INTRODUCTION TO POLIOVIRUS
Poliovirus is the causative agent of poliomyelitis, an acute infection that affects the CNS i.e. the brain and spinal cord.
MORPHOLOGY OF POLIOVIRUS
⇒ Polio virus belongs to Enterovirus genus of Picornaviridae
⇒ Polio virus is a Spherical, Enveloped virus, composed of 60 subunits, each consisting of four viral Proteins – VP1 – VP4, arranged in icosahedral symmetry.
⇒ Size: 27-30 nm.
⇒ Genetic material – ssRNA.
RESISTANCE SHOWN BY POLIOVIRUS
⇒ Polio virus is resistant to Ether, Chloroform, Bile, Proteolytic enzymes & detergent, is Stable at pH 3.
⇒ In feces, it can survive for months at 4oC and for years at -20oC, readily inactivated by heat – 55oC for 30 minutes.
⇒ Polio virus doesn’t survive lyophilization well.
ANTIGENIC PROPERTIES OF POLIOVIRUS
⇒ By neutralization test, Polio virus strains have been classified into three types as: 1, 2 & 3.
⇒ Type 1 is the most common and causes most epidemics. Type 2 usually causes endemic infections. Type 3 strains have caused epidemics.
HOST RANGE & CULTIVATION OF POLIOVIRUS
⇒ Natural infection occurs only in humans. Experimental host is monkey, may be infected by intracerebral or intraspinal inoculation.
⇒ Polio virus grows readily in primary monkey kidney cultures, Vero, HeLa and Hep-2 cell lines, used for diagnostic cultures and vaccine production.
PATHOGENICITY OF POLIOVIRUS
⇒ Mode of Transmission: Polio virus is transmitted primarily by Fecal-Oral route through ingestion, other routes includes Inhalation & conjunctiva.
Initially virus multiplies in epithelial cells of GIT &
Lymphatic tissues, from the tonsils
Spreads to regional lymph nodes- enters bloodstream.
Carried to the spinal cord & brain.
In CNS, virus multiplies in neurons & destroys them and
Degenerates the Nissl Bodies.
Lesions are mostly in anterior horn of spinal cord, cause paralysis.
CLINICAL FEATURES OF POLIOVIRUS INFECTION
⇒ The incubation period ranges 4 days to 4 weeks; average 10 days.
⇒ Minor illness characterized by primary viremia – Fever, Headache, Sore throat and malaise lasts for 1-5 days.
⇒ Major illness, following minor illness after 3-4 days, characterized by Fever, Headache, Stiff neck – marks the stage of viral invasion of CNS, proceeding to paralytic poliomyelitis, flaccid paralysis develops.
LABORATORY DIAGNOSIS OF POLIOVIRUS
⇒ Specimen : Blood, CSF, Throat swab and Feces; autopsy specimen includes brain & spinal cord.
⇒ Direct demonstration of virus : virus can be demonstrated in feces y direct electron microscopy or by immune electron microscopy.
⇒ Isolation of virus : specimens are inoculated into HeLa and Hep-2 cell line, virus growth indicated by typical cytopathic effect seen in cells within 2-3 days.
⇒ Serology : Complement Fixation Test and neutralization test can be used for serotyping of poliovirus.
PROPHYLAXIS OF POLIOVIRUS
⇒ Both active and passive immunization is available but passive immunization by administration of human gamma globulins is of little value
⇒ Active immunization by vaccination is effective against poliovirus; two types of vaccines are available, killed polio vaccine (Salk) and Live attenuated oral polio vaccine (Sabin).
⇒ The Salk vaccine is given by deep subcutaneous or intramuscular injection, induces only systemic antibody response but do not provide intestinal or mucosal immunity.
⇒ The Sabin vaccine administered orally, commonly called as oral polio vaccine (OPV), induces both local secretory IgA antibodies in the intestine or mucosa and also humoral antibodies (IgM & IgG).