Surveillance underpins the entire polio eradication initiative. Without surveillance, it would be impossible to pinpoint where and how wild poliovirus is still circulating, or to verify when the virus has been eradicated in the wild. Surveillance identifies new cases and detects importations of wild poliovirus.
The four steps of acute flaccid paralysis (AFP) surveillance
1. Finding and reporting children with acute flaccid paralysis (AFP)
The first links in the surveillance chain are the staff in all health facilities – from district health centres to large hospitals. They must promptly report every case of acute flaccid paralysis (AFP) in any child under 15 years of age. In addition, public health staff make regular visits to hospitals and rehabilitation centres to search for AFP cases which may have been overlooked or misdiagnosed.
In areas with few formal health workers, some countries use “community” surveillance, where pharmacists, traditional healers or clerics may serve as a source of information on paralysed children.
The number of AFP cases reported each year is used as an indicator of a country’s ability to detect polio – even in countries where the disease no longer occurs. A country’s surveillance system needs to be sensitive enough to detect at least one case of AFP for every 100 000 children under 15 – even in the absence of polio.
2. Transporting stool samples for analysis
In the early stages, polio may be difficult to differentiate from other forms of acute flaccid paralysis, such as Guillain-Barré Syndrome, transverse myelitis, or traumatic neuritis.
All children with acute flaccid paralysis (AFP) should be reported and tested for wild poliovirus within 48 hours of onset, even if doctors are confident on clinical grounds that the child does not have polio.
To test for polio, faecal specimens are analysed for the presence of poliovirus. Because shedding of the virus is variable, two specimens – taken 24-48 hours apart – are required.
Speed is essential, since the highest concentrations of poliovirus in the stools of infected individuals are found during the first two weeks after onset of paralysis.
Stool specimens have to be sealed in containers and stored immediately inside a refrigerator or packed between frozen ice packs at 4–8 degrees celsius in a cold box, ready for shipment to a laboratory. Undue delays or prolonged exposure to heat on the way to the laboratory may destroy the virus. Specimens should arrive at the laboratory within 72 hours of collection. Otherwise they must be frozen (at -200 degrees celsius), and then shipped frozen, ideally packed with dry ice or cold packs. The procedure is known as the “reverse cold chain”.
3. Isolating poliovirus
In a laboratory, virologists begin the task of isolating poliovirus from the stool samples.
If poliovirus is isolated, the next step is to distinguish between wild (naturally occurring) and vaccine-related poliovirus. This is necessary because the oral vaccine consists of attenuated live polioviruses and resembles wild virus in the laboratory. If wild poliovirus is isolated, the virologists identify which of the two surviving types of wild virus is involved. Wild poliovirus type 2 has not been recorded since 1999.
4. Mapping the virus
Once wild poliovirus has been identified, further tests are carried out to determine where the strain may have originated.
By determining the exact genetic makeup of the virus, wild viruses can be compared to others and classified into genetic families which cluster in defined geographical areas.
The newly-found poliovirus sequence is checked against a reference bank of known polioviruses, allowing inferences about the geographical origin of the newly found virus. When polio has been pinpointed to a precise geographical area, it is possible to identify the source of importation of poliovirus – both long-range and cross-border. Appropriate immunization strategies can then be determined to prevent further spread of the poliovirus.
Environmental surveillance involves testing sewage or other environmental samples for the presence of poliovirus. Environmental surveillance often confirms wild poliovirus infections in the absence of cases of paralysis. Systematic environmental sampling (e.g. in Egypt and Mumbai, India) provides important supplementary surveillance data. Ad-hoc environmental surveillance elsewhere (especially in polio-free regions) provides insights into the international spread of poliovirus.