Mites usually don't stay more than a week or two. They bite the skin to suck blood and other fluids, but this causes the skin to scab. Since mites cannot bite through the scab, they don't get enough food anymore and either starve or leave their host to find a new one. This means that ear mites are contagious!
Additionally, cats often become extremely irritated and stressed by mites and often cannot sleep because they hear the mites in their own ears.
Apart from mites, there are several more ways cats can have thick black wax in their ears.
- Cats and dogs excrete toxins through the skin in their ears. It could be that your cat is allergic to its food or any medication he's receiving.
- His ears could be infected with yeast or bacteria
- There could be polyps or other tissue growth in the ear
- It could have had mites in the past which now causes the ears to produce more wax in general.
In any case, if the first vet didn't analyze the ear wax, you should definitely ask a second vet. The wax needs to be examined under a microscope and in a laboratory to find out which pathogen caused the infection or to exclude an infection as the cause.
A word of warning: Cats are very sensitive to problems with their ears. Should the wax or mites or whatever completely block the ears, your cat will scratch it's cheeks rather than the ears.
We have a cat who was lost for several months before coming back, teeming with all kinds of parasites, including ear mites. He scratched all the fur from both cheeks and ears. Most of the fur grew back, but he now has very thick scar tissue (because he wouldn't stop scratching, even if he had scratched himself bloody) and several polyps in his ears. He's constantly producing too much wax and we have to clean his ears regularly or he starts scratching again.
I hope it doesn't come to this for your furry friend.