LOS ANGELES – A potentially lethal fungus normally found only in the tropics has established a foothold on the Pacific Coast of British Columbia and has slowly made its way southward into Washington, Oregon and Idaho, researchers said Thursday.

Health authorities are not unduly alarmed by the fungus because the number of cases so far remains small, but both federal and state officials are monitoring its progress out of fear that it will spread more rapidly as it reaches warmer climates.

“Overall, I don’t think it is a large threat at this time,” said molecular biologist Edmond J. Byrnes III of the Duke University Medical Center, the lead author of the report appearing in the online journal PLoS Pathogens. “But the fact that it is continuing to spread geographically and the number of cases is rising makes it a concern.”

The spread is also a concern because the strain of the fungus that moved into the United States in 2004 has mutated to become more lethal than the original strain that invaded British Columbia in 1999.

Five of the 21 people who contracted the fungus in the United States have died — about 25 percent — compared to 8.7 percent of the 218 infected people in Canada. The fungus also has infected many different species of mammals.

“This is really big news in the world of people who study (fungi),” said mycologist Karen Bartlett of the University of British Columbia, who was not involved in the current research but who is one of the leading experts on the outbreak. “It’s of clinical health importance because of the fact that the diagnosis can potentially be missed by physicians and veterinarians” because they don’t know about this new fungus.

The fungus in question is called Cryptococcus gatii, which is found primarily in tropical and subtropical regions. It is a close relative of Cryptococcus neoformans, which kills more than 600,000 people annually in such regions. C. gatii’s primary habitat in Canada is about 10 species of trees, primarily Douglas fir and western hemlock.

C. gatii seems to infect primarily healthy people, while its close relatives primarily infect those with a compromised immune system. Physicians seeing its symptoms in an otherwise healthy person might not think to look for it.

It is not transmitted from person to person or carried by insect or animal vectors. C. gatii, like all fungi, forms spores that are blown in the wind or moved around by disturbances of the soil. The spores are inhaled and colonize the lungs, then spread throughout the body.

Primary symptoms include prolonged coughing, night sweats, pneumonia and weight loss over a period of weeks. Meningitis can occur, which produces severe headaches. Treatment involves six to eight weeks of intravenous antifungal medications, such as amphotericin B, followed by up to six months of oral fluconazole.

The primary victims have been people who spend a lot of time outdoors, often in contact with soil, and those who do woodwork and construction.