Why Most Megaesophagus Dogs with Pneumonia Don’t Cough:
From Dr. Kathy Stilwell-Morris on the Yahoo group:

In an attempt to provide owners documentation as to why any megaesophagus patient who has increased regurgitation, vomiting, lethargy, or loss of appetite should collect 3-view chest x-rays to rule out pneumonia, this information has been created. Please consider printing out this information for your veterinarian if they are unwilling to take x-rays, so that they may understand what has taken me a long time to find out.

Since I began participating on this advice board (Yahoo Megaesophagus) 12, or so, years ago, I have been perplexed as to why megaesophagus dogs do not frequently cough when they have Aspiration Pneumonia. And, when we veterinarians auscultate (listen to) the lungs of these dogs, who may have even severe lung disease, frequently we hear no abnormalities…no wheezing, no rales, etc. Recently, I was able to attend an all day respiratory seminar, at which I asked the specialist about this, and also asked the same question of cardio-pulmonary specialists on VIN (Veterinary Information Network).

Below, I have attempted to share what I have learned, in hopes that when megaesophagus dog owners encourage their dvm’s to take 3-view x-rays, they can provide this information as validation for the request.

For an individual to cough, there are certain “sensors” that must be irritated, most of which are located in the trachea. There are none of those sensors within the lung tissue, bronchioles or alveoli. So, if the trachea is not affected, no cough, even if there is severe disease. Why this is so common in dogs with megaesophagus is not known. In humans, this would be called “walking pneumonia” – i.e., this is the diagnosis in people with pneumonia who have no cough.

The following article discusses many of the mechanisms that affect coughing.
*****https://coughjournal.biomedcentral.com/articles/10.1186/1745-9974-1-2 This was the original item but is no longer coming up when searched so probably archived at above link.

This extensive article discusses, in depth, the “cough reflex,” which is extremely complicated and can be affected (increased or decreased) by a variety of mechanisms.

This is a brief introduction to the article:

The cough reflex represents a primary defensive mechanism for airway protection in a variety of mammalian species, [AND, THE WAY THAT IT WORKS INVOLVES MANY NERVES, MUSCLES AND THE BRAIN AND IS NOT WELL UNDERSTOOD].

Coughing is generally characterized by a reflex-evoked modification of breathing pattern in response to airway irritation. Reflex cough occurs when subsets of airway afferent (sensory) nerves are activated by inhaled, aspirated or locally produced substances. These afferent nerves provide modifying inputs to the brainstem neural elements controlling respiration, and consequently help generate the cough respiratory pattern. Although widely studied for many years, there has been much debate surrounding the identity of the airway afferent nerve subtype that precipitates reflex coughing. In addition, cough can also be initiated voluntarily. Little is known about the cortical [BRAIN] pathways responsible for voluntary coughing, although they likely share similarities with those pathways responsible for voluntary breath holding and other conscious modifications of respiration.

After much discussion, the article concludes: “The recent increased interest in cough reflex sensory neurobiology has unveiled a previously unrecognized complexity in the interacting roles of multiple afferent nerve subtypes in regulating this defensive reflex. However, further careful dissection of the cough sensory pathways is still required for the identification of future therapeutic targets for the effective treatment of cough disorders.”

For a veterinarian to perceive abnormalities in the lungs just by listening (auscultation), there must be turbulence – i.e. bubbles being formed as air within the bronchioles passes through debris/pus. If there is no fluid within the bronchioles (as when there is solid material), or no bubbles are passing through fluid, there will not be any noise. So, just because we are unable to HEAR abnormalities doesn’t mean that none exist.

So, the bottom line is that we do not understand all of the reasons why some dogs/cats/humans cough with lung disease, and others don’t. And, we cannot always hear abnormalities with a stethoscope. But, because Aspiration Pneumonia is so common in dogs with megaesophagus, any change in behavior (increased regurgitation, vomiting, loss of appetite, lethargy) should alert an owner and veterinarian to consider collecting 3-view chest x-rays.