Bovine Viral Diarrhoea Virus

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Description

Virus Characteristics

Transmission and Epidemiology

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.


Pathogenesis

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


Diagnosis

Clinical Signs

Laboratory Tests

  • 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

Pathology

Small erosions of MDV/BVDV - vesicles are microscopic (Courtesy of Alun Williams (RVC))
Coalescing lesions of BVDV (Courtesy of Alun Williams (RVC))
  • Mucosal Disease: erosive condition produces small multiple, cleanly punched out lesion in mouth
  • Neutrophils invade the ulcer and if bacterial colonisation occurs, further excavation follows. Either:
  1. This lesion develops a granular base and becomes diphtheritic.
  2. If bacterial colonisation does not take place, healing occurs within fourteen days.
  • Seen in most parts of mouth (or maybe on muzzle) e.g. dental pad, cheeks, sides of tongue
  • Lesions extend throughout gut with particularly big ulcers in small intestine over Peyers patches. Necrosis occurs in lymph nodes and spleen
  • No vesicular stage, prickle cells die off from surface resulting in layer of necrotic debris over epithelial layer
  • Infection penetrates inward through stratum germinativum.
  • Epithelium does not recover as animal does not recover

Treatment and 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

Links

References