Animal programs

Bovine respiratory diseases have bad origins

If there is one academic discipline in which veterinarians are well versed, it is microbiology. Some version of this topic was almost constantly on our class schedules, from undergrad through vet school. General microbiology was the aperitif of the suite: specific courses in bacteriology, virology, mycology, immunology and – finally – infectious diseases, where we learned how the body reacts to these germs.

For future veterinarians – especially those who care for food animals – it is appropriate to spend some time learning about these “insects”. Our animal patients are more likely to be affected by infectious diseases than human patients treated by a physician.

This microbiological focus leads us veterinarians, as we review the lab results of a producer’s sick or dead animals, to regale pet owners with exotic-sounding germ names. Some of these terms go over the heads of people who haven’t taken five semesters of microbiology, but I’ve found over the years that growers have become familiar with many of them. This unfortunately includes Mannheimia hemolytica.

Mannheimia is frequently cultured by the veterinary laboratory from specimens of calves with pneumonia. It is not the only type of bacteria implicated in bovine pneumonia, but it is the most important.

When we catch pneumonia ourselves, the question is, “Where did we catch it?” We were fine before we inhaled someone else’s nasty germs, and now we’re sick.” It’s tempting to assume the same thing happens with Mannheimia in cattle, but that’s not the case.

It might surprise you that the majority of calves – healthy calves – already have this nasty bacteria in their upper respiratory tract. Mannheimia hemolytica has a “commensal” relationship with cattle, living on body surfaces like the nasal passages, but without harming or benefiting the calf.

If Mannheimia is present in normal cattle, how can it cause such problems? When Mannheimia is confined to the upper respiratory tract, this is not the case.

The game changes, however, if Mannheimia hemolytica reaches the lower lungs. When the bacteria get there, the body reacts violently to their presence. Mannheimia is really good at getting the calf’s immune system to pour white blood cells and fluid into the lung tissue, a task made easier by the tight connection between blood vessels and air sacs in this part of the lungs.

Normally, an influx of bacteria-attacking white blood cells is exactly what the calf wants when an infection strikes. However, Mannheimia has a bad counter move. The bacteria produces a toxin that specifically kills these white blood cells. Not only is this defense mechanism neutralized, but when the white blood cells break down, their enzymes damage normal lung tissue. This results in more white blood cells being called in, only to fall into the Mannheimia trap, leading to more lung damage, and so on. Before long, the calf’s lungs are filled with fluid and pus where there should be air. Fever and breathing difficulties quickly ensue.

The key, then, is to ensure that Mannheimia does not enter the lungs. A good diet and good water intake reinforce the non-specific resistance offered by the upper respiratory tract. Less stress – long transport, mixing and processing – means lower cortisol levels and less chance of bacteria growing deeper in the lungs. Proper immunizations against viruses like IBR and BVD mean the lungs will be more resistant to bacteria like Mannheimia.

There are also vaccines against Mannheimia. It is best to give them well in advance of stresses like weaning and transport. Most of these vaccines have a new approach. They mobilize the calf’s immune system against this bacterial toxin that kills white blood cells. Therefore, they only really come into play after the rest of the body has already let the bacteria enter the lungs and begin their damage. Giving feeder cattle Mannheimia vaccines when they arrive is usually too late.

Mannheimia is a prime example of how a pathogenic bacterium has adapted and innovated to cause disease and evade an animal’s immune system. Ensuring calves are healthy enough to control it is key to avoiding its disastrous effects.

Russ Daly, DVM, is an extension veterinarian at South Dakota State University. He can be reached by email at [email protected] or 605-688-5171.