Bovis ISSN 2398-2993

Bovine Viral Diarrhea: the disease

Synonym(s): BVD

Contributor(s): Jonathan Statham , James Breen

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Introduction

  • Cause: bovine viral diarrhea virus (BVDv). 
  • Signs: pneumonia, abortion & returns to service, gastro-intestinal ulceration, diarrhea, ataxia and blindness.
  • Diagnosis:
    • Antibody presence in blood or milk – may be due to viral challenge or vaccination.
    • Presence of BVD virus on PCR or antigen ELISA indicates transient or persistent infection (PI)  in blood, milk or tissue samples.
  • Treatment: none.
  • Prognosis: poor if PI; transient animals will recover.
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Pathogenesis

Etiology

  • BVDV BVD: the virus is a small enveloped virus, a member the of the Pestivirus genus within the family Flaviviridae.
  • It has a genome consisting of a single positive RNA strand of around 12-5 kb. The genome encodes a variety of structural and non-structural proteins.
  • The structural proteins ultimately assemble to form the virion, in which the major envelope glycoprotein (E2) is anchored in a host-derived lipid bilayer surrounding a capsid containing the newly synthesised viral RNA. E2 is the major target for neutralising antibodies, which confer protection following infection or vaccination.

Predisposing factors

General

  • Purchased or straying animals represent the greatest risk of BVDv infection entering a herd, although both sheep and contaminated clothing and vehicles may be responsible.

Pathophysiology

  • BVDV may exist as two distinct biotypes: 
    • Non-cytopathogenic (BVDV nc).
    • Cytopathogenic (BVDV c). 
    • The 'nc' biotype causes no cytopathology in cell culture, whereas the 'c' biotype does.
      • Recent work has demonstrated that this also happens in the live animal.
Pregnant females
  • The biotype responsible for in utero infections is non-cytopathogenic. 
    • Experimental infections during the first trimester have shown that up to 30 per cent of fetuses are aborted even though the majority of the surviving fetuses go to full term and are born persistently infected (PI).
    • It is the non-cytopathogenic biotype that persists in the cattle population (mainly within PI calves) and thereafter gives rise by mutation to the cytopathogenic biotype.
    • In persistently infected cattle conversion of the non-cytopathogenic biotype of BVDV to the cytopathogenic biotype results in the development of mucosal disease.
  • BVDV rarely infects the fetuses of seropositive cattle.
  • It is only during the viremia of acute or persistent infections in seronegative dams that the virus invades the placentome and replicates in the trophoblast before crossing to the fetus.
  • Fetuses born to PI dams likewise become persistently infected with near 100 per cent vertical transmission from dam to fetus.
    • However, the proportion of P1 calves that are born to PI dams is reportedly only 7 per cent. This infers that the remaining 93 per cent arise as a result of acute infection of the seronegative dam in early pregnancy.
  • The outcome of fetal infection is dependent on two main variables:
    • The age of the fetus at the time of infection.
      • Infection during the FIRST TRIMESTER (0 to I 10 days) of fetal life can result in abortions, congenital damage or the birth of PI calves.
      • Infection during the SECOND TRIMESTER (111 to 180/200 days) can result in congenital damage and fetal loss.
      • During the THIRD TRIMESTER, the fetus is immunocompetent and able to mount an active immune response.
    • The biotype of the infecting virus. 
Bulls
  • In bulls, the virus infects the testes and causes reduced sperm production and infertility which can last several weeks.
  • Infected bulls can pass the disease through semen.
  • Some infected adults (male or female) may show a transient fever and slight diarrhea, but many do not show any overt clinical signs. 
  • A rare condition has been reported in which BVD infection may localise to the testes. Affected animals may test as BVD negative but are in fact positive. This has been referred to as the Cumulus condition – named after the first recognised case in a bull called “Cumulus”.  Vets are advised to test semen from bulls to have full confidence that they are BVD free.
Young stock
  • More commonly, acute infection produces profound leukopenia and so lead to exacerbation of concomitant infections such as:
  • It is believed that high virulence BVD viruses can down-regulate genes, leading to immuno compromise and hence BVD often presents with concurrent disease.
  • In young animals, the virus can rarely cause primary respiratory disease seen as pneumonia. 
  • BVDv is commonly found as part of the bovine respiratory disease syndrome where several viruses can combine to cause severe outbreaks of pneumonia.
  • Animals that are acutely infected with BVDv (as compared to persistently infected) eventually become immune and mostly recover from the acute signs. They remain immune for life and when tested for antibody, they remain positive.
  • Persistently infected (PI) animals are born to mothers who were infected in the first third of pregnancy. 
    • PI animals can appear clinically normal and live a normal life. But, they will be constantly spreading infection in the herd.
A typical 250kg, 9 month old PI animal will release 250, 000, 000, 000 virus particles. However, only 10 to 1000 virus particles are required to infect another animal.
  • Some will become pregnant and if they have offspring, their calves will always be PI.
  • Many succumb to disease, and occasionally become “superinfected” with a mutation of the BVDv to show as Mucosal Disease.  
    • Typically, these animals are 12 – 18 months old, and show as acute diarrhea with ulcers in the mouth, and nasal discharge and fever. 
    • They are incurable and invariably die.

Timecourse

  • The time taken for the passage of virus in cattle from dam to fetus is variable.
    • Abortions due to BVDV have been shown, experimentally, to occur within 10 to 18 days of intramuscular infection.
    • Abortions can take place several months after fetal infection.
  • With the acute infection, there is inevitably a pyrexia, a leucopenia from about days 3 to 7 post-infection and a limited recovery of virus from both blood and nasal secretions during the first three to 10 days. 

Epidemiology

  • The importance of acute infections in the transmission and maintenance of BVDV within a herd or population should not be underestimated. They are responsible for 93 per cent of all in utero infections that result in the birth of PI calves.
  • BVDV can persist in the environment, in feces or cattle slurry, for up to 3 weeks at temperatures of around 5oC, this decreases to approximately 3 days at temperatures of 20oC.

Diagnosis

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Treatment

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Prevention

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Further Reading

Publications

Refereed Papers

  • Recent references from PubMed and VetMed Resource.
  • Booth R E & Brownlie J (2012) Establishing a pilot bovine viral diarrhoea virus eradication scheme in Somerset. Veterinary Record 170 (3) pp 73 PubMed.
  • Brownlie J, Thompson I & Curwen A (2000) Bovine virus diarrhoea virus – strategic decisions for diagnosis and control. In Practice 22 pp 176-187.
  • Lindberg A, Brownlie J, Gunn G J, Houe H & Moennig V et al (2006) The control of bovine diarrhoea virus in Europe: today and in the future. Rev Sci tech Offic Int Epiz 25 pp 961-979 PubMed.
  • Rikula U, Nuotio L, Laamanen U I & Sihvonen (2008) Transmission of bovine viral diarrhea virus through the semen of acutely infected bulls under field conditions. Vet Rec 162 (3) pp 79-82 PubMed.
  • Bennett R & IJpelaar J (2005) Updated estimates of the costs associated with thirty four endemic livestock diseases in Great Britain: a note. J Agric Econ 56 pp 135–44.

Other sources

  • BVD lectures by Prof. Joe Brownlie at World Buiatrics Conference 2018 (Sapporo).
  • BVD lecture by David Andrew Graham at World Buiatrics Conference 2018 (Sapporo).


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