Emerging and Re-emerging Enterovirus Infections: The Known Unknowns

INTRODUCTION TO THE ENTEROVIRUSES The enteroviruses are a genus of viruses displaying high genetic and antigenic diversity, belonging to the Picornaviridae family. They share common features including a single-stranded, positive sense RNA genome of around 7500 kb—encapsulated into a nonenveloped protein particle.1,2 Early classification was based on serotype (the ability of a specific antiserum to neutralize a virus) or on clinical features. The resulting nomenclature was often confusing and rarely consistent. The enteroviruses are now classified based on genetic characteristics alone, first into species Enterovirus A to L (A to D containing the clinically relevant viruses for human health) and the Rhinoviruses A to C. Within these species sit the various enterovirus types, including well-recognized viruses such as the coxsackieviruses and polioviruses2 EPIDEMIOLOGY AND SURVEILLANCE Information on the circulation patterns of the nonpolio enteroviruses by type is largely obtained as a byproduct of the global systematic poliovirus surveillance systems, from bespoke studies, or from regions that have introduced routine typing. The data from this ad hoc surveillance suggest that enterovirus circulation is complex, with dominant types circulating in a background of other types. Some have been shown to have biannual epidemics, but those epidemics can also be highly regional in nature.3,4 This lack of global systematic surveillance leaves a void in our understanding of what the circulation patterns and typical clinical features are for different types and all vital background information to help inform when cases and epidemics of emerging and re-emerging enteroviruses occur. ROUTINE DIAGNOSIS AND TYPING OF ENTEROVIRUSES Diagnosis of acute infection is problematic due to little guidance on sampling and testing, the wide spectrum of disease, and the use of molecular-based methods for diagnosis, which are often not specific, nor sensitive for all types5 The genetic similarity between the enteroviruses and rhinoviruses means that they warrant a single genus for classification, despite differences in symptomatic disease. However, this genetic similarity means that there are often specificity issues in molecular tests. This problem is especially apparent in respiratory samples. Most commercial tests for rhinoviruses in respiratory samples report positive results as rhinovirus/enterovirus positive, with an onus on the testing laboratory to determine which is present if discrimination is required. A more specific enterovirus polymerase chain reaction is the preferred method to diagnose acute infection from sample types such as CSF, blood, vesicular fluid, and stool samples. A specific enterovirus polymerase chain reaction can also be used on respiratory samples although there may be ongoing specificity issues with some rhinoviruses, which can occur even with the best designed assays.5 The name “entero” comes from the Greek enteron meaning intestine, relating to the primary replication site for most of the enteroviruses. Fecal samples, therefore, play an important role in the diagnosis of acute infection. Fecal samples can also play a role in the diagnosis later in the course of infection due to the prolonged shedding of enteroviruses into the gut after symptoms have stopped. This is one reason that waste water plays an important role in the surveillance of polioviruses as we head for global elimination status and eventually eradication. Typing methods can be used to identify the enterovirus detected, and several methods have been described. Most frequently, genome sequencing is performed using classical Sanger methods targeting diverse regions of the genome such as the viral protein (VP) VP1 and the junction between VP2 and VP4. Next-generation sequencing (NGS) can also be used; however, this is most often used for single-family members or as part of a metagenomics approach to finding the cause of infections of uncertain etiology or in specific surveillance studies. Currently, neither NGS nor metagenomics is yet widely deployed routinely for enterovirus typing. Type-specific polymerase chain reaction is a rapid and useful approach for laboratories without any typing capability, with tests described for enterovirus D68 (EV-D68), enterovirus A71 (EV-A71), and poliovirus amongst others.5,6 ENTEROVIRUS DISEASE AND VIRUSES OF RECENT CONCERN There is a wide spectrum of diseases associated with enterovirus infections. Importantly, asymptomatic infection is common, and this combined with transmission largely through the fecal-oral route means that enteroviruses are common infections during childhood. Hand Foot and Mouth Disease and Coxsackievirus A6 and EV-A71 The onset of enterovirus disease often begins with a mild upper respiratory tract infection and/or diarrhea with high fever. This can then progress to other clinical features including nonspecific rashes, which are easily confused with other rash-causing infections in childhood. The typical vesicular hand foot and mouth disease (HFMD) while associated with many coxsackie A viruses is also seen in EV-A71 infection. HFMD is often epidemic in nature, and transmission is common in nursery and school settings. Coxsackievirus A6 emerged as a significant pathogen in the mid-2000s and is associated with a widespread vesicular rash, including in the nappy region in babies.7,8 This is commonly misdiagnosed clinically as eczema herpeticum or chickenpox. Treatment with acyclovir and flucloxacillin is often started empirically, with samples not being sent for laboratory confirmation, meaning that it is likely to be underdiagnosed and often treated inappropriately. Coxsackievirus A6 has also been confirmed as present in the CSF of cases of aseptic meningitis when there are epidemics in children; however, it is not clear how many present with a vesicular rash prior to the CNS signs and symptoms. This contrasts with EV-A71, a significant epidemic pathogen, particularly in Asia, where presentation is classically HFMD with rapid CNS involvement, sometimes with acute flaccid paralysis (AFP). Outbreaks are often extensive and severe, and consequently, it is one of the first enteroviruses beyond poliovirus for which a vaccine has been developed.9,10 In other countries in the West, HFMD is not always a precursor to the CNS infection with EV-A71 epidemics more often associated with aseptic meningitis with infrequent reports of AFP.11 Poliomyelitis and EV-D68—Causes of AFP and Acute Flaccid Myelitis Most of the clinically important enteroviruses have tropism for the central nervous system and are leading causes of aseptic meningitis and more rarely meningoencephalitis. AFP/acute flaccid myelitis (AFM) has been associated with fewer enterovirus types such as the polioviruses, some of the coxsackieviruses, EV-A71, and, more recently, EV-D68. Polioviruses are among the most well-known enteroviruses. Once commonplace, they caused epidemics of severe paralytic disease (poliomyelitis) with long-lasting sequelae and fatalities. They have been brought to the brink of elimination through a sustained global effort to prevent infection and circulation through vaccination and close monitoring of progress through enhanced surveillance.12 Of the 3 polioviruses, which previously circulated and caused disease, only wild-type poliovirus type 1 is still circulating in specific regions of 2 countries, Afghanistan and Pakistan.13 Robust surveillance is a core requirement of the elimination program, with member states required to document that adequate surveillance is in place to achieve and maintain polio-free certification by the World Health Organization. In recent times, there have been small numbers of limited outbreaks of infections caused by vaccine-derived polioviruses, often detected through enhanced surveillance (such as waste-water surveillance).14 Ensuring high levels of population protection through vaccination and maintaining robust surveillance to track transmission have been key in minimizing complications of infection and stemming these outbreaks. In countries where elimination status has been achieved, moves from live vaccine to inactivated vaccines have limited potential for circulation of vaccine-type viruses; however, surveillance is important to identify shed virus imported from areas still using live vaccines, which may have to the potential for circulation in the local population.12 In 2014, cases of paralysis were reported in the USA, later defined as AFM by a combination of AFP and evidence of gray matter spinal cord lesions on MRI. Typical features on MRI include T2 hyperintensity restricted to the anterior horn. These cases appeared to be linked with increased circulation of a respiratory enterovirus EV-D68. The paralysis was unusual and shown to be descending in nature, often with the facial nerves, eyes, and upper limbs affected. This is atypical when compared to poliomyelitis, where the paralysis is generally ascending in nature.15 Because EV-D68 is unusual for the enteroviruses in being primarily a respiratory infection, the best sample for detection is from the respiratory tract. It also means that detection in fecal samples is less reliable than for other enteroviruses and detection in CSF is very rare. Because detection of EV-D68 in CSF is rare, confirming it as a cause of AFM remained controversial. This is despite poliovirus being rarely detected in CSF and the clear temporal and spatial association with EV-D68 and AFM. Animal studies eventually showed that mice developed AFM when infected with strains of EV-D68 that emerged in 2014 but not from those isolated before suggesting a change in tropism to the brain, without losing tropism to the respiratory tract.16 Several countries continue to test for EV-D68 routinely, and this has added further evidence around EV-D68 and its clinical syndromes, which are not only limited to AFM but also severe respiratory disease and viral-induced wheeze. This level of surveillance also identified a biannual circulation pattern with EV-D68 epidemics and AFM cases reported in both 2016 and 2018. AFM remains a rare complication of EV-D68 infection, but the outcome is often poor with long-term sequelae and ongoing paralysis reported in follow-ups from many identified cases17,18 This biannual pattern was somewhat disrupted, as was the case for many infections with seasonal/temporal patterns, perhaps due to social restrictions and nonpharmaceutical interventions set in place to combat COVID-19. With few EV-D68 cases reported in 2020, higher numbers reported again in the following years as restrictions were lifted.18,19 Neonatal Enterovirus Infection and Coxsackievirus B In the neonate, enterovirus infection is usually systemic, with the virus detected in multiple sites during the acute infection. Congenital enterovirus infection is rare, with most infections acquired postdelivery. Fortunately, most neonatal infections are mild, and recovery is often complete. A small proportion of babies, especially those who acquire enterovirus infection within the first week of life, can develop life-threatening complications. As enterovirus case numbers began to increase following the COVID-19 pandemic, reports of severe neonatal disease in several European countries were published, including fatal echovirus 11 disease.20 More unusual was a cluster of neonatal acute myocarditis reported in the Southwest of England and South Wales in the United Kingdom between June 2022 and April 2023. Over this period, 20 babies were diagnosed with acute myocarditis. Most babies presented critically ill with evidence of myocarditis, 16 of the babies required prolonged intensive care admission, and 2 babies died. All cases were positive for enterovirus and, where enterovirus typing was performed, were infected with either coxsackievirus B3 or B4. Limited historical typing data from the United Kingdom suggested that previously the six serotypes of coxsackieviruses B1-6 circulated in a biannual pattern with limited cocirculation of types. Reports of enterovirus myocarditis in neonates prior to this were rare, and as the presentation of the cases in the 2022–2023 cluster was so severe, it was felt that overascertainment following years of previously unrecognized cases was an unlikely explanation for the increase. However, even during this epidemic, the submission of samples for testing was not consistent across all the cases, highlighting the need for better sampling guidance.21,22 The association with coxsackieviruses and myocarditis especially in neonates and young children was made in the 1950s. Clustering of acute myocarditis cases before this event had not previously been reported, with most epidemics associated with sporadic case reports. The cause of the 2022–2023 cluster has not been conclusively ascertained. A review of the UK enterovirus clinical data at the time did not show similar clustering elsewhere in the United Kingdom. Unfortunately, the opportunity to investigate further the cause was largely missed.22 In contrast to the efforts deployed to determine the underlying cause of the similarly severe presentation associated with adenovirus, AAV2 and acute hepatitis in children also occurred when COVID-19 restrictions were lifted across the United Kingdom.23 FUTURE DIRECTION AND DISCUSSION Despite the common and often severe presentations associated with the enteroviruses, and evidence that shows the nonpolio enteroviruses can be associated with poor outcomes, they remain largely neglected with limited treatment options and poor understanding of host risk factors. The severity of disease associated with the coxsackieviruses has been reported since the 1950s, and emerging infections, such as EV-D68 and EV-A71, have not in any way increased global research and spending to shore up advancements in detection, surveillance, management, or treatment. While it is commendable that we are on the verge of seeing poliovirus eradicated, the wider circulation of other nonpolioviruses remains a significant public health threat. Without increased awareness of the severe outcomes seen not only in emerging enterovirus but also in those types that have always been circulating, it becomes difficult to respond when outbreaks do occur, as historical data to contextualize any event are largely missing. There is then a paucity of data on treatment and outcomes, with very few clinical trials, or antivirals in development, particularly for children. This means that when outbreaks or severe sporadic disease occurs, the off-license use of antivirals often occurs without robust clinical data to support effectiveness.24,25 Research groups such as the European Non-Poliovirus Enterovirus Network are being formed, with increased surveillance activities happening globally, along with collaborative efforts on clinical presentations and detection of enteroviruses that cause AFM/AFP.26 As a result, often with little financial support or infrastructure, there are now published guidelines on enterovirus diagnostics and best practices; however, none are officially standardized or accepted by leading societies.5 The World Health Organization, while mandating enhanced surveillance for poliovirus, currently has not yet fully supported efforts to establish a similar global network of laboratories willing and able to perform systematic surveillance for the nonpolio enteroviruses.27 Until this happens, enteroviruses will continue to circulate and emerge, causing high rates of morbidity and mortality globally. Only by fully understanding that impact, we might then facilitate better treatment, vaccines, and outcomes for children infected with enteroviruses.