What is a PI calf?
A PI, or persistently infected, calf is one that is born infected with BVD virus and is a lifelong carrier and shedder of the virus.
How do calves become PI?
The unborn fetus is targeted by BVD virus when its dam (cow or heifer) is exposed to the virus while pregnant. The virus enters the bloodstream of a susceptible (not vaccinated or inadequately vaccinated) female and immediately gains entrance into the developing fetus. The risk period for the formation of PI calves is thought to be from approximately the 40th day of pregnancy through to the 150th day of pregnancy.
What happens to calves born PI?
The calf is often born normal, but carries the virus and spreads millions of virus particles daily to the rest of the herd.
PI calves are usually destined to die early. Most of them die between 7 and 14 months of age, either from BVD virus itself or due to a secondary virus or bacteria entering the animal.
What does the virus do?
The virus is very immunosuppressive, which allows other viruses and bacteria to enter the calf and cause disease. PI calves have been referred to as “walking time bombs” due to the massive amounts of virus being shed to herd mates or pen mates. BVD virus has severe negative production effects on animals exposed to the virus, resulting in poor weight gain, poor performance and increased susceptibility to both disease and dying.
What are the potential signs that a PI calf is in a herd?
If a PI calf is in the herd, you may see signs indicative of BVD, including:
- Increased number of open cows
- Delayed or strung-out calving periods
- Weak or dead calves at birth
- Birth defects
- Increased percentage of calves with sickness including scours, pneumonia and lameness
- Runty calves or those doing poorly
- Calves dying post weaning.
Facts about BVD
What is BVD?
BVD stands for bovine viral diarrhoea.
Despite its name, BVD is not just diarrhoea; it is a group of diseases caused by the BVD virus. It gets confusing because some people use the word "BVD" to refer to the virus itself.
What diseases are parts of BVD?
The BVD virus causes different diseases, depending on what animal is infected. The BVD virus can cause many different diseases because it damages the immunity of the cattle it infects.
In feedlot calves, BVD usually shows up as pneumonia (bovine respiratory disease, or BRD) or undifferentiated fever (UF). Sometimes, BVD causes diarrhoea, too, but that isn’t very common.
In suckling beef calves, BVD can cause diarrhoea or pneumonia. It can cause calves to be unthrifty without other signs of illness except “not doing well.” Research in Saskatoon, SK, shows that calves born in cow herds in which there is BVD tend to have lower weaning weights than in cow herds without BVD. In pregnant cows or heifers, BVD can cause the cow or heifer to have a fever, but the biggest problem is that BVD can infect the unborn calf. If it infects the unborn calf, it can kill it, leading to cows that are open, or cause outright abortion.
If the unborn calf is infected late in pregnancy, it can be stillborn. Sometimes, BVD doesn’t outright kill the unborn calf. In this case, the calf will be born stunted or weak. If BVD infects the unborn calf in the first five months of pregnancy and doesn’t kill it, the calf may be born and be a carrier of BVD. See What Is Persistent Infection?
In bulls, BVD virus may cause mild sickness, as it does in cows. Most of the time, you wouldn’t be able to tell it was BVD that caused the bull to get sick. The big problem for bulls is that BVD can infect their testicles. If it does infect the bull’s testicles, the bull will have BVD virus in its semen for weeks to months after it recovers from the BVD infection.
What are BVD carriers or PI calves?
When a pregnant cow is infected in the first five months of pregnancy, her unborn calf may become infected with the virus and be born a carrier. These carriers, called PIs, will be infected with BVD virus for their entire life. They spread the BVD virus to other cattle that they contact. If they are sold into a feedlot, the calves will spread the BVD virus throughout the pen.
These carriers tend to die before they are 2 years old, but they can survive for years. If a heifer calf is born a carrier and she is kept as a replacement, she will always give birth to persistently infected calves.
How do you control BVD?
How you control it depends on what type of beef operation you have:
In cow herds, the goal is to make sure that you protect the pregnant cows. You need to make sure that the cows and heifers are vaccinated with an approved BVD vaccine according to label directions. The vaccine should have both Type 1 and Type 2 BVD viruses in it. The bulls should be vaccinated, too, to protect them from getting infection in their testicles.
In seed stock cow herds, you can test for carriers and remove them to make the herd “BVD carrier free.” Even if you do that, you still need to protect the breeding herd to make sure new carriers aren’t formed. You also need to make sure you keep BVD out of the herd.
In the feedlot, you need to make sure that calves are vaccinated against BVD. That way, they will be protected if a carrier ends up in their pen. Ideally, calves should be fully vaccinated before they go through the sale process, rather than on arrival at the feed yard.
Now people are testing calves on arrival at feed yards to see if they are carriers. The idea is to find and remove these carriers before they spread BVD throughout the pen or into the pens on either side.
How do Express®
Verification and the CCIA control BVD?
See Express Verified®
Economics of the BVD virus
BVD virus (BVDV) is arguably one of the most important infectious agents of cattle. BVDV causes a wide variety of clinical syndromes in cattle including:
- congenital defects in calves
- weak and dying calves
- production losses from poor performance.
Losses due to a BVD PI
Losses in the Cow Herd
A landmark investigation into the economics of BVD virus in cow herds was published by a group of researchers from the United States. Their data showed that the negative effects of PI calves on pregnancy percentage, preweaning mortality and weaning weights varied between $14.85 U.S. and $24.84 U.S. per calf, depending on the year investigated. For a 200-cow beef herd, these costs would be $2,970 and $4,968 U.S. in direct economic costs.
Losses in the Feedlot
Feedlots incur large economic costs due to BVD as well. The PI calves often die in the feedlot. Those that do not die have drastically decreased performance and increased health costs.