The threat of cord-cutters and cord-nevers to the cable and satellite TV industry just got a little more real.

After years of tracking the ebb and flow of pay-TV subscribers every quarter, analysts Craig Moffett and Michael Nathanson now see a turning point. "It's too soon to panic," they wrote in a research note released Monday for investors in cable and satellite television stocks. "But it's not too soon to be genuinely worried."

For the first time, the industry reported a net loss in subscribers in the first quarter, which has always been a strong time of year for pay TV. Although the number was tiny—31,000—MoffettNathanson Research calculates that the pay-TV industry, which includes cable, satellite and telecommunications providers such as Verizon, has contracted by 0.5% during the past 12 months.

That's particularly striking and a big step up from the 0.1% and 0.2% declines of the previous eight quarters, the analysts say, in light of the increase in the number of new households during the past year.

The 0.5% dip "may not sound dramatic, perhaps, but it's the fastest rate of decline on record, and it represents by far the largest sequential acceleration we have seen to date," Messrs. Moffett and Nathanson wrote.

Though cable and satellite TV companies have been seeing slight declines every quarter for the past two years, the first quarter of 2015 was marked by slowing growth for telecom pay-TV providers, which also include AT&T and Frontier. Their net subscriber additions grew by 8.4% in the quarter, compared with 13.6% a year ago.

MoffettNathanson blamed the overall decline on all the television shows that broadcasters and cable TV networks have licensed to the likes of Netflix and Hulu. The analysts also pointed out that the first-quarter results don't take into account the recent launches of streaming services such as HBO Now and Sony Vue—and the coming launch of Apple's over-the-top television service—which they expect will accelerate cord-cutting.

At the same time, there may be nuances in the pay-TV-subscriber numbers that paint a more complicated picture, according to cable analyst Bruce Leichtman, who has long been a skeptic of the cord-cutting scenario. He says the declines reflect a shift toward targeting higher-value customers, who spend more on premium programming packages and high-speed broadband.

Those customers are of particular interest to smaller cable operators, for whom pay TV is a lot less profitable than broadband services because they spend more than the big operators on programming costs.

"They want the customer who is going to provide the most revenue and stay with them longer," Mr. Leichtman said. Typically, that would mean targeting homeowners rather than renters.

As for the impact of streaming services like HBO Now, Mr. Leichtman acknowledged they could be a game-changer. "That's an offering that didn't exist a year ago," he said. "It sets up a different dynamic for the industry as a whole."