Minnesota is reporting a seventh case of a rare, paralyzing illness in children that has now been detected in 22 states.

Federal health officials are trying to confirm that the child has acute flaccid myelitis, or AFM, a polio-like illness that causes paralysis or limited mobility, said Kris Ehresmann, who directs the infectious disease section of the Minnesota Department of Health.

The symptoms match those of the other children in the state who have been diagnosed, Ehresmann said Tuesday.

"It's one more case too many," she said.

The additional child was announced by the state Tuesday while the U.S. Centers for Disease Control and Prevention (CDC) reported an increase in cases nationally and slight progress in narrowing down the mysterious causes of AFM.

The condition gained national attention in 2014, when outbreaks in California and Colorado led to a total of 120 cases in U.S. children. Another 149 cases were reported in 2016. This year, the CDC has received reports of 127 cases from 22 states, but has only confirmed 62 of them.

The paralyzing symptoms of AFM are caused by inflammation in the spinal cord. Experts suspect a combination of viral, genetic and environmental causes at work.

CDC officials said Tuesday that they have ruled out the polio virus, which caused similar disabling symptoms until it was eradicated in North America through vaccination. Just to be sure, they checked the children suffering from AFM and found no trace. Mosquito-born West Nile virus had also been suspected as a cause, but it isn't showing up in the latest cases either.

Despite these findings, the CDC has been unable to identify a common cause, said Dr. Nancy Messonnier, who directs the CDC's national center for immunization and respiratory diseases. "There is a lot we don't know about AFM, and I am frustrated that despite all of our efforts, we haven't been able to identify the cause of this mystery illness."

All of the Minnesota cases involved children who first experienced common viral cold symptoms, but Ehresmann said that is about all they had in common. The children, all age 10 and younger, came from different parts of the state.

"We're talking about a temporal cluster, a cluster in time," Ehresmann said, "but we didn't see any commonalities for location — the same city, the same school, even the same part of the state."

The presence of viruses in the Minnesota children only adds to the mystery. Not all children nationally have first suffered from viruses before having AFM symptoms. And among those who did, testing found different kinds of viruses at work.

AFM cases appear to spike in August and September, and in an every-other-year cycle. Why isn't understood, but Messonnier said this pattern is not being seen in other countries.

State and federal health officials said concerned parents should follow good public health practices, such as washing hands and covering coughs, and seek medical attention if their children report sudden limb weakness. But they also urged calm, when considering that only one in 1 million children suffers this affliction.

"As a parent myself, I understand what it's like to be scared for your child," Messonnier said. "Parents need to know that AFM is rare even with the increase in cases we are seeing now."

AFM can be fatal if it causes paralysis to the stomach muscles that support breathing, but none of the Minnesota children have died, Ehresmann said.

Recovery has been varied. Some of the Minnesota children regained control of their hands and wrists, for example, but still cannot move their upper arms.

Five of the Minnesota children are receiving physical therapy through Gillette Children's Specialty Healthcare in St. Paul and Minnetonka. Exercises sometimes include electrical stimulation to spark nerves and muscles into action. Some children with AFM undergo nerve transplants to regain lost mobility.

"We do the best we can to see how much function they can gain back or how much we can stop the progression" of nerve damage, said Dr. Angela Sinner, a pediatric rehabilitation specialist at Gillette.

AFM cases aren't formally tracked, so states have been voluntarily reporting them to the CDC. The public disclosure of the six Minnesota cases earlier this month caused national interest in AFM, Ehresmann said.

Ehresmann said she hopes that will accelerate public reporting of cases and research and understanding of the causes and treatments, because the state didn't have enough cases to figure it out. The state has been interviewing the children and families, and providing their medical records and lab specimens to the CDC for investigation.

"Six cases is too many," Ehresmann said, "but it's not enough necessarily to get to the bottom of this."

Jeremy Olson • 612-673-7744