Feb. 25 2017 08:00 AM

Steps can be taken to keep bovine viral diarrhea (BVD) out of your herd.

Miller is a senior professional services veterinarian-dairy, Boehringer Ingelheim Vetmedica Inc., St. Joseph, Mo. Ridpath recently retired from the ruminant diseases and immunology research unit, NADC/ARS/USDA, Ames, Iowa.
Animals that leave the farm and then return, from a custom grower or a cattle show, for example, should be quarantined for two weeks to help prevent the spread of BVD and other diseases.

Many dairy producers are under the false impression that all that is needed to prevent BVD (bovine viral diarrhea) from entering their herd is to vaccinate with a proven BVD vaccine. Although vaccination will help prevent acute disease and reduce the incidence of persistently infected (PI) calves, vaccination by itself will not completely prevent or control BVD. Similarly, testing and eliminating persistently infected cattle is only effective if a biosecurity program is in place that reduces the risk of the pathogens that cause BVD (BVDV1, BVDV2, and HoBi-like viruses) from being reintroduced into the herd.

Preventing disease entry is always a more cost effective approach than reacting to disease once it has entered a herd. No dairyman wants to learn that they have BVD within their operation, in particular, that a PI animal or PI animals are present. The intent of a biosecurity program is to reduce the risk of introducing or reintroducing disease into a herd.

Bringing home risks

An effective biosecurity program is based on periodic re-evaluations of practices as changes in procedure or management occur. Although intended to improve farm profitability or efficiency, these changes may have the unintended consequence of reintroducing pathogens that were assumed to be under control. For example, in the last several years, there has been an evolution from smaller closed herds where replacement heifers infrequently left the farm of origin to larger dairy operations where replacement animals frequently leave the home farm to be raised off-site at a calf and/or heifer grower.

In the aforementioned scenario, replacement heifers are often commingled with heifers from multiple operations and geographic locations. This “trafficking” of heifers invites risk of bringing several diseases back into your herd, including BVD. Most commonly, BVD returns to the farm of origin in the form of a PI calf or a pregnant heifer carrying a PI calf.

If your heifers are raised off-site, to help protect your home operation, only contract with calf/heifer growers who follow a strategic BVD vaccination program that utilizes an efficacious BVD vaccine and where a mandatory BVD-PI screening program is in place. The goal of a BVD screening program is to test and remove all PI animals prior to entry into a facility. Otherwise, BVD-PI positive animals lurking within the facility not only raise the risk of infectious diseases but can also lead to the formation of additional PI calves.

Carriers of the disease

How does PI calf formation occur? Most PI animals are the result of an acute infection of a pregnant naïve dam. Heifers are more likely to be naïve (unexposed) than older cows and thus the majority of PIs generated are the result of infections of first-calf heifers.

The most common source of BVD exposure to the pregnant dam is direct contact with a PI animal(s) while housed together in the breeding pen. However, PI animals may also occur when naïve dams are exposed to acutely infected cattle, tissues expelled during abortion or contaminated housing and equipment (see sidebar). While the BVD virus can infect the fetus at any stage of pregnancy and often kills the fetus, exposure at approximately 40 to 125 days of gestation can lead to the birth of a PI calf if conditions are ideal.

During this specific time frame, the fetus does not recognize the BVD virus as foreign since its own immune system has not yet fully developed. If a PI calf results from this exposure, the risk to the owner begins once the replacement heifer carrying the PI calf returns to the farm of origin. In addition to this example, another common scenerio that occurs is the purchase of pregnant heifers or cows that may be carrying PI calves. This is another means of bringing BVD onto a dairy operation.

There are four outcomes for a PI calf:

1. The calf appears normal and can live to adulthood if not detected and removed.

2. The calf appears normal but becomes an unthrifty poor doer that succumbs to disease in the first year of life.

3. The calf is born weak and small.

4. The calf is born dead (stillbirth).

A PI calf that is born alive can cause many problems. This calf acts like a BVD factory because it secretes virus in all bodily secretions 24 hours a day, seven days a week. These animals are infected for life and vaccination will not cure them.

Although these animals represent an economic loss to the owner, the real cost to an operation results from constant viral shed to unsuspecting herdmates. Exposed animals have to expend energy to respond to this constant viral challenge; energy that could be used for growth. The viruses that cause BVD suppress the immune system and the constant viral shed to penmates results in an elevated risk of diarrhea, respiratory disease, and pinkeye in these animals.

Most dairy producers are familiar with postvaccination milk production drops. The production drop is due to cows diverting energy from milk production in response to vaccination. Living with a PI herdmate is the equivalent of being vaccinated every day, 365 days a year. In addition, adult herdmates housed in close proximity of a PI animal or animals have a higher risk of uterus infections, mastitis, pneumonia, diarrhea, and may also experience reproductive inefficiencies.

Developing a biosecurity program relies on careful assessment of risks and reducing those practices that raise the risk of bringing BVD pathogens into the herd (see sidebar). In practice, a good biosecurity program would include the following safeguards as standard practices:

All new herd additions should be PI test negative prior to herd entry.

All animals returning to the herd, such as those returning from cattle shows, must be quarantined for a minimum of two weeks.

Pregnant heifers, purchased or returning to the herd from heifer raisers, must be kept quarantined from the herd until they have calved and the calf has tested negative for persistent infection.

Loaned equipment must be disinfected before being returned to premises.

Semen must be screened for BVD pathogens (BVDV1 and BVDV2 in the U.S.).

Embryo transplant procedures must use reagents free of BVD pathogens.

It has been stated that BVDV is the gift that just keeps on giving. When it comes to biosecurity, it is only as strong as the weakest link in the chain. Don’t let your guard down regarding biosecurity or BVD will find a way to get back into your dairy operation. Work with your herd veterinarian and implement solid biosecurity practices to protect your herd.

Risky practices that spread BVDV

  • Use of heifer raisers who do not have BVD biosecurity protocols in place
  • Purchase of bred heifers of unknown origin
  • Embryo transplant using contaminated fetal bovine serum (FBS) — particularly using FBS originated outside of North America or if implanting untested recipients
  • Contaminated semen
  • Shared equipment — stomach tubes, nose tongs, tattoo pliers, ear taggers, OB sleeves
  • Not using two weeks’ quarantine for animals that have been off-site (such as fairs and cattle shows)

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Final issue: Strengthen your BVD fight