Helping heifers grow
Dairy farmers can easily track the performance of their replacement heifers and ensure they reach their genetic potential.
BVD’s most costly impacts are on pregnant cattle and their unborn calves, and it is estimated in recent economic modelling that the cost of a BVD outbreak in a naive dairy herd is set at $22 per mixed age cow per year.
When pregnant cows are infected with BVD prior to 35 days of gestation, the pregnancy is usually lost, which impacts next season’s calving spread and/or days in milk.
Between 35 and 125 days of gestation, BVD infection most often causes the calf to become persistently infected with the virus (a “PI”); PIs go on to shed the BVD virus in their saliva, faeces and reproductive secretions for their entire lives, and are the source of future BVD transmission to other naïve cows or herds. Non-pregnant animals are also affected by BVD exposure and infection, young stock that are infected have reduced growth rates and animals in the milking herd that are exposed can have reduced milk production. Affected cattle are also more likely to be susceptible to other diseases, therefore a BVD incursion affects all animals on the farm.
When a pregnant cow is infected with BVD beyond 125 days of gestation, sometimes the calf is born normal, but it could also suffer from birth defects, such as stunting, eye abnormalities and immune suppression, which may limit its productivity and increase the risk of being culled.
So, protecting pregnancies is critical for BVD control; there is never a good time for a pregnant cow to get BVD. You can protect pregnant cattle with vigilant biosecurity, or by vaccinating cows and heifers prior to mating with a BVD vaccine that has a label claim for fetal protection against BVD.
Recent work revealed that 40% of New Zealand dairy herds monitoring bulk milk are considered “actively infected” with BVD, meaning that more than about 20% of the milking cows have had recent exposure to the virus. Often, the source of the recent BVD exposure isn’t obvious, e.g. no PIs are in the milking herd to be easily identified and culled. Instead, most dairy farms have biosecurity risk factors that allow intermittent BVD exposure from outside the milking herd. Examples include heifers away grazing, over-the-fence contact with other cattle and onto-farm cattle movements.
Biosecurity for pregnant animals means preventing them from having contact with any cattle of an unknown BVD status (i.e. any potential PIs).
To ensure good BVD biosecurity, all animals which pregnant cattle have contact with should be BVD virus tested negative.
Also implement barriers, such as boundary fences with outriggers, and always separate pregnant cattle from untested animals.
If you share yards or equipment with cattle of an unknown BVD status, cleaning and disinfecting between mobs, and spelling facilities you can’t disinfect for 7 days will minimise the risk of BVD transmission.
Most farms in New Zealand are not biosecure, in fact from New Zealand research into New Zealand risk factors for BVD, only 2.1% of farms in New Zealand were completely closed (MSD funded research).
This means there is risk for diseases, including BVD, to be introduced onto farms. If strict biosecurity isn’t always possible, nor is testing and culling of all PI calves before the planned start of mating, then vaccinating cows and heifers prior to mating each year with a BVD vaccine is the best option. A vaccine that provides fetal protection will prevent transmission of BVD virus through the placenta to the calf, which means that even if the dam is exposed to BVD during pregnancy, the pregnancy should be protected.
Bovilis BVD vaccine has demonstrated six months of fetal protection following the initial sensitiser and booster, and 12 months of fetal protection following a third dose (for example, an annual booster). This is the longest demonstrated duration of fetal protection available in New Zealand. Twelve months of fetal protection covers the herd’s entire risk period for PI formation and continues to protect pregnancies from the late gestation effects of BVD.
Furthermore, vaccinating so your herd has 12 months of fetal protection provides flexibility around the timing of the annual booster; if you need to give your annual herd booster pre-calving rather than pre-mating, the subsequent mating season’s unborn calves will still be protected from BVD.
There’s never a good time for a pregnant animal to get BVD; protecting pregnant cattle throughout gestation with biosecurity and/or vaccination, and testing calves is the only way to break the cycle of BVD transmission in the long-term, and to minimise the impact of BVD exposure now.
Dr Penny Mehrtens is a veterinary advisor, for MSD Animal Health
