BVD disease is one of the most complicated viral diseases in dairy cattle worldwide. Terms associated with BVD such as “cytopathic” vs. “noncytopathic,” “persistent infection” vs. “transient infection,” “types I and II” and “immunosuppression” make understanding this virus a difficult task.
However, the results of this disease are very clear – economic losses due to poor performance, loss of milk production, open cows, abortions, calf sickness and death. In order to control this problem, producers must first know if the virus is circulating in their herd.
If so, then efforts must be focused on finding those infected individuals, minimizing their harmful effects and eliminating them from the herd at the right time. If the herd is free of BVD virus, then it is of the utmost importance to keep the virus out and minimize losses if it is accidentally introduced.
A basic understanding of the disease is essential to finding the appropriate strategy for your farm. The virus is picked up by a cow or calf by breathing it in (inhalation) or swallowing (ingesting) the organism. The virus can survive in the environment up to seven days and may be brought in on trucks, tractors, boots or gloves or carried in by wild animals.
It can also travel in the air, including across fences from a neighboring farm. However the most important source of the virus is a cow or calf already infected with BVD.
Once exposed, what happens next depends on the immune system of the cow or calf (vaccination status), aggressiveness (“virulence”) of that strain of virus and the overall stress level present in the herd. Symptoms of disease range from a mild fever to severe diarrhea and death.
Clinical signs of acute BVD (also known as “transient” or “primary” BVD) disease in young dairy animals three to 12 months old may include:
• Unthrifty/rough hair coat (calf looks “wormy”)
• Ulcers in the mouth (slobbering)
• Hemorrhage (bleeding) and death
Perhaps the most important symptom of BVD virus in calves is one we cannot see – immunosuppression. The virus actually keeps the calf’s immune system from functioning properly so other infectious diseases such as pneumonia and calf scours are much more severe and often lead to death.
Clinical signs in dairy cows are generally mild and seldom noticed, but the infection causes serious consequences if it occurs during breeding season or early pregnancy (first and second trimesters). Reproductive effects of the BVD virus include:
• Failure to conceive
• Early embryonic loss (long time to return to heat)
• Abortion (up to six months)
• Mummified fetuses
• Developmental defects or “dummy calves”
• Calves with eye defects such as cataracts and retinal degeneration
• From 180 days on, a normal calf is produced.
The most important consequence occurs if the cow is infected with the BVD virus between 42 and 125 days of gestation. In this short window of time, the unborn calf contracts the virus and is never able to get rid of it for the rest of its life. When this calf is born, it is “persistently infected” or a “PI” calf.
The “PI” animals are the reason BVD disease continues. They are the primary source of virus transmission because they shed an extremely high number of virus particles throughout their lives. A PI animal is a virus factory, producing millions of virus particles in all body secretions including feces, urine, saliva, nasal discharge, milk, semen, uterine secretions, and aborted membranes.
These contaminated body fluids are deposited on the grass, in ponds, watering troughs, feed troughs – virtually everywhere the animal goes. Although it is often assumed that PI calves are stunted, will grow poorly and usually die young, some are absolutely normal and will survive well into adulthood and have calves.
Unfortunately, if a PI female gets pregnant, her offspring will be a PI calf 100 percent of the time. This is important, because if one PI heifer survives and is out in the pasture constantly shedding the virus, many (if not all) of the heifers in her group will be exposed during the highest-risk time for creating more PI animals. This risk continues within the cow herd when the heifer freshens and enters the milking string. PD
—Excerpts from University of Kentucky newsletter, Kentucky Dairy Notes, February 2011