SNAP ANALYSIS: New Canadian swine virus may be dead-end

WASHINGTON (Reuters) - Canadian health officials said late on Tuesday they had discovered two farm workers were infected with a new swine flu virus -- one unrelated to the current pandemic H1N1 virus.

They said both workers had recovered and the risks of the virus spreading were low. Here are some reasons why:

* People and pigs can both catch the same strains of influenza, but usually when people become infected by an animal, the virus stops there. The new H1N1 strain of swine flu is an exception, causing the first pandemic of the 21st century, because it acquired the ability to pass from person to person.

* Canadian health officials say the two workers, who never became seriously ill, had likely been infected by pigs. Some pigs in the herd also tested positive for influenza.

* The pandemic H1N1 swine flu virus, which has killed 429 people globally and likely now infects millions, was never traced to any specific pig. It was found only after it began spreading person to person.

* In contrast, the 50 or so known cases of human infection direct from pigs never moved into the general human population. They are known as dead-ends.

* From December 2005 to February 2009, the U.S. Centers for Disease Control and Prevention has documented 12 cases of human infection with swine flu.

* Cases of human infection with swine flu are almost certainly more common than this. Swine flu acts like any other influenza, causing chills, fever, sore throat, muscle pains, severe headache and coughing -- in other words, flu-like symptoms. Most people with those symptoms are never tested to see what they have.

* Since the new H1N1 broke out in March in Mexico and the United States, many countries have stepped up surveillance in both people and pigs. Officials are very likely to find cases of flu that would never have been detected without such heightened awareness.

(Editing by Peter Cooney)