The canine nasal mites, Pneumonyssus caninum, only live in dogs. Swedish studies show that it is found in 2% of autopsied dogs.
A similar frequency can be expected from Danish dogs. The largest of the mites measures 1.5 mm x 0.9 mm, which is quite big for a mite. They do not they eggs but give birth to live nymphs. What they feed on is not known. The nasal mites live in the nasal passages and paranasal sinuses of the dogs. When the dogs are sleeping (or are anesthetized), they can sometimes wander out of the nostrils, so you see them.
Usually, the infected dogs do not have any, but it is believed that the mites can cause sneezing, chronic rhinitis, etc. in dogs. No treatment is known. If control is necessary, a systemic preparation is the most obvious option. The canine nasal mite cannot infect humans. Its existence may have inspired the popular notions of “mites” in the sinuses as a cause of sinusitis in humans.