BVD - what do we need to know?

[Duncan MacIntyre]

What do we need to know about BVD?

BVD is a virus disease of cattle. In a way its name is very unfortunate, because it is a complicated disease and diarrhoea is one of the least important features of it. But we are stuck with the name, an abbreviation for Bovine Viral Diarrhoea, because the virus was first identified from cattle showing diarrhoea. Other similar viruses are Classical Swine Fever and Border Disease of sheep. It is known that BVD virus can be found in sheep but they are not thought to be an important source of infection – the main source is invariably other cattle.

Although BVD was identified quite some time ago, the whole story of how it affects cattle has not been known till the latter years of the 20th century, and a vaccine has only been licensed in UK for a few years.

So apart from diarrhoea, why are we so concerned about it? The virus causes a wide range of signs, including transient diarrhoea, a form of fatal diarrhoea with ulcers in mouth and on feet called Mucosal Disease, reproductive problems such as abortion, stillbirth, neurological and ocular abnormalities in newborn calves, birth of persistently infected calves, and suppressing the immune system leaving cattle more susceptible to lots of other things such as calf pneumonia and mastitis. One of the nastiest features is the production of PI’s, ie persistently infected calves, when a cow or heifer first becomes infected between about 40 and 130 days in calf. The resultant calf does not recognise the virus, produces no antibody to it, but is a veritable virus factory for its entire life. That life may well be short, as they are often poor doers, and may develop mucosal disease before dying. Once in a while such a beast does live long enough to become pregnant herself, and if this happens any calves from her will be PI.

With a fuller appreciation of the different manifestations of the disease it is now seen as one of the major health threats to our cattle. It is estimated to be present in over two thirds of herds in the UK, and the Scottish Agricultural College estimate of cost some years ago was that if BVD entered a previously free herd of 100 suckler cows, the cost over a 10 year period would be around £46000. Of course, Dexter herds of 100 cows are few and far between, and the disease will be less likely to persist in small herds – but there may be more frequent movements between our small herds, be it for getting cows served, or for showing, or because we keep a few Dexters as a hobby part of a larger beef or dairy herd.

Recognising the presence of BVD is not as easy as you would think, given the range of disease it can cause. Dairy herds can have bulk milk tested, and an estimate of the extent of the disease in the herd made. In beef herds the usual method to screen for the disease it to test 10 calves from each separate group, usually left till 9 months of age so that being weaned they are not affected by maternal antibodies. Whilst the bulk milk test gives a general picture of the herd, the blood samples are used to pinpoint the individual animals status
Antibody negative Antigen negative Unexposed /vulnerable
Antibody negative Antigen positive Persistently infected
Antibody positive Antigen negative Previously exposed
vaccinated maternal
antibodies

The use of representative groups from the herd is not so useful in the smaller herds. Many Dexter herds should be looking at all animals in the herd over 9 months to get a reliable picture. Having done that, we are going to find one of a number of situations –
• Completely free of BVD
• Evidence of exposure to disease
• Presence of PI animals in herd

Those lucky enough to find that they are free of BVD are indeed fortunate, but they must ask some serious questions on how to maintain that freedom. Biosecurity needs to be looked at very carefully, and freedom from the disease can only be maintained without vaccination if biosecurity is extremely good. I suspect that few Dexter herds can claim that, we all tend to want to show animals, send cows for service, take other cows in for service, etc..If your herd is free of BVD and you intend to keep it that way I think a vaccination policy is the only way unless you can be absolutely sure of your biosecurity. So I would recommend vaccination of the herd after testing, with the one proviso that any bulls likely to have semen collected should not be vaccinated as positive tests would be a barrier to collection especially for export.

If you discover evidence of exposure to disease at all, it would be wise to test the whole herd looking for PI animals, remove them, and vaccinate as above. However this can be costly. If the budget is limited it would be possible to carry out a gradual programme of testing breeding replacements before bulling age, eliminating any PI animals found, and vaccinating those with no antibodies. Whilst vaccinating an already antibody positive animal will do no harm, it is almost certainly unnecessary.

If PI animals are present it is most important to root them out as some may survive for quite a long time and wreck all attempts to rid the herd of the virus. Once this is done those with negative tests should be vaccinated.

Showing of animals and returning them to the herd presents a risk to many Dexter herds but probably not quite as high as might be inferred from the percentage of the national herd which is infected at any one time, simply because the great majority of animals at shows are there because they are in good health. However this cannot be guaranteed and those handling them may even carry virus on clothing etc. so animals returning from shows should ideally be quarantined for a three week spell before mixing with the rest of the herd. A show team might well be segregated for the showing season if you go round a large number.

As a final thought I wonder if Dexter owners should be looking around their area for similar herds whose owners have similar ideas to “group” with for purposes of bull sharing – in effect function as separate “cells” with less worry of disease from cell members?

Duncan, written some time ago for a Dexter Group newsletter

[Issy]

Thank you Duncan, Something I wish I had known about when looking for calves originally and in hindsight (a wonderful thing I know) we should have spotted Heff a mile off.

[Inger]

Both our bulls must have been exposed to the desease previously as they both have antibodies to BVD. So I'm hopeful that at least our bulls can't spread the desease. When we buy a new bull calf, we should put him with the two older bulls for his quarantine shouldn't we? That way no harm can come to the rest of the herd. A newly weaned calf would panic if it is kept by itself. I've put a weaned bull calf in with the two older bulls before and they were quite good with him. So hopefully they'll be fine with a calf from outside the herd?

[Duncan MacIntyre]

Inger,

the new arrival quarantined with the bulls already exposed to BVD is fine from the BVD point of view, but of course you should think about other things too such as IBR, leptospirosis, salmonella etc etc.

Duncan

[Penny]

This is fantastic information that should appear in the Bulletin ( or its replacement) every so often, plus other health issues eg Ringworm, warts, mineral deficiencies. Members come and go with alarming frequency in the DCS, so this and many other health issues, should be repeated every so often.

Perhaps I should use tiredness as an excuse(!), but I would like to clarify a few things:
When an animal has been exposed to the disease, do the symptoms such as abortion, stillbirth, diarrhoea etc continue intermittently or are these no longer a risk once the animals have developed antibodies? ie are the problems of abortion etc only likely when first exposed to the virus ( unless a PI)?
What about grazing ground that has been previously grazed by a PI? Does that need to be rested for a time period, and is there any risk from brought in haylage/silage that has been fertilised by manure?

I had other questions too, but the brain has gone foggy......

Duncan, I guess this one is for you!

[Mark Bowles]

We have decided to test a sample batch this week to give us an idea on our health status. The vet says he may be able to send some samples ( poss 10) thru a drug company free of charge, the test i suppose not his labour costs.We will test for Lepto, IBR and bvd, plud Johnnes desease i think.
With the herd as a whole i would like to acheive a high health status, not sure how possible it will be as we do enjoy showing.
Mark

[Inger]

Yes as you mention showing. It does pose risks doesn't it. The bull calf we are getting has been shown all season. His parents and grandparents were shown as well. I'll check with the breeder as see what innoculations the calf has had so far. I can get a bloodtest done and get him innoculated against BVD if he hasn't got any antibodies.