaps febrile) for a few days and then recover
  • Due to primary exposure to cytopathic strain of virus
• PI cows:
  • 100% vertical transmission to offspring
  • Are infected with BVDV-1nc and NEVER BVDV-1c
  • Are often antibody-negative (though they can show low levels of Ab to heterologous virus)
  • Show a wide range of clinical signs:
    • Severe congenital damage (ataxia)
    • Poor body condition
    • Increased susceptibility to enteric and respiratory disease
  • Act as the herd reservoir of BVDV
  • Can ONLY be identified by blood testing
• Transfer via semen, direct contact with acutely infected animals, or vertical from dam to offspring
• Transfer can be iatrogenic: repeated use of needles and gloves, etc.

Diagnosis

• Traditional test: virus isolation followed by serology on infected cells
• ELISA for virus antigen in animals with persistent viremia (will show up 3-8 days post-infection)
• PI calves often appear virus negative as a result of receiving neutralizing Ab in colostrum: can be countered by RT-PCR
• Paired serum samples from cows with acute BVDV
• Herd sampling by ELISA for antibody on bulk milk

Control

• No known treatment to reverse persistent infection or to cure mucosal disease
• BUT, without exposure to BVDV, the whole herd is at risk as there is no developed immunity
• Vaccination of dams before pregnancy will prevent PI calves being born
  • Beta-propiolactone inactivated vaccine
  • Combine with screening for antigen and removal of PI animals