Mandatory bovine viral diarrhoea screening: guidance for vets

PART 3 MINIMUM TESTING METHODS

This part of the guidance explains which testing methods may be used to screen for BVD.

Rather than only have one testing method available such as ear tissue tag testing, as has been used in BVD eradication schemes in Switzerland, Germany and Ireland, a range of methods has been developed. This reflects Scotland's relatively low disease prevalence; as most herds in Scotland are free of BVD virus at any one time, it is important to allow such herds to demonstrate that as cost-effectively as possible. It also reflects the diversity of farming practices, from crofting to large commercial breeders.

For those with BVDV infections, the priority is that the testing method should help them move towards identifying and removing PI cattle.

You should work with your clients to ensure the most appropriate testing method is chosen for their circumstances.

The methods for screening herds are based on the Cattle Health Certification Standards ( CHeCS) ( www.checs.co.uk), though they do not precisely reflect them. This means that those in CHeCS schemes will not need to do anything more than fulfil the requirements of their health scheme.

All herds must be tested using one of the following six methods. The first three methods can be used in both dairy and beef herds.

1. Sampling calves - the check-test (Antibody)

Whenever possible for a check test, use Option (a) below. Options (b) or (c) are only for herds that don't have calves aged 9 to 18 month old to test.

For all three such options shown below the calves should ideally not have had any vaccine with a BVD component, and certainly not have had a BVD vaccine within the preceding month. If they have been vaccinated, when submitting the samples please ensure that their vaccination dates and vaccine name are provided to the laboratory. The bloods must be tested for BVD antibody. If all the samples are negative for BVD antibody this will constitute a negative result for that year.

Each separately managed group must be identified and surveyed, otherwise the test may fail to detect infection when it is present - please see Part 4 for more advice. You should speak to your client before cattle are selected for sampling to ensure that all separately managed groups have been identified.

Where there is a very small number of calves in a herd or group, and they all test negative for antibodies, you may wish to consider asking the testing laboratory to test one sample for BVD virus as they may all be PI calves.

(a) 9 to 18 months

Take samples of blood from not less than five calves in the age range 9 to 18 months in each separately managed group. If there are fewer than five calves in a group, then test all calves in the group.

(b) 6-9 months

If any of the calves sampled in a group are aged 6-9 months, then take a sample of blood from not less than ten calves in the age range 6 to 18 months in each separately managed group. If there are fewer than ten calves in a group, then test all the calves in the group.

(c) Over 18 months and on the holding since birth

If there are no calves in either of the above age categories then, but only then, you can choose to use the following method:

Take a sample of blood from not less than five animals aged over 18 months that have been on the holding since birth in each separately managed group. If there are fewer than five animals in a group, then test all the animals in the group.

2. Test all calves (Antigen)

Individually test all calves born in the herd within the preceding year for BVD virus by blood or tissue sample. Calves can be tested as they are born, or all at once later in the year. Testing at birth has the advantage that PI calves can be identified and removed before the breeding season begins.

If all the calves are negative for BVD virus this will constitute a negative result for the year.

3. Test all animals (Antigen)

Individually test all animals in the herd within the year for BVD virus by blood or tissue sample, regardless of age and including bulls. The youngest age at which calves are old enough to be blood tested will be advised by the testing laboratory; this is usually one month. If all the animals are negative for BVD virus this will constitute a negative result for that year.

This testing method is the most expensive option, but it may suit herds where BVDV infection is suspected or herds where no other method is entirely suitable.

Dairy Testing Methods

The remaining testing methods are for dairy herds only.

4. Single bulk tank milk ( BTM) and blood tests (Antibody)

Test a single bulk tank milk ( BTM) sample plus individual bloods from any cows that did not contribute to the bulk tank that day, including dry cows and in-calf heifers, for BVDV antibody. If the milk goes into more than one tank a separate sample from each tank should be tested. If all the samples are negative for BVDV antibody this will constitute a negative result for that year.

5. Four quarterly bulk milk tests (Antibody)

A representative milk sample from the bulk tank(s) should be tested for BVDV antibody. Four quarterly BTM tests in one year, each not less than 80 days and not more than 100 days apart, are needed.

Four consecutive negative BTM tests are needed to constitute a negative result for that year. Following one positive result, it may be preferable to discontinue quarterly testing and switch to a more focussed testing method, such as a first lactation milk test (detailed below), or testing calves.

Where BTM is monitored quarterly, and the herd is vaccinated but has no animals that have been infected, the sample may be positive, but at a low level. If this does not change over the course of the year, this will also constitute a negative test.

6. First lactation composite milk test (Antibody)

A composite sample made of milk from each of the cows in the herd in their first lactation can be tested for antibody. This is a useful test in herds where the BTM is positive for antibody.

Testing options summary table:

TESTING OPTION	DAIRY/ BEEF	ANTIBODY/ ANTIGEN
1a: 5 animals between 9-18 month, separately managed groups	Either	Antibody
1b: 10 animals between 6-18 months, separately managed groups	Either	Antibody
1c: If neither above are possible - 5 animals 18m+ on holding since birth, separately managed groups	Either	Antibody
2: All calves	Either	Antigen
3: All animals	Either	Antigen
4: Bulk milk plus dry	Dairy	Antibody
5: Quarterly bulk milk	Dairy	Antibody
6: First lactation	Dairy	Antibody

TESTS AVAILABLE

Blood serum or plasma and milk are used to test for antibody to BVDV.

Blood is used to test for virus antigen. Tissue samples collected during ear-tagging can also be used. Milk may be tested for virus but only by the polymerase chain reaction ( PCR), a test that is best suited to screening BTM for the presence of a PI. Because high levels of maternally derived anti- BVDV antibody in colostrum can block the detection of BVD virus, blood can only be tested for antigen if the calf is over 4 weeks old. This restriction does not apply to skin samples collected during ear tagging which can be tested for antigen from calves of any age

Tests for BVDV antibody

Antibody detection ELISA

Antibody detection ELISAs are accurate and suited to high-throughput testing. They are available in a variety of formats so that results are given in different ways, which can be confusing. Labs will provide interpretation and explanatory notes specific to the test that has been used. Not all serum/milk samples are clearly negative or positive. Those with readings around the negative/positive cut-off value may be classified as inconclusive. The SG scheme has had to accommodate the inconclusive results and has included them with positive results to classify a group as 'not-negative'. A 'negative' group contains only animals with no anti- BVDV antibody.

Low or inconclusive levels of antibody in one or two animals in a group will require careful interpretation. The herd's vet after consultation with the laboratory may be justified in changing the herd's status from 'not-negative' to 'negative'. Assistance with interpretation can be found on the BVD CPD course ( www.scotland.gov.uk/bvd). Vets who have successfully completed the CPD course can sanction such changes in a herd's BVD status.

Other tests for BVD antibody

Other tests for BVD antibody (eg serum neutralisation test ( SNT)) are available and may be used occasionally to resolve unusual ELISA results.

Tests for BVD virus

Virus antigen detection ELISA

Samples will be reported as negative or positive for the presence of BVD virus. Only rarely will a sample be classed as inconclusive necessitating a retest.

A virus positive animal will usually be PI, but it is recommended that to confirm an animal as PI it should be sampled for a second time after at least three weeks. True PI animals will be virus positive in both tests. Any animal undergoing an acute infection at the first test will be virus negative and antibody positive at the second test.

BVD vaccination will not interfere with any test to detect BVD virus. Any PI animal that has been vaccinated against BVD continues to shed high levels of infectious virus and is unlikely to produce enough antibody in response to vaccination to block the detection of the virus in the blood.

Other BVD virus detection tests

Other tests to detect virus are available. The most sensitive of these is the reverse transcriptase polymerase chain reaction ( RT- PCR) which detects virus RNA. The test is used to detect virus in bulk milk from up to 300 cows, pooled blood samples and foetal tissues. There is some indication that pooling blood samples is not advisable in the very young calf and the age of calves less than two months of age should always be provided with the samples. It is advisable to consult with your laboratory on the age threshold for pooling before submitting samples.