Describe poliovirus and the other enteroviruses. Compare the two polio vaccines and discuss eradication.

The Enteroviruses

• Picornaviruses — "pico" (small) plus "RNA"; the smallest RNA viruses. naked, icosahedral, with single-stranded positive-sense RNA, so the genome acts directly as messenger RNA
• Being naked they are acid-stable and resistant to bile, detergents and the environment, which is exactly what a virus transmitted by the faecal-oral route through the stomach requires
• Members — poliovirus types 1, 2 and 3; coxsackie A and B; ECHOviruses; newer enteroviruses including EV71; rhinovirus (which is acid-labile and confined to the nose); and hepatitis A virus

Poliovirus — Pathogenesis

Ingestion; the virus is acid-stable and survives the stomach → Primary multiplication in the oropharynx and the lymphoid tissue of the gut — tonsils and PEYER patches → Minor viraemia, which in most people is contained here → In a minority, spread to the reticuloendothelial system and a major viraemia → Invasion of the central nervous system → Selective destruction of the anterior horn cells of the spinal cord, and of brainstem motor nuclei in bulbar disease → asymmetrical flaccid paralysis with NO sensory loss, since only motor neurons are affected

• The outcome is overwhelmingly favourable — about 90 to 95% are asymptomatic, 4 to 8% have a minor non-specific febrile illness, 1 to 2% develop non-paralytic aseptic meningitis, and fewer than 1% develop paralytic disease. But every infected person excretes virus and can infect others
• Provocative factors that increase the risk of paralysis and determine its site — intramuscular injections during the incubation period, strenuous exercise, tonsillectomy (predisposing to bulbar disease), pregnancy and immunodeficiency
• Post-polio syndrome — new weakness developing decades after recovery, from attrition of the surviving enlarged motor units

The Two Vaccines

Polio Eradication

• INDIA was certified polio-free in 2014, the last case having occurred in West Bengal in January 2011 — an achievement widely thought impossible, given the population, density and sanitation
• How it was done — National Immunisation Days ("pulse polio") delivering OPV to every child under 5 on a single day; mopping-up rounds; social mobilisation to overcome refusal; and above all acute flaccid paralysis (AFP) surveillance, in which every child under 15 with sudden flaccid weakness is investigated with two stool samples
• Wild type 2 was declared eradicated in 2015 and type 3 in 2019; only wild type 1 remains, endemic in Pakistan and Afghanistan
• The remaining obstacles are conflict, inaccessibility, vaccine refusal and circulating vaccine-derived poliovirus, which now causes more cases in some regions than wild virus does
• Why polio was a plausible target — man is the only reservoir, there is no chronic carrier state in the immunocompetent, and effective cheap vaccines exist; the obstacle was silent transmission, not biology

Laboratory Diagnosis of Poliomyelitis

• Stool is the specimen of choice — two samples collected at least 24 hours apart, within 14 days of the onset of paralysis, and transported in a reverse cold chain to a national laboratory
• Virus is excreted in stool for weeks, far longer than it is present in throat secretions, which is why stool rather than a throat swab is used
• Cell culture in human or monkey kidney lines, with typing by neutralisation; and intratypic differentiation by molecular methods to distinguish wild virus from vaccine virus and from vaccine-derived poliovirus — the critical distinction in a certified polio-free country
• Cerebrospinal fluid shows a lymphocytic pleocytosis with normal glucose, but the virus is rarely isolated from it
• Environmental surveillance of sewage detects circulating virus even where no paralytic case has occurred, and is now an important part of the endgame

Applied Aspects

• Investigate every case of acute flaccid paralysis in a child and notify it; the differential includes Guillain-Barré syndrome, transverse myelitis and traumatic neuritis, but surveillance depends on all of them being reported
• Avoid unnecessary intramuscular injections in a febrile child in an endemic setting, because of provocative paralysis — a specific and often forgotten precaution
• Two stool samples 24 hours apart are required for virological investigation of acute flaccid paralysis, collected within 14 days of onset
• Give IPV to immunodeficient children and their household contacts, never OPV, since the live vaccine can cause paralysis and prolonged excretion in them
• Enteroviruses are a leading cause of aseptic meningitis in children, and recognising this avoids prolonged unnecessary antibiotics in a self-limiting illness
• Hand, foot and mouth disease is usually benign, but EV71 outbreaks require vigilance for brainstem encephalitis, which can progress rapidly
• Polio-free does not mean polio-proof; importation remains possible while wild virus circulates anywhere, which is why India maintains immunisation, surveillance and border vaccination posts
• Environmental sewage surveillance detected poliovirus in India after the last clinical case, demonstrating that silent circulation can be found without a paralysed child
• Coxsackie B myocarditis should be considered in a young patient with chest pain and a raised troponin after a viral illness, since it is a leading infectious cause of cardiomyopathy
• The rhinovirus has over 100 serotypes, which is why the common cold recurs indefinitely and why no vaccine is possible — a useful contrast with measles, which has one
• Enteroviruses circulate most in the monsoon and post-monsoon months in India, and cluster in children; recognising a seasonal outbreak of aseptic meningitis prevents a great deal of unnecessary investigation
• Distinguish poliomyelitis from Guillain-Barré syndrome — polio is asymmetrical, febrile at onset, purely motor and without sensory loss, whereas Guillain-Barré is symmetrical, ascending, afebrile and often has sensory features
• Vaccine-derived poliovirus is now the commoner cause of paralytic polio in several regions, which is the argument for moving to IPV once wild virus is gone
• Never give OPV to a child known to be immunodeficient, or to the household contact of one; prolonged excretion and paralysis have both occurred
• Report acute flaccid paralysis within 24 hours and arrange the two stool samples; the target rate of non-polio AFP reporting is itself a measure of how sensitive surveillance is
• Post-polio syndrome affects survivors decades later, and India has a large cohort of people paralysed before eradication who need rehabilitation and orthotic support rather than antiviral treatment
• Hand hygiene and safe water interrupt enterovirus transmission, which is why polio was a disease of poor sanitation and why its eradication required vaccine rather than waiting for sanitation to improve
• Explain to parents why a healthy child needs repeated polio drops; refusal in some communities was a genuine obstacle, and it was overcome by engagement rather than by insistence
• Maintain routine immunisation as well as campaign doses; campaigns raise population immunity quickly but routine coverage is what sustains it