Enterovirus D68 (EV-D68), also known as enterovirus 68 (EV-68 or EV68), is a non-poliovirus, nonenveloped, positive-sense single-stranded RNA virus that belongs to the Picornaviridae family. See the images below. EV-D68 is transmitted person to person by contact with respiratory and gastrointestinal secretions.
While most of the approximately 100 species of enteroviruses primarily infect the gastrointestinal (GI) tract, EV-D68 has tropism for the respiratory tract. It causes acute respiratory disease ranging from mild upper respiratory tract symptoms to severe pneumonia. In an outpatient setting, EV-D68 disease has manifested most commonly among persons younger than 20 years and adults aged 50-59 years. Inpatients have predominately been children. [1, 2]
EV-D68 was first isolated in 4 pediatric patients with lower respiratory tract infection in California in 1962. 
In August 2014, EV-D68 emerged as a cause of severe respiratory infections. Hospitals in Illinois and Missouri reported an increased incidence of rhinovirus and enterovirus infection, with 30 of 36 isolates from the nasopharyngeal secretions of patients with severe respiratory illness identified as EV-D68. Following these reports, an unusually high number of patients with severe respiratory illness were admitted to these facilities, presumably with EV-D68 infection. 
Susceptible patients generally have underlying asthma and wheezing. In addition to respiratory illness, EV-D68 has also been associated with rare and lethal central nervous system (CNS) infection with polioviruslike manifestations.
Enteroviruses typically spread through the fecal-oral route, although some serotypes, like EV-D68, also spread via respiratory secretions. The enteroviruses invade the submucosal tissues of the distal pharynx and alimentary tract. The incubation period varies from 3-10 days, leading to a transient minor viremia that delivers virus to lymphoid tissues. Ongoing viral replication results in major viremia that spreads the virus to targets such as the CNS.
Enterovirus neurovirulence is believed to result from direct cellular destruction. [6, 7] EV-D68 pathogenesis was examined by Schieble et al  in suckling mice by intrathecal and intraperitoneal inoculation with 4 strains of EV-D68. CNS pathogenicity was observed only in mice inoculated with the Rhyne strain, showing limb tremor, weakness, paralysis, and death. This experiment is consistent with case reports of fatal meningoencephalitis resulting from EV-D68, serving as evidence of the neurovirulence of the virus. No animal models for EV-D68 respiratory infections have been documented. 
The immune response to enteroviruses is mainly humoral, mediated by secretory immunoglobulin A (IgA) in nasal and alimentary secretions. Serum neutralizing immunoglobulin G (IgG) is detected 7-10 days after infection and persists for life after natural enteroviral infection. Macrophages also play an important role in viral clearance. [6, 7]
Human EV-D68 is a small nonenveloped virus with single-stranded positive-sense RNA genome of 7.5 kilobases. It belongs to the enterovirus D species, within the Enterovirus genus in the Picornaviridae family. [10, 3] EV-D68 is acid-labile and behaves more like human rhinoviruses in its affinity for the respiratory tract.
Enterovirus D68 was first identified in California in 1962 in 4 pediatric patients with bronchiolitis and pneumonia.  Enterovirus surveillance in the United States between 1970 and 2005 identified only 26 cases of EV-D68 infection. In autumn 2009, in the context of pursuing respiratory virus surveillance during the H1N1 pandemic, a cluster of EV-D68 cases were detected in New York. 
A similar increase in detection of EV-D68 has been reported worldwide, although the increased incidence was believed to have resulted from the use of more highly sensitive virus identification methods such as polymerase chain reaction (PCR). Verifying stored respiratory samples from Yamagata, Japan and the Netherlands revealed EV-D68–positive samples in recent years.  In 2010, 40 cases were reported in Yamagata and 24 in the Netherlands.
The frequency of outbreaks may vary among countries.  In the Philippines, for example, EV-D68 outbreaks in the last decade appear to be cyclic in 2-year intervals: 2008-2009, 2011, and 2013-2014.  This may be partly due to differences in regional climate. In South Africa, 8 cases were detected between May 2000 and May 2001 in children aged 5-23 months. Three cases were reported from February to March 2010 in Senegal and 5 cases in June 2008 in Gambia. The climate in these countries consists primarily of dry and rainy seasons, but a much larger sample will be needed to assess the seasonal circulating patterns in non-temperate regions. 
From mid-August to October 2014, a total of 691 cases of EV-D68 infection had been reported in 46 states in the United States, especially among children with severe respiratory illnesses.  The ongoing 2014 EV-D68 outbreak in the United States is the largest and most widespread identified to date. Seven co-circulating strains of EV-D68 have been identified in this outbreak by extracting genome sequences generated from RNA from nasopharyngeal swab supernatant or from virus isolates obtained by inoculation of nasopharyngeal supernatants into human rhabdomyosarcoma cells. The VP1 gene sequences of the 7 EV-D68 strains are most closely related to EV-D68 viruses detected previously in the United States, Europe, and Asia in recent years. 
In August 2014, increased pediatric hospitalizations of children with severe respiratory illness were reported by Children’s Mercy Hospital in Kansas City, Missouri, and the University of Chicago Medicine Comer Children's Hospital in Illinois. Enterovirus D68 was identified in 19 of 22 patients in Kansas City and 11 of 14 specimens in Illinois. After these reports, an unusually high number of admissions for acute severe respiratory dysfunction continued. 
In September 2014, a case of acute flaccid paralysis following enterovirus D68–associated pneumonia was reported in France. 
From August 8 to September 15, 2014, 9 children aged 1-18 years (median age, 8 years) evaluated at Children’s Hospital Colorado presented with acute neurological illness manifesting as weakness, cranial nerve dysfunction (eg, diplopia, facial droop, dysphagia or dysarthria), or both following a nonspecific febrile upper respiratory illness. Using reverse-transcription PCR (RT-PCR), 6 of the 8 nasopharyngeal swabs were positive for rhinovirus/enterovirus. 
A few cases of human enterovirus as a cause of respiratory disease in Latin America have been reported. A study was conducted in 8 countries throughout Central and South America, where 3,375 nasopharyngeal swabs from subjects aged 1 month to 25 years (median age, 3 years) with influenzalike symptoms were collected; 84 samples (3%) were positive for human enterovirus via RT-PCR, of which EV-D68 was among the isolated serotypes. 
Between the 1960s and mid-1990s, EV-D68 underwent some genetic rearrangements. Such resulting phenotypic changes are thought to have a role in immune response modulation to the virus, which may aid in viral persistence.  Prior studies indicated that different phylogenetic lineages of EV-D68 co-circulate because of increased variability of the VP1 genomic region, leading to reduced cross-neutralizing antibodies raised against viruses of the major groups. 
EV-D68 circulates between summer and fall and appears to spread by close contact with infected people (cough, saliva, mucus, fomites).
Pediatric patients with a history of asthma, wheezing, or other underlying respiratory disease were shown to be at risk for more severe disease.  Children, especially those aged 1 month to 16 years, appear to be the most susceptible population based on most reports. They represented more than 80% of cases in both the Missouri and Illinois outbreaks (mean age, 4-5 years, respectively).  A 2014 report from the Netherlands found 95% of cases were in children.  Neurological disease has been described most commonly in males. 
The prognosis of EV-D68 infection varies, ranging from mild, self-limited respiratory infection to severe respiratory disease, especially in individuals with asthma.
In patients with CNS involvement, which is rare, residual neurologic deficits can persist. From a series of 23 cases of acute flaccid paralysis reported in California between 2012 and 2014, only 2 tested positive for EV-D68. The remaining cases had no clear etiology. Most cases require extended hospital stays (median, 17 days). Thirteen of 23 patients with available data had prolonged paralysis persisting at 60 days follow-up. In contrast, most patients who presented with mental status changes returned to baseline.