Shares of pharmaceutical companies working on a drug to treat Ebola spiked Tuesday after the Center for Disease Control and Prevention confirmed the first case diagnosed in the United States.

Shares of Tekmira Pharmaceuticals surged more than 20 percent in extended-hours trading shortly after the news. BioCryst Pharmacuticals and Sarepta Therapeutics also rallied sharply.

The CDC is scheduled to hold a press conference at its Atlanta headquarters at 5:30 p.m. ET. (CNBC.com will be streaming this event live.)

Canadian drugmaker Tekmira said last week that U.S. and Canadian regulators have authorized the use of its Ebola treatment in patients who have confirmed or suspected infections from the disease. The drug works by preventing the virus from replicating.

Tekmira's drug was given to Rick Sacra, a doctor who contracted the virus in West Africa, according to the Nebraska Medical Center.