Parechoviruses

Historical Background

The initial two serotypes of the genus Parechovirus were isolated in 1956 from children with diarrhea. On the basis of the scheme used at that time for the classification of the enteroviruses, they were classified as echovirus (E) serotypes 22 (Harris strain) and 23 (Williamson strain). After their classification as enteroviruses, it became evident that they differed from other serotypes within the genus. Their growth in cell culture produced a cytopathic effect that differed from that of the enteroviruses. E22 and E23 produce incomplete cytopathic effects of the cell monolayer and a distinctive nuclear cytopathology consisting of thickening of the nuclear membrane and the disappearance of the nucleolus, and of nuclear chromatin. As molecular methods for the evaluation of viral replication became available, additional discordances were identified. Salient among these were the inability of E22 and E23 to cleave the eukaryotic translation initiation factor 4G (eIF4G) and thereby shut off host cell protein synthesis—a notable characteristic of the enteroviruses—and the efficient translation of their genome in rabbit reticulocyte lysates, something not observed in the enteroviruses. Furthermore, the cleavage of the nascent capsid proteins of E22 resulted in three rather than four mature capsid proteins, as with the enteroviruses. With the advent of tools for direct and computational analysis of the viral genome, it became evident that E22 and E23 were clearly dissimilar to other members of the genus. Nuclease digestion of the RNA genome of E22 revealed that it possessed a greater mean relative resistance to digestion than that of poliovirus type 1, suggesting that it contained significantly more secondary structure (i.e., double-stranded or base-paired regions). Failure to hybridize subgenomic probes to the E22 genome, known to be reactive with other serotypes of the Enterovirus genus, or amplify subgenomic segments using pan-enterovirus reactive primers, added further support that it was not a true enterovirus. Ultimately, the sequencing and phylogenetic analysis of the complete genomes of E22 and E23 confirmed that they were indeed genetically distinct from the enteroviruses and led to their assignation to a new genus: Parechovirus, members of the Picornaviridae family. The genus is currently comprised of 4 (A—D). Only members of species A and B infect humans.

Human parechoviruses (HPeVs) 1 and 2 (formerly E22 and E23, respectively) were soon joined by 17 newly identified members of the species bringing the total number of types to 19. Potentially new types continue to be identified.

Virology

Only the major differences between HPeVs and enteroviruses are discussed here. Several reviews of HPeVs have been published in recent years. Readers are directed to those sources, and to Chapter 170 in this text, for a more detailed discussion of the virology of HPeVs and enteroviruses.

HPeVs are small (28 nm in diameter), nonenveloped viruses possessing a single-stranded, positive (messenger)-sense RNA genome. The external appearance of the virion is much smoother than that of the enteroviruses owing to shorter surface loops on the capsid protein, VP1, resulting in a less pronounced plateau at the fivefold axis of symmetry and a shallower canyon ( Fig. 173.1 ). The HPeV genome is approximately 7.35 kb in length and organized in a manner comparable with that of enteroviruses. The open reading frame (ORF) codes for a polyprotein of approximately 2180 amino acids, depending on HPeV type and strain, that is shorter than that of enteroviruses.

Computational support has been generated for the existence of linear regions of RNA and higher-order RNA structures within the HPeV 5′ and 3′ nontranslated regions (NTRs) analogous to those of enteroviruses (see Chapter 172 ). Analysis of the HPeV internal ribosome entry site (IRES), a region essential for efficient translation of the picornavirus genome, reveals it to be more closely related to that of the Cardiovirus and Aphthovirus genera. Conserved regions of nucleotide identity exist among the 16 types within the HPeV 5′ NTR and are used in nucleic acid amplification tests (NAATs) for their detection.

Translation of the ORF yields three capsid (VP0, VP3, and VP1) and seven nonstructural proteins essential for replication and viral assembly. The structures and functions of all of the HPeV proteins have not been extensively studied. Many have functions similar to cognate enterovirus proteins, but differences are emerging. Unlike the enteroviruses, the HPeV genome codes for three, rather than four, capsid proteins as a result of failure to cleave the precursor protein, VP0, of capsid proteins VP2 and VP4. The result is a capsid composed of VP0, VP3, and VP1. The VP0 coding sequence contains a stem-loop structure that may function as a cis -acting replication element (CRE) critical for replicaton. The C-terminal region of the VP1 protein of several HPeVs contains an arginine-glycine-aspartic acid (RGD) motif functional in binding to host cell integrins. ^a

a References .

Evidence points to the integrins α _v β ₃ and α _v β ₆ as receptors for some HPeVs. The cell receptors used by HPeVs lacking the RGD motif are currently unknown. The VP3 of HPeV varies from that of the enteroviruses as the result of the N-terminal extension of 28-34 amino-acid residues.

Among the nonstructural proteins, 2A protein differs functionally from that of the enteroviruses in that it lacks proteolytic activity and is incapable of mediating the cleavage between the 1D and 2A proteins . The HPeVs fail to shut off host cell protein synthesis, a function mediated in enteroviruses by 2A cleavage of eIF4G. The 2A protein also binds to RNA and may play a role in RNA replication. The HPeV 2C protein also appears to differ from its cognate enterovirus protein. Like the enterovirus 2C, it localizes to endoplasmic reticulum membrane–associated replication complexes, supporting a role in viral replication. However, it is also associated with cellular structures not directly involved in replication. 2C also has AMP (adenosine monophosphate) kinase activity, a property whose significance is unknown.

Epidemiology

Our understanding of the epidemiology of the HPeV is lacking for multiple reasons: continued discovery of new types ; inability of all HPeV types to be identified through cell culture ; and an NAA that is pan-amplifying for all HPeVs.

HPeVs have been identified worldwide. HPeV infections occur throughout the year; however, they exhibit seasonal epidemiology. Worldwide, the peak incidence for HPeV infection is during the summer and autumn months.

In the case of HPeV3, a biennial pattern has been reported, but several reports specifically have not supported this finding. Cocirculation of multiple types within a community or geographic region is common. ^b

b References .

The HPeVs have been associated with community and nosocomial outbreaks in pediatric and neonatal units.

The contribution of the HPeV to the disease burden, and the relative proportion due to each of the types, is yet unclear. With use of traditional cell culture methods, the HPeVs accounted for approximately 2% of all “enteroviruses” isolated by clinical laboratories. HPeV1 is the most frequently identified member of the genus, followed by HPeV3. In the United States, HPeV1 (E22) accounted for 1.8% and HPeV2 (E23) for 0.1% of all typed enterovirus isolates reported to the Centers for Disease Control and Prevention (CDC) over a 39-year period (1970–2008) and ranked among the top 15 isolates isolated for 74% all years. Reports from two clinical laboratories in Japan found that HPeV accounted for 2.2% to 2.8% of all enteroviruses isolated over a period of 15 and 9 years, respectively. Typing revealed that HPeV1 and HPeV3 accounted for 85% of the isolates, whereas types 6 and 4 accounted for the remainder in decreasing order of frequency. A Canadian report found that over a 20-year period, of 28 HPeV isolates that were identified, types 1, 2, and 3 accounted for 71.4%, 10.7%, and 17.9%, respectively, of the total number.

However, with use of molecular techniques for the detection of these viruses, it appears that HPeV may play a more significant role in disease burden than previously was thought. Surveillance by the CDC (2009–2013) found that HPeV3 accounted for 12.3% of 1819 reports with a known enterovirus or HPeV type. However, most HPeV3 detections originated from a single hospital that routinely tested for HPeVs. A report from Northern Italy documented that HPeVs were detected in 5.2% of 812 samples collected from children younger than 5 years over a 4-year period. For comparison, enteroviruses were identified in 16.1% of samples. HPeV1 accounted for 66.7% of the HPeVs identified, followed by HPeV3 (20%) and HPeV6 (13.3%).

HPeV infections occur early in life. Seroprevalence data and longitudinal studies of the shedding of HPeV in stools of healthy infants suggest that approximately 70% to 80% of infants have evidence of at least one HPeV infection by 2 years of age, increasing to approximately 90% by 5 years in some studies. In the United States, approximately 70% of cases of HPeV1 and HPeV2 infection reported to the CDC occurred in infants younger than 1 year. Children younger than 5 years accounted for 95.6% and 88.2% of all HPeV1 and HPeV2 isolates, respectively. The seroprevalence for some HPeV types approaches 100% in adults. However, in some geographic areas, certain HPeV types fail to circulate or do so at low levels. A limited serologic survey of the prevalence of HPeV3 among adults in the Milwaukee, Wisconsin, area failed to detect any seropositive individuals

Clinical Manifestations

Transmission of HPeV occurs primarily by the fecal-oral and, less frequently, the respiratory routes. Fecal-oral transmission is favored by high viral titers in stool. After infection, HPeVs are shed from the gastrointestinal and upper respiratory tracts. The duration of shedding is still not well defined but may range from less than 2 weeks to as long as 5 months in stool and 1 to 3 weeks from the upper respiratory tract. In one report, after symptomatic infection, shedding of HPeV lasted for 2 to 24 weeks (median, 58 days) and most of the shedding occurred while infants were asymptomatic. Reports of HPeV infections occurring in the first 2 days after birth may support the possibility of in utero transmission. Identification of genetically identical HPeV3 in infected patients and in asymptomatic children in their families have suggested that family members may be a source of infection in neonates and young infants.

The finding of HPeV in the stool of asymptomatic infants, along with the high seroprevalence among young infants and children in the general population, provides evidence that the majority of HPeV infections are subclinical. Given the high prevalence of HPeV infections, it is likely that the reported cases of severe HPeV infections represent infrequent events.

Undifferentiated Febrile Illness/Sepsis Syndrome

HPeVs have been isolated from the stool, nasopharyngeal swabs, urine, and blood of neonates and infants with an undifferentiated febrile illness (UFI). The overwhelming majority of reported cases have occurred in male infants younger than 2 months. Seven percent of infants younger than 36 months evaluated for fever without a source in the emergency department of a children's hospital had a HPeV infection. In some cases, cardiovascular and respiratory dysfunction is severe enough to warrant a clinical diagnosis of sepsis. Although multiple HPeV types have been associated with UFI, cases of sepsis have been most frequently associated with infection due to HPeV3. HPeV4 infection may also be associated with serious, sepsis-like disease.

In addition to fever, irritability, and poor feeding, tachycardia and tachypnea are typically present. ^c

c References .

A rash may be present on the trunk and extremities, including the palms and soles, which has been described as macular, maculopapular, or erythematous. Mild respiratory (rhinorrhea, cough) and gastrointestinal symptoms (diarrhea, abdominal distention) may be present. Some may present with or develop a sepsis-like syndrome with severe tachycardia, retractions, apnea, oxygen desaturation, poor capillary filling, mottling, or shock. In these patients, hepatitis may be present.

Admission to the intensive care unit may be necessary for monitoring and cardiovascular or respiratory support and may be more common for infants younger than 1 to 2 months. In a review of three recent series involving very young infants, totaling over 200 cases, only a single death was reported. A recent study raised concerns regarding normal neurodevelopment among infants hospitalized with HPeV sepsis-like presentation. With use of a standard questionnaire, follow-up was performed 1 year after hospitalization in 42 infants who had been hospitalized with HPeV sepsis-like syndrome, 12% of whom had encephalitis. The investigators found that half of the infants may have had some abnormal neurodevelopment, including possible significant abnormalities in one-fifth of the infants, most prominently in the gross motor and problem solving domains. These findings suggest that careful follow-up of young infants hospitalized with HPeV disease may be warranted.