What causes BVD?
It is an endemic disease (regularly found among particular people or in a certain area), is widespread and has a great economic impact.
The virus has two genotypes (the genetic constitution of an individual organism), BVDV-1 and BVDV-2.
There are also two biotypes (a group of organisms having an identical genetic constitution).
The two biotypes are cytopathic (producing damage to living cells) and non-cytopathic (not producing damage to the cells), based on the occurrence of apoptosis in infected cells (Grummer et al. 2002).
What is a naive cow?
A naive cow is a cow that has never had BVD before.
What is a TI cow?
A TI cow is a transiently infected cow. When a naive cow gets the BVD virus, they are called transiently infected cows, a TI cow.
During this time when a naive cow gets sick with BVD, it can have symptoms such as:
High somatic cell count
Abortion - If she is pregnant at the time of infection, she could abort the foetus
If she is pregnant she could also go on to have an abnormal calf, experiencing birth defects. An example of this is cerebellar hypoplasia - you can read a case report on cerebellar hypoplasia in BVD here.
Immunosuppression (a reduction of the activation or efficacy of the immune system). BVD makes animals more prone to other infections like salmonella, mastitis, infectious bovine rhinotracheitis (IBR) and respiratory diseases.
Remember that some cows may be sub-clinical (relating to or denoting a disease that is not severe enough to present definite or readily observable symptoms).
Unless the cow picks up a secondary infection and it causes a more lengthy recovery, a TI cow should get better from BVD within 2 or 3 weeks when the immune system helps to clear the infection.
After the initial infection, BVD virus is cleared from the host within 2–3 weeks and this is referred to as transient infection (Liebler-Tenorio et al. 2004).
When you test the milk or blood from a cow that has had BVD, you can measure the antibody levels to establish the level of immunity the animal has to the disease.
Immunity for these animals who have been infected can last at least 3 years (Fredriksen et al. 1999). The reason you want to avoid infections is that it is costly to your farm.
NOTE: a TI cow doesn't shed a lot of the virus to make other cows sick, while it is possible that naive cows can get BVD from TI cattle, naive cows are much more likely to become infected and become a TI from being in contact with a cow that is persistently infected (PI).
Compared to PI animals, transiently infected (TI) animals are considered to have a limited impact on BVD virus transmission, as several studies found that TI animals cannot propagate BVD virus in a herd, presumably due to the short duration and smaller amounts of the virus being shed (Niskanen et al. 2002; Niskanen and Lindberg 2003).
So what is a Persistently Infected (PI) cow?
A PI is a persistently infected cow, it is born this way and will die this way.
A PI cow will have become a PI while inside its mother. Its mother would have been a transiently affected cow who was pregnant and would have been infected with the BVD virus at 40 to 150 days (roughly over 1 month to 5 months gestation).
During this time, the foetus' immune system would have been developing and if the immune system made the mistake of identifying the BVD virus as part of itself, a persistently affected or PI calf is born.
This PI calf excretes high levels of BVD virus all through its life in saliva, faeces, mucous and body tissues.
If you were to test the blood, milk or skin of the PI calf/cow, you would find high levels of the virus but no antibodies because its immune system never learned to fight it, it sees it as part of itself.
PI cattle are a real problem because they infect naive cows through direct or indirect contact.
Introduction of a PI animal can result in devastating consequences in a naïve herd as the virus can spread rapidly resulting in up to 97% of susceptible cows seroconverting (Houe and Meyling 1991).
You might think that if you had a PI in your herd, other cattle would just become immune, but this is not the case, there are always naive cows that will suffer a transient infection.
Scours and pneumonia cases are common in infected calves because of their weakened immune system due to BVDv.
Most PI cattle die before they are 2 years old either because they are immunosuppressed and have picked up an infection from another source, or they may die from a fatal condition called mucosal disease (MD) when the virus turns cytopathic.
Fatal Mucosal Disease - this is when the non-cytopathic strain of BVDv changes into the cytopathic strain where it damages cells. For more detailed accounts of mucosal disease in a paper by Lanyon et al (2014) click here. There's also an older paper by Tautz et al (1999) here.
Even though PIs are usually unhealthy, poor doers and will die, sometimes that's not the case, some do live normal lives.
It is important to be aware of this and control it because if you don't, PIs will infect your herd causing illness, costing money and impacting welfare.
Active BVD control is necessary to avoid this costly disease.
To read about controlling BVD click here.
Han et al. 2018. Elimination of bovine viral diarrhoea virus in New Zealand: a review of research progress and future directions. (References for quotes were extracted from this paper and will be found here)
Lanyon et al. 2014. Bovine viral diarrhoea: Pathogenesis and diagnosis. The Veterinary Journal. https://www.sciencedirect.com/science/article/pii/S1090023313003614
Tautz et al. 1998. Pathogenesis of mucosal disease, a deadly disease of cattle caused by a pestivirus. Clinical and Diagnostic Virology. https://www.sciencedirect.com/science/article/pii/S0928019798000373