Cattle Health and Persistent Infection of BVD Virus

As fall approaches, many livestock producers are preparing to stock anticipated wheat pasture. Health issues are one of the annual battles that they must face as they try to get the stocker cattle straightened out and ready to grow. Respiratory disease is the primary health issue that must be dealt with in these cattle. It is the same issue that is faced daily in the feedlots and dairies around the country.

BVD (Bovine Viral Diarrhea) virus has been identified as one of the leading causes of respiratory disease in cattle. Although it poses no human health concerns, it is an economically devastating disease when it is present in a cattle herd.

What is a PI calf?
When present, BVD has been determined to negatively impact conception rates in a breeding herd by as much as 5%. This means fewer pregnant cows. BVD also causes abortion in cows so that although they may have become pregnant, there is no calf to sell. It also is a contributing factor to weak calves. Many calves born live only a few days and then die. Their immune system has been compromised while in their mother’s womb when she was exposed to BVD.

BVD is unique in that it also can cause calves to become persistently infected (PI) while in the womb. It is a permanent infection that occurs if the cow is exposed to the BVD virus during the first trimester of pregnancy. When she is exposed, the BVD virus begins to circulate within her system and thus within the calf’s system. If the fetal calf is exposed to the BVD virus prior to immune competency (that point at which the immune system begins functioning), the BVD is recognized as part of “self” and therefore ignored. The calf’s immune system “assumes” that the BVD virus which was present at the point of “self-recognition” is normal and so it ignores it throughout the lifetime of the calf.

The challenge of PI cattle.
These persistently infected calves are the primary means of spreading the BVD virus throughout the cattle industry. Because their immune system ignores the virus, the BVD proliferates within their body. They often look healthy because their immune system makes no attempt to combat the virus. When this happens, they shed viral particles at an extremely high rate that has been estimated to be as much as 1,000,000 to 10,000,000 viral particles daily. This extreme viral shed occurs every day throughout the calf’s life.

When healthy cattle come into contact with these PI calves, their immune system is challenged by the virus particles that begin to attack them. If the healthy animal has a thoroughly functioning immune system that has developed resistance to the BVD virus, it is likely that they will not succumb to the onslaught. However, if their immune system is unable to combat the BVD attack, they will become sick with the disease.

Healthy cattle that have been conditioned with proper vaccinations and who are in excellent health and are not subjected to extraordinary stressing factors will usually resist the BVD virus. If these healthy cattle are stressed though, such as through gathering, weaning, shipping, processing, or co-mingling with other cattle, their immune system likely is not working at its peak. In such case, there is an increased probability that they will be impacted negatively by exposure to the BVD virus. Sometimes this impact is manifested in clinical signs but often it is a sub-clinical infection that may not cause the exhibition of symptoms, but will impact their performance. If an animal is expending energy fighting off the BVD attack, he is using energy that should be building muscle.

BVD not only attacks the animal directly as a disease, but is also an immuno-suppressant. This means that it causes the immune system to temporarily become unable to adequately fight against other diseases that may be present. This opens the animal up to sickness if he is exposed to bacteria or other viruses during the suppressed period. This is frequently the case when the animals are in a confined condition such as a feedlot, or when they have been mixed with other cattle that may be carrying diseases to which they have not yet built immunity such as assembling stocker cattle.

The economic impact of PI cattle.
There have been several studies throughout the world that were conducted to determine the economic impact of BVD on cattle. These studies indicated a negative impact that ranged from $10 per head to as high as $60 per head. The studies looked at the impact at the cow herd level all the way through the feedlot and in dairies. One particular study focused on bringing calves into the feedlot and carrying them through to finish. The impact in that particular study was in excess of $42 per head across the entire feedlot population.

Although there have been few studies conducted on yearling cattle in the feedlot, one must wonder at the impact of the constant exposure to PI cattle on performance. Again, if an animal is expending energy fighting off disease, he is utilizing energy that should be building muscle.

Data collected by one large commercial laboratory devoted to testing for the persistently infected animals indicates that in 700 lb. yearling cattle, the prevalence of PI animals is approximately 0.2%. That is 2 animals per every 1,000. That doesn’t seem like many until you think of the typical feedlot configuration of 100 head pens in alleys of adjacent pens where there are frequently shared water tanks between pens. In such a scenario, 2 per 1000 could potentially affect as much as 60% of the population of animals in the yard through in-pen and cross-fence exposure. When one considers the viral shed rate of 1,000,000 to 10,000,000 particles daily, there are a lot of animals being continually exposed to the BVD virus.

In lighter calves, the number of PI’s per 1,000 head is much higher. In 300 lb. salebarn calves, it is approximately .75%, or 7-8 animals per 1,000 head. This higher percentage of PI calves significantly increases the risk of exposure to the BVD disease.

Testing for PI animals.
Testing for these PI animals is a simple procedure. Utilizing a small V-type ear notcher, a piece of ear tissue is sampled from each animal and placed in a properly labeled plastic vial for transport to the laboratory for testing. The test takes anywhere from a few hours to several days to run at the lab, depending on the type of test used, and the results are reported back to the producer. The producer then can remove any persistently infected animal from the general population thus eliminating the continual exposure of the remaining animals to the BVD virus.

There are several testing methods available for detecting the PI animals. They are: 1) Viral isolation which requires the culturing of the BVD virus from the animal’s blood or other tissue. This is an extremely time-consuming and expensive test. It requires that a second sample be taken from the animal after about 3 weeks. It is not practical for detecting PI animals but is useful in determining the presence of BVD in an animal and is the first step in determining the particular strain of BVD infecting the animal. 2) Immuno-histo-chemistry is another type of test for BVD. It is done from a thin slice of skin tissue that is stained and examined manually under a microscope for the BVD. It also requires a second sample from an animal in about 3 weeks. 3) Pooled PCR or polymerase chain reaction sampling is a technology that has recently been applied to BVD. One of the inherent problems of this technology is that it has been shown to miss PI animals a significant percentage of the time. This is because of the fact that samples from multiple animals are pooled together before being tested.This problem has been overcome by including internal controls in the pools that are indicators of whether the reaction was influenced by inhibitors.  If those inhibitions are detected, the pools are further tested under different conditions.  There is now a USDA licensed test on the market that includes a pooling claim.  If your lab is using PCR, make certain they are using the USDA licensed test and methodology.  4) ACE or antigen capture enzyme-linked immunosorbent assay, is another test that is available for testing for the PI animals. It is reliable, inexpensive, and provides for quick turnaround of results. Usually results are obtained within 24 hours of submission of the samples.  There are multiple ACE or ELISA tests available on the market.  There is high variability in the quality of results obtained.  The very first test to obtain USDA licensing was an antigen capture ELISA or ACE test.  It's pioneering technology was created by Syracuse laboratories, licensed by a large diagnostic company and marketed under their brand.  They have since replaced that technology with their second generation test which incorporates patented technology developed for the European markets where government regulation rather than economics drives the decision for testing.  It is extremely sensitive and may indicate a positive status for animals that are transiently infected with the BVD virus but are not true PI's.  The quality of results obtained are often closely related to the experience of the laboratory technician.

What do I do with them?
Proper disposition of animals identified as persistently infected is critical. They should be either 1) humanely slaughtered once any drug withdrawal periods are completed, or 2) destroyed immediately. It is unethical to return such animals to the market via an auction or private sale. The meat from such animals is perfectly safe for human consumption. Their identification and removal from production may also decrease antibiotic usage among remaining animals which both saves cost for the producer and limits the chances of the development of antibiotic resistance.

Risk to cow herds.
The ideal place for PI testing to occur is at the cow-calf level. Many ranchers are concerned about realizing a return for the testing and opt to let it occur at some later point in the production chain. By doing so, they ignore the positive impact that the elimination of BVD can have on their herd. By removing PI animals, BVD then becomes a matter of bio-security. The rancher must continually monitor his herd due to potential cross-fence exposure from the neighbor’s cattle (such as yearlings) and from the introduction of untested cattle into his herd. With a potential increase of 5% to conception rates as well as the reduction of the number of weak calves, the increase in weaned calves for sale should compensate the producer many times over for the cost of testing and monitoring.

Because of the fragmentation of production within the cattle industry in the United States, it is unlikely that BVD will ever be completely eliminated. That is why testing is important at all phases of production – whether it be at the cow-calf, stocker, or feedlot level. The positive economic impact of removing PI animals and of implementing a good bio-security program provides the opportunity for significant benefit at all points of production through the reduction in the number of sick animals, fewer dead animals, and better performance because the animals are able to devote their energy to growth rather than fighting the constant exposure to the BVD virus.