I think the folks over at petpartners put it the best:
Some Rules for Interacting with People with Service Dogs:
1. Speak to the person first. Do not aim distracting or rude noises at the dog .
A service dog and its handler are a team. And should you want to approach a service dog team, please speak to the person first. Speaking, touching, or making rude noises to the dog may only confuse him or her.
2. Do not touch the service dog without asking for, and receiving, permission.
It may be tempting to pet or touch the service dog, but only do so after receiving permission from the dog's handler. Moreover, don't be insulted if your request is denied. Releasing the dog in order to greet you may distract his or her attentiveness to the handler.
3. Do not offer food to the service dog.
Do not offer a service dog food or dog treats. Even though service dogs are trained to ignore food on the ground and not beg for treats, your offerings may serve as a distraction for the dog. Besides, you can't be sure that the dog food or treat you are offering will not inadvertently cause the dog to become sick.
4. Do not ask personal questions about the handler's disability, or otherwise intrude on his
or her privacy.
Asking the handler about his or her disability is impolite and an intrusion of privacy. You should also never assume an animal is not a service dog if he or she does not wear a vest, patch, or any other item that would identify the dog as a service dog. This is not required by U.S. federal law.
5. Don't be offended if the handler does not wish to chat about the service dog.
If you have a dog with you, do not let him or her approach the service dog without first consulting with the handler. This can distract the service dog and, despite his or her training, may lead to an unwanted altercation between the animals.
Finally, there are many different types of Service dogs. Also, a service dog and a therapy dog are two different things. As defined by the Americans with Disabilities Act :
"Service animals are defined as dogs that are individually trained to do work or perform tasks for people with disabilities. Examples of such work or tasks include guiding people who are blind, alerting people who are deaf, pulling a wheelchair, alerting and protecting a person who is having a seizure, reminding a person with mental illness to take prescribed medications, calming a person with Post Traumatic Stress Disorder (PTSD) during an anxiety attack, or performing other duties. Service animals are working animals, not pets. The work or task a dog has been trained to provide must be directly related to the person’s disability. Dogs whose sole function is to provide comfort or emotional support do not qualify as service animals under the ADA."
As defined by wikipedia, a therapy dog is a dog trained to provide affection and comfort to people in hospitals, retirement homes, nursing homes, schools, hospices, disaster areas, and to people with learning difficulties.;
My dog is the people person, not me.
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