Changing Patterns Of Bovine Respiratory Disease – for Sep. 6, 2010

New vaccines, long-acting antimicrobials and effective treatment protocols have largely eliminated illness and deaths resulting from acute bovine respiratory disease (pneumonia) caused by the bacteria, Mannheimia hemolytica. It was generally known as shipping fever because, at first, it was associated with transporting cattle and young cattle shortly after arrival at the feedlot.

Today, feedlot operators in Western Canada will tell you that animals chronically affected with a relatively new type of pneumonia far outnumber cases of acute bacterial pneumonia. This shift in prominence from acute to the chronic type of pneumonia has been most noticeable in the past 10 to 15 years.

Dr. Eugene Janzen, assistant dean of clinical practice and community partnerships with the University of Calgary’s faculty of veterinary medicine, provides some background regarding this changing pattern and what seems to be behind it.

In the past, it was usually easy to identify animals with acute pneumonia because the pathogen produced a toxin that made them appear ill and depressed. Symptoms usually started to appear in the first week after arrival at the feedlot and shortly after weaning in ranch calves. Affected animals were often standing off by themselves, reluctant to move. Without timely treatment, the bacteria produced massive amounts of toxin and the animal died from toxemia rather than any pathology in the lungs.

The term undifferentiated fever is now commonly used to describe a fever when no specifipathogen or disease can be pinpointed without a considerable diagnostic workup.

Research improved the understanding of acute bacterial pneumonia and the capacity of producers and veterinarians to successfully manage and treat this disease. Klaas Jericho at the Canadian Food Inspection Agency’s Animal Disease Research Institute in Lethbridge developed a disease model that was eventually used by other researchers to study bovine respiratory disease. Researchers at the Ontario Veterinary College developed a vaccine that protected the animal from the effects of the toxin. Together, the Western College of Veterinary Medicine and Vaccine and Infectious Disease Organization at the University of Saskatchewan evaluated treatment protocols. They found that the timing of administering an antimicrobial — either the conventional daily treatment or a product with sustained action — was so critical that if treatment was delayed by a mere 24 hours, a high percentage of the affected animals would die. These developments greatly reduced the prevalence and severity of acute bacterial pneumonia.

Cases of the chronic type of pneumonia typically begin to appear or die about 75 days after the calves have settled into the feedlots. The occurrence seems to be quite sporadic and the disease doesn’t spread rampantly throughout the pen. Often animals show no outward signs of illness — it is observed resting quietly, then found dead on the next pass through the pen.

It is suspected that there is a mix of bacteria and viruses involved. When the toxin-producing Mannheimia hemolytica bacteria is markedly reduced, the animal won’t show the symptoms commonly associated with acute pneumonia. The disease silently progresses until most of the lung is affected and the functioning of the lung is seriously compromised. Death occurs due to lack of oxygen rather than toxemia. Necropsies consistently reveal that there is very little normal lung tissue left even though the animals never appeared to be ill.

The other scenario is animals that show subtle signs of illness but don’t respond to the feedlot’s protocol for managing undifferentiated fever. The lack of response is because the loss of lung tissue caused by the pathogen(s) is permanent and the damage has advanced to the extent that the animal will never be able to fully recover. They either die or end up being treated multiple times and usually have to be euthanized. The unpredictability of chronic pneumonia in both the timing of its onset and its progression makes it a very difficult disease for feedlots to manage and the case mortality rate is high.

On a day-to-day basis at the feedlot, it is too costly and time prohibitive to collect samples and carry out tests on every animal that comes down with pneumonia to identify the pathogen and organs involved. The tests that have been done to date indicated that the predominant pathogen involved is a Mycoplasma species. In addition to having difficulty breathing, the affected animals often show signs of other complications such as diarrhea, or polyarthritis.

There is always some pneumonia-related research going on elsewhere in Canada, such as the projects studying Mycoplasma bovis at the University of Saskatchewan. There may be pieces to the puzzle of chronic pneumonia coming to light here and there, but to date there has been no research specifically looking into the cause and progression of the chronic type of bovine respiratory disease.

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