Taxonomy and Classification
Etymologically, the genus name derives from the Greek word enteron (gut), directly pointing to the primary habitat of these pathogens. The genus Enterovirus belongs to the family Picornaviridae.
Two approaches exist for classifying this group:
- Historical classification divides them into polioviruses, coxsackieviruses (groups A and B, named after the American town where they were first isolated), ECHO viruses (an acronym for Enteric cytopathogenic human orphan viruses), and numbered serotypes (68, 69, 70, and onward).
- Modern nomenclature groups them into four main species: Enterovirus A, B, C, D, each comprising various serotypes.
Morphology and Resistance
Enteroviruses are spherical and rank among the simplest organized viruses. Their capsid exhibits cubic symmetry enclosing a single-stranded positive-sense RNA (+ssRNA) genome.
A key structural feature is the complete absence of a lipoprotein envelope (envelope/supercapsid), as well as lipids and carbohydrates. This confers specific resistance properties:
- Chemical resistance: Viruses are completely insensitive to ether, lipid solvents, alcohols, phenol, and detergents.
- Sensitivity: Oxidizing agents, formalin, and chlorine-containing compounds are virucidal.
- Environmental survival: They persist for months in water bodies, soil, and on fomites, tolerating an extreme pH range from 2.5 to 11.
- Physical factors: Rapidly inactivated by drying and ultraviolet radiation. Heating to 50 °C inactivates them within 30 minutes, while boiling destroys virions instantly.
Cultivation Features
In laboratory settings, most enteroviruses successfully replicate in primary and continuous human cell cultures. Their presence is identified by a pronounced cytopathic effect (CPE) or plaque formation under an agar overlay.
A notable exception is Coxsackie A virus, which replicates extremely poorly in cell cultures. Newborn mice are used for its identification, as well as for differentiating Coxsackie viruses into groups A and B, based on pathogenicity patterns. Conversely, ECHO group viruses are entirely non-pathogenic to laboratory animals.
Epidemiology and Pathogenesis
Infection is widespread and frequently occurs in mass outbreaks, especially among children. Enteroviruses display a distinct summer-autumn seasonality.
Sources of infection include symptomatic patients and asymptomatic viral shedders. Transmission occurs via two routes:
- Fecal-oral (primary): Mediated by unwashed hands, contaminated water, food, and household items. Mechanical vector transmission by flies is also possible.
- Droplet / Airborne: Relevant during the first 1–2 weeks of illness when the pathogen is actively shed in nasopharyngeal secretions.
Pathogenesis: The virus enters through the mucosal surfaces of the nasopharynx or small intestine. Initial replication occurs locally (in the epithelium and regional lymph nodes). The pathogen then enters the bloodstream (viremia), disseminates throughout the body, and targets specific organs.
Clinical Manifestations, Immunity, and Diagnostics
Enteroviral infections are characterized by pronounced clinical polymorphism. Depending on the tropism of a specific serotype, manifestations may include:
- CNS involvement: Poliomyelitis, aseptic (serous) meningitis.
- Cardromuscular pathologies: Myocarditis, epidemic pleurodynia.
- Respiratory syndromes: Acute respiratory infections, "summer flu".
- Gastrointestinal issues: Diarrhea and gastroenteritis.
- Cutaneous manifestations: Exanthems (rashes) and conjunctivitis.
Recovery confers lasting immunity. However, it is serotype-specific: protection is limited strictly to the serotype that caused the infection.
Virological and serological methods are used for diagnosis. Clinical specimens include nasopharyngeal swabs, stool, or cerebrospinal fluid (CSF). In serology, the hallmark diagnostic criterion is a 4-fold or greater rise in antibody titer between paired sera (the first sample taken on days 4–5, the second on day 14).