The respiratory system in animals serves a vital function — without the ability to breath, animals will succumb within minutes.
Clean air with oxygen is breathed in through the nostrils and transported down the trachea and through the tree-like branches, where it enters the lungs. Here, oxygen is exchanged for carbon dioxide, refreshing the blood before the carbon dioxide is breathed out.
Each breath brings air from outside the body to deep in the lungs, creating an ongoing entry point for potentially damaging materials. To counteract this continuous onslaught, the respiratory system is stocked with a number of defence mechanisms that prevent disease from occurring.
First, there is the nasal cavity itself. It contains thin, scrolled bones that are lined by a special layer known as the mucosa, a slimy mucous membrane. The intricate nasal structure creates air turbulence that smashes large airborne particles such as bacteria, dust and viruses against the outer mucous layer. Trapped particles can then be “snotted out.”
Air next enters the trachea, which is lined by a similar sticky mucosa that traps the smaller particles that make it past the nose. This lining tissue has tiny hair-like projections called cilia that sweep mucus and trapped debris up into the throat where it can be safely swallowed or coughed out. The mucus layer in the nose and airways also contains antibodies and other immune secretions to trap and inactivate pathogens.
Deeper in the lungs, the defences change. Specific types of white blood cells hide out in the deepest part of the tissue, waiting for rogue pathogens to turn up. Known as alveolar macrophages, these function as the “generals” in the army against infection. If things start to go wrong, for example, if bacteria are inhaled, macrophages can call up foot soldiers, known as neutrophils, to back them up. The macrophages and neutrophils can eat bacteria and release enzymes to attempt to kill them.
This elaborate system is highly capable of dealing with normal inhaled pathogen challenges when an animal is healthy. But there are circumstances where the defence mechanisms are compromised, and that is when disease can occur.
One of the main ways the defence system is breached is by respiratory viruses. These pathogens directly damage the lining tissue when they infect cells and start replicating. This hinders the mucous clearance and prevents the hair-like cilia from doing their job. This allows large numbers of bacteria to enter deep in the lungs.
In cattle, a major virus that does this is bovine herpes virus 1 (infectious bovine rhinotracheitis), which sets the stage for bacterial pneumonia that can be life threatening.
Stress is another factor that can reduce the effectiveness of the normal defence mechanisms. The stress hormones, including adrenalin and cortisol, have a negative effect on the immune system throughout the body, including the lungs. With the lung immune cells in dysfunction, bacteria that enter these deep tissues can run rampant. In cattle, this is how mixing groups and shipping can predispose to pneumonia.
There are also the circumstances where the immune system overdoes it. In feedlot cattle with shipping fever, the immune system response to the bacteria causes catastrophic damage to the lungs, more so than the bacteria itself.
While there is plenty that can go wrong, we can also game this incredible system to our advantage for disease prevention. Good nutrition, managing stress and vaccinations can all optimize the respiratory defense systems for better animal health.
Dr. Jamie Rothenburger, DVM, MVetSc, PhD, DACVP, is a veterinarian who practices pathology and is an assistant professor at the University of Calgary’s Faculty of Veterinary Medicine.