Poliomyelitis: Pathogenesis, clinical features and Laboratory diagnosis

Poliomyelitis is a contagious disease which spreads through person to person contact (the virus infects only human) mainly via fecal oral route. The virus affects the young children (mainly children under 5 years of age) and can cause permanent disability.  Polio virus has been eradicated from most of countries of the world but still 2 countries, Pakistan and Afghanistan are endemic. Vaccination is the best way to protect people and stop 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.
 PATHOGENESIS–         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 destruction of nerve cells.
Clinical findings
–         Incubation period 7-14 days.
–         Inapparent infection: 90-95% cases; only seroconversion.
–         Abortive poliomyelitis: 4-8%; minor illness.
– Fever, malaise, drowsiness, headache, nausea, vomiting, constipation.
Recovery in few days.
–         Non- paralytic poliomyelitis ( aseptic meningitis) – 1-2%
– above symptoms + stiffness and pain in back and neck; 2-10 days.
–         Paralytic poliomyelitis: 0.1-2%
– flaccid paralysis resulting from lower motor neuron damage.
– Maximal recovery with in 6 months, with residual paralysis lasting much longer.
–         Progressive post – poliomyelitis muscle atrophy: rare
– recrudescence of paralysis and muscle wasting.
– Decades after experience with paralytic poliomyelitis.
– doesn’t appear to be a consequence of persistent infection rather a result of physiologic and aging changes.
Lab diagnosis
specimen: throat swabs, rectal swabs / stool, blood, CSF ( uncommon).
No permanent carriers known.
Specimen should be kept frozen during transmit to lab.
A. Isolation of virus.
– can be isolated from blood and pharyngeal aspiration during 10 viraemia,(3-5 days after infection). From feces → upto 5 weeks.
–         After processing specimen, inoculated into tissue culture.
      •Primary monkey kidney cells, typical CPE in 2-3 days.
       • Identification by neutralization tests with pooled and specific antisera.
–         Virus isolation from feces must be interpreted along with clinical and serological evidence.
B. direct demonstration of virus by electron microscopy
c. serology: less often employed.
– paired serum specimens to show a rise in antibody titer
–         Only 1st infection produces strictly type- specific responses.