What is BVD?
BVD is a viral disease of cattle. The virus is excreted in body fluids (urine, dung, saliva etc) and therefore is a risk when one group of animals has “nose to nose” contact with another. The virus is mainly excreted by Persistently Infected (PI) animals (see later), and to a limited extend by animals currently experiencing disease.
Animals who have never come across the disease before will test antibody negative, however they have no protection from the virus if they are ever exposed to it. These are termed “naïve” animals.
Naïve animals that meet the virus for the first time will, for a short period (days to weeks), test positive for virus (also called antigen). Thereafter they produce antibodies to fight off the virus and thereafter test positive for antibodies but negative for virus (antigen).
Non–pregnant naïve animals that meet the disease may develop pneumonia, display reduced fertility or may scour although this is uncommon, but will in time produce antibodies to fight the disease off and thereafter become “immune” (for an indeterminate period of time).
Naïve cows that meet the disease for the first time during pregnancy will also produce an antibody response but the fate of the calf is more uncertain. The calf may be aborted, born with deformities such as blindness, born with its own immunity, or become a PI calf.
During the last part of pregnancy the calf has a developed immune system of its own and therefore responds in the same way as the mother (becoming antibody positive but virus negative). During the first part of pregnancy however, the calf has no immune system and therefore cannot recognise the virus as such. There is no antibodies produced and the virus is allowed to persist within the calf. This is the persistently infected (PI) calf.
|Status of animal||Antibody||Antigen (virus)|
|Immune(following natural infection or vaccination)||+ve||-ve|
|Persistently Infected (PI) animal||-ve||+ve|
Why is it a concern for my herd?
Persistently infected animals will shed the virus in huge quantities, spreading it to the rest of the herd and to any other animals with which it has “nose to nose” contact. PI animals may appear completely normal but most go on to develop Mucosal Disease before they are 2 years of age. These animals have ulceration of the nose, mouth, entire digestive system, reproductive and urinary tracts. They scour dramatically, suffer severe pain, and die. They cannot be treated.
A small percentage of PI animals do not develop mucosal disease but go on to adulthood and may join a breeding herd whilst shedding disease.
If your herd is naïve to BVD there is no herd immunity to the disease, which would have a devastating effect on overall herd health, pneumonia, scour and fertility, if it were introduced at this stage. Any bought in animals may be carrying the disease or may be PI’s but appear completely normal.
How can I screen my herd and how does this help?
By sampling 5 homebred animals (from each separately managed group) aged 9 – 18 months, we can establish if there is active infection. Animals under 9 months of age may have antibodies from their dams if they are vaccinated or had come across the disease before (although this may be many years ago). Animals over 18months of age may test antibody positive indicating that they have encountered BVD, however this could indicate a historic infection and may not be relevant to your current breeding herd.
What are my options for control/prevention?
Vaccination of the breeding herd – two vaccinations given 3 – 4 weeks apart forms the primary course. This is given initially to all adult animals and thereafter to only heifers before they go to the bull, and to new animals joining the herd. Adults are given a single annual booster. This approach would protect a naive breeding herd from virus coming in with purchased animals.
If the breeding herd is naïve and not vaccinated, it is absolutely essential to prevent the virus entering the herd. This involves very strict biosecurity, especially in preventing contact with other animals at grazing and when buying in animals for breeding or store.
Purchasing animals direct rather than through a market reduces the chance of acquiring animals that are carrying the virus currently.
Purchasing animals only from BVD accredited herds would remove the possibility of buying PI animals.
If this is not an option, animals should be isolated on arrival on farm and screened for BVD.
What about joining a health scheme?
Accreditation with a health scheme would be advantageous when selling animals for breeding. Purchased animals, however, would have to be screened on arrival and isolated until test results were received. Boundary fencing would need to be capable of preventing nose-to-nose contact with other livestock. Currently Biobest laboratories offer a “BVD plus” scheme, membership of a CHeCS scheme solely for BVD. Seewww.biobest.co.uk/herdcare or www.cattlehealth.co.uk for further details.
In 2010 the Scottish Government introduced a BVD eradication scheme with the aim of removing persistently infected animals from the national cattle herd, and therefore reducing spread of the disease.
From December 2011 it will become compulsory for all breeding herds to know their BVD status (infected or clear). This can be done by annual blood testing of 5 unvaccinated calves from each separately managed group aged 9 – 18 months.
From December 2012 movement controls will be in place so that animals may only move from the holding of origin if that holding is annually tested clear or alternatively if the individual animal’s BVD status is known. Newborn calves can be individually tested using an ear tagging system available from Biobest (via the practice) and this may be appropriate for herds that have no 9-18 month old animals on the holding available for annual screening.
In dairy herds bulk milk tank testing may be appropriate – either 2 bulk milk antibody tests 3 months apart in unvaccinated herds or quarterly bulk milk PCR screens in vaccinated herds.
Please contact us at Forth Valley Vets to plan the most suitable approach for your herd.
Further information is also available on the Scottish Government website
or call 0300 244 9823