Clinical signs that would lead a producer to suspect BVD include:
Bovine viral disease, or pestivirus, presents a collection of symptoms that range from diarrhea to respiratory disease.
“The biggest loss leader for the industry is probably a reproductive disease,” said Julia Ridpath, who has recently retired from her role as a leading BVD researcher at the National Animal Disease Centre at Ames, Iowa.
A number of strains and subtypes have been discovered around the world. To properly control the disease, it is important to know which species of virus is present, Ridpath said during a talk at the University of Calgary faculty of veterinary medicine.
BVD 1 was first written about in 1946.
“Since then, it has been found anywhere we have cattle,” she said.
BVD 2 was found in 1987 following some huge outbreaks in Canada. Another form known as HoBi-like virus has also been found in some parts of the world. It is sometimes called BVD 3.
BVD1 and BVD2 are divided into two groups and it is difficult to differentiate them in the field.
Researchers have found 11 different genetic groups within the BVD1 strain, four groups in BVD2 and three genetic groups in HoBi-like virus.
Vaccines are available but matching them to the disease presents challenges.
“Vaccination increases herd immunity but we have to go in with all the information,” she said.
These viruses replicate in immune cells and cause immunosuppression.
“Even the cells they don’t kill don’t work as well after they got infected. It has this ability to cross the placenta, which causes a lot of problems,” she said.
Pregnant cows with the virus can deliver a persistently infected (PI) calf that passes on the disease to others. Every tissue is infected and it is constantly shedding the virus through feces, mucus, urine and other body fluids.
The birth of a PI animal is unusual because many do not survive when infected in utero. Live PIs are fairly rare at .1 or .2 percent of the herd but they can cause acute infections in other animals.
“A viral infection rarely does you a favour. They are obligate, inter-cellular parasites,” she said.
“Even if you clear that virus, it probably wiped out other cells with important jobs to do.”
Prevalence of the disease is hard to measure in North America. A study in the U.S. starting in 2014 looked at slaughter cows and found many had been exposed to the disease because they were carrying antibodies against BVD1 and BVD2.
Control includes good biosecurity, as well as vaccination. Producers need to consider which animals are to be protected.
“Who are you protecting: fetus, neonate, breeding stock or feedlot animals? All have different requirements when they need protection in place. Cattle should have protection before they arrive at the feedlot, not the day they arrive,” she said.
“If you want protection of the fetus, you need protection of the dam.”
An immune response takes about 10 to 14 days to develop and sometimes protection levels are not high enough. There could be differences among animals or the vaccine may not match the virus.
Cattle may not respond well to a vaccine when many other stresses are happening at the same time like weaning, castration, dehorning and deworming.
For unknown reasons, some calves do not respond well to the vaccine. If five out of 100 do not respond, that is a production loss of five animals.
Frank van der Meer, a professor of infectious diseases at the University of Calgary, studies disease prevalence and vaccine efficacy .
There is limited information on prevalence in Canada.
“As far as we can guess BVD type 1 is the most prevalent in North America,” he said.
There are 11 different genetic groups within the BVD1, so van der Meer and his team have studied virus variation to find differences among the strains and the patterns of change. That work included examining PI animals to see what variations exist there.
“We are at the stage where we can start to analyze what these variations actually mean for vaccine efficacy.”
In addition, there is never a problem finding PI calves.
“We have never had an issue with finding new PI calves. Unfortunately for the industry, they keep me in business,” he said.
The best way to make progress within a herd is to get rid of these viruses.
“These viruses are not something you want to have in your animals. They are always a negative influence that are resulting from the infection,” he said.
“If you want to make progress in your production system, you want to get rid of those viruses like BVD, IBR and bovine leukosis.”
The modified live vaccine seems more effective than the killed form but not everyone vaccinates.
“If the modified live are used properly, they have the best efficacy that you can wish for,” he said.
“Not everybody is inclined to vaccinate but it is important to understand the consequences if you don’t vaccinate, especially when you have an industry that is well connected. The cow-calf owners who are vaccinating their calves for BVD do not see the positive benefits if they send them to a feedlot,” he said.
However, feedlot owners do not necessarily want to pay more for vaccinated animals.
“It is tough to motivate people to vaccinate if they don’t see the benefits in the long term. If the industry wants to make any progress on this bovine respiratory disease, and BVD is a big player in that whole story, you have to ensure that the calves that are delivered to the market and the feed yards are well vaccinated,” he said.
Cattle can be vaccinated at a relatively young age for BVD but the protocol should be discussed between the veterinarian and the farmer to determine the diseases involved, when cattle are to be marketed and goals of the farm.