The current outbreak across North America has been unusual in terms of both symptoms and scale, says enterovirus expert Rafal Tokarz of Columbia University in New York.
Four deaths have been linked to the virus, though not conclusively.
"The virus has never been associated with such severe symptoms," notes Tokarz. "But at the same time there has never been such a large scale outbreak."
Tokarz says little is known about enterovirus D68 and very few scientists have worked with it.
What are enteroviruses?
Scientists have identified more than 120 types of enteroviruses, ranging from severe strains such as poliomyelitis and aseptic meningitis to the rhinovirus that causes the common cold. From 10 to 15 million infections take place annually in the U.S. according to Susan Gerber, the team lead for respiratory viruses at the Atlanta-based Centers for Disease Control and Prevention.
A mix of enteroviruses circulate every year, although not all of them circulate every year.
Besides respiratory illnesses, Gerber said enteroviruses can cause "febrile rash illnesses such as hand foot and mouth disease, and neurologic illnesses such as aseptic meningitis."
"Most infected people are asymptomatic or have mild symptoms," she said during a conference call.
Some enteroviruses go into the gastrointestinal tract and some infect the central nervous system, Tokarz points out.
Enteroviruses tend to circulate in the summer and fall, the Public Health Agency of Canada said.
What's known about enterovirusD68?
Gerber describes enterovirusD68 as "similar to rhinoviruses both genetically and by the illness that it causes."
At the National Microbiology Laboratory in Winnipeg, scientists have confirmed there are five sub-strains or "clades" of enterovirusD68 circulating in multiple provinces across Canada.
"Through genetic analysis of patient samples, the ... National Microbiology Laboratory has detected EV-D68 strains in Canada that are very similar to strains circulating in the United States, as well as other parts of the world, such as Europe," a spokesman told CBC News.
Scientists at the lab need to sequence the genome of the virus to distinguish enterovirusD68 from other types of enteroviruses and other common cold viruses, which can take three to five days after a sample is received, he added.
Tokarz says, "It remains to be seen whether this virus that's circulating in North America right now, whether it's a new clade, or whether it's a variant of a new clade, and what mutations it may have that may allow it to be more transmissible and pathogenic."
He describes D68's cellular appearance as not circular but with many different sides.
How long has D68 been around?
D68 was first identified in 1962 in California, from four children with pneumonia and bronchiolitis (an infection that clogs the tiny airways that lead to the lungs).
The CDC says it has received reports of D68 regularly since 1987, but the numbers have been very small.
Outbreaks of D68 grew increasingly more common in the past decade, as documented by RafalTokarz and his colleagues in 2012 in the Journal of General Virology.They flagged D68 as "an emerging respiratory pathogen."
"In Japan, the virus has been detected each year since 2003," they wrote. There was an outbreak among military recruits in San Diego starting in 2004, outbreaks in the Philippines in 2008 and 2009, resulting in two deaths, and in France, Italy, the Netherlands and New York City during the same period.
Tokarz told CBC News that he feels that the D68enterovirus behind the current infections in North America "is probably a newer strain of this virus, that's maybe more transmissible and perhaps more pathogenic than other strains." He stresses that he is only presenting an hypothesis, and "there's a lot more research that needs to be done."
How does enterovirusD68 spread?
The virus likely spreads from person to person when an infected person coughs, sneezes or touches a surface that is then touched by others, according to the CDC.
That’s why hand washing with soap and water for 20 seconds is recommended to prevent the spread of not only enterovirusD68 but other cold-like viruses, too.
Clusters of cases have been reported recently among children in British Columbia, Alberta, Saskatchewan, Manitoba, and parts ofOntario.
What treatment is available for enterovirusD68?
There is no vaccine or specific treatment besides how a common cold is treated — stay home when sick with lots of rest and fluids.
Beyond that, "there's really nothing that one can do," Tokarz laments.
There are antiviral drugs that work against other enteroviruses. The CDC has tested pleconaril, pocapavir,and vapendavir against the current strain but none of them has worked well.
Tokarz says enterovirusD68 "is going to become one of the mainstay respiratory disease-causing viruses, very much like influenza."
Why are symptoms more severe in some children?
When the lower airways become compromised by something like D68, there can be wheezing and trouble breathing that demand immediate medical care, doctors say.
Children who have asthma or who have weakened immune systems can have more serious symptoms.
"Children may be having more severe symptoms because of the combination of lack of immunity and smaller airways," said Dr. Danuta Skowronski, a physician epidemiologist with the B.C. Centre for Disease Control.
"It really is children with underlying conditions, and in particular those who have a history of asthma or wheezing who are having the most difficult time with this virus."
How widespread is the current outbreak?
In the U.S., from mid August to Oct. 6, CDC or state public health departments have confirmed 594 people from 43 states and the District of Columbia had respiratory illness caused by enterovirus D68.
The first reports of D68 infections were from Missouri and Illinois in August.
Does the virus cause paralysis?
Health officials are also investigating what role, if any, the virus has played in the paralysis of a handful of young patients in British Columbia and Alberta.
"It's a possible link," said Dr. Gregory Taylor, Canada’s chief public health officer, said Friday. "We have no evidence that ... this virus causes paralysis."
Taylor said there is nothing to suggest that the D68 virus has changed, but researchers continue to investigate that possibility.
When children go to hospital, doctors order a battery of tests to try to figure out what’s going on. "We’re detecting D68," Taylor said.
Can enterovirusD68 kill?
Deaths from enterovirusD68 rarely happen.
In the current outbreak, four deaths have been linked to D68, but it's not clear what, if any, role the virus had in those deaths, according to the CDC.
However, in the death of a four-year-old New Jersey boy, Eli Waller, the local medical examiner's office attributed his death to an enterovirusD68 infection.
In another case involving a 10-year-old Rhode Island girl, her doctors think the cause was the combined effects of two infections: Staphylococcus aureus sepsis bacteria and D68.
Why is the infection more severe in children?
This year, the vast majority of cases of enterovirus D68 have been young children, and that's usually the pattern.
Scientists say that may be because adults have built up immunity to the virus, and usually don't get sick enough to get tested, or even seek medical help.
As to why there seems to be more cases this year, Taylor suspects there’s a simple answer.
"Up until this year, we only had 82 positive samples for many, many years. This year we have 72 positive samples that we've detected. I'm convinced that's simply because people are requesting more tests."
Tokarz agrees that better and more frequent testing has resulted in increased findings of D68, but only up to a point.
He anticipates that "with increased screening, EV-D68 may ultimately be recognized as a respiratory pathogen in all age groups."
How long is this outbreak expected to last?
Tokarz says he expects "This outbreak's going to taper off soon, probably." That's based on the normal pattern of enterovirus outbreaks, which usually show up in the late summer and early fall, and then the season ends about the end of October.
"However, since we know so little about D68, it's difficult to predict," he adds.
In Kansas City, where one of the first two outbreaks began, doctors were seeing "in excess of 100 children a day," Dr. Mary Anne Jackson of Kansas City's Children's Mercy Hospital and Clinics said last month.
A few days ago she told the New York Times that the Kansas City numbers peaked in early September and recently began to fall, although slowly, and that she suspects, "at least in my community, we’re going to be out of enterovirus season soon, and we won’t be seeing it by the end of the month."