More than 200 patients in more than 55 UK hospitals were discovered by healthcare workers to be infected or colonized by the multi-drug resistant fungus Candida auris, a globally emerging yeast pathogen that has experts nervous.

Three of the hospitals experienced large outbreaks, which as of Monday were all declared officially over by health authorities there. No deaths have been reported since the fungus was first detected in the country in 2013, but 27 affected patients have developed blood infections, which can be life-threatening. And about a quarter of the more than 200 cases were clinical infections.

Officials in the UK aimed to assuage fear of the fungus and assure patients that hospitals were safe. “Our enhanced surveillance shows a low risk to patients in healthcare settings. Most cases detected have not shown symptoms or developed an infection as a result of the fungus,” Dr Colin Brown, of Public Health England's national infection service, told the BBC.

Yet, public health experts are uneasy about the rapid emergence and level of drug resistance the pathogen is showing. In a surveillance update in July, the US Centers for Disease Control and Prevention said that C. auris “presents a serious global health threat.”

It was first identified in the ear of a patient in Japan in 2009. Since then, it has spread swiftly, showing up in more than a dozen countries, including the US, according to the CDC. So far, health officials have reported around 100 infections in nine US states and more than 100 other cases where the fungus was detected but wasn’t causing an infection.

Fungal foe

Though many people who pick up the fungus don’t develop an infection or develop a mild one, the fungus can be deadly in patients with compromised immune systems or other underlying conditions. More than a third of people who develop an invasive infection die.

Those invasive infections are often hard to halt because C. auris is unusually resistant to anti-fungal drugs. For instance, every C. auris case in the UK has shown reduced susceptibility to the first line antifungal fluconazole, and many are resistant to multiple drugs. Some are resistant to all three main classes of antifungal drugs used to treat Candida infections, azoles, echinocandins, and polyenes.

C. auris is also oddly good at spreading among patients and lurking in environments, particularly healthcare settings. One of the three UK hospitals hit with an outbreak reported having trouble stamping it out over more than a year. Environmental sampling revealed the fungus was on “the floor around bed sites, trollies, radiators, windowsills, equipment monitors and key pads, and also one air sample.” At least 50 patients were involved in the outbreak that began in April of 2015.

Healthcare officials have since adopted new protocols, including having healthcare workers wear more protective equipment and isolating all patients that are infected or colonized by the fungus. This month, Public Health England released new guidelines for managing the yeast’s emergence.