Bloomberg News Humana Inc. headquarters in Louisville, Ky.Walmart Inc. is in preliminary talks to buy insurer Humana Inc., according to people familiar with the matter, a deal that would mark a dramatic shift for the retail behemoth and would be the latest in a recent flurry of big deals in health-care services. It isn’t clear what terms the companies may be discussing, and there is no guarantee they will strike a deal. If they do, the deal would be big: Humana currently has a market value of about $37 billion. It also would be Walmart’s largest deal by far, eclipsing its 1999 acquisition of the U.K.’s Asda Group PLC for $10.8 billion. Walmart, which in addition to being the world’s biggest retailer is also a major drugstore operator, has a market value of about $260 billion. The two companies are discussing a range of options, including an acquisition, one of the people familiar said. Shares of Humana HUM, +0.48% surged 10% to $297 in after-hours trading after The Wall Street Journal first reported the talks. Walmart shares WMT, +1.37% slipped 1% to $88.10 in late action. An expanded version of this report appears on WSJ.com.