Poliovirus

Poliovirus
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Name	Poliovirus
Caption	Structure of the virion
Taxon	Poliovirus
Family	Picornaviridae
Genus	Enterovirus
Species	Enterovirus C

Poliovirus. It is a human enterovirus belonging to the Picornaviridae family and the species Enterovirus C. This highly contagious virus is the causative agent of poliomyelitis, a disease that can lead to irreversible paralysis. While most infections are asymptomatic, the virus's ability to invade the central nervous system has driven global public health initiatives for its control and elimination.

Virology and structure

Poliovirus is a small, non-enveloped virus with an icosahedral capsid approximately 30 nanometers in diameter. Its genome consists of a single-stranded, positive-sense RNA molecule, which is translated into a single polyprotein that is subsequently cleaved by viral proteases. The capsid is composed of 60 copies each of four structural proteins: VP1, VP2, VP3, and VP4. These proteins determine the virus's serotype, with three distinct wild types identified: Poliovirus type 1, Poliovirus type 2, and Poliovirus type 3. The virus binds to host cells via the poliovirus receptor (CD155), a member of the immunoglobulin superfamily. Research on its structure was advanced by teams including those at the University of Wisconsin–Madison and the National Institutes of Health.

Transmission and infection

The virus spreads primarily through the fecal-oral route, often in areas with poor sanitation and contaminated water supply. It can also be transmitted via the respiratory tract through respiratory droplets. After ingestion, initial replication occurs in the oropharynx and the intestinal mucosa, specifically within M cells and enterocytes of the small intestine. The virus then drains into local lymphatic tissue, such as the tonsils and Peyer's patches, and enters the bloodstream, causing a viremia. In a small percentage of cases, it crosses the blood-brain barrier or invades the central nervous system via neural pathways, leading to infection of motor neurons in the spinal cord and brainstem.

Clinical manifestations

Approximately 70% of infections are entirely asymptomatic. Around 25% of cases result in minor illness known as abortive poliomyelitis, with symptoms like fever, sore throat, headache, and vomiting. A smaller proportion, about 1-5%, develop non-paralytic aseptic meningitis. The most severe form, paralytic poliomyelitis, occurs in roughly 0.1% to 2% of infections. This manifests as flaccid paralysis, which is often asymmetric and most commonly affects the legs. When the brainstem is involved, it can lead to bulbar poliomyelitis, affecting functions like swallowing and breathing, potentially requiring intervention with an iron lung. A post-polio syndrome, characterized by new muscle weakness, can occur decades after the initial illness.

Prevention and vaccination

The primary defense is vaccination. Two main vaccines have been instrumental: the inactivated poliovirus vaccine (IPV), developed by Jonas Salk and first used widely in the 1950s, and the oral poliovirus vaccine (OPV), developed by Albert Sabin and introduced in the 1960s. IPV, an injectable vaccine, induces strong humoral immunity and prevents paralysis but does not fully stop intestinal infection. OPV, a live-attenuated oral vaccine, induces both humoral and mucosal immunity, providing better community protection but carrying a rare risk of vaccine-derived poliovirus. Global immunization programs are coordinated by entities like the World Health Organization and UNICEF.

Eradication efforts

The global initiative to eradicate the disease was launched in 1988 by the World Health Assembly and is led by the World Health Organization, Rotary International, the U.S. Centers for Disease Control and Prevention, and UNICEF. This effort, the Global Polio Eradication Initiative, has reduced global cases by over 99%. Wild poliovirus type 2 was declared eradicated in 2015, and type 3 was declared eradicated in 2019. As of the early 2020s, endemic transmission of wild poliovirus persists only in regions of Afghanistan and Pakistan. Challenges include vaccine-derived poliovirus outbreaks, conflict zones hindering vaccination campaigns, and vaccine hesitancy. The program employs strategies like National Immunization Days and surveillance for acute flaccid paralysis.

Category:Enteroviruses Category:Eradicated diseases