Bovine viral diarrhoea virus (BVD) is a member of the Pestivirus family that primarily infects cattle, but can also be found in other species such as sheep, deer, camelids, and pigs. The first cases of BVD in New Zealand were reported in the 1960s, but the virus has likely been present for much longer. Previous research studies have estimated that 65% of beef herds and 35% of dairy herds are currently actively infected with BVD.
When susceptible (S) cattle are exposed to the virus, they become transiently infected (TI) for a period of 2 to 3 weeks. Most adult cattle will not show any obvious signs of being infected although they can still experience poor appetite, depression, diarrhoea, nasal discharge, decreased milk production, and reduced fertility for several days. The virus likes to multiply in lymphoid tissue (the cells responsible for making white blood cells), which can cause immunosuppression and increased susceptibility to other infectious diseases particularly in young calves. Once animals start producing antibodies to BVD, they rapidly recover and gain lifelong immunity to the virus. Any calves born to recovered (R) dams will get maternal antibodies (MA) to BVD through colostrum, which can remain in their bloodstream for up to 10 months. After this time, they become susceptible to the virus and can become infected after exposure.
Problems During Pregnancy
If a susceptible dam is pregnant when she becomes transiently infected, the virus can cross the placenta and cause damage to the developing foetus. During early pregnancy (days 0 to 40), the foetus may die and become mummified or aborted. If the infection occurs in mid-pregnancy (days 40 to 120) before the foetus develops a functional immune system, it may not be able to recognize the virus as “foreign” and can’t produce antibodies to clear the virus. These calves are born persistently infected (PI) with BVD and shed very large quantities of virus through saliva, faeces, milk, semen, and other secretions for the rest of their life. If the infection occurs in late pregnancy (days 120 to 280), the calf may be born weak or with birth defects. There are currently no reliable diagnostic tests that can be performed on pregnant cattle to determine if they are carrying a PI calf. These dams are called "Trojan dams" because they look normal on the outside, but are carrying a dangerous source of BVD on the inside.
Persistently infected (PI) animals are the main reservoir of BVD in cattle populations and are responsible for infecting most other susceptible animals in the herd. An estimated 1 to 3% of calves in an actively infected herd are expected to be PI and any calves born to a PI animal will always be PI themselves. Although PI animals often have many health problems such as poor growth, ill thriftiness, frequent infections with other diseases, and poor fertility, they can also appear completely normal and so you may not be aware that these animals are in your herd.
Most PI animals end up leaving the herd by 18 months of age due to either poor performance or the development of fatal mucosal disease (MD), which occurs when PI cattle become infected with a different (cytopathic) strain of the BVD virus usually between 6 and 12 months of age. Affected animals develop fever, depression, profuse bloody diarrhoea, and purulent discharge from the eyes and nose, which progresses to death within 5 to 10 days.
If the persistently infected (PI) animals are not removed from the herd fast enough or if the herd is exposed to virus from other sources during the risk period in pregnancy, there is a good chance that another PI calf will created and the herd will become chronically infected with BVD.