(Bloomberg) -- Argentine health authorities are investigating a cluster of pneumonia cases from an unknown cause that has led to three deaths among 10 cases identified so far. 

The illnesses have been centered around a private medical clinic in the city of San Miguel de Tucumán, Argentina’s fifth-largest city, located in the northwest part of the country. Health officials there have been giving daily updates as they investigate the cases.

Of the 10 known patients, three have died. That includes a 70-year-old suspected to be “patient zero,” who had been admitted for a surgical procedure and developed a lung infection around Aug. 20., according to Luis Medina Ruiz, the minister of public health of Tucumán. The onset of the patient’s symptoms coincided with those of the health-care workers who also fell ill. 

Symptoms have included fever, muscle aches, abdominal pain and shortness of breath, according to the Pan American Health Organization, which acts as the regional office for the World Health Organization in the Americas. 

Tests for respiratory viruses including Covid-19 and flu, as well as other known viral, bacterial and fungal agents, have all come back negative, according to a statement from the PAHO, meaning the cause of the pneumonia hasn’t been identified yet. Health officials said the use of antibiotic treatment may have wiped out evidence of the pneumonia’s cause, making it more difficult to find the culprit.

An Aug. 30 report first identified six patients, all of whom had contact with one another at the clinic. That group consisted of five health-care workers and one patient who was hospitalized at the clinic’s intensive care unit. On Thursday, another three health workers were found to have the illness. 

On Friday, authorities said they found a 10th patient, an 81-year-old man with other health conditions, bringing the number of hospitalized patients in critical condition to four. Three other patients were getting outpatient care, the officials said in their update, one of whom had been hospitalized earlier.   

Health authorities are conducting a review of clinic personnel who were in close contact and sharing workspaces with those who have fallen ill. About 20 people have been identified and are being followed closely, Medina Ruiz said. All patients who have developed the still-unknown illness had underlying health conditions, such as smoking, obesity, diabetes, high blood pressure or a history of respiratory symptoms. 

Several infectious disease experts said that the concentration of cases among health-care workers could indicate human-to-human spread of a pathogen, which would be a cause for concern. 

The PAHO and the WHO are assisting in the investigation and providing support to local health authorities, the organizations said. Contact tracing and control actions related to the health-care clinic where the outbreak occurred are being performed by local health authorities, the WHO said.

“Argentina has lots of experience in investigating and managing outbreaks of severe acute respiratory illness, particularly those caused by infections (e.g. pandemic H1N1 influenza, COVID-19, Andes hantavirus infection),” said Jake Dunning, a senior research fellow at the University of Oxford’s Pandemic Sciences Institute. “Hopefully this cluster will remain relatively small and the underlying cause will be identified quickly.” 

The US Centers for Disease Control and Prevention didn’t respond to a request for comment on Friday.

(Adds 10th case starting in first paragraph.)

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