(Bloomberg) -- US health officials cautioned Tuesday that monkeypox led to neurological complications in two otherwise healthy men in their 30s.

The first patient, a resident of Colorado, developed an itchy rash and then nine days later began experiencing numbness and weakness in his lower extremities, as well as issues with urination, according to a report from the Centers for Disease Control and Prevention. He was hospitalized and magnetic resonance imaging tests revealed lesions on various parts of his brain and spinal cord. The patient had a past syphilis infection, and monkeypox wasn’t found in spinal fluid tests, the report said. The skin lesions resolved over three weeks with treatment, but the man still needed assistance walking after a month.

The second patient, from Washington, D.C., was hospitalized after experiencing bladder issues and limpness in his legs five days after developing a rash, a more common symptom of monkeypox. The man then began showing signs of an altered mental state. MRI tests revealed lesions affecting his spinal cord and internal imaging showed signs of inflammation in the pelvis and rectum. The skin lesions healed after five weeks, and the man was discharged to an inpatient rehabilitation unit, the CDC said.

“Clinicians and public health professionals should be aware of the range of possible clinical presentations of MPXV infections and potential treatments,” the agency said in its report. Neither man had been vaccinated against monkeypox. 

It’s unclear whether the neurological issues were caused by the virus, or by an autoimmune reaction to the virus, something that warrants further investigation, the CDC said. 

The virus primarily causes painful lesions but some patients have experienced other, less common symptoms. Previous studies have also found rare links between monkeypox and neurological issues. One, published in the Journal of Infectious Diseases in 2004, found that one patient had a severe neurological infection during a 2003 outbreak in the US.

The first US monkeypox case was identified in May, and has spread rapidly, mostly among men who have sex with men. The US leads globally in confirmed cases, with nearly 22,000, according to the CDC.

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