Epidemiology of Poliomyelitis
Poliomyelitis, commonly known as polio, is an acute viral illness that primarily targets the human gastrointestinal tract. While the majority of infections remain subclinical or mild, a very small fraction—less than 1%—advance to infect the central nervous system, resulting in flaccid paralysis. Thanks to concerted global health efforts, the disease has been largely eradicated in many regions; however, its complete elimination remains an ongoing challenge, especially due to the threat of vaccine-derived variants and persistent endemic areas.
Introduction
Poliomyelitis is caused by a single-stranded RNA virus that belongs to the Enterovirus C species in the Picornaviridae family. Its transmission is predominantly via the fecal-oral route, though droplet infection can also occur. The disease is particularly notorious for its potential to cause lifelong disability and, in severe cases, death.
Problem Statement
India achieved a significant public health milestone when it was declared polio-free on March 27, 2014. Despite this success, the risk of reintroduction remains due to ongoing transmission in Pakistan, Afghanistan, and Nigeria—collectively known as PAN countries. Compounding the challenge is the oral polio vaccine (OPV), which contains live attenuated viruses. These viruses can, under rare circumstances, mutate and regain neurovirulence, leading to vaccine-derived polioviruses (VDPVs) capable of causing paralytic polio.
Epidemiological Determinants
The classic epidemiological triad of disease—agent, host, and environment—offers insights into polio transmission.
Agent Factors
Poliovirus exists in three serotypes: types 1, 2, and 3. Type 1 is the most virulent and responsible for most outbreaks. The virus can survive in water for 4 to 6 months in cold environments, and humans are the only known reservoir. It is found in feces and oropharyngeal secretions, and infected individuals are most contagious from 7 to 10 days before and after the onset of symptoms.
Interestingly, most polio infections are subclinical, contributing significantly to community transmission. For every clinical case, there may be up to 1,000 subclinical cases in children and 75 in adults, making it difficult to detect and control without widespread immunization.
Host Factors
Polio affects individuals of all ages, but children are particularly vulnerable. In India, 50% of cases occur in infancy, with the highest risk between 6 months and 3 years of age. Maternal antibodies offer protection only up to 6 months. There is also a gender disparity, with males being three times more likely to be affected than females. Immunity after infection is usually long-lasting, but it is serotype-specific—immunity to one type does not confer protection against the others. Other risk factors include trauma, intramuscular injections, and surgical procedures like tonsillectomy.
Environmental Factors
Environmental conditions play a crucial role in polio transmission. The disease is more prevalent during the monsoon season (June–September) in regions like India. Risk factors include contaminated food and water, poor sanitation, overcrowding, and the presence of disease vectors such as flies. The incubation period typically ranges from 7 to 14 days.
Clinical Spectrum
Polio can manifest in several ways, ranging from inapparent infections to paralytic disease. A distinctive sign in affected children is the “tripod sign,” where a child sits with hands braced behind for support and hips and knees partially flexed.
There is no specific antiviral treatment for poliomyelitis. Effective management involves supportive nursing care from the early stages of illness, which can help prevent severe disability. Physiotherapy is essential for strengthening weakened muscles and aiding in rehabilitation.
Prevention and Control
Immunization remains the cornerstone of polio prevention. It is crucial to ensure all infants are fully immunized by six months of age. Two types of vaccines are currently used:
- Inactivated Polio Vaccine (IPV) – Developed by Salk, it is administered via injection and contains killed virus.
- Oral Polio Vaccine (OPV) – Developed by Sabin, this live attenuated vaccine is easy to administer and plays a vital role in interrupting transmission.
In special cases, Normal Human Immunoglobulin (Ig) may be used at doses of 0.25–0.3 ml/kg to offer short-term protection against paralytic polio.
Strategies for Eradication in India
India has adopted several aggressive strategies to maintain its polio-free status:
- Conducting annual Pulse Polio Immunization (PPI) campaigns.
- Maintaining high routine immunization coverage.
- Monitoring OPV coverage at district and sub-district levels.
- Enhancing Acute Flaccid Paralysis (AFP) surveillance to detect all polio and non-polio causes.
- Rapid investigation and collection of stool samples from suspected AFP cases.
- Ensuring follow-up of AFP cases at 60 days to assess for residual paralysis.
- Launching outbreak control measures promptly upon confirmation or suspicion of poliomyelitis.
A single case of polio is now treated as an epidemic. Since 2015, India introduced IPV on November 30, and switched from trivalent OPV (tOPV) to bivalent OPV (bOPV) in April 2016 to reduce risks associated with type 2 vaccine-derived poliovirus.
Conclusion
While India has made remarkable progress in eradicating polio, vigilance must be sustained. The potential for reintroduction and the emergence of vaccine-derived strains underscore the need for continued surveillance, immunization, and public health preparedness. With global cooperation and adherence to strategic eradication programs, a polio-free world is within reach.
Introduction & Problem Statement
- What is the causative agent of poliomyelitis?
a) DNA virus
b) RNA virus
c) Bacterium
d) Fungus
Answer: b) RNA virus - Which part of the body does the poliovirus primarily infect?
a) Central nervous system
b) Respiratory tract
c) Alimentary tract
d) Skin and mucous membranes
Answer: c) Alimentary tract - India was declared polio-free on:
a) 15th August 2012
b) 30th January 2013
c) 27th March 2014
d) 1st January 2015
Answer: c) 27th March 2014 - Which countries are still reporting wild polio cases?
a) India, Bangladesh, Sri Lanka
b) China, Indonesia, Pakistan
c) Pakistan, Afghanistan, Nigeria
d) Iran, Iraq, Syria
Answer: c) Pakistan, Afghanistan, Nigeria
Agent Factors
- Which poliovirus serotype is most associated with outbreaks?
a) Type 2
b) Type 3
c) Type 1
d) Type 4
Answer: c) Type 1 - Poliovirus can survive in cold water for up to:
a) 1–2 weeks
b) 1–2 months
c) 4–6 months
d) 1 year
Answer: c) 4–6 months - How many subclinical cases may exist for every clinical case in children?
a) 10
b) 100
c) 500
d) 1000
Answer: d) 1000
Host Factors
- What is the most vulnerable age group for polio in children?
a) Birth to 6 months
b) 6 months to 3 years
c) 4 to 6 years
d) Above 6 years
Answer: b) 6 months to 3 years - What is the male to female ratio for polio susceptibility?
a) 1:1
b) 2:1
c) 3:1
d) 4:1
Answer: c) 3:1 - Immunity after polio infection is:
a) Temporary and partial
b) Long-lasting and serotype-specific
c) Complete and cross-protective
d) Short-term for all types
Answer: b) Long-lasting and serotype-specific
Environmental Factors
- In which season is polio more likely to occur in India?
a) Winter
b) Summer
c) Rainy (June–September)
d) Spring
Answer: c) Rainy (June–September) - What are the main modes of poliovirus transmission?
a) Airborne and droplet
b) Bloodborne and vertical
c) Fecal-oral and droplet
d) Direct contact and fomites
Answer: c) Fecal-oral and droplet - What is the typical incubation period of poliovirus?
a) 1–3 days
b) 3–5 days
c) 7–14 days
d) 21–28 days
Answer: c) 7–14 days
Clinical Features & Treatment
- Which sign indicates difficulty in sitting due to muscle weakness in polio?
a) Kernig’s sign
b) Tripod sign
c) Brudzinski’s sign
d) Lasegue’s sign
Answer: b) Tripod sign - Which of the following is true regarding polio treatment?
a) Antiviral therapy is highly effective
b) There is a specific vaccine-based cure
c) Supportive care and physiotherapy are crucial
d) Antibiotics prevent progression
Answer: c) Supportive care and physiotherapy are crucial
Prevention & Eradication
- Which of the following vaccines is inactivated?
a) OPV (Sabin)
b) IPV (Salk)
c) BCG
d) MMR
Answer: b) IPV (Salk) - What is the role of Human Normal Immunoglobulin (Ig) in polio?
a) Long-term immunity
b) Prevents subclinical infection
c) Temporary protection against paralytic polio
d) Used for diagnostic confirmation
Answer: c) Temporary protection against paralytic polio - Which strategy is NOT part of India’s polio eradication program?
a) Pulse polio immunization
b) Stool sample collection from AFP cases
c) Seasonal outbreak forecasting using AI
d) Surveillance of all AFP cases
Answer: c) Seasonal outbreak forecasting using AI - When was IPV introduced in India?
a) April 2014
b) November 2015
c) March 2016
d) January 2017
Answer: b) November 2015 - What change was made in polio vaccines in April 2016 in India?
a) Introduction of new serotype
b) Switching from tOPV to bOPV
c) Discontinuation of IPV
d) Replacement of OPV with DNA vaccine
Answer: b) Switching from tOPV to bOPV
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