Think like a pig farmer

The key to successfully controlling BVD in a herd is to get rid of Persistently Infected (PI) animals, vaccinate, and, think like a pig farmer. The former removes the source of infection, vaccination reduces the effects of disease, and the latter prevents re-infection through top notch biosecurity.

Intervet’s recent BVD Symposium in Hungary revealed that the Scandinavian countries were able to eradicate BVD through following such a systematic approach, once herd BVD status had been established using bulk milk tests to detect antibodies. But the crucial final part in a control programme is to operate sufficient biosecurity standards to prevent re-infection. And according to Dr Jeff Laureyns, from Belgium’s Department of Veterinary Medicine, Ghent University, biosecurity means providing washable clothing for farm visitors, quarantining new stock, and preventing nose to nose contact with stock on neighbouring farms.

“Farmers should have boots and overalls in several different sizes, that can be washed and disinfected between use, for the people coming on farm: the vet, AI technician, foot trimmer – just like on pig and poultry units,” he said. “People are a major source of spreading BVD infection, however, it also helps reduce the spread of other diseases such as Johne’s. It isn’t necessary to have a wheel wash for vehicles, but stop livestock lorries from coming into the farm – keep them at the boundary.” On farms with a regular collection of calves, Dr Laureyns suggested converting calf hutches to make them mobile and use these to transport calves to the farm gate, where they can stay until pick-up.

“Quarantining new stock until blood test results come back (showing that animals are free from disease) is very important. Buying in freshly-calved animals isn’t easy as they have to be milked, however, they could be milked last and should be housed so that nose to nose contact with the rest of the herd is avoided,” he said.

Another method to reduce transfer of infection is to use disposable needles and syringes for all injections. While he agreed this is expensive and not very environmentally friendly, Dr Laureyns pointed out this must be weighed up against the cost of treating cows for BVD and the number of culls involved. “That’s not environmentally friendly either,” he added.

With 90% of Britain’s dairy herds, and 60% of cows now infected with BVD, the initial cost of severe and obvious effects of first infection are soon forgotten. Yet producers should take note of French statistics showing the annual cost to their business from long lasting infection and the consequences of undiagnosed extra cases of mastitis or abortion. “The virus is very clever [at survival] and can stay in a herd for more than 10 years. There is also a high risk of re-introduction to a BVD-free herd, so we need to assess the effects in cases of long-lasting infection,” Dr Christine Fourrichon, from the Veterinary School in Nantes, told Intervet’s BVD Symposium.

Using data from 7000 dairy herds in Brittany, Dr Fourrichon revealed the damage from underlying poor performance. She reported up to 0.6kg/day of milk lost in the year following infection (see Table 1), 11% of embryo loss after service and cell counts up to 25,000 cells/ml higher. “The yield effect is still significant and the effect on cell counts lasts at least two years, not just after the initial infection. Even though we assume a large majority of animals have immunity, there is still a decrease in performance. The average economic losses associated with BVD infection where the virus is present are comparable to those associated with mastitis: the farm gross margin fell by 11 euros/1000 litres and the average cost of mastitis is 11 euros/1000 litres. Vaccination against BVD won’t eliminate these costs, but it will reduce them.”

The drop in herd performance (Table 2) is not only due to the effects of the virus, but other diseases on the farm to which the infected cow becomes more susceptible, added Dr Fourrichon. With greater culling rates in infected herds, and quota trading prohibited in France, French farmers rear more heifers to allow for losses yet still fill quota, she said. This means that variable costs increase and opportunity costs begin to appear in budgets. More land is needed to rear replacements and although on a family farm there is no salary value, farmers end up with less family and free time.

TABLE 1

Production effects of BVD infection in a dairy herd.

TABLE 2

AVERAGE EFFECT IN PERSISTENTLY POSITIVE HERDS

+ 1 abortion

+2% more AI returns

+ 27,000 cells/ml SCC

-8500 litres total herd (40 cows)

+1 more case retained placenta

+ 3 more cases mastitis

+ 5 more calf deaths

+ 8 more heifer treatments