Aetna has become the latest health insurer to retreat from the Affordable Care Act's public exchanges, and the decision will have a major impact in Arizona.

Aetna is the nation's third-largest health insurer and announced Tuesday it would exit public exchanges in all but four states. This leaves Pinal County without a company offering marketplace health insurance plans next year. 

Aetna was the only insurer planning to offer ACA plans in Pinal County for 2017. It currently only sells in Maricopa County, but had planned to expand to Pinal.

Blue Cross Blue Shield Arizona and UnitedHealthcare currently cover about 8,500 people in Pinal County. United is pulling out of Arizona and Blue Cross is not offering plans there next year.

Arizona Insurance Department spokesman Stephen Briggs said Blue Cross filings would allow it to offer Pinal County plans.

A Blue Cross spokeswoman had no immediate comment on whether the company might reconsider.