There are many theories surrounding the origin of the breed, however one thing that most archaeologists and historians can agree on is where they come from. Named after the city Chihuahua, Mexico these breeds were kept in the temples of the Aztecs to help control the rodent population. Some historians believe that Chihuahuas came from the island of Malta in the Mediterranean. Either way the early depictions of this breed had them as a bigger dog with a longer life span. What could of effected the Chihuahua's growth and life span could be the diet they adapted to over the centuries.
Since this breed's origins come from the rain forests and jungles of Mexico and South America, they would of ate a lot of fruits such as mangoes and avocado, meats such as rodents or wild boar and poultry.
We would recommend you feed your Chihuahua foods that contain avocado, blended with poultry and rice. You should avoid beef and beat pulp.