ACUTE POLIOMYELITIS

After successful eradication of smallpox in 1978, poliomyelitis is the second infectious disease on the

verge of eradication. Sporadic cases of poliomyelitis due to wild poliovirus are being reported only from two countries—Pakistan and Afghanistan.

Epidemiology: Polioviruses are the small RNA entero­viruses with three serotypes—types 1, 2 and 3.

All global cases presently are due to type 1, as wild type 2 and 3 virus have been eradicated in 1999 and 2020 respectively.

Reservoir of infection is a symptomatic or carrier case, excreting virus in stools. As the virus cannot survive outside human body for gt;48 hours, presence of large number of non-immunized hosts is essential for propagation of infection.

Infection is mainly transmitted feco-orally due to con­taminated water supply, rarely by contaminated fingers or as droplet infection in close contacts.

Poliomyelitis largely affects young children lt;5 years. Infants lt;3 months are relatively protected by trans­placental immunity.

Maximum cases occur in late summer or rainy season. Poor sanitation and inadequate immunization coverage are important determinants of polio outbreaks.

Pathogenesis: After ingestion, virus replicates in intestines and regional lymph nodes for few days, followed by transient primary viremia, during which the reticuloendothelial system is infected. Subsequently, sustained viral replication in reticuloendothelial system leads to secondary viremia and CNS involvement. Virus traverses to the CNS, either along the nerve pathways or through the blood stream. Local IgA-mediated intestinal immunity is the first line of defence against polio virus invasion, while systemic immunity may neutralize the virus before CNS involvement.

While ~90% of infections are subclinical, some provoking factors increase the risk of symptomatic disease, by increasing spinal vascularity and risk of viral invasion and include: (a) intramuscular injections, (b) surgery, (c) muscle trauma and (d) strenuous physical exercise.

CNS injury is caused by direct viral invasion as well as immunological mechanisms and selectively affects the anterior horn cells of spinal cord, cranial nerve nuclei and vital centers in the brain stem and motor cortex, sparing rest of the cerebral cortex and spinal white matter.

Clinically, symptomatic cases present after an incubation period of 7-14 days, with:

• Abortive poliomyelitis (4-8%), as self-limiting non­specific febrile illness,

• Non-paralytic poliomyelitis (1-2%), which may or may not progress to paralytic stage, with intense headache, nausea/vomiting and soreness/stiffness of neck and back muscles. Nuchal-spinal rigidity is the clinical hallmark of these cases,

• Paralytic poliomyelitis (lt;1%) presents as acute spinal, bulbar or spino-bulbar paralysis.

Spinal poliomyelitis is characterized by acute flaccid paralysis of different group of muscles involving neck, trunk or limb, with/without diaphragmatic involve­ment. Paralysis is typically asymmetric and patchy with variable severity. Involves muscles are quite tender urinary retention and constipation is common (20%) due to autonomic involvement. New areas of weakness may appear till next 3-7 days, followed by gradual recovery to some extent. Involvement of diaphragmatic/intercostal muscles indicates impending respiratory failure.

Bulbar poliomyelitis due to involvement of cranial nerve nuclei and/or respiratory center in the brain stem may also develop in some cases, presenting with feeding and breathing difficulties. Isolated encephalitic poliomyelitis is extremely rare.

Diagnosis: Each case of acute flaccid paralysis (AFP) is considered as potential poliomyelitis for surveillance purpose, however unlikely it may be, and needs to be reported (discussed later).

Clinical diagnosis rests on—(a) epidemiological history, (b) unimmunized child, and (c) acute onset of asymmetrical/patchy flaccid paralysis with pain/ tenderness of involved muscles.

Laboratory diagnosis depends on viral culture from stools, which should be collected (~8 gm or thumb size) and transported to designated laboratory in ice pack (reverse cold-chain) as early as possible, under AFP surveillance program.

D/D of acute paralytic poliomyelitis includes: (a) vaccine associated illness, e.g. VAPP or VDPV, (b) other causes of AFP (Table 10.36), or (c) pseudoparalysis due to pain, e.g. in scurvy, osteomyelitis, trauma, etc.

Possibility of vaccine related disease (VAPP/VDPV) should be considered in any case of AFP with history of: (a) OPV vaccination in preceding 30 days, (b) contact with recently immunized child in preceding 60 days, or

(c) mass immunization program, i.e. pulse polio in the community, in preceding 60 days.

Management is non-specific, aims to: (a) provide supportive care during acute phase, (b) treat compli­cations, e.g. respiratory failure, and (c) minimize residual functional loss with physiotherapy, orthotic corrections and occupational rehabilitation. All suspected cases should be hospitalized and notified to public health authorities

Prevention largely depends on adequate immuni­zation, using an oral live-attenuated Sabin vaccine or a parenteral, inactivated Salk vaccine (Ch 9.2.1).

Polio Eradication in India

In May 1988, WHO launched a global polio eradication initiative and wild poliovirus 2 and 3 have been

TABLE 10.36: D/D common causes of acute flaccid paralysis (AFP)

Associated involvement

*2-3 weeks before. **Flaccidity rarely lasts for gt; 72 hours, changes to spasticity.

eradicated in 1999 and 2020 respectively. However, sporadic cases of WPV1 are still being reported from two countries - Pakistan and Afghanistan.

Despite being polio-free since 2014, eradication measures need to be continue here till disease is eradi­cated globally due to potential risk of spread through travellers from endemic countries.

Strategies for polio eradication in India included: (a) routine immunization, (b) supplementary immunizations, e.g. pulse polio immunization, (c) AFP surveillance, (d) environmental surveillance, and (e) outbreak control measures.

Pulse polio immunization (PPI) is a mass immunization campaign to supplement routine immunization and to facilitate development of herd immunity, conducted annually and simultaneously (pulse) throughout the country on pre-fixed dates (national immunization day/s or NIDs).

Principle: Since the vaccine-virus is excreted in stools for 4-6 weeks, simultaneous OPV administration to large population leads to extensive dissemination of vaccine virus in community, which competes with wild-virus for gut uptake. As wild-virus cannot survive in external involvement for long, it is expected to be flushed-out from community, replaced by non-virulent vaccine virus.

Target population for PPI is all children lt;5 years of age (including newborns), irrespective of previous immunization status. OPV doses during PPI are additional and should not replace regular immunization dose. There are no contraindications for PPI, even if routine dose was given a few days back.

Immunization days: First country-wide pulse polio campaign was launched on 9^th December, 1995 and presently one polio immunization day is held nationally (NID) and with 1-2 additional state immunization days (SIDs) in high risk-states. These days are decided in advance using a Sunday and during winter season, as the wild-virus circulation is least common in these months and consequently, vaccine uptake is better.

Acute flaccid paralysis (AFP) surveillance is an essential component of global polio eradiation initiative, quot;to detect all cases of wild polio virus disease and eliminate remaining foci of polio transmission”, undertaken in India since 1997 as National Polio Surveillance Project (NPSP). Surveillance is carried out for all cases of AFP and not only for polio. What is AFP? AFP is defined as quot;any illness presenting with acute onset of flaccid paralysis in a child lt;15 years, for which no obvious cause such as trauma or electrolyte imbalance is found or a person of any age, with clinical suspicion of polio”. Generally, AFP of lt; 4 weeks is considered as acute.

Why all cases of AFP should be investigated? Since it is often difficult to exclude the diagnosis of poliomyelitis in a suspected case at the time of presentation, it is essential to investigate each case of AFP, to ensure that all cases are detected, reported and investigated and no case is missed out.

Components of AFP surveillance include, regular reporting system, investigation of suspected case and outbreak-control measures after suspected case report.

• Reporting: All field-reporting units, e.g. district health centers or pediatric units of major hospitals collect and send weekly reports of suspected case/s. If no case is seen during reporting week, the report has to be sent as 'nil or zero report'

• Investigation of suspected AFP case includes: (a) immediate reporting to the designated person, (b) detailed data collection and line-listing of all reported

cases with a unique EPID number, to prevent dupli­cation of reports, (c) collection of stool cultures for virus isolation, as specified earlier.

• Outbreak-containment measures are activated for every reported case of AFP with: (a) immunization of under-5 children in vicinity of suspected case, (b) intensified collection of random stool and water/sewage samples to detect the presence of wild-virus, and (c) active case surveillance by door-to-door survey.

Classification of an AFP case: All cases of reported AFP are classified after detailed epidemiological and viral studies as polio or non-polio cases.

An AFP case is confirmed as poliomyelitis only if: (a) wild polio virus isolated from stool sample, or in absence of adequate stool sample, (b) cases continues to have residual weakness after 60 days of onset or 60 days-follow-up not available due to death or absence and expert review concludes that these cases could not be discarded as non-polio based on available data.

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