Infectious diseases are not at the top of the list of dairy and beef farmers’ concerns. That is especially true with the weather/climate and trade issues that seem unrelenting. Almost 30 years ago, the Production Limiting Disease Committee managed to work with university researchers to estimate the extent that four infectious diseases occurred in Canada’s dairy and beef cow-calf herds. The four diseases were BVD (Bovine viral diarrhea), Johne’s Disease, Enzootic Bovine Leucosis (EBL) and Neospora. Many provinces developed on-farm programs to address Johne’s Disease and several are now considering if there is enough interest in a program to attempt to control EBL.
Farmers in many European countries have a very different view of major infectious diseases. For example, farmers in several countries have made a decision to eradicate BVD. With the help of government, they have managed to achieve eradication in at least eight countries. The BVD virus is carried by cattle that were infected before they were born: They are called persistently-infected (PI) carriers. BVD will be eradicated if you find and remove all the PI carriers.
Even though there doesn’t seem to be much interest in eradicating BVD in Canada, we can learn some lessons about how BVD works from the eradication programs in the EU. Ireland recently shared what they learned from their eradication program. Their approach to eradication was similar to the method adopted by Switzerland — they tested all cattle in the country and came back to test the calves born to the cows that were pregnant during the first test.
Ireland has a large beef and dairy cattle population on both large and small farms. Irish agriculture is not exactly the same as Canada’s but it is still worthwhile to be aware of the lessons they learned.
They found that roughly 20 % of farms had at least one PI carrier. This was at the lower end of the estimate of the number of dairy herds that likely contained PI carriers in Canada back in the early 2000s. Part of that difference may be due to the fact that the surveys in Canada did not actually test for PI carriers. They just looked to see if there was evidence of natural exposure to BVD virus in the dairy herds. In Ireland, they actually tested every animal in the herds so they knew that PI carriers were present at the time of the test. The Canadian method would have been less accurate because it looked at exposure that might have occurred over several months.
The majority of herds in Ireland that contained PI carriers only contained one carrier. We have no data about the number of carriers in herds for Canada but my personal experience is that there are usually few carriers in a herd — it just appears to be that you need many to have an impact on the health and productivity of the herd. It helps to understand when you start testing for PI carriers that there are likely only a few in the herd. I have seen farmers who get upset when we only find one or two carriers. It helps if you know that from the outset.
In Ireland, they found that ‘Trojan’ cattle commonly introduced BVD into herds. Trojan cattle are cows or heifers that are themselves healthy but are pregnant with a PI carrier. There is no way to test those pregnant cattle before they give birth — so biosecurity rests on either testing calves of purchased pregnant cattle or in being sure that the cows have been properly vaccinated before they were bred.
The Irish eradication program made two other important discoveries. The first was that all PI carriers should be removed to slaughter as soon as they are found. Delaying removal increased the risk that another PI carrier would be born later. The second was that herds adjacent to a farm with a confirmed carrier were more likely to also end up with a carrier. In other words, BVD likely spreads across property lines by cows touching each other across fence lines.
More countries are likely to eradicate BVD in the EU and, as a result we are likely to increase our knowledge of how BVD acts in cattle populations. That could help us if we decide to act against BVD as an industry.
