We all have watched the dairy industry feel the pressure of increased input costs. Maintaining profitability is even more challenging in times of rapid changes. Producers need to continue to find areas where they can make improvements that increase their margins. Disease control is critical to maintain optimal milk production and reduce added costs associated with treatments, loss of production, early culling and death loss.

One of the major diseases in need of attention is bovine viral diarrhea virus (BVDV or just BVD), which continues to cause losses between $20 and $160 per adult cow in the average dairy. The reality is well accepted and documented that the persistently infected (PI) animal is the main reservoir and culprit for continued spread of the virus to the herd.

The challenge is that many producers believe that PI animals are not present in herds because of widespread use of vaccination programs to control the spread of BVD virus. Even though many vaccines have label claims for various levels of fetal protection to reduce the formation of PI calves, we are still finding PI animals in vaccinated herds. Although vaccination is a critical part of a BVD control plan, vaccination alone is not sufficient to eliminate the risk of BVD causing problems in your dairy herd. BVD control involves proper vaccination, PI animal testing and a biosecurity program designed to keep PI animals from entering the herd.

BVD basics
BVD was first identified in the U.S. in 1946. A large range of clinical signs associated with BVD infection includes subclinical to severe acute disease. Noticeable signs include fever, loss of appetite, respiratory and gastrointestinal symptoms, infertility, increased embryonic mortality and fetal death, mummification and abortion. This obviously will have negative production effects reflected in young stock as increased sickness, increased mortality, reduced performance and higher culling rates. In adult cattle this is expressed as increased abortion rates, reduced conception and pregnancy rates and reduced milk production.

Maybe as important as any of these overt signs are the immunosuppression caused by the virus that can increase a whole range of problems including mastitis, lameness and predisposition to other pathogens. This virus can affect animals of all ages but also can infect fetuses still in the uterus.


How a persistent infection forms
If the BVD virus infects a pregnant cow and is passed through the placenta to the calf before 125 days in gestation, the developing calf’s immune system does not recognize it as “foreign” and allows the virus to maintain in the calf and reproduce the virus at high rates because the immune system is fooled to believe the virus is normal. This is what is called persistently infected (PI). Once an animal is born, it is either a PI animal or not. An animal cannot become a PI animal after birth. The PI animal will shed huge amounts of virus every day of its life to its herdmates. Approximately 50 percent of PI animals die before they reach mature age, but the ones that live are responsible for the major part of new infections.

The prevalence based on testing done by Animal Profiling International is currently about three in every 1,000 (0.3 percent) calves born on dairies are PI animals. This rate of PI animals is higher than earlier estimates of 0.11 percent. This may not sound like a high number, but the amount of virus that is shed daily by just one animal can be devastating to a dairy operation. The number of Western dairy operations identified as having a PI animal born is greater than 50 percent using current calf ranch data where we are able to trace the source dairy.

Why are PI animals increasing?
As we investigate the reason for an increase in PI percentage, we see two major reasons. First, because of the expansion in the dairy industry, dairymen, especially in the West, are buying animals from unknown and multiple sources to increase the number of milking animals in their herds. This reduced focus on biosecurity has allowed visually normal PI animals to enter a herd and shed virus. This agrees with the 2007 National Animal Health Monitoring System’s Dairy 2007 study where only 23 percent of dairies required any disease testing prior to introducing new cattle to the herd. In many cases the pressure from a PI animal overwhelms even vaccinated animals’ ability to stop disease or infection.

Are Western dairies alone experiencing the costs of increasing BVD PI incidence? No, yet we have determined that BVD testing is significantly less common at these facilities than on smaller operations or in the Midwest and East. Continued testing and biosecurity is needed in all dairies because the cost is the same to everyone if they introduce BVD.

Preventing the introduction of PI animals is as simple as testing them on arrival, or better yet, requiring that animals be tested BVD PI-negative prior to purchase. A dairyman must understand that a PI test on a bred animal only determines the status of the adult, not the fetus. Testing of newborn calves on the dairy is still important to eliminate PI calves. We estimate testing on positive herds for BVD PI animals return a minimum of $4 to $8 per head in cost-savings and added-value above the cost of the test.

A second change and risk related to the incidence of PI animals is associated with the number of heifers that are raised and bred off-site from the dairy. In Western operations 11.5 percent are raised off the dairy. Professional heifer developers are growing an increasing number of heifers for dairymen to allow them to maximize their permits with just milking animals. This commingling of heifers from multiple sources increases the likelihood of exposure to a PI animal.

As heifers are bred in larger pens with multiple sources, having one PI animal present in the pen can cause increased number of PI calves born. So even if you send your negative- tested PI heifers to a breeding facility, if all heifers have not been tested negative they could have an increased risk of acute infection from a PI penmate. The heifer could then come home and have a PI calf born which sheds virus to animals, starting in the fresh pen. In our (API) data, fresh heifers have the highest incidence of PI calves.

We need to get aggressive at identifying those herds that harbor BVD PI animals. Vaccination is an excellent adjunct approach for control, but testing and surveillance will be required to reduce this insidious problem. If all newborn calves are tested and PI calves are removed before exposure to the breeding herd, the number of additional PI animals will be reduced. New testing protocols are now available utilizing PCR technology that is accurate and affordable. A simple ear notch can be used to identify a PI calf as early as one day of age.

In the dairy industry we need to always look for ways to be more efficient. Having a good BVD control plan involving proper vaccination, PI animal testing and a level of biosecurity that keeps PI animals off the dairy is critical to reduction of disease risk from BVD. Work with your veterinarian and management team to implement a program that keeps the risk of costly BVD virus from your herd. Now is the time to get started. The tools and knowledge are available. PD

References omitted but are available upon request at editor@progressivedairy.com

Bruce W. Hoffman
Animal Profiling International, Inc.