For most of us, mid-November brings a reminder that it's time to shift our thinking into winter mode. For food animal veterinarians, it's the time of year when calls about sick calves come in. The changing weather, and the stresses of shipping and weaning, have a lot to do with this.
Bovine respiratory disease (BRD, or calf pneumonia) is the most common cause of sick calves this time of year, and many different viruses (IBR, BRSV, BVD) and bacteria (Mannheimia, Histophilus, or Mycoplasma, for example) are involved in this complex. When vets show up on the farm to aid a producer with sick calves, however, the focus is usually first on getting the calves better instead of determining exactly which germs are involved. Often, the calves are successfully treated without the specific germs being identified at all.
But sometimes it does become necessary to try to figure out just which germs are causing problems in a BRD outbreak. This information is useful when investigating treatment failures or for designing future vaccination and prevention programs.
In the unfortunate case where cattle have died from BRD, samples of lung and other organs can be submitted to the diagnostic lab at SDSU for identification of the bacteria and viruses associated with the outbreak. Diagnostic techniques such as culture or PCR are ever evolving and are becoming more refined all the time. But what if no calves have diet yet from the BRD outbreak?
Samples from live calves can be taken and submitted to the lab, too. These samples are not from tissue, they're from the fluid that coats the lining of the respiratory tract, similar to samples a nurse might take from your sore throat at the doctor's office. How could this be done in calves?
The first sampling site to consider is the nose of the calf - the gateway to the respiratory tract. While probably the easiest to obtain, swabs taken from the nasal passages might be the least useful. The problem is that healthy calves usually have some of these nasty germs hanging around their nose, too. So just because we can identify a germ like Mycoplasma or Pasteurella in the nose, it doesn't necessarily mean that that is the germ causing the problem deep in the lung. On the other hand, finding certain germs like BRSV or IBR, or consistent results across a large number of calves might be significant.
To get the best picture of the germs making a calf sick from pneumonia, veterinarians would prefer a sample from the depths of the lungs. There are techniques that can get us closer to this type of sample, but they are best left to veterinarians to perform as they require skill and special equipment. One technique involves passing a long tube down into the lower airways either through the nose or through a surgical site in the windpipe, and sucking up fluid through the tube with a syringe. Another involves using longer swabs that pass through the nose and into the back of the throat. The bacteria and viruses found back here tend to be more representative of what's going on in the lungs compared to samples obtained from the nose.
Even after the right samples are taken and results obtained, those results still need to be successfully interpreted. This is where an experienced veterinarian with their boots in the feedlot and a good relationship with the diagnostic lab comes in. For example, with viruses like IBR and BRSV, our diagnostic techniques are becoming so sensitive that recent use of certain modified live and intranasal vaccines could cause positive results. And as mentioned, healthy calves can harbor some of these pathogenic germs as well.
Diagnostic techniques for food animals are becoming more sensitive and advanced as time goes on. But they do not diminish the roles of the veterinarian and animal caretaker. In fact, these newer techniques probably beg more than ever for skilled veterinarians that can navigate a lab report in the context of the cattle right there in front of their eyes. Clinical common sense meshed with technology from the lab is a great combination for cattle producers and the animals that depend on them.