This post is going to have a bit of human medicine and a bit of veterinary medicine in it. Mainly because my husband was diagnosed with eosinophilic esophagitis.
What are Eosinophils?
Eosinophils are inflammatory cells that are in the family of the white blood cell population. These types of cells are usually elevated in conditions that are allergic or parasitic, and in cats, many inflammatory conditions cause an eosinophilia, such as inflammatory bowel disease or allergic bronchitis. They are called eosinophils because of their eosinophilic granules, that is a pink or reddish colour. Eosin is the part of the most common staining procedure used (H&E).
Eosinophils can infiltrate tissues, including the skin, the inside of the airway, the lining of the gut - usually areas that are exposed to the outside world - and yes, the gut lining is "exposed". Inflammatory cells respond to antigens - which are foreign proteins that are not supposed to penetrate the body. While a hypersensitivity reaction is an allergic reaction where the body has a hyper response to that antigen stimulus. Hypersensitivity responses can come in different forms.
What is Esophagitis?
Anything that ends in -itis just means inflammation of that structure, so esophagitis is inflammation of the esophagus.
Esophagitis in Cats
Cats have a different amount of skeletal muscle versus smooth muscle in their esophagus compared to dogs, with the lower portion having more smooth muscle fibers.
The anatomy of the cat esophagus has circular rings see on endoscopy, which in human medicine has now been called "feline esophagus" when the circular rings are seen in human endoscopy. This was a bit confusing when I was trying to look up esophagitis in cats because I kept getting photos of the esophagus of humans.
Esophagitis in cats can be due to drugs or caustic substances that have been swallowed. Likely things like licking those topical flea pesticide treatments off their cats causes esophagitis. Administration of tablet doxycycline (an antibiotic medication) can lead to esophagitis. But chronic intermittent vomiting and gastric reflux can cause inflammation of the esophagus. Foreign objects, bones, toys, garbage, etc can lead to this as well. Sometimes the cause is unknown, and then we call this idiopathic. If we cannot find an underlying cause, we may try management through a diet change and promotility medications.
Some cats develop inflammatory bowel disease (IBD) due to a food intolerance, or hypersensitivity.
Eosinophilic Esophagitis in People
Diagnosis is based on histological findings of eosinophils infiltrating the tissue of the esophagus, however, there may be gross visualization of pathognomonic characteristics suggesting the diagnosis, prior to histology. This is the "feline esophagus" of circular rings seen on endoscopic evaluation of patients with esophagitis.
As mentioned above, the eosinophils react to allergies. Therefore, if a person (or cat) has a food allergy, then the eosinophil reacts to these allergens present in the gastrointestinal tract. My husband is allergic to tree nuts. Tree nuts causes his mouth to become itchy, yet, he occasionally will consume almond milk, almonds in cereals or granola bars, or other treats. He also has an intolerance to dairy, though he has not been tested, the gastroenterologist suggests that he cut out dairy to see if this helps his esophagitis. I suspect that this condition as been going on too long, and he will need a balloon placed to relieve the stricture, but at least now he has a diagnosis of the problems he has been experiencing. The lesson learnt through this is, if you or your pet is regurgitating food, this could be a sign of esophageal disease. An endoscopy may be warranted.
However, in cats, since IBD can be related to these conditions, I start by doing an elimination diet, using a hypoallergenic diet, as roughly 50% of cats with IBD can be relieved of their symptoms of vomiting just through diet change alone.