What is Pestivirus?
Pestivirus is an extremely common virus in cattle herds, and it can cause a range of disease syndromes including reproductive failure, birth of abnormal calves, respiratory and diarrhoeal disease. Cows first exposed to the virus and infected during early pregnancy may abort, have deformed calves, or give birth to calves that appear normal but act as infectious carriers – these persistently infected (PI) animals perpetuate the disease within the herd.
What is a PI?
A persistently infected animal, or “PI” is one that was exposed to the virus while in the uterus, at a stage when the developing immune system mistakenly assumes the virus is a normal part of the animal (between about 1 and 4 months of pregnancy), this means the calf doesn’t develop a normal immune response and the virus continues to live on in the PI undetected, reproducing and shedding in huge amounts for the entire life of the animal. These animals later develop the clinical signs of Mucosal disease which causes chronic, antibiotic non-responsive diarrhea and ultimately death.
‘Classic’ PI calves are poor-doers and:
- Are responsible for most of the production-limiting outcomes of pestivirus
- A reservoir for year to year re-infection
- Often carry the infection between properties
Transiently infected animals will also shed virus, but for only a limited period of time, about 2 weeks.
Testing
Serology: Blood testing
Serology tests for antibodies produced by the animal as a result of Pestivirus infection. It can take up to 2 weeks from initial infection for antibodies to be produced, and these will remain for years after the virus has been eliminated.
A seropositive result means the cow has been infected at some time, but does not reveal whether it is currently infected with or shedding virus.
A seronegative result means the cow has never been infected with the virus.
NOTE: A PI animal will also be seronegative – because the immune system thinks the virus is a normal part of the animal’s body, antibodies are never produced.
Virology: Ear-notch testing
Virology detects the presence of the virus itself. The small vessels in the ear notch are analysed for the presence of virus particles. Samples are sent to a pestivirus-specific lab in Western Australia.
The lab will reports results as:
- Strong positives – extremely likely to be exposed to virus CULL or can retest in 2 weeks to be sure they are not transiently infected.
- Weak Positives – may be low shedding PIs or transiently infected retest in 2+ weeks to determine persistent from transiently infected.
- Negatives – not currently infected with the virus, but may have been in the past (in that case will be blood test positive)
Management
Management is centered on location and removal of PI animals, combined with a vaccination programme for the rest of the herd, and there are various approaches which can be taken to achieve this aim. If pestivirus is a problem in your herd, or you would like further information on Pestivirus–please feel free to contact us at the clinic.