Bovine Viral Diarrhea Flashcards
how is BVD transmitted
Direct = nasal secretion, semen, urine, milk, saliva, tears, fetal fluid (not much in diarrhea)
- Nose to nose
- Sharing feed
- Sexual contact
Indirect = ET, VETs, visitors
what is the main source of virus spread
persistently infected animals
what happens after infection depends on what
- immune status (defined by prev exposure or vaccine)
- pregnancy status
what occurs when infected and not pregnant if the animal is immune
Nothing
Or very mild transient infection
what occurs when infected and not pregnant if the animal is not immune
Transient infection (TI)
A bit like fresher’s flu
Rarely noticed
Viremia day 3-14 only = antigen + (nasal shedding)
But then, mount immune response from day 14, clear the virus and recover
- = Antibody + (persists)
what happens when infected if immune and pregnant
Nothing
Antibodies clear the virus
Or
Very mild transient infection
Fetus unaffected
what happens when infected if not immune and pregnant
Cow — TI and antibodies but bad news for the fetus
what occurs to the fetus when dam is not immune and infected during 1st trimester
1st trimester: (0-120 days)
Persistently infected or abort
93% of PI’s produced this way
what occurs to the fetus when dam is not immune and infected during 2nd trimester (120-240 days)
Congenital defects
- Cerebellar hypoplasia, ocular degeneration, undershot jaw, thymus/bone/growth retardation
Abort
what occurs to the fetus when dam is not immune and infected during 3rd trimester (240-280 days)
Still born
Stunted
Abort
how do persistent infections come about
Fetus infected in first trimester of pregnancy
Offspring of PI
how do PI’s occur in first trimester of pregnancy
Before immunocompetent
BVD virus seen as ‘self’
Fetus/calf NEVER fights the virus (no antibodies)
When the calf is produced it is a huge shedder! Never produce any antibodies
why does a PI cow always produce a PI offspring
A PI cow will ALWAYS produce a PI calf (BVD gets into oocyst)
Antigen positive, antibody negative (if MDA used up) forever!
what are the clinical signs of a transient infection (TI)
Often very little… all temporary
- ? Pyrexia
- ? Diarrhea
- ? Anorexia
- ? Infertility
- ? Immunosuppression
Make antibodies = recovery and virus eliminated
how long after transient infection do antibodies peak
Antibodies peak 12 weeks after infected and remain for years (? lifelong)
are TI’s antigen +/- on day 1 and 21
are TI’s antibody +/- on day 1 and 21
what are the clinical signs of a persistent infection
Stunted, poor doing
Poor DLWG
Prone to secondary infections — scours, pneumonia, ringworm
90% under 2 years old, most of them die
Sometimes they look completely normal
- Don’t presume if it’s a healthy animal that it can’t be a PI
Develop mucosal disease
are PI’s antigen +/- on day 1 and 21
are PI’s antibody +/- on day 1 and 21
what is mucosal disease due to BVD
Only happens in PI
Mutation of own virus from non-cytopathic (NCP) to cytopathic strain (CP) or superinfection with CP virus from another PI in pen
CP virus can circulate causing an outbreak of mucosal disease within PI’s
- Kills cells AND the host
Ulceration of all mucosa, salivation, lameness, hemorrhagic diarrhea (probably where BV’D’)
how do you treat mucosal disease due to BVD
6 months to 2 years
Euthanize asap
No treatment
what are ddx for oral ulcerations
Foot and mouth disease (notifiable)
Bluetongue (notifiable)
Bovine viral diarrhea (BVD)/mucosal disease
Malignant catarrhal fever
Bovine papular stomatitis (BPS)
Calf diphtheria
Actinobacillus — wooden tongue
Actinomycosis — lumpy jaw
Trauma
(IBR)
what occurs to TI bulls
Sperm motility and morphology affected temporarily (2 weeks viremia + 9 weeks spermatogenesis)
Virus also shed in semen (Up to 5 months ie longer than nasal secretions)
what occurs to PI bulls
Secrete virus in semen continually
Sperm quality reduced, fetus may/may not be PI (compared to mother)
No reports of this phenomenon causing an outbreak
Advise farmers not to use bulls for 2 months after purchase
- Even it has a negative antigen test
what are the clinical manifestations in the herd if the herd is immune
If herd immune (+ not massive viral load):
Very little
what are the clinical manifestations in the herd if the herd is not immune
Abortion, still birth, congenital problems, diarrhea, infertility (immunosuppression), poor growth rates, decreased semen quality, mucosal disease cases, increase disease in general
what type of virus causes BVD
Flaviviridae (family)
Pestivirus (genus)
Species:
- Bovine viral diarrhea
- Border disease virus (BDV in sheep)
- Classical swine fever virus (CSFV)
what are the 2 genotypes of BVD
BVD 1 - classic, UK
BVD 2 - more severe TI
subgenotypes and variation
what are the 2 biotypes of BVD
noncytopathic (NCP)
cytopathic (CP)
what are the differences between the noncytopathic and cytopathic genotypes
NCP causes the TI’s
CP superinfects PI’s to cause mucosal disease and kills cells –> mutation of NCP
is the BVD virus prone to mutation
yes so strain variation
vaccines not 100%, variable virulence
what makes the BVD an effective virus
Multiples in dividing cells without killing them (NCP)
Crosses the placenta (PI and congenital diseases)
Causes inflammation of ovary and lowers LH (infertility)
Interrupts GI neurones (diarrhea)
Causes immunosuppression:
- Hides from the immune response
- Down regulates interferon
- Decreases leukocyte number (lymphopenia)
- Effect on B and T cell response
what are key points to remember about BVD testing (4)
You can not ‘become’ PI —> only born a PI
PI cow will ALWAYS give birth to a PI calf
PI calf MIGHT have a PI mother (or mother had TI when pregnant)
If calf not a PI, mother definitely not PI — presumed negative status
what might antibody results be affected by
MDA lasts 9 months (less in PI)
Vaccination
how do you test an individual animal to determine if they are a PI
test for antigen
PI result = antigen positive
- But it could be TI
- Re-test in 3 weeks
- If TI animal will clear virus and mount immune response
3 weeks later:
- PI result = antigen + (antibody +/-)
- TI result = antigen - (antibody +)
how do you determine if an animal has been exposed
Not often done for a single animal
- Ex. buying in calf cows
Test blood (or milk) for antibody
- Negative = not exposed… or is this a PI?
- Positive = exposed or vaccinated but… when?
Calf is complicated due to maternally derived antibody
how do you test calves for antibodies and interpret resuts
Acquired from mother’s colostrum, lasts 9 months
Antibody testing:
- Can give false positive result if test calves <9 months
- In other words, the animal appears to have been exposed but wasn’t
how do you test calves for antigens and interpret resuts
MDA can neutralize antigen in PI — false negative
In other words, a PI animal is missed
But antibody used up really quickly in PI as so much virus
So blood NOT used for antigen <30 days
Tissue okay from birth (MDA doesn’t get into ear so well)
where is the diagnostic gap in testing calves
Where MDA might give false negative for tissue antigen
MDA effect in PI peaks at 3 weeks
Tag test early or test late
how do you determine if a calf was exposed in utero
First check antigen test (fetal fluid or tissue) to see if PI:
If positive (and calf alive), re-test in 3 weeks
- If antigen positive = PI calf
- If antigen negative = calf is not a PI but… was it exposed in utero?
If not exposed in utero = antibody negative
If was exposed in utero (after 120 days) = antibody positive because immune system of fetus can produce antibodies at stage of pregnancy
But as soon as calf suckles it acquires MDA from colostrum and becomes antibody positive (ie false positive) so need pre-colostral sample? Not practical
If antibody negative (and antigen negative) = calf was not exposed in utero (rare to get this result due to vaccine/exposure of mother)
how do you determine if a cow is carrying a PI calf in utero (aka trojan cow)
No easy way to work this out
Can’t test the unborn fetus
Can test pregnant mother for antigen and antibody but…
Don’t buy in pregnant animals (testing confusing)
how do you determine if a herd has been exposed to BVD using a screen test
5 calves 9-18 months in each management group
- Test blood for antibody ex. exposure
- Tells you if group exposed/immune to BVD (not what calf is PI)
If positive need to hunt for the PI
how does a screen test work
PI secretes massive amounts of virus
Nose to nose contact allows spread of virus
Those in contact = TI and become antibody positive
If you happened to sample the PI it would be antibody negative and the rest would be antibody positive
when doing a screen test, what is a management group
nose to nose contact
together for 2 months+
not as effective as grass
does this herd have BVD
what are the issues with a screen test
Relies on farmers honesty re management problems
Need accurate DOB for MDA
Vaccination interference
Need distinct calf groups
If positive — maybe have a PI on farm for 9 months already causing problems
Can test more calves
Can test more often
Scottish Govt BVD scheme — dairy, screen test 10 calves every 6 months as not distinct management groups
how do you use a milk body antibody test to determine if a herd has been exposed to BVD and what are the issues with this test
Not that useful
5% herd need to be seropositive before BT positive
Lag between infection and + BMT (up to 36 months)
If positive = exposure or vaccination (no idea when, how many, if any PI’s)
Next step is hunt for PI
If negative = no BVD (rare as most vaccinated)
Removed from Scottish Govt scheme
how do you hunt for the PI
Test the whole herd for antigen
Next step following positive screen test
Tissue, blood, milk sample (individual or bulk but NOT
£££ but tells you if any PI’s
Make sure you have test result for every animal (bulls, in calf heifers)
But what if the PI has been sold or died
how can you hunt for the PI and be strategic/cost saving
Handy tricks:
- Presumed negative status (saves sampling all cows) but need to make sure a calf for every cow
- If cow is a PI her calf will be a PI
- Can pool blood samples (not <6 weeks)
- Bulk milk can detect 1 x PI in 300 cows
what are the possible test outcomes and what do you always need to consider when interpreting results
always think about
age
mda
vaxx status
what does BVD cost the farmer
estimated £37 per annum per cow in beef herd
£200 per cow over a few months in an outbreak in a dairy herd
PI calves 43-73kg less at weaning
what are the areas of control of BVD
how is BVD controlled
- biosecurity
- hunt and remove PI’s
- vaccination
- monitor
how does hunting and removing the PI’s help control BVD in a herd
Fundamental to any control programme
Make sure ALL animals tested for antigen
Need to have at least 12 months of no PI’s born before can consider stopping antigen testing
- Calves in-utero
Farmers keen to keep/attempt to fatten PI’s
- It doesn’t work!
what biosecurity points are key to controlling BVD
Reason for most breakdowns
What are the common risks?
- Bought in animals — maintain a closed herd (if possible)*
- Rented grazing away from farm*
- Heifer rearing away from farm*
- Showing/bringing home from market (if do treat as bought in animal)*
- Contact with neighbours stock (double fencing)
People:
- Vets, staff, foot trimmers, AI technician
Equipment
- Vehicles
- Knacker lorry, milk tanker, feed deliveries
***especially if pregnant animals
if a farmer has an open farm what are key biosecurity points
If open herd:
- Test bought in animals for antigen
- Buy ‘accredited free’ animals
- Isolate 4 weeks
- Avoid buying in calf animals (can’t test unborn calf)
- Trojan cow — carrying PI high antibody
how can vaccination be used to control BVD
complete primary course before insemination
not 100% effective in the face of massive challenge
what are caviets to vaccine and controlling BVD
often incorrectly administered on farm
vaccine antibody complicates testing
prev vaccine used as primary control without removing PI’s
what are the key elements of Scottish Gov BVD scheme
all breeding herd must test
- establishes ‘negative’ or ‘not negative’
if herd not negative:
- Further testing to ID the issues (compulsory if >15 months not negative)
- Can’t move animals off farm without an individual antigen test
- Can’t move animals onto farm
- Status of herd communicated online ‘name and shame’
illegal to move PI’s except to slaughter (can get special licence)
vets have authority to change status
if negative herd brings in animal of unknown or not negative status they will lose negative status
what is the CHeCS accredidation scheme
regulatory body for cattle health scheme
various reasons to join (pedigree sotck, eradication of disease)
