AFP surveillance

Acute Flaccid Paralysis (AFP) surveillance is the gold standard for detecting cases of poliomyelitis. It is a critical strategy of the Global Polio Eradication Initiative (GPEI) to detect poliovirus transmission.

I. Definition and Case Definition

Acute Flaccid Paralysis (AFP) is defined as a syndrome characterized by the sudden onset of weakness and floppiness (flaccidity) in any part of the body.

Standard Case Definition for Surveillance:

• Any child under 15 years of age presenting with sudden onset of floppy paralysis or muscle weakness due to any cause.
• OR any person of any age with paralytic illness if poliomyelitis is suspected by a clinician.

II. Approach to a Child with AFP

The clinical approach involves a detailed history and neurological examination to distinguish poliomyelitis from other causes of AFP, such as Guillain-Barré Syndrome (GBS), Traumatic Neuritis, and Transverse Myelitis.

1. Clinical History

A comprehensive history is essential to establish the diagnosis and rule out mimics.

• Onset and Progression: Polio typically presents with a rapid progression of paralysis (24–48 hours) from onset to maximum paralysis. GBS may progress over days to weeks.
• Fever: High fever at the onset of paralysis is a hallmark of poliomyelitis. In GBS, fever is usually absent at the onset of neurological signs, though there may be a history of antecedent illness.
• Site of Injection: History of intramuscular injection in a limb prior to the onset of paralysis suggests Traumatic Neuritis.
• Sensory Symptoms: Polio is a pure motor neuron disease; sensation is preserved (though muscle pain/myalgia is common). GBS and Transverse Myelitis often present with sensory loss or paresthesia.
• Prodromal Illness: History of gastrointestinal (diarrhea/vomiting) or respiratory illness suggests a viral etiology.
• Immunization History: Number of OPV/IPV doses received (Routine and SIAs).

2. Physical Examination

• General: Assessment for toxicity, respiratory distress (bulbar/respiratory muscle involvement), and signs of meningeal irritation.
• Neurological Examination:
  • Tone: Hypotonia (flaccidity) is the cardinal sign.
  • Power: Assess for asymmetry. Polio typically causes asymmetric paralysis (e.g., one leg more affected than the other). GBS usually presents with symmetrical ascending paralysis.
  • Reflexes: Deep tendon reflexes are decreased or absent (areflexia) in the affected limbs.
  • Sensation: Objective sensory loss rules out polio (except for subjective myalgia). A definite sensory level indicates Transverse Myelitis.
  • Cranial Nerves: Bulbar polio can affect cranial nerves (IX, X), causing difficulty swallowing or speaking. GBS may cause facial palsy.
• Musculoskeletal: Check for muscle atrophy (rapid in polio) and severe muscle tenderness/spasm.

3. Differential Diagnosis of AFP

The following table summarizes the clinical characteristics distinguishing the major causes of AFP:

III. Components of AFP Surveillance

AFP surveillance is a comprehensive system designed to detect and investigate every case of AFP to rule out poliovirus. It consists of specific strategies, case activities, and performance monitoring.

A. Surveillance Strategies

To ensure no polio case is missed, surveillance relies on a multi-tiered approach:

1. Routine (Passive) Surveillance:

   • Designated health facilities (Reporting Units) send regular reports of AFP cases.
   • "Zero Reporting": Sites must report even if no AFP cases are seen (reporting "zero" cases) to ensure the system is active and staff are alert.

2. Active Surveillance (AS):

   • Designated surveillance medical officers (SMOs) or government staff physically visit high-priority health facilities (hospitals, rehabilitation centers) to search for unreported AFP cases.
   • They review patient registers (pediatrics, neurology, physiotherapy) for diagnoses like GBS, transverse myelitis, or symptoms like "floppy weakness".
   • Prioritization: Sites are categorized (e.g., Very High Priority, High Priority) to determine visit frequency (weekly/monthly) based on the likelihood of seeing AFP cases.

3. Community-Based Surveillance (CBS):

   • Utilized in hard-to-reach areas, nomadic populations, or conflict zones where facility-based surveillance is weak. Community informants (healers, leaders) report suspected cases.

4. Environmental Surveillance (ES) (Supplementary):

   • Testing sewage/wastewater for poliovirus to detect circulation in the absence of paralytic cases (silent transmission).

5. iVDPV Surveillance (Supplementary):

   • Surveillance for poliovirus excretion among patients with Primary Immunodeficiency Disorders (PIDs) who are at risk of prolonged excretion of vaccine-derived polioviruses.

B. Case Activities (The Surveillance Cycle)

Once an AFP case is identified, a standardized sequence of activities must be followed:

1. Detection and Notification:

   • Any case meeting the AFP case definition must be notified immediately (within 24 hours) to the district surveillance unit (DIO/SMO).

2. Investigation and Validation:

   • Investigation must occur within 48 hours of notification.
   • The investigator verifies the case definition, takes a detailed history, performs a physical exam, and assigns a unique EPID number.
   • "Hot Cases": Cases <5 years with fever at onset and asymmetric paralysis are flagged as "Hot Cases" requiring prioritized attention.

3. Stool Specimen Collection (The Cornerstone):

   • Polio is diagnosed by isolating the virus from stool.
   • Requirement: Two adequate stool specimens must be collected.
   • Definition of Adequate: Two samples collected 24–48 hours apart and within 14 days of paralysis onset.
   • Transport: Specimens must be transported under reverse cold chain (2–8°C) to a WHO-accredited laboratory within 72 hours of collection.

4. Laboratory Testing:

   • Specimens are tested for poliovirus isolation, intratypic differentiation (wild vs. vaccine), and sequencing (to determine genetic origin).

5. Outbreak Response and Active Case Search:

   • Following case identification, an Active Case Search is conducted in the community (house-to-house) to find missed cases.
   • Outbreak Response Immunization (ORI) may be conducted in the immediate area to boost immunity.

6. 60-Day Follow-Up Examination:

   • Required for cases with:
     • Inadequate stool specimens.
     • Isolation of vaccine virus.
     • Residual paralysis suspected.
   • The child is re-examined 60–90 days after onset to check for residual paralysis (a hallmark of polio).

7. Contact Sampling:

   • If stool specimens from the index case are inadequate (late collection, poor condition), stool samples are collected from 3–5 close contacts (children <5 years) to increase sensitivity.

C. AFP Case Classification (Virological Scheme)

The final classification of an AFP case depends on laboratory results and the Expert Review Committee (ERC):

1. Confirmed Polio: Wild poliovirus or VDPV isolated from any stool specimen (case or contact).
2. Non-Polio AFP (Discarded):
   • Adequate stool specimens are negative for poliovirus.
   • OR Inadequate stools but no residual paralysis at 60-day follow-up.
3. Polio Compatible:
   • Stool specimens were inadequate (or not collected).
   • AND There is residual paralysis at 60 days (or the child died/was lost to follow-up).
   • AND The ERC cannot rule out polio based on clinical evidence. (These represent surveillance failure).

D. Performance Indicators

The quality of the surveillance system is monitored using standard indicators:

1. Non-Polio AFP Rate: Detects sensitivity.
   • Target: ≥2 cases per 100,000 population under 15 years of age (in endemic/risk regions like India).
2. Stool Adequacy Rate: Detects quality of investigation.
   • Target: ≥80% of AFP cases with two adequate stool specimens collected within 14 days of onset.
3. Timeliness of Investigation: ≥80% investigated within 48 hours.
4. Non-Polio Enterovirus (NPEV) Isolation Rate: Target ≥10% (Indicates the lab is capable of growing viruses and the cold chain was maintained).

IV. The AFP Surveillance Network (India Context)

In India, the surveillance network is highly structured to maintain polio-free status:

• National Polio Surveillance Project (NPSP): A WHO-Govt of India collaboration providing technical oversight.
• Surveillance Medical Officers (SMOs): Responsible for active surveillance, training, and monitoring at the district level.
• Reporting Network: Consists of Reporting Units (RUs) (hospitals/clinics that submit weekly reports) and Informer Units (IUs) (practitioners/quacks who notify cases on occurrence).
• Regular Review: The network is dynamically updated to include new health facilities.