Entropion is when your eyelids get turned in, and your eyelashes rub against your eyeballs all the time, which hurts a lot. It can happen to dogs and cats and people. Maybe it can happen to some other animals, too, but I don't have time to research whether it does or not. So mostly I am going to talk about entropion in dogs.
Some dog breeds are more likely to get entropion than others, like for example shar peis, chow-chows, and bulldogs. Other breeds that get entropion fairly often are bloodhounds, Irish setters, Labrador retrievers, St. Bernards, cocker spaniels, boxers, springer spaniels, golden retrievers, Cavalier King Charles spaniels, Neapolitan mastiffs, bull mastiffs, shiba inus, rottweilers, poodles, Great Danes, and Chesapeake Bay retrievers.
Entropion may show up in puppies when they are only a few weeks old. In this case, it is often treated by tacking the puppy's eyelids so that the corneas are not damaged as the puppy grows. After 3 or 4 weeks, the stitches can be removed, and the eyelids may grow normally. This technique is mostly used on very young puppies.
Dogs that are older than 6 months will probably have surgery to take out some of the eyelid skin. There are several ways to do this, and the method used depends on how bad the entropion is. When this surgery heals up, the eyelid should be in a more normal position so that the eyelashes don't rub on the eye. If the correction is done too much, the dog will end up with ectropion, which is the opposite of entropion. In ectropion, the eyelid turns out instead of in, and this lets the inside of the eyelid hang out, which it is not supposed to do.
Usually it is the lower eyelid that turns in and rubs on the cornea, but sometimes it is the upper one. A dog can have entropion in one eye or in both eyes. One way to get entropion is to inherit it, but you can also get it because of having an eye disease, infection, or injury.
When cats get entropion, it is usually because of an injury or infection that causes a change in the eyelid. But sometimes those breeds with smooshy faces are born with the condition.
If entropion is not fixed, the animal will keep getting its cornea damaged by having hair rubbing over it. Then after a while, the cornea will get so wounded and scarred that the animal will go blind. So if you have a dog or cat with entropion, you need to get it taken care of right away.
Mom has known some dogs at the shelter who had entropion, but the nice vets there did surgery and made those dogs' eyelids all better. Usually the dogs who had it were some kind of shar pei mix, but not always. Anyway, I have never heard of a basenji getting it, but I suppose it's possible. At least I'm glad that I don't have it. I have enough other health problems to worry about!