This is because the infection is deep in the ear canal and has not come to the outside where it can be seen yet. The veterinarian can see the early infection in the canal with an otoscope. Then a sample is taken from the canal to determine what is causing the problem. Sometimes ear mites can be seen with the otoscope alone if the infestation is bad enough. Other times the vet may look under the microscope to look for the parasite.
Ear mites look like tiny ticks and are indeed in the tick family.
They also feed on blood like a tick. If ear mites are diagnosed, then an ear drop containing an insecticide is prescribed to kill the mites. Also an ear cleaner is prescribed to clean the ear of the debris the mite leaves behind and to flush out the dead mites and old medicine. If ear mites are not the cause, the vet may take the sample from the ear and stain it with a special stain to look for bacteria or yeast. Some bacteria can be roughly identified this way and this can help the vet determine what medication to use.