Bovine viral diarrhoea virus (“pestivirus”) is one of the most significant viral diseases in cattle. Clinically, there are three forms of the disease:
- A persistently infected (PI) form which may or may not have clinical signs
- An acute transient form characterised by fever and diarrhoea and short term immunosuppression. These animals will mount an immune response and clear the virus in 10-14 days.
- Mucosal disease (MD) only occurring in PI animals. PI animals are infected by a non-cytopathogenic strain of the virus. A subsequent spontaneous mutation of the virus to a cytopathogenic strain within the PI animal results in MD, characterised by sero-mucoid nasal secretions, severe erosive lesions in the oral and intestinal mucosa, diarrhoea and death.
A key element for persistence of infection in cattle populations is the ability of the virus to cross the placenta in non-immune animals and infect the foetus. Infection of pregnant cows has different outcomes depending upon the gestation period:
< 40 days
> 150 days
Foetal abortion or birth of persistently infected calves immunologically tolerant to the virus (virus positive and antibody negative).
Foetal abortion or birth of calves with congenital defects, especially of the CNS, such as cerebellar hypoplasia (usually antibody positive with or without detectable virus).
Birth of normal calf which may be small and weak, but has a competent immune response (antibody positive and virus negative).
Infection is widespread in Australia. Persistently infected (PI) animals are considered the most important source of infection in a herd, as these animals excrete large amounts of virus continuously throughout their lives from nasal discharges, saliva, semen, urine, tears, and milk.
Disease often occurs when a susceptible animal is introduced into an infected herd or a PI animal is introduced into a susceptible herd.
Gribbles Veterinary Pathology is able to detect BVD virus in blood, milk and skin using the following tests:
- Reverse transcription polymerase chain reaction (RT-PCR). PCR is a sensitive methodology and individual or pooled serum and milk samples can be tested.
- Antigen capture ELISA (PCR is preferred due to greater sensitivity)
- Antibody ELISA to detect BVDV antibodies in serum and milk.
Defining the BVDV status of animals in a herd and identifying any PI animals are the first stages toward controlling or eradicating the disease. This can now be done economically and rapidly using a combination of RT-PCR, antigen and antibody ELISA tests.
1. To see if BVDV is present in the herd
- Lactating animals: collect a bulk milk sample from the vat and test for antibody by ELISA.
If the antibody result is high, a PI could be present and a milk sample should then be tested by RT-PCR. If a milk sample is positive on RT-PCR, stratify the herd by production and serum sample the poorest performing 10% of the herd first. Test for virus by RT-PCR, eliminate any PI animals, then recheck another milk sample by RT-PCR.
- Non-lactating animals/non-dairy: collect serum samples from 15 animals (nine minimum) in each age group of cattle you wish to investigate, and test for antibody by ELISA (15 serum samples are pooled together in the laboratory and tested giving a 95% chance of finding a seropositive animal). If negative and BVDV infection is strongly suspected, test all animals by pooled antibody ELISA samples.
2. To identify a PI animal in a seropositive herd, or where a PI is suspected in the herd
- Lactating animals: collect a bulk milk sample from the vat and test by RT-PCR to confirm a PI is present. If a milk sample is positive on RT-PCR, stratify the herd by production and serum sample the poorest performing 10% of the herd first. Test for virus by RT-PCR, eliminate any PI animals, then recheck another milk sample by RT-PCR.
- Non-lactating/non-dairy: Take serum samples from all cattle. Gribbles Veterinary can pool the submitted serum samples into batches of 20 to be tested by RT-PCR. When a positive pool is found the samples making up the pool will be tested individually. This will identify both PI and transiently infected animals. Skin can also be tested by PCR.
- Suspected PI animals can be tested individually by RT-PCR
- If all serum pools or a milk sample from a herd test negative by BVDV RT-PCR, then the animals sampled can be considered clear of infected animals and a biosecurity/vaccination program put in place.
- Annual herd testing by antibody ELISA (individual serum samples from 15 yearling animals) and a bulk milk antibody test on the lactating animals.
- Virus screening of all keeper calves by PCR or antigen capture ELISA
- Pre-testing new introductions to the herd for virus, with possible vaccination for on-going protection.
- Quarantine a newly bought pregnant cow until she calves because her calf could be PI even if she isn't (so-called Trojan calf). Then test the calf for virus, once born.
- Serum (1 - 10 ml), preferably in a gel tube.
- Milk sample – 50 mL in a sterile container.
- Serum or whole blood
- Milk sample - 50 ml in a sterile container
- Skin – > 3mm kept fresh in a sterile container (eg: ear notch)
- Serum or whole blood
- Skin - > 3mm kept fresh in a sterile container (eg: ear notch)
Plain or gel tube (serum)
EDTA (whole blood)
Plain yellow-top pot (skin tissue, milk)