Respiratory disease, pneumonia, is still one of the most common reasons why dairy calves, veal calves and beef calves end up getting sick. Because most of these calves are treated with an antibiotic drug, and many of them get better after being treated, we tend to focus on the bacteria that are part of the complex of microbes that cause pneumonia.
Trying to figure out why calves get pneumonia in the first place is not easy. We believe that environment and stress play big roles. They tend to impact the calf’s general health and innate resistance to disease. We also think that viruses play a big role because some viruses can weaken the calf’s immune system and some can damage the mechanisms that the airways and lungs have in place as a first line of defence against bacteria.
We know that BVD can be important because it can damage the calf’s immune system. BRSV is also important and it is much more common than BVD. BRSV can cause pneumonia on its own but it also damages the lung, making it easier for other viruses and bacteria to invade the lung.
We have known for a long time that there are other viruses that are likely as important as BRSV. Bovine respiratory coronavirus is another virus that shows up in outbreaks of pneumonia. You may have heard of coronavirus because it is one of the causes of scours but the virus that causes pneumonia is likely different from the coronaviruses that cause scours.
Using antibody reactions on blood samples taken at the start of an outbreak of pneumonia and again a few weeks after it is over, you can get some good evidence of what caused the outbreak. That may not help you during the outbreak but finding out what viruses were involved may help in planning preventive programs going forward. In Ontario, it is not unusual to see BRSV and bovine coronavirus (BCV) likely causing outbreaks of pneumonia even in adult cattle.
It has been frustrating that outbreaks that look like they might have been started by a virus may test negative for BRSV and BCV. That created a suspicion that there might be other viruses that we didn’t know about. With that in mind, a few experts began to speculate that a cattle flu virus — bovine influenza D — might be important. Nobody really knew for sure.
Recently a research group made up of scientists from the veterinary schools in Calgary and Saskatoon looked at viruses that they could identify in beef calves with pneumonia. As everyone expected, they found BRSV and BCV. They also found bovine influenza D. But they found two other viruses: Bovine rhinitis viruses A and B.
Because the researchers compared their ability to find viruses that were in both sick and healthy cattle, they were able to get an idea whether the viruses could be involved in causing pneumonia. Because sick cattle were more likely to have one of these five viruses, it was proposed that these viruses played some role in the calf’s disease. The technology was able to detect several other viruses too but their pattern of isolation from sick and healthy calves did not conclusively point to them as important causes of pneumonia.
Nobody knows the exact role that influenza D and the two rhinitis viruses play in causing pneumonia or even how important they are. One obvious next step is to develop better diagnostic tools so we will be able to routinely look for them. Ultimately, if they turn out to be common and important, they may be added to current vaccines or be included in newly developed vaccines.