The dairy industry continues to change as the number of farms are decreasing and the number of cows per farm are increasing. Introducing new animals to a herd can increase the risk for a bovine viral diarrhea (BVD) persistently infected (PI) animal to enter the herd and negatively impact reproduction, health and performance.

This makes biosecurity increasingly important. However, fewer than 20 percent of farms have established biosecurity protocols for new animals entering the herd, according to the National Animal Health Monitoring System (NAHMS).

There is a new approach that makes BVD monitoring easier and more cost-effective, thanks to the sensitivity of a real-time reverse transcriptase polymerase chain reaction test. In lieu of sampling individual cows, dairy producers can take pooled bulk milk tank samples.

Because PI cows shed a large amount of BVD virus in milk, the PCR test effectively identifies BVD in a bulk tank milk sample. This is a good first step in identifying the presence of a PI animal in the milking herd and an economical way to periodically monitor the herd to assure a negative status.

BVD monitoring pays dividends
Identifying and removing PI animals from milking strings can increase productivity between $20 to $88 per cow per year by reducing negative effects of this endemic, subclinical disease in a herd. Negative effects include impact on reproduction, mastitis, culling rates, slaughter value and mortality, as well as reduced milk production.

A positive bulk milk tank sample can be reflective of the number of calves that are born persistently infected. Identifying and removing these calves at birth, as well as removing adult PIs, would eliminate the primary source of BVD in dairy herds.

Incidence highest on large dairies
In 2007, NAHMS conducted bulk milk tank tests in 527 dairies across the U.S. and found a 1.7 percent positive BVD rate. Large dairies (more than 500 cows) had a positive finding 12.8 percent of the time.

Small dairies (fewer than 100 head) had no positives, and medium dairies (101 to 499 cows) tested positive 3.5 percent of the time. Regionally, there was a higher percentage of positive results in Western dairies (7.7 percent) versus Eastern dairies (1.1 percent).

In 2011 to 2013, Merck Animal Health – in conjunction with Animal Profiling International, a testing laboratory in Portland, Oregon – performed a similar bulk milk tank testing program targeting mostly large dairies (more than 500 cows) from Vermont to California.

Bulk tank samples were collected from dairies in 18 states: Arizona, California, Florida, Idaho, Illinois, Indiana, Iowa, Kansas, Minnesota, New York, Ohio, Oregon, Pennsylvania, South Dakota, Utah, Vermont, Washington and Wisconsin. Veterinarians and herdsmen collected samples and submitted them to Animal Profiling International for processing.

The actual size of the dairies represented ranged from 45 to 15,000 cows. Some dairies submitted quarterly samples to help ensure all cows in the herd would have at least one sample tested during the course of the year. A data sheet, which accompanied the first sample, included key questions related to management, herd health and BVD control measures.

In addition, individual samples from neonatal calves were collected from some of these same dairies, as well as regional calf facilities over time. These samples were tested in a pooled fashion, and individual BVD- positive animals were identified.

PI animals have profound effect
The program included 182 dairies, of which 18 percent tested positive for BVD. Looking just at the 118 large dairies (more than 500 cows), the testing revealed 28 percent were positive for BVD. Positive BVD findings were 23 percent for Western dairies and 8 percent for Eastern dairies.

The results of the study showed an increased rate of positive BVD PI animals across all tested dairies, which was appreciable compared to the 2007 NAHMS test. Some of the dairies in the survey conducted multiple tests over time to reflect a more representative population. Of those dairies, one in 10 tested negative a single time but then tested BVD-positive on a subsequent test.

The incidence of PI calves was reflective of the percentage of positive bulk tanks by region. Where positive bulk tank tests are found, the number of calves born persistently infected varied from less than two out of 1,000 to levels approaching five out of 1,000.

Although these numbers may seem small, PI calves can have profound effects on their developing cohort, and in group-management conditions they have the potential to affect exponentially more animals.

Conclusions
With the advent of real-time reverse transcriptase polymerase chain reaction testing for BVD, sampling costs are much less expensive.

We can now test the whole herd or groups of calves by pooling individual samples for screening. This has allowed us to “pull back the covers” on a subclinical disease that manifests itself in insidious and economically adverse ways.

Eliminating BVD from our dairy herds requires effective vaccination programs, reasonable biosecurity to reduce reintroduction and cost-effective monitoring and management. Only by combining all of these factors can we obtain the benefits of increased production efficiency and, thus, a higher return on investment.

Future articles will chart the costs, return on investments and do’s and don’ts required to deal with those herds that have been found BVD-positive. PD

Tom Shelton, DVM, M.S., is a senior technical services veterinarian for Merck Animal Health from Washington, Utah. He specializes in calf health and performance, vaccinology and immunology. Michael Coe, DVM, Ph.D., is the vice president of animal health for Animal Profiling International from Logan, Utah. He specializes in health management, nutrition, microbiology and production medicine.

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Tom Shelton
Senior Technical Services Veterinarian
Merck Animal Health