Child being examined in hospital
60 cases of acute flaccid myelitis have been reported in California since 2012 Jim Young/Reuters

The number of cases of a rare virus, which causes muscle paralysis and polio-like symptoms, is rapidly on the rise in California. Researchers found nearly 60 cases of the acute flaccid myelitis syndrome have appeared since 2012, while there were no cases in the 14 years previous.

Acute flaccid myelitis causes the sudden onset of weakness in at least one arm and/or leg, and nerve cells become inflamed in the spinal cord. A study has shown this virus suddenly increased in cases across California, with all but nine of them occurring in people under the age of 21.

The results, published in the Journal Of The American Medical Association, showed 59 cases of the virus were identified, with two deaths; 50 of those cases occurred in individuals younger than 21 years old. Cases were most recorded in young children, with the median age at nine years old.

The most common symptoms recorded included respiratory or gastrointestinal illness (54 cases), fever (47 cases) and limb muscle pain (41 cases).

Two of the 59 cases died within 60 days of first symptoms. Of 45 patients who came back for re-evaluation after nine months, 38 still had muscle weakness.

Researchers are speculating that the cause of the cases is due to an outbreak of Enterovirus 68, between August 2014 and January 2015. Its pathogens were present in one in three people showing symptoms of acute flaccid myelitis.

Although the numbers have continued to rise, the source of the virus is still unclear. The authors of the paper wrote: "Although the syndrome described is largely indistinguishable from [polio] on clinical grounds, laboratory studies have effectively excluded poliovirus as [a source]."

The report, by Keith Van Haren of Stanford University, was helped by the California Department of Public Health, who recorded all reported cases in the state since 2012. The CDPH started recording the cases after three separate reports of acute flaccid myelitis occurred in autumn 2012. There had been zero cases between 1998 and 2012.

The authors wrote: "Ongoing surveillance efforts are required to understand the full, and potentially evolving, levels of infectious agent-associated morbidity and mortality."

The Centers for Disease Control and Prevention reported in January that 103 children across 34 US states were infected with acute flaccid myelitis.