More children have been diagnosed with a mysterious paralyzing illness in recent weeks, and U.S. health officials said Tuesday that they still aren't sure what's causing it.

This year's count could surpass the tallies seen in similar outbreaks in 2014 and 2016, officials said. Fortunately, the disease remains rare: This year, there have been 90 cases spread among 27 states, the Centers for Disease Control and Prevention said.

It's not clear what's causing some children to lose the ability to move their face, neck, back, arms or legs. The symptoms tend to occur about a week after the children had a fever and respiratory illness.

Health officials call the condition acute flaccid myelitis. No one has died from it this year, but CDC officials say at least half the patients do not recover from the paralysis and some have serious complications.

Polio and West Nile virus have been ruled out. Doctors have suspected the cause might be some kind of enterovirus, which in most people causes cold symptoms. But CDC officials say that's not clear.

The first mysterious wave of paralysis cases in 2014 coincided with a wider spike in illnesses connected to an enterovirus called EV-D68, CDC officials said. But there was no such spike during the waves in 2016 or this year.

There's also a lack of clinical evidence: CDC officials have checked the spinal fluid of about three-quarters of the 90 patients, and found EV-68 in only one. Another type of enterovirus called EV-A71 was found in only one other patient.

But there are questions about that, too. If a virus is the cause, it's possible the test is not good enough, or the germ left the spinal fluid by the time the tests were taken, said the CDC's Dr. Nancy Messonnier. It's also possible the culprit is hiding elsewhere in the body.

Or perhaps the paralyzing illnesses are caused by some new germ for which no lab test has been developed. Or maybe there's some predisposing factor in some patients that cause their immune systems to react so severely to a germ or other trigger that the immune response causes paralysis, CDC officials said.

On Tuesday, Minnesota Senator Amy Klobuchar released the following letter she sent to CDC Director Robert Redfield:

“Dear Director Redfield:”

“I write to express my ongoing concern about the continuing rise in the number of children suffering from acute flaccid myelitis (AFM), and to request an update on the Centers for Disease Control and Prevention's (CDC) progress in working to identify the etiology of AFM and issuing updated clinical guidelines for treatment.”

“While I appreciate your response to my previous letters and willingness to provide staff briefings, important questions remain regarding the CDC's efforts to address the 80 cases of AFM that have now been confirmed in 25 states. Accordingly, I respectfully request answers to the following additional questions:”

“1. The CDC has identified many possible causes of AFM. Is there an update as to whether the CDC has identified a common etiology for the confirmed cases of AFM?”

“2. Which states have reported cases of AFM, and what is the breakdown of cases for each state? Do you have any other information as to the geographic areas that have been more affected by AFM? Which states mandate reporting of potential AFM cases to the CDC? Do these or other states only require AFM reporting if an illness is first identified as an outbreak by state and local health departments or other state authorities?”

“3. Concerns have been raised that AFM may be under-reported. Does the CDC have a view as to whether incidences of AFM are currently being under-reported? If so, what factors contribute to the potential incidence of under-reporting?”

“4. Is the CDC working with scientific experts to consolidate the existing research on AFM from various universities and other research institutions in an effort to provide both the families of those affected by AFM and clinicians the most updated information?”

“a. When will the CDC issue updated clinical guidance on the treatment of AFM?”

“b. I understand that you recently expressed support for the establishment of a task force to assess and respond to the recent increase in AFM cases. What would be the benefits of a task force like the one you described, and has the CDC taken action to establish such a task force? If not, would the CDC need additional authority, funding, or resources to convene a task force?”

“5. A child who became paralyzed from AFM recently underwent an innovative surgery that established connections between newly transferred nerves and the affected muscles, ultimately enabling the child to walk again. Has the CDC studied the safety and efficacy of the procedure for potential use in treating other former AFM cases? What diagnostics and other tools are the CDC and other agencies exploring to enable the earlier detection of AFM and potentially minimize the impact of the condition once it is discovered?”

“6. Does the CDC have an estimate of the overall financial burden of AFM to a family with a child who develops the condition and requires long-term health care services?”

“Thank you for your continued work to ensure that vulnerable young patients battling AFM and their families have hope for the future. I look forward to your response.”

Klobuchar previously sent a letter on this subject to the CDC in October and was joined days later by fellow Minnesota Senator Tina Smith and Iowa Senators Chuck Grassley and Joni Ernst in pressing for more information.

About 120 cases of AFM were confirmed in 2014, the first time such a wave occurred. Another 149 were reported in 2016. In 2015 and 2017, the counts were far lower, and it's not clear why.

The illnesses have spiked in September each year there's been a wave and tailed off significantly by November. But it can take weeks to determine which cases should be counted in the outbreak. More than 160 cases are still being investigated, and some of those may join the count, CDC officials said.