The 73-year-old was passing an animal, believed to be a Staffordshire bull terrier, in Walton on 25 June when it bit his leg, Essex Police said.
The victim suffered a small wound and returned home. He became unwell two days later and was admitted to hospital, where he later died.
The force is now appealing to find the owner of the dog involved in the incident in Naze Court, Old Hall Lane.
Det Insp Gary Biddle said: "We are treating the dog owner as a witness at this time and are keen to talk to him to establish what occurred."
An elderly man has died after being bitten by a dog.