Bovine Viral Diarrhoea (BVD) Flashcards
BVD Background
- The disease was reported in 1946 in the USA
- Initially, clinical cases at that time were associated with high fever, diarrhoea, mucosa lesions and leukopenia
- Until the late 1980s, effective BVD control was hindered by its widespread distribution and lack of cost-effective diagnostic tools.
- The stealthy nature of the disease prevented farmers and veterinarians from recognizing its full economic impact
- It took at least 40 years to fully recognize the extent of the effects caused by BVDV infections
Why is BVD important?
- poor fertility
- high level of dz in calves
- poor weight gain
- scours
- low milk yields
economic impact:
- Poor reproductive performances
- Growth retardation
- Reduced milk yield
- Increased susceptibility to other diseases
- Early culling increased mortality among youngstock
BVD - Aetiology
- Pestivirus Single-stranded RNA virus
- Antigenic variations of the viral surface proteins result in two types of BVDV: type 1 and type 2
- BVDV can be cytopathogenic (CP) or Non -CP
UK:
- BVDv Type 1: prevalent
- BVDv Type 2: rare
BVDv subtypes
- BVDV-1 (Pestivirus A) and BVDV-2 (Pestivirus B), which are further divided into subtypes
- There are currently 17 recognized subgenotypes of BVDV type 1 (designated 1a–1q) and 3 subgenotypes of type 2 (designated 2a–2c)
UK:
- BVDv Type 1: prevalent
- BVDv Type 2: rare
BVD - host range
- BVDV includes most even-toed ungulates
- Many species can be affected, domestic cattle seem to be the primary host
BVD transmission
- Nose-to-nose contact is the main route for the transmission of infection
- Most commonly by introducing infected animal(s) to herd
- And/or contact at boundaries, shows etc
- Virus survives very poorly in the environment, on fomites etc.
– Survives ~4w in the environment - PIs are generally much more important sources of infection
- Most transmission is via nasopharyngeal secretions (faeces are a poor source)
BVD - Pathogenesis
- Lymphoid tissue is the primary target of BVDV
- BVD is associated with a reduction in the number of T lymphocytes in blood and diminished function of T lymphocyte (thymocytes, lymphocytes, monocytes, macrophages, and dendritic cells)
- However, viral replication occurs in a variety of cell types located in the integument, alimentary canal, nervous system, respiratory tract, and immune system
BVD infection: non-pregnant animals - CS
Not pregnant = transient infection
- Subclinical or clinical (dull, decreased appetite, D+)
- No clear/specific CS
- Shed low levels for up to 3 weeks
- Antibodies present from 2-4 weeks pi and persist for several years
Respiratory infections:
- Worse with BVD
- e.g. BVDV + other viruses (IBR + RSV) + bacteria (M.haemolytica)
Enteric infections:
- Worse with BVD
- e.g. BVDV + other viruses (corona + rota) + bacteria (Salmonella spp)
BVD infection during pregnancy - CS
- Lowered herd fertility
- Increased number of barren cows
- Decreased number of newborn calves
- Can cause an inflammation of the ovaries -> impaired ovarian function
Wide variety of possible CS:
- PI calf
- Normal calf
- Abortion
- Congenital defects
– e.g. e.g. cerebellar hypoplasia arthrogryposis, microphthalmia, cataracts, hydrocephalus, musculoskeletal malformations, and alopecia
- Embryo loss
Stage of gestation (~) & outcome:
- 0-30d = embryo loss
- 30-120d = PI calf (Ab -ve, Ag +ve)
- 90-210 = normal calf or congenital defects or abortion (Ab +ve, Ag -ve)
- 190-270 = normal calf (Ab +ve, Ag -ve)
PI animal
= persistently infected
= persistent transmission
- BVDv seen as part of normal self
- no antibodies ever produced
- may go undetected or may show CS
- shed large quantities of virus
- 0.5-2% of national population are PI animals
BVD infection during pregnancy - PI
- PI dam -> PI calf (100% chance)
– ~7% of all PI calves come from a PI cow - Pregnant, BVD naive cow + BVD -> PI calf
– 93% of all PI calves come from acute infection of the dam
Acute BVD infection during pregnancy - PI CS
- PI’s often small and stunted but can be clinically completely normal until they develop MD
BVD mucosal dz - CS
- Mucosal dz = transmitting whilst alive
- Fatal condition
- Clinical signs: weight loss, bloody D+, dehydration and ulcerated lesions in mouth, nose and interdigital space
- Virus mutates from non-cytopathic to cytopathic form (ie cell killing form)
- Transmission of cp strain = potential for MD in other PIs
BVD infection in bulls - CS
- Can be PI
- Can be acutely infected
- Rare: the virus can ‘hide’ in testicles “immunologically privileged site”
– Hence, it can be blood Ab positive, antigen negative, yet still spreading viru
– Only identified by semen sample - Potential to introduce virus to herd
- Test before bringing new bull in
BVD 1 CS
- Acute infection
– Mild systemic illness
– Reduced reproductive performance
– Immunosuppression - Mucosal disease (PI animals)
BVD 2 CS
- Acute severe haemorrhagic syndrome
- Has been seen in UK but not very recently/widely
Acute infection:
- generally severe disease often fatal, usually adults
- virus clearance 28 – 38 days
- thrombocytopenia, diarrhoea, haemorrhagic disease
- weak cross-protection to Type 1 BVDV antibodies
BVD – Diagnosis
- Viral antigen and antibody tests
- Both can be identified in blood and milk
– Ag also skin, hair, semen, tracheal fluid, follicular fluid - Both can be used on bulk milk (dairy herds)
– Viral Ag can be detected in bulk milk from 1 PI in 300 cows
Early identification of PI calves
- Virus can be readily identified in blood samples pre-colostrum or from older calves (> 8 months)
– Detection in blood can be hampered in younger animals by maternal antibodies - Possible to look for viral Ag in calves of any age by using ear tissue samples, where high levels of antibody are less likely
BVD diagnosis on individual level
- Presence of virus (i.e. Ag) can mean
– Animal is a PI
– Animal is acutely infected
-> Viraemia following acute infection lasts up to 2 weeks - So, to demonstrate PI animal:
– 2 positive antigen tests >3 weeks apart
– Usually, PIs have low/no antibody levels in both samples - PI animals represent the major reservoir of infection
– Cull if discovered - Sensitivity and specificity of both Ab and Ag test rel. good
– But no test is perfect… always bear this in mind, especially important when screening high numbers - Paired serology in acute infections can be difficult due to a slow response
– After abortions with BVD – it won’t necessarily give a rising titre - Care with buying pregnant cattle (the calf may be a PI)
– The “Trojan horse” effect - Some PI animals can make antibodies (rare)
BVD diagnosis at herd level
Main questions:
- Is BVD present in the herd?
- Are PI calves being produced?
Antibody level:
- <0.1 = naive animal
- 0.1-0.35 = low level of exposure
- 0.35-0.70 = moderate level of exposure
- >0.7 = recent or active exposure
BVD diagnosis at herd level - tag & test
– Newer way to tell whether PIs are being produced
– More expensive (as test every animal)
– But also shows which individuals are PIs
–> Allows culling
–> Normal also to test dam where PI revealed
– Becoming a more common approach
BVD diagnosis at herd level - blood sampling a heifer cohort
- Blood sample for Ab from ~ 8 months of age (maternal Ab)
– Usually, sample 8-10 from the group
– It should be Ab negative (and Ag if it was tested) - If Ab positive
– must have seroconverted due to infection from a PI
OR - Contact with an adult (acute infection or PI)
- A useful measure of whether the infection is active in a herd
Basic approach to BVD control
- Evaluate current herd status
- Assess the routes by which virus could enter the herd and spread within the herd
- Eradication by removal of PIs and preventing new cases
BVD – Diagnosis
- Initial screening commonly…
– Bulk milk Ab +/- Ag test (dairy)
– Youngstock cohort tests (dairy and beef herds) - Calf tag/test results useful
– But normally only have these once the farmer embarks on a control programme - PM of the spleen
- Abortion results -> can use aborted foetuses
BVD – Management - Eradication programme
Requires a committed farmer and excellent biosecurity
Need to identify and remove any PI animals
- Often via tag and test (i.e. from “now” onwards)
– Generally also test bulk milk for antigens to identify adult PIs
- Time-consuming and costly (lab fees)
- Even when PIs have gone, the virus can still be present and slow spread via acute infection is theoretically possible
Herd will become completely naïve; re-infection could result in large losses
Commonly stop Ag testing calves at some point (often 1 year after the last positive) and rely on bulk milk antibodies to detect the incursion
Protect the breeding herd and stop the birth of PI animals
- With time, any existing PIs will be culled out naturally
- Must ensure protection before first breeding
- Bovilis BVD (Intervet), Bovidec (Novartis) and Bovela (Boehringer) currently available in the UK
Care: PI animals are sometimes still produced even after a long period of vaccination
- As PI cows will still have PI calves
- Can be important for calf health
BVD – Management - Vaccination & eradication
Adopt vaccination and monitor for Pis
- Usually vaccinate now plus tag & test calves born from now on
– Again, usually stop this 1 year after the last positive
- +/- adult herd testing to detect/remove PIs at the start
Leads to the most rapid and complete resolution of BVD problems within the herd and protects against re-infection: especially useful if…
- Hard/impossible to adopt biosecurity measures to keep disease out (e.g. “flying herd”)
- Large number of adult PIs and/or farmers unwilling to test/cull adults
Two main types of vaccines:
1. Inactivated (CP bovine viral diarrhoea (BVD) virus type 1)
- No replication in vivo, no viraemia following vaccination
- Modified live (BVDV Type 1 & 2 non-cytopathic)
- MLV result in transient viraemia following vaccination o MLV vaccinated animals will test antigen positive for 2wks following vaccination
BVD – Management - Do nothing
- A proportion of the herd will acquire natural immunity
- But this is random, and immunity will wane over time
- Losses that occur as a result of infection can be significant – but are very variable and can be hidden
- If the herd is only monitored through bulk milk Ab and is consistently low/medium positive, this is still a fairly common option, but is this appropriate?
BVD – National control programmes
- Voluntary national control schemes in England and Wales
- Compulsory scheme in Scotland
– Mandatory screening, restrictions on untested or “non-negative” herds
BVD summary
- aetiology: infectious dz caused by a pestivirus
- transmission: horizontally and vertically
- diagnosis of the individual: skin biopsies and blood samples
- diagnosis of the herd: bulk milk
- control strategies:
– rigorous biosecurity measures (i.e. quarantine)
– systematic testing
– culling of PI animals
– maintaining high herd immunity through vaccination
