Three more people in the US have died from fungal meningitis in an outbreak linked to tainted surgeries in Mexico, bringing the total deaths to seven, the Centers for Disease Control and Prevention reported Thursday.

The total case count remains unchanged from an update earlier this month, with 34 cases in the US: nine confirmed, 10 probable, and 15 suspected. Health officials are investigating 161 others who may have been exposed.

The outbreak is linked to cosmetic surgeries involving epidural anesthesia at two clinics in Matamoros, Mexico, just across the border from Brownsville, Texas. Mexican and US officials suspect that a component of the anesthetic was contaminated, resulting in the pathogenic fungus Fusarium solani being injected directly into people's spinal cords. The tainted surgeries are thought to have occurred between January 1, 2023, to May 13, 2023, around when the clinics were shut down by local health officials.

In an interview earlier this month, Dr. Tom Chiller, Mycotic Diseases Branch chief for the CDC, told Ars that Mexican officials suspect that morphine used in the epidural may be the culprit. The painkiller is mixed in with a widely used anesthetic, but morphine is in short supply in Mexico currently, which may have led to the use of gray- or black-market drugs.

Another possibility is simply poor hygiene practices by anesthesiologists. In Mexico, anesthesiologists do not use clinic or hospital stores for drugs; they procure their own medications and bring them into the facilities where they practice. This makes it hard to track down the specific drugs used in the outbreak cases.

Fungal meningitis is also difficult to test for and treat, Chiller noted. Tests looking for genetic traces of the fungus in the spinal fluid often miss it when it's present, he said. "The fungus itself isn't hanging out in that fluid; it's getting into the tissues," Chiller said. "It's wedging itself into the meninges and into the brain." There, it can stay indolent and quiet for weeks to months, "but then it can boom, then it can move rapidly," Chiller said. In the current outbreak, officials have seen cases with symptoms beginning 102 days after a contaminated surgery.

The CDC is urging anyone who had procedures at either of the two clinics—River Side Surgical Center or Clinica K-3—from January 1 to May 13, 2023, to get medical care immediately, even if there are no symptoms of infection. Health care providers are advised to give anyone exposed a lumbar puncture to look for signs of infection.

If symptoms have developed, they can include fever, headache, stiff neck, nausea, vomiting, sensitivity to light, and confusion.