Thirteen cases of Candida auris fungus have been recently confirmed in the United States. Scientists aren’t sure how it’s spreading.

The severe fungal infection emerging globally, Candida auris, has international public health officials worried.

Thirteen cases of C. auris have been identified in the United States in the past three years. The infection, often drug-resistant, can be fatal.

Seven of the U.S. cases occurred between May 2013 and August 2016. They were described this month in a Morbidity and Mortality Weekly Report from the U.S. Centers for Disease Control and Prevention (CDC).

The CDC report is the first to examine the U.S. cases of C. auris, which shared similar circumstances.

Cases of the infection also have been identified throughout England since 2013, according to Public Health England. Since April 2015, an adult critical care unit in England has been managing an outbreak of C. auris. More than 40 patients have been colonized or infected (plus two from another hospital), according to an announcement on the health agency’s website.

Read more: Get the facts on candida fungus skin infections »

In an interview with Healthline, Dr. Tom Chiller, M.P.H.T.M., chief of the CDC’s Mycotic Diseases Branch, said the seven U.S. patients all had serious underlying medical conditions and had been hospitalized an average of 18 days when C. auris was identified.

“In two instances, two patients had been treated in the same hospital or long-term care facility and had nearly identical fungal strains,” Chiller said. “This suggests that C. auris could be spread in healthcare settings. Similarly, most of the international patients were very sick and in the hospital when they got infected.”

Six of the seven cases were identified through retrospective review of hospital and reference laboratory records.

Chiller said that identifying C. auris requires specialized laboratory methods because it can easily be misidentified as another type of Candida infection. In those cases, patients may not receive appropriate treatment.

Most patient samples in the current CDC report were initially misidentified as another species of Candida, he said.

The CDC and its international partners conducted a collaborative analysis, which found that most patients had central venous catheters or urinary catheters, and half had recent surgery, he said.

Read more: Fungi plays a role in Crohn’s disease »

Some people have been treated successfully.

The major types of antifungal drugs are azoles, echinocandins, and polyenes. In the United States, some strains of C. auris have been shown to be resistant to one or more of these drugs, but none have been resistant to all three types of drugs, Chiller said.

“Patients have been able to clear C. auris infections after being treated with available antifungal drugs,” he said. “But four of the patients died, and it is not clear whether the deaths were associated with C. auris infection or the underlying health conditions.”

“Patients in the United States had strains of C. auris that were related to strains in South America and South Asia,” Chiller said. “However, a majority of these patients did not travel to or have any direct links to these parts of the world, suggesting they picked up the strain locally, in the United States.”

“We need to act now to better understand, contain, and stop the spread of this drug-resistant fungus,” Dr. Tom Frieden, M.D., M.P.H., the director of the CDC, said in an agency press release. “This is an emerging threat, and we need to protect vulnerable patients and others.”

Read more: Is there a fungal jungle lurking in your dishwasher? »

The fungus first emerged in Asia.

C. auris was first described in 2009 as a yeast found in the ear canal of a patient in Japan, Chiller said.

Since then, it has been identified in several other countries on four continents, with different strains among the continents. Other countries that have published reports of the infection include Colombia, India, Israel, Kenya, Kuwait, Pakistan, South Africa, South Korea, and Venezuela.

“This suggests that the strains are emerging independently and not spreading from region to region,” he said. “The earliest known infection with C. auris, based on retrospective testing, occurred in South Korea in 1996, but it is likely that other infections have occurred in the past as well.”

Chiller said there is no known “patient zero.”

“Nor is there likely to be one identified,” he said. “We know of no reports of C. auris being isolated from the natural environment, and the ultimate source of this pathogen remains a mystery.”

Much remains unknown about C. auris, including why it is resistant to one or more antifungal drugs.

Chiller said his agency does not know why C. auris has emerged recently in so many different locations. It may be emerging because of changes in the use of antifungal drugs in people or animals, or antifungal chemicals in the environment, he said.

The infection also may be something related to the organism mutating and adapting to become a “better” pathogen, he added.

Read More: Get the Facts on Parasitic Infections »

Strict hospitals procedures help to guard against the spread of the infection.

Chiller said the CDC recommends that healthcare personnel in hospitals and nursing homes place patients with C. auris in single rooms and follow strict Standard and Contact Precautions to control the spread of the infection.

Facilities should conduct thorough daily and after-discharge cleaning of rooms of C. auris patients with an EPA-registered disinfectant that is active against fungi.

Calculating the worldwide infection toll is proving complicated.

“It is difficult to know the total number of people worldwide who have been affected by C. auris,” Chiller said. “A review of the published literature and through communication with colleagues has turned up hundreds of cases from several continents. However, given that C. auris is difficult to identify without specialized laboratory methods — and that many cases likely go unpublished — the true number of cases is certainly much higher.”

“We do not have information on the total number of deaths,” he added. “But, in the recent collaborative study between CDC and its international partners, more than half of people with C. auris infection died before discharge from the hospital, though it is not known whether C. auris contributed to their death.”