Bovine viral diarrhea (BVD), frequently a silent disease, is caused by bovine viral diarrhea virus (BVDV) and has long-term consequences.
BVDV is one of the infectious agents involved in shipping fever.
The immune system is compromised by BVDV and as a result, the virus makes animals more susceptible to other diseases.
Controlling BVDV alone will not prevent infection from the other shipping fever pathogens, but it can reduce its severity.
Occurrences of this expensive disease in the U.S. have been steady over the past 50 years despite the availability of improved vaccines and diagnostic tests.
This is probably because symptoms are frequently mild and not easily recognized, and producers don’t realize their herd is infected. Due to not recognizing the disease, control measures aren’t exercised.
“Respiratory, immune, intestinal, blood and reproductive systems are affected by BVD,” says Dr. Tom Hairgrove, a DVM with Texas A&M AgriLife Extension Service.
“Scientists first believed that the disease was related to diarrhea and intestinal tract damage – hence, they named it bovine viral diarrhea. About 70 to 90 percent of all BVD infections show minimal symptoms. The most economically important consequence of this disease to cow-calf producers is reproductive loss, although economic loss due to immunosuppression is probably underestimated.”
“Although there are usually no outward signs of BVD, the infection greatly damages the immune system,” states Dr. Julia Ridpath, who is with the USDA Agricultural Research Service (ARS) at the National Animal Disease Center in Ames, Iowa. “
BVD destroys portions of the immune system and makes animals very susceptible to other diseases. The immune system may repair itself over time or it may have long-lasting deficits, depending on the age of the infected animal and strength of the viral strain.
“Many respiratory pathogens infect the upper respiratory system without causing disease. If these pathogens migrate from the upper respiratory system to the lung, severe disease will result.
Loss of immune tissues and function following BVDV virus infection makes it easier for respiratory pathogens to get into the lung and cause disease.
“Another economically important consequence of this disease is reproductive loss through open cows, abortions, stillborn calves or delivery of weak calves.”
The other viruses
Shipping fever losses can be limited by vaccinating for IBR (infectious bovine rhinotracheitis), PI3 (parainfluenza-3 virus) and BRSV (bovine respiratory syncytial virus) in addition to BVDV.
“IBR, also known as red nose, is a complex of diseases occurring throughout the U.S.,” explains John Cothren with North Carolina State University Cooperative Extension. “Some of the symptoms include abortions, respiratory and eye infections.
“The IBR virus is one of the most common causes of abortion in cattle. Often abortion is preceded by other IBR symptoms.
Abortion has the potential to occur at any stage of pregnancy but usually occurs in the second half. Calves may be born infected with IBR, evidenced by diarrhea, weakness and difficulty in nursing.
“The respiratory form of IBR usually affects concentrated groups of cattle as in feedlots. The first signs of the disease (rapid and labored respiration, nasal discharge, fever, loss of appetite) appear about a week after infection.
The pinkeye form of IBR causes reddened, swollen eyes with clear, watery discharge and sometimes ulcers on the eye.”
“PI3 is another costly disease found in cattle,” says Cothren. “Affected animals exhibit watery to yellow-colored discharges from eyes and nose, labored breathing and fever.
This organism works in combination with other respiratory virus and bacterial infections, so death from PI3 infections alone is rare.
The combinations are more dangerous because PI3 can enhance the damage of other disease organisms. PI3 vaccines are almost always incorporated with IBR, BVD and BRSV.”
According to Purdue University, BRSV has been recognized as a pathogen in cattle since 1970. Cattle most susceptible are calves 6 weeks to 13 months old.
In younger calves, mortality rates are increased by secondary bacterial infections.
BRSV infections tend to be acute, with discharge from the nose and eyes, fever, coughing and labored breathing.
Understand BVD persistence
Hairgrove states, “There are two types of BVD virus infection – transient or persistent.
Transiently infected animals (TI) are exposed to BVDV after they are born.
They will eventually clear the virus and recover. While they are infected, they can spread the virus to other animals.
“When BVDV is passed from dams to their fetuses in the first third of their pregnancy, the calf becomes PI because of an undeveloped immune system.
If the fetuses live, the calves shed virus continuously for life. Understanding BVDV persistence is essential to designing an adequate disease control program.”
“Abortions by BVDV-infected cows can occur at any time during gestation,” Hairgrove says. “At the beginning of the calving season, premature births and stillbirths are common.
Some calves are born alive but they may not thrive. A high percentage of them may die within the first 4 to 6 weeks. If a calf lives, it will be PI the rest of its life.”
BVDV eradication
“A BVDV eradication program has three important parts,” says James England, a veterinarian with the University of Idaho. “The first part is to enhance herd immunity with vaccination.
A producer should vaccinate all replacement heifers with a modified live virus (MLV) vaccine after heifers are 6 months old but at least one to two months before breeding.
Protecting heifers is essential. If possible, vaccinate cows annually 30 days before the beginning of the breeding season.”
The second part of the program is to eliminate persistently infected (PI) carriers. These animals are the main reservoirs of BVDV infection in the herd. Test to identify all PI animals. Then cull them from the herd and sell for slaughter.
“Implementation of a BVDV biosecurity plan is the third step,” states England. “Prevent direct contact of BVDV-tested cattle with non-tested cattle by avoiding commingling. Test all incoming animals to ensure they are not PI carriers.”
“PI animals perform poorly in the feedlot and reduce performance of penmates,” says Ridpath. “For these reasons, feedlot managers normally test for and eliminate PI cattle.
However, removing PI calves from the feedlot doesn’t stop their production in the next calving cycle. Generation of BVDV PI cattle can only be stopped by the cow-calf producer. Producers do not want to build their reputation on selling PI calves.” 
Robert Fears is a freelance writer based in Texas.
PHOTO
TOP: Untested or BVD-infected cattle should not be commingled with BVD-free cattle.
BOTTOM: If a calf is exposed to BVD as a fetus, it will be born as a PI and will shed the virus the rest of its life. Photo courtesy of Robert Fears.
