Generally speaking, dogs and cats will NOT be quarantined as long as they are healthy and have the required paperwork to enter the US. However, according to the Pets and Wildlife Division of US Customs, all domestic pets once taken out of the US are subject to the same requirements upon returning as pets entering the US for the first time. More detailed information about returning to the US with an animal may be found on the Center for Disease Control (CDC) website.
Pet dogs and cats may be examined at the border into the US for any sign of diseases that could be transmitted to other animals or humans. Dogs and cats that are entering the US from Mexico must have a valid rabies certification.
Thank goodness, some good news! There is no "duty" or fee for dogs or cats re-entering the US.
As with all other domestic cats and dogs entering the US, you will have to bring proof of your pet's vaccinations and a recent certificate of health with you. The good news is that when your dog is originally from Mexico, and you have a health certificate issued in Mexico from the Mexican Consulate, you can bring your dog back to Mexico by using those same documents as long as you do so within 6 months. You will not be required to get a new certification or proof of vaccination while you are in the US - as long as you don't stay for more than half a year.
Importation of dogs and cats into the US is regulated by the CDC.
According to the CDC: Adult dogs older than 15 months of age that have previously received a rabies vaccination given no earlier than 3 months of age and that has since expired may be imported immediately following booster vaccination, without the need to wait for 30 days.
All domestic cats and dogs must show no evidence of disease that can be communicable to humans at the port of entry.
Even if all of your paperwork is in order, if your dog or cat does not look like it is in good health, further examination by a licensed vet (at your expense!) will be necessary before your dog or cat is allowed to re-enter the US!
According to the CDC, the youngest a puppy can be to legally enter the US is 4 months old. Puppies can not get vaccinated before 3 months of age, and once vaccinated they need to wait 30 days before traveling so the vaccine can take effect.
Unfortunately, even if you depend upon your dog for sight and constant assistance, it will not be allowed re-entry to the US if it appears to US Customs inspectors to be ill in any way, or if you do not have the proper paperwork. All of the same rules and restrictions apply to Seeing Eye dogs that apply to regular domestic pets.
Center for Disease Control