A virus that hunts down and treats cancer was used in patients for the first time, with encouraging results, Canadian researchers said Thursday.

Doctors at the Ottawa Hospital Research Institute found that an intravenous injection allowed the JX-594 virus to spread through the bloodstream and infect tumor cells anywhere in the body.

Healthy tissue was unharmed by the virus, which was derived from a strain used in the smallpox vaccine and engineered to enhance its anti-cancer properties.

"We are very excited because this is the first time in medical history that a viral therapy has been shown to consistently and selectively replicate in cancer tissue," according to the institute's cancer scientist, Dr. John Bell.

The ground-breaking trial was intended to test only safety -- but in six of the eight patients given the highest doses, the tumors shrank or stopped growing.

The 23 patients in the trial had a range of advanced cancers that no longer responded to existing treatments. Some suffered flu-like symptoms after being given a one-time infusion of the virus, but the treatment was otherwise well tolerated.

The trial was reported in the journal Nature, and further studies are planned.

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