(Adds analyst comments, background on Humira, byline, updates stock price)

By Ransdell Pierson

April 25 (Reuters) - U.S. drugmaker AbbVie Inc reported better-than-expected quarterly sales and earnings, fueled by a sizzling performance from its Humira treatment for rheumatoid arthritis, the world's top-selling prescription medicine.

Company shares rose 2.6 percent in early trading.

Humira sales soared 17.5 percent to $2.64 billion, and represented almost 58 percent of the company's total sales, it said on Friday, accelerating from a 13 percent gain for the injectable drug in the prior quarter.

"Humira's growth was a little more than we expected and price increases in the United States are probably a big reason," said Morningstar analyst Damien Conover. Humira sales came in $90 million above his forecast.

Conover said Humira is one of the most effective treatments for rheumatoid arthritis and other inflammatory diseases, and has milder side effects than many rival drugs. "So it can get away with big price increases," without great resistance from insurers.

The big question is how long will Humira be AbbVie's cash cow and earnings driver.

Although Humira's U.S. patent lapses in late 2016, AbbVie officials have predicted it will take years for other drugmakers to develop their own version of Humira and win approval. To lessen its reliance on Humira, the company is attempting to develop treatments for hepatitis C, cancer and other diseases.

It expects approval later this year for a multi-pill treatment for hepatitis C, a potentially fatal liver disease that affects more than 3 million Americans. Analysts believe it eventually could rake in peak annual sales of $3 billion.

The regimen would compete with highly effective new oral drugs from other drugmakers, including Gilead Sciences Inc's hot-selling Sovaldi, which can wipe out the hepatitis C virus months faster than older treatments, with far milder side effects.

AbbVie said it earned $980 million, or 61 cents per share, in the first quarter, up from $968 million, or 60 cents, in the year-ago period.

Excluding special charges, AbbVie earned 71 cents per share. Analysts, on average, expected 68 cents, according to Thomson Reuters I/B/E/S.

Global revenues for AbbVie, spun off from Abbott Laboratories Inc early last year, rose 5.4 percent to $4.56 billion, above Wall Street expectations of $4.33 billion. Sales would have risen 6.7 percent, if not for the stronger dollar, which lowers the value of sales outside the United States.

The suburban Chicago-based company still expects full-year 2014 earnings of $3.00 to $3.10 per share, excluding special items.

AbbVie shares rose $1.31 to $50.63 on the New York Stock Exchange.

(Editing by Jeffrey Benkoe)