Animal Health Matters: Bovine respiratory disease and the nastiest germ that causes it

Russ Daly
Special to the Farm Forum
Russ Daly

If there’s one academic discipline that veterinarians are well-versed in, it’s microbiology. Some version of that subject was almost constantly on our class schedules from undergrad through vet school. General microbiology was the appetizer for what was to come: 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 caring for food animals – it’s appropriate to spend time learning about those “bugs." Our animal patients are more likely to be affected by infectious diseases compared to human patients treated by a physician.

This microbiology slant leads us veterinarians, as we pore over laboratory results from a producer’s sick or dead animals, to regale animal owners with exotic-sounding germ names. Some of those terms go over the heads of people who haven’t taken five semesters of microbiology, but I’ve found over the years that producers have become more familiar themselves with many of them. This unfortunately includes Mannheimia hemolytica.

Mannheimia is frequently grown by the veterinary lab from specimens from calves with pneumonia. It’s not the only type of bacteria involved in cattle pneumonia, but it is the most important one.

When we get pneumonia ourselves, the question is, “Where did we catch it? We were fine before we inhaled nasty germs from another person, and now we’re sick." It’s tempting to assume the same 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 tracts. Mannheimia hemolytica has a “commensal” relationship with cattle, living on body surfaces like the nasal passages, but neither harming nor benefiting the calf.

If Mannheimia is present in normal cattle, then how can it cause such problems? When Mannheimia is confined to the upper respiratory tract, it doesn’t.

The game changes, however, if Mannheimia hemolytica gets to the lower lungs. When the bacteria get down there, the body reacts fiercely 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 close connection between blood vessels and air sacs in that part of the lungs.

Normally, an influx of bacteria-attacking white blood cells is just what the calf wants when an infection occurs. However, Mannheimia has a nasty counter move. The bacteria produces a toxin that specifically kills these white blood cells. Not only is this defense mechanism neutralized, when the white cells break apart, their enzymes damage the normal lung tissue. This results in more white blood cells being called in, only to fall into Mannheimia’s trap, which results in 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 difficult breathing soon ensue.

The key, then, is making sure Mannheimia doesn’t get down into the lungs. Good feed and water intake bolster the non-specific resistance offered by the upper respiratory tract. Diminishing stress – long transportation, comingling, and processing – means lower cortisol levels and less of a chance the bacteria will proliferate deeper into the lungs. Proper immunizations against viruses like IBR and BVD means the lungs will be more resistant to bacteria like Mannheimia.

There are vaccines against Mannheimia, also. They’re best given well ahead of stresses like weaning and transport. Most of these vaccines have a novel approach. They mobilize the calf’s immune system against that white blood cell-killing bacterial toxin. Therefore, they really only come into play after the rest of the body has already let the bacteria get into the lungs and begin their damage. Giving feedlot cattle Mannheimia vaccines on arrival is usually too late.

Mannheimia is a great example of how a pathogenic bacteria has adapted and innovated to cause illness and evade an animal’s immune system. Making sure calves are healthy enough to keep it in check is the key to avoiding its dire effects.

Russ Daly, DVM, is the Extension Veterinarian at South Dakota State University.  He can be reached via e-mail at russell.daly@sdstate.edu or at 605-688-5171.