True statement regarding poliomyelitis is: March 2010

Ans. C: Commonly spreads by faeco-oral Poliomyelitis is caused by infection with a member of the genus Enterovirus known as poliovirus (PV). This group of RNA viruses prefers to inhabit the gastrointestinal tract. PV infects and causes disease in humans alone. Its structure is very simple, composed of a single (+) sense RNA genome enclosed in a protein shell called a capsid. Three serotypes of poliovirus have been identified-poliovirus type 1 (PV1), type 2 (PV2), and type 3 (PV3)- each with a slightly different capsid protein. All three are extremely virulent and produce the same disease symptoms. PV1 is the most commonly encountered form, and the one most closely associated with paralysis. The virus may be rapidly inactivated by pasteurization. Individuals who are exposed to the virus, either through infection or by immunization with polio vaccine, develop immunity. In immune individuals, IgA antibodies against poliovirus are present in the tonsils and gastrointestinal tract and are able to block virus replication; IgG and IgM antibodies against PV can prevent the spread of the virus to motor neurons of the central nervous system. Infection or vaccination with one serotype of poliovirus does not provide immunity against the other serotypes, and full immunity requires exposure to each serotype Poliomyelitis is highly contagious and spreads easily by human-to-human contact. Incubation period is usually between 7 to 14 days, with a maximum range of 3 to 35 days. Virus paicles are excreted in the feces for several weeks following initial infection. The disease is transmitted primarily the fecal-oral route, by ingesting contaminated food or water. It is occasionally transmitted the oral-oral route, a mode especially visible in areas with good sanitation and hygiene. Polio is most infectious between 7-10 days before and 7-10 days after the appearance of symptoms, but transmission is possible as long as the virus remains in the saliva or feces Man is the only known reservoir of infection. Most infections are subclinical. They form the submerged poion of the iceberg. It is estimated that for every clinical case, there may be 1000 subclinical cases in children and 75 in adults.