Update 8/22: Carmen and Mike Zapata have mapped the locations where Theo was last seen.
They've also created a Facebook page for those that might have seen him.
As of 10:22 p.m. Tuesday, Carmen said the couple has been unable to locate Theo but "we are still optimistic."
"We spent hours today putting flyers up in Mclean Hamlet and speaking with a variety of organizations," she said in an email.
Original 8/21: It was a puppy named Theo that helped Carmen Zapata recover from an emergency procedure in December 2006.
Zapata's husband, Mike, gifted his wife the puppy with hopes the 12-pound yorkie mix would serve as a therapy dog — and it worked.
"Theo has been a great member of our family since," Zapata wrote in an email to Patch.
This week, it is Theo that needs help after he went missing Sunday during a walk in the Colonies of McLean. The dog, who turns 6 this October, is diabetic and has missed more than four shots of insulin.
Zapata said Theo escaped from his collar in the neighborhood around 3:30 p.m. Sunday. Calls to the family have indicated he "managed to run up Chain Bridge and enter McLean Hamlet."
He was spotted there around 4:30 p.m. Sunday, but "nobody has seen him since," Zapata said.
Without insulin, the couple, who has lived in McLean since 2007, is "afraid that if he is not found immediately his chances of survival are slim to none."
If you have seen Theo, call 917-637-0562 or 917-528-4587 or click here for more information.