Some basic facts to ponder:
AKC only recognizes 158 breeds. There are rougly 45 AKC-recognized breeds in which megaesophagus is noted to be most likely genetic. There is no DNA test.

Most breeders do not jump @ the chance to tell their peers they have an affected litter. In other words, the megaesophagus topic can fly under the radar for a very long time. Think along the terms of decades/multiples of generations.

There are, known to me, a few breeds that are not counted in that list of 47, that should most likely be counted in.

It is not just the affected that should not be bred. The accepted ‘rule of thumb’ in the genetics circle is that, minimally, parents producing megaesophagus should never be bred to one another again (breeding to others remains questionable – no DNA test – one carrier or two?). Siblings can be carriers. Which ones? W/o a DNA test… Megaesophagus Roulette. In any one of the scenarios above, there is increased probability that the disorder will express again; plus more carriers are created.

Rule of thumb: As per Padget, highly revered geneticist:
The criteria for including a disease as genetic includes *any or* all of the following:
The disease has been reported as being genetic in the breed involved.

The disease has been reported to be genetic in other breeds.

The disease has been reported as being genetic in other species.

The disease follows family lines, and it occurs in multiple generations of a given line.”