Despite the requirement for a chiro to work under a veterinarians supervision in many states, this generally doesn't happen. Just the same as a lay equine dentist is required to work under a vet's supervision in most states but they don't. Just like farrier's aren't legally allowed to give any sort of sedation to another person's horses but they do.
For example, the OP here asked if she should have her horse seen by a vet or a chiro...... Many horse owners call a chiro expecting a diagnosis. And many of those chiro's are not vets.
Licensed Veterinary Technician