Poliomyelitis is a contagious disease which spreads through person to person contact (the virus infects only human) mainly via the fecal-oral route. The virus affects young children (mainly children under 5 years of age) and can cause permanent disability. Poliovirus has been eradicated from most countries of the world but still 2 countries, Pakistan and Afghanistan are endemic.
Vaccination is the best way to protect people and stop the transmission of Poliomyelitis. There are two types of vaccine; inactivated poliovirus vaccine (IPV) and oral poliovirus vaccine (OPV).
– destruction of motor neurons in spinal cord → flaccid paralysis
– very restricted host range → natural infection occurs only in humans.
– Grows readily in tissue cultures of primate origin.
Transmitted by faeco- oral route.
– Virus multiplies initially in epithelial cells of AC (Oropharynx or intestine) and the lymphatic tissue (tonsils/payers patches).
– Regularly present in throat and stools before the onset of illness.
– Then spreads to the regional lymph nodes and enters blood stream (minor / 20 viraemia) across the BBB.
– Direct neural transmission may also occur eg after tonsillectomy..
– In the CNS, Virus multiplies selectively in the neurons destroys the anterior horn cells of spinal cord.
– Earliest change is degeneration of Nissils bodies.
– When degeneration becomes irreversible, necrotic cell lyses or is phagocytosed.
– Viruses don’t multiply in muscle in vivo.
– Changes that occur in peripheral nerves and voluntary muscles are secondary to the destruction of nerve cells.
The incubation period is 7-14 days. The manifestations may range from the asymptomatic stage (>90 % cases) to the most severe paralytic disease (<1%).
- Inapparent infections: Following infection, the majority (91-96%) of cases are asymptomatic.
- Abortive infection: About 5% of patients develop minor symptoms such as fever, malaise, sore throat, anorexia, myalgia, and headache.
- Nonparalytic poliomyelitis: It is seen in 1% of patients, presented as aseptic meningitis.
- Paralytic poliomyelitis is the least common form (<1%) among all the stages and is characterized by descending asymmetric acute flaccid paralysis (AFP). Proximal muscles are affected earlier than the distal muscles; paralysis starts at the hip and proceeds towards extremities; which leads to the characteristic tripod sign (child sits with flexed hip, both arms are extended towards the back for support). Sites involved can be spinal, bulbospinal, and bulbar. Accordingly, the nature of paralysis varies (e.g. respiratory insufficiency or dysphagia are common in bulbar involvement).
Sample: Poliovirus can be detected in specimens from the throat, feces (stool), and occasionally blood or cerebrospinal fluid (CSF).
Virus Isolation and Detection
Virus isolation from stool specimens is the most sensitive method to diagnose poliovirus infection. To increase the probability of isolating poliovirus, the collection of at least two stool specimens in 24 hours apart from suspected patients of poliomyelitis is recommended. Poliovirus may also be isolated from pharyngeal swabs. Isolation is less likely from blood or CSF. Samples should be collected as early in the course of the disease as possible (ideally within 14 days after onset).
Primary money kidney cells are the most recommended cell lines. Virus growth can be identified by various methods.
- Cytopathic effects appear in 3-6 days; described as crenation and degeneration of the entire cell sheet.
- Isolated virus can be identified and serotyped by neutralization with specific antiserum.
- Specific gene sequence of the virus can be detected by polymerase chain reaction (PCR) assays.
For the patients who did not receive polio vaccination, the serological test may be helpful in supporting the diagnosis of paralytic poliomyelitis. An acute serum specimen should be obtained as early in the course of the disease as possible, and a convalescent specimen should be obtained at least 3 weeks later.
A four-fold rise in antibody titer in paired sera confirms the diagnosis. Neutralization tests and complement fixation tests can be used to measure the concentration of antibodies.