Health Net Inc. said it may appeal a U.S. Department of Defense decision to award Aetna Inc. its longtime $2.8 billion annual contract to provide health care to active and retired military personnel and their families.

The new contract, including a 10-month base period and five one-year options, is valued at $17 billion and would begin April 1. The contract covers active-duty military personnel, National Guard and Reserve, retirees and dependents in 20 East Coast and Midwest states.

About 900 employees work on the contract out the Woodland Hills company’s Rancho Cordova offices.

Steven Tough, president of the Health Net Federal Services unit, said that the company would decide what action to take after a Defense Department briefing expected in the next few weeks.

“We will consider the information provided at the debriefing, and within two weeks following, we will determine whether we will accept or challenge the award decision,” Tough said in a statement late Monday.

Health Net was part of a pilot military program to have the private sector provide health coverage that began in 1988 under a predecessor program. It has held the contract for all but three years since then.

Humana Inc. also lost its $3.73 billion annual contract to cover 10 southern states to United Health Group. The only insurer retained the new round of contracts was TriWest Healthcare Alliance Corp., a Phoenix company which handles 21 Western states in a $2.9 billion annual contract.

Health Net shares closed down $2.07, or 14.6 percent, to $12.06 on the New York Stock Exchange.