Model Answer
0 min readIntroduction
Poliomyelitis, commonly known as polio, is a highly infectious viral disease that primarily affects young children. Caused by the poliovirus, a member of the Enterovirus genus, the disease can lead to paralysis. While significant progress has been made towards global eradication, polio remains endemic in a few countries, necessitating robust surveillance programs. Understanding the pathogenicity of the virus and the methods used for its isolation are crucial for effective disease control and eradication efforts. The Global Polio Eradication Initiative (GPEI), launched in 1988, has dramatically reduced the incidence of polio worldwide, but continued vigilance is essential.
Pathogenicity of Poliovirus Infection
The pathogenicity of poliovirus is a complex process involving several stages, from initial infection to potential paralysis. There are three serotypes of poliovirus – PV1, PV2, and PV3 – each capable of causing paralytic polio. The virus enters the body through the fecal-oral route, or less commonly, through contaminated water or food.
Viral Replication and Spread
Once ingested, the virus initially replicates in the oropharynx and the intestinal tract. From there, it can spread to other sites, including the central nervous system (CNS). The replication cycle involves:
- Attachment: The virus attaches to the poliovirus receptor (CD155) on the surface of cells.
- Entry: The virus enters the cell via receptor-mediated endocytosis.
- Replication: Viral RNA is replicated, and new viral particles are assembled.
- Release: The virus is released from the cell, ready to infect other cells.
Clinical Manifestations
The clinical presentation of poliovirus infection varies widely. Approximately 90-95% of infections are asymptomatic. The remaining 5-10% experience varying degrees of illness:
- Abortive Polio: This is a mild, non-specific illness characterized by fever, fatigue, headache, sore throat, and vomiting. Symptoms typically last for a few days and resolve completely.
- Non-Paralytic Polio (Aseptic Meningitis): This involves inflammation of the meninges, causing symptoms such as headache, stiff neck, and fever. It does not result in paralysis.
- Paralytic Polio: This is the most severe form of the disease, affecting less than 1% of infected individuals. The virus attacks motor neurons in the spinal cord, leading to muscle weakness and paralysis. Paralysis can be asymmetrical and may affect the limbs, trunk, or respiratory muscles.
Factors Influencing Pathogenicity
Several factors influence the severity of poliovirus infection:
- Viral Serotype: PV1 is generally associated with more severe disease than PV2 or PV3.
- Viral Load: Higher viral loads are associated with a greater risk of paralysis.
- Host Factors: Age, immune status, and nutritional status can influence susceptibility and disease severity.
Virus Isolation During Surveillance Programme
Effective surveillance is critical for monitoring the circulation of poliovirus and detecting outbreaks. Virus isolation is a key component of this surveillance.
Sample Collection
The primary samples collected for poliovirus surveillance are stool samples from individuals presenting with acute flaccid paralysis (AFP). AFP is defined as a sudden onset of weakness in any part of the body, lasting for at least 24 hours. Stool samples should be collected within 14 days of the onset of paralysis, ideally within the first 7 days, as viral shedding is highest during this period. Two stool samples, collected 24-48 hours apart, are recommended to maximize the chances of detection.
Laboratory Methods for Virus Isolation
Several laboratory methods are used for poliovirus isolation:
- Cell Culture: This is the traditional method for poliovirus isolation. Stool samples are inoculated onto susceptible cell lines, such as Vero cells or RD cells. If poliovirus is present, it will replicate in the cells, causing a cytopathic effect (CPE), which is visible under a microscope.
- PCR (Polymerase Chain Reaction): PCR is a highly sensitive and specific molecular technique used to detect poliovirus RNA in stool samples. Real-time PCR is commonly used for rapid detection and quantification of the virus.
- Virus Neutralization Test (VNT): This test is used to identify the serotype of the isolated poliovirus.
- Sequencing: Genetic sequencing of the isolated virus helps to determine its origin and track its spread.
Surveillance Network
The surveillance network involves a coordinated effort between healthcare workers, laboratories, and public health authorities. Suspected polio cases are reported, stool samples are collected and transported to designated laboratories, and the results are analyzed to monitor the circulation of poliovirus.
Conclusion
The pathogenicity of poliovirus is a multifaceted process, ranging from asymptomatic infection to severe paralytic disease. Effective surveillance, relying on prompt detection through stool sample analysis using cell culture and molecular techniques like PCR, remains paramount in the global effort to eradicate polio. Continued investment in surveillance infrastructure, coupled with widespread vaccination campaigns, is essential to achieve a polio-free world. The emergence of vaccine-derived poliovirus (VDPV) underscores the need for maintaining high vaccination coverage and strengthening surveillance systems.
Answer Length
This is a comprehensive model answer for learning purposes and may exceed the word limit. In the exam, always adhere to the prescribed word count.