Study connects enterovirus A7 to a large outbreak of neurological illness in the US in 2018


The Lancet Infectious Diseases recently published the results of an observational study led by researchers on Children’s Hospital Colorado Infectious Disease and Neurology teams, along with counterparts at the Centers for Disease Control and Colorado Department of Public Health and Environment.

The study was conducted from March 1 to November 30, 2018, and led to a discovery of the largest outbreak of enterovirus A71 (EV-A71) in the United States.

Since the 1990s, every 1 to 3 years, EV-A71 has caused large-scale, and sometimes deadly epidemics in the Asia-Pacific region, which has prompted the development of EV-A71 vaccines.

In the United States, detections of this virus have been small-scale and sporadic.

However, the unique symptoms, unusually high number of cases, and the geographic clustering of children who were observed during this study, indicated an outbreak.

“We need to watch this very closely,” said Kevin Messacar, MD, pediatric infectious disease physician and researcher at Children’s Hospital Colorado and University of Colorado Anschutz Medical Campus .

“Enhanced surveillance is needed in order to determine whether this outbreak was an isolated event, or a warning of impending cyclic outbreaks of EV-A71 neurological disease in the U.S.”

Finally, it’s important to note that these viruses tend to appear in seasonal waves. If through additional surveillance efforts, the United States continues to see enteroviruses circulating that cause neurological illness, the development of antivirals and vaccines may need to become a priority.

In addition to highlighting the need to improve enterovirus surveillance, the observational study also helped identify what other medical providers should be looking for.

Children with EV-A71 disease were best differentiated from children with other enteroviruses by the neurological findings of myoclonus (quick, involuntary muscle jerks), ataxia (dizziness), weakness and autonomic instability (dysregulation of heart rate, blood pressure and perfusion).

Often times these symptoms can be misunderstood or misattributed to other diagnoses – especially among young children.

Finally, it’s important to note that these viruses tend to appear in seasonal waves. If through additional surveillance efforts, the United States continues to see enteroviruses circulating that cause neurological illness, the development of antivirals and vaccines may need to become a priority.

“Were it not for Children’s Hospital Colorado’s ongoing interest and commitment to the study of enteroviruses, this outbreak would probably not have been detected,” noted Drs. Carol Glaser and Mike Wilson in a commentary published alongside the Lancet study.

“The USA has yet to have large-scale epidemics of enteroviruses as are seen in Asia and other countries, but it should take steps to become better prepared.”

Infection by an enterovirus such as enterovirus A71 (EV-A71) can be asymptomatic or manifest as a self-limiting influenza-like illness. However, EV-A71 is one of the most important neurotropic viruses known.

It is highly transmissible and infection results in hundreds of thousands of hospitalizations of children annually throughout the world, many of whom experienced severe or fatal neurologic consequences.

EV-A71 has been recognized as the most common pathogen of the hand, foot, and mouth disease (HFMD), which is highly contagious and frequently affects young children below 5 years of age.

EV-A71 can also occasionally cause serious neuropathology and cardiopulmonary complications, including aseptic meningitis, acute flaccid paralysis, brainstem encephalitis, and fatal myocarditis and pulmonary edema [12].

EV-A71 is a member of the genus Enterovirus within the family Picornaviridae. EV-A71 belongs to the species A enterovirus, which includes 25 other serotypes [3]. Although other members of the genus can infect multiple animals, EV-A71 only infects humans.

The virus has a positive-sense, single-stranded RNA genome encapsidated in a non-enveloped capsid virion. The viral genome is approximately 7500 bases in length and is flanked by 5′ and 3′ untranslated regions (UTR) and a polyadenylated tail of variable length [4].

The single open reading frame (ORF) encodes a large polyprotein, which is proteolytically cleaved by the viral protease into structural protein P1 (VP1-VP4), and nonstructural proteins P2 (2A-2C) and P3 (3A-3D).

EV-A71 is transmitted predominantly via oral-fecal route, but also through contact with virus-contaminated oral secretions, vesicular fluid, surfaces and fomites. It can also be transmitted through direct contact with patient’s aerosolized respiratory droplets [5].

EV-A71 can infect a wide range of cell types with different replicative capacity. Virus entry into susceptible host cells involves surface attachment, receptor binding and particle uptake into host cell through an endocytic pathway.

The specific host cellular receptor for EV-A71 remains unknown, but until recently, there has been at least five different types of human cellular receptors identified so far. The first characterized receptor is the human scavenger receptor class B member 2 (SCARB2), also known as the lysosomal integral membrane protein II or CD36b like-2 [6]. SCARB2 was also identified to be a receptor for coxsackievirus genotypes A (CV)-A7, A14 and A16 [7].

The second characterized receptor is the human P-selectin glycoprotein ligand-1 (PSGL-1), a membrane protein expressed on leukocytes. Several studies have shown, however, that only some strains of EV-A71 utilize this receptor for cell entry [8].

The third characterized receptor is the sialic-acid-linked glycan, which is express in abundance in the respiratory and gastrointestinal epithelium cells [9]. The fourth receptor is human annexin 2 protein, which was identified as cellular host factor that interacts with EV-A71 during viral entry into human rhabdomyosarcoma (RD) cells [10].

The fifth attachment receptor is heparan sulfate glycosaminoglycan, which is widely expressed in all cell types [11]. Heparan sulfate was also observed to facilitate infection of RD cells by CV-A16, thereby serving as its receptor [12]. However a recent study identified KREMEN1 as an entry receptor for CV-A10 and other coxsackievirus A [13].

A brief history and diagnosis of EV-A71

Historically, EV-A71 was first isolated from the feces of a female encephalitis patient in 1969 in California [14]. A retrospective analysis by a group in The Netherlands suggests that it could have emerged there as early as 1963 [15], consistent with records of probable epidemic of EV-A71 in the late nineteenth century in the United States, Europe, Australia, and Asia [16].

Between 1972 and 1990, EV-A71 outbreaks were reported in New York (1972 and 1977) [1718], Sweden (1973) [19], Bulgaria (1975) [20], Hungary (1978) [21], The Netherlands (1986) [22], and Brazil (1988–1990) [23].

More recently, EV-A71 and other enterovirus A infections are recognized as a major public health concern, especially after yearly HFMD outbreaks in several Asia-Pacific countries.

Clinical manifestation and severity of EV-A71 and other enterovirus infections are very similar, but their genetic background and pathogenic potential are notably different. As such, early and effective diagnostic techniques are required to differentiate these enteroviruses necessary for appropriate clinical management. Virus isolation has been the traditional diagnostic method to detect EV-A71, which involves taking clinical samples from patients and culturing in a variety of cell lines of human (RD, HEK293, HEp-2, HeLa cells) or other primate origin (Vero and COS-7 cells) [24].

However, this method has gradually been replaced by more sensitive and rapid molecular diagnostics. Work pioneered by Oberste et al. [2526] utilizes reverse-transcription polymerase chain reaction (RT-PCR) assay to examine the VP1 region, which combined with nucleotide sequencing could reveal the viral serotypes.

Nowadays, most diagnostic laboratories follow three basic techniques to definitively identify EV-A71, primarily isolation using tissue cultures, conventional immunological methods (indirect immunofluorescence and/or neutralization assay) [27], and nucleotide sequences from gene amplification using conventional and/or real-time RT-PCR [28].

The evolving methodologies used to diagnose and identify EV-A71 over the past decades have therefore confound the comparison of regional and temporal prevalence of EV-A71.

In addition, variation in the detection methods used in different countries and settings may also influence the effectiveness of disease surveillance and ultimately mortality and morbidity rate reported in the literatures.

Children’s Hospital Colorado


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