BVD testing: What's happening in the herd—acute, chronic, transient or PI? (Proceedings)
When I report back to a cattle owner that his animal is "BVD positive," before I can even begin to explain what that means, I am all to often asked "what is BDD, BV . . . what?" since I am usually talking to the rancher about a normal calf , not an obviously sick animal. So, here is a little client educational aid to help in addressing, and potentially mitigating, the various production problems that can occur in cattle operations exposed/infected with BVDV.
It is BVD, just like the underwear, and the animal is infected with a virus named Bovine Virus Diarrhea Virus. It is rarely associated with any type of diarrhea although it was originally identified in diarrhea of weanling calves. What this BVD means is that this animal is a source of infection and problems for other animals in the herd.
BVDV is a common and widespread virus that infects cattle and wild ruminants and this virus is a member of a group of viruses called Pestiviruses—very appropriate. BVD is not one virus, but 200 or more closely related strains divided between two BVD serotypes. Even the two serotypes are quite closely related.BVD virus is spread by respiratory secretions and has an incubation of about 10 days. After the virus infects the cells of the respiratory system, it invades the bloodstream and spreads to all tissues. Its favorite blood cell is the lymphocyte which transports the virus to all other organs and tissues. Unfortunately, the lymphocyte is the basis of the immune system and immune response of mammals. The immune system of the infected animal also may be compromised by BVD infection.
BVD virus has the ability to permanently infect some cattle; we call these cattle persistently infected—PI animals or BVD PI! PI calves occur when a non-immune, pregnant cow is exposed to the virus between 40 - 120 days of gestation. This may not happen with all exposed fetuses. However, during this time in the uterus, the fetal immune system may not recognize BVDV as foreign so the fetus accepts the BVD virus as part of it's normal makeup. These calves are usually normal at birth, but they are able to spread the virus to contacts through respiratory fluids. Very few PI calves reach 2 years of age! This should not be reassuring because PI animals can shed the virus until death or removal.
If the BVD invades a susceptible or non-vaccinated (not immune) cow or calf, one of several different situations may occur. 1) An exposed cow/calf can become transiently infected and shed the virus for days or weeks. This occurs more often than not without clinical signs but she still is a source of infection for herd mates during the infection. These individuals routinely develop immunity and clear the infection and are then safe to keep in the herd. 2) A susceptible (non-immune) animal, especially a calf, may develop acute signs of a respiratory infection or may just be a chronic poor doer because the virus can affect the immune system making the calf susceptible to other infections while shedding virus from 10-30+days. These animals may clear the infection and return to normal. 3) If a pregnant, non-immune cow is exposed to BVDV any of the following can occur: a) fetal resorbtion (0- 90 days gestation), b) abortion (60-200days gestation), c) a PI calf (90-150 days gestation) or d) elimination of the virus and birth of a normal calf (> 150 days of gestation).
If an immune cow is exposed to BVDV, none of the above disease situations will occur. A transient, acute, chronic or persistent infected animal coming into the lot/herd presents a danger to all herd mates. PI animals excrete more virus than the acute, transient or chronic infected animal and occasionally excrete sufficiently high quantities of virus and overcome immunity.
The most worrisome situation associated with BVD in cattle is abortion. Other presentations are chronic poor doers, weak calves and calves with signs of brain involvement. In the feedlot, poor performance and increased respiratory disease and an occasional diarrhea are associated with BVD infection.
While a PI animal is often a signal that others are present in the cow-calf herd, the prevalence of PI animals within a herd is variable but generally quite low ranging from 0.1 - 6% with the average less than 2%. Surveys have shown that upwards of one quarter of beef herds have PI animals present.