(CNN) Public health officials in the United Kingdom are investigating a sudden unexplained increase in the number of cases of a polio-like condition that can cause weakness or paralysis and largely affects children.
Twenty-eight cases of the rare and potentially debilitating condition called acute flaccid paralysis, or AFP, have been reported in England in 2018, most of the cases since September.
A record number of cases of a similar condition, acute flaccid myelitis or AFM, have been seen in the United States this year. The US Centers for Disease Control and Prevention reported 165 confirmed cases of AFM in 36 states, up from 35 confirmed cases in 2017. There were 149 cases reported in 2016; the condition peaks in the fall every other year.
AFP includes a number of conditions characterized by a sudden onset of limb weakness, such as myelitis and Guillain-Barré syndrome.
Public Health England, the UK government's executive health agency, says anyone experiencing weakness in the limbs should seek medical care immediately.
"AFP is very rare," Dr. Mary Ramsay, head of immunizations at Public Health England, said in a statement. "We are investigating potential causes and working hard to build better awareness amongst health-care professionals about how to test and manage patients with AFP."
The condition is typically associated with a virus infection affecting the central nervous system. Epidemiologist Dr. Shamez Ladhani of Public Health England said the most common of these is the enterovirus, of which there are more than 100 types.
"The vast majority of those with enterovirus infection, and especially young children, either develop no symptoms at all, or they develop mild self-limiting respiratory illnesses," he said.
The US CDC says the cause of AFM cases there is still unknown. Last month, the agency said evidence suggests a "viral association" and noted that more than 90% of patients with AFM experienced mild respiratory illness or fever before weakness or paralysis began.
Public Health England noted that enteroviruses typically cause mild infections with symptoms including colds, coughs and diarrhea. Ladhani added that there is no treatment for the viruses, and the majority of people recover independently. However, in a small number of cases, individuals may "develop neurological symptoms, including AFP."
The agency is monitoring the cases to try to find the cause of the spike in diagnoses and to ensure that no cases are directly caused by polio, which is in the same virus family as enteroviruses.
"At present, it is not possible from this small number of cases to determine whether this represents an increase in cases or simply an increase in awareness and diagnosis," Ladhani said.
Ladhani nevertheless urged parents to seek medical advice if children develop muscle weakness and health care professionals to contact Public Health England if they suspect AFP in patients.
The agency warns that the condition can be difficult to diagnose, as there are many other causes of weakness.
Doctors will typically look at images of a patient's spinal cord and brain, along with examining their nervous system, in order to diagnose the condition. They can also test fluid around the brain and spinal cord or check nervous system conduction.
Dr. Beate Kampmann, director of The Vaccine Centre at the London School of Hygiene and Tropical Medicine, noted that paralysis of a limb is very rare but a "very worrying condition."
"There can be a number of reasons for such symptoms, and it is essential to determine the causes as quickly as possible, as the available treatments might vary a great deal," she said.
"A new type of enterovirus has recently emerged, and there was a larger outbreak in the United States in 2014. Doctors working with children and infectious diseases are alert to this strain, which has been associated with the limb paralysis and will be sending the appropriate specimens to the Public Health laboratories, where the specific testing is available and rapidly done."
She noted that not all of these cases will be connected to the EV-D68 strain of enterovirus, as reported by Public Health England, but warned that health officials "need to be on the watch and exclude other conditions."