Susan Wilson-Sanders, DVM, MS, DACLAM, Emeritus Professor, University of Arizona; Owner, Lazy S Ranch Willcox, LLC
BVD is a viral disease with worldwide distribution. Infection occurs in two forms: acute, where the animal sheds virus for a short period of time and recovers, or the persistently infected (PI) form from which the animal never recovers and sheds the virus throughout its lifetime. The most severe impacts occur when a pregnant cow is infected, as the calf may also become infected. The outcomes of fetal infection are based on the immune status of the cow and the stage of gestation during which she becomes infected. Potential outcomes include failure to conceive, abortion, congenital malformations in the calf, and most significant for the herd—fetal infection leading to persistent infection. A 2016 report in Beef Magazine indicates that losses due to persistently infected animals cost the American cattle industry $1.5-2.5 billion/year. Animal welfare is also affected through illness, death, and euthanasia of infected animals. Control of BVDV (the virus) has been difficult due to viremia (virus in the blood) remaining constant in the acutely infected individual coupled with the continual viral shedding from PI animals. PI animals must be recognized and eliminated from the herd to block continual disease transmission.
In cattle, transmission occurs in utero (leading to PI) and also through nose-to-nose or sexual contact with PI or acutely infected bovines. Other means of transmission include insect vectors, contaminated equipment or pens that have not been properly disinfected, and human “fomites”, such as hands and clothes. When exposed, BVDV naïve animals develop a short-term infection lasting from 10-14 days. Clinical signs observed during acute infection include decreased appetite and milk production, depression, diarrhea, ocular and nasal discharge, and oral ulcerations. Acutely infected bulls have changes in the semen, leading to lowered conception rates and transmission of virus to cows. BVDV can cause respiratory infections and contribute to enhancement of the Bovine Respiratory Disease (BRD) Complex. However, many animals will show minimal clinical signs, and the producer will not know that the virus is in the herd.
PI occurs when a pregnant cow is exposed to the virus during early to mid-pregnancy. The virus avoids the calf’s immune system, and the calf will tolerate the virus, becoming a PI animal. PI animals may appear clinically healthy; others will be small and weak. They will show reduced weight gain with stunted growth. These “poor-doing” calves are often culled early to the sale barn, where they pass infection to healthy animals destined for farms, ranches and feed lots--spreading the disease to naïve populations. Most PI’s have shortened life spans; however, about 30% of them survive to 2 years or more, allowing them to continue to infect clean animals.
A variety of diagnostic tests are available to help identify infected animals. Tests are based on either recognition of BVDV specific antigen (virus) or BVDV specific antibodies produced by the body following exposure to the virus. When exposed to the viral antigen, the animal launches a response through the body’s immune cells to create an antibody response to fight the virus.
Antigen recognition tests identify live virus in tissues, blood, or milk. These tests recognize that the virus is CURRENTLY present in the animal. One such test is the RT-PCR test for BVD, which is highly sensitive. Another, highly sensitive and efficacious test is IDEXX Laboratory’s SNAP test, which recognizes the virus in blood or tissue samples--typically used with ear notch samples. Test costs range from $5-$10/animal.
Antibody detection methods recognize previous exposure to the virus. In an unvaccinated animal, a positive test indicates previous exposure and that the animal is not a PI. In a pregnant cow, a positive result means that the cow may be carrying a PI fetus. Unfortunately, a negative Ab outcome does not mean the animal has never been exposed to BVDV. Additional viral antigen testing must be performed to determine PI, as PI animals do not mount an antibody response because of their immune tolerance to the virus. When considering antibody results, if the population is predominately negative, there are, most likely, no PI animals in the population. While this is a very beneficial result, it also means the population is at high risk for infection and severe consequences if BVDV enters the population. These herds will greatly benefit from vaccination programs. Conversely, a high number of positive animals indicates high probability that the population is infected; thus, containing one or more PI animals. These herds need to be culled of positive animals with continued retesting until all PI’s are eliminated. Simultaneous vaccination of the infected herd should begin and continued after PI animals are eliminated. Highly positive herds serve as a major risk to any clean animals: the home herd, neighboring herds, to sale yards, backgrounders, or finishers where PI animals meet their final destiny.
Producer Steps for Recognition and Eradication
Effective use of vaccines has significantly reduced the financial impacts of BVDV. A 2015 study found that regular vaccination of breeding cattle decreased abortions by 45% and yielded an 85% decrease in fetal PI’s in comparison to unvaccinated cattle. Clearly, vaccination is vitally important in preventing the reproductive impacts of infection with BVDV and help block the BVDV cycle in the herd. Consultation with your veterinarian regarding use of killed and/or Modified Live (MLV) vaccines is recommended to make the correct choices for your herd situation.
Because of the infectious nature of BVD and the economic impacts it can have on individual herds and all of the cattle industry up the chain where PI cattle may go, each producer needs to include BVDV as a disease for recognition and eradication. Whether the herd is determined to be positive or negative, a plan for vaccination and/or removal needs to be in place, along with an overall plan for ranch biosecurity. BVD eradication requires a three-pronged approach: diagnosis/removal of PI’s; vaccination to aid in protection of all ages and sexes of cattle; and biosecurity to prevent introduction of PI cattle. The results achieved for the producer by faithfully following this approach include increasing the productive performance of the herd, improving weaning weights/number weaned, lowering costs of treatments and death loss, and providing highly marketable cattle from a BVD-free herd. All of these will allow the producer to reap the benefits of increased profitability as well as the satisfaction of knowing that the operation is contributing to the overall good of the industry.
Minimizing practices, which can contribute to introduction of BVDV, is important to the operation. Animals of unknown BVDV status can be minimized by careful selection of animals added to the herd. Exercise caution in buying replacement cattle from commercial auction sales, which contain animals from many unknown sources. Know and trust the source of the sale barn animals you purchase. New animals from other ranches or seed stock producers should be from BVD free sources. Regardless of the source of animals you introduce to your herd, an initial quarantine period should be followed with testing of animals to ensure BVD negative status before entry into the herd.
Bulls serve as an important source of herd contamination, and only BVD-negative bulls should be used. Virus in semen of PI bulls is extremely high, and clean cattle will experience a reduced fertility rate and potential infection, if the cow does not have immunity to the virus. Studies have shown that young bulls can acquire a long-term testicular infection following vaccination with MLV vaccines. Thus, it may be wise to vaccinate potential breeding bulls with Killed vaccines.
The most important factor in determining whether a herd is clean or ridding BVDV from a herd, if infection is found, is for the producer to have a close working relationship with their veterinarian. The veterinarian can evaluate the herd situation and provide the producer with the best strategy for the individual herd. Recently, a new option for BVD prevention has become available—the “BVD Consult” Website. The site is available for both veterinarians and producers. Through a series of questions, which the veterinarian and producer answer on-line, recommendations are provided for the individual herd situation.
Since 2016, our ranch has embarked on a new testing and vaccination program for our herds, which includes registered and commercial cattle. We utilize IDEXX SNAP kits and annually test all of our herd bulls, a sampling of older cows, and a number of new born calves, as well as all breeding animals sold at sale or private treaty. So far, all are negative. We follow the following vaccination program for BVD:
1. All calves receive Killed BVD vaccine a month following birth (along with other vaccines)
2. 2-3 months later, all calves receive a booster of Killed BVD
3. At weaning:
a. Steers receive MLV BVD
b. Heifers and bull calves receive a booster of Killed BVD
4. Sale heifers and bulls receive an additional killed BVD vaccination prior to sales.
5. All adults receive Killed BVD semi-annually, when we work cattle in the spring and fall.
We primarily utilize killed vaccine based on the safety of this vaccine in all ages of cattle. Nursing calves cannot pass live virus to their mothers, which may have missed a vaccination. We avoid the possibility of testicular infection of bulls by live virus. Heifers, which may have been bred prior to weaning, cannot pass BVD on to their offspring, creating PI’s. However, steers going to market can be safely vaccinated with live vaccine, meeting the standards of most buyers.
The investment to ensure that a herd is BVD negative is an expense that ensures the future of the herd and the profitable operation of the ranch. I hope that this information will assist you in the process of evaluating your herd and protecting your investment of time, heart, and money.