Poliomyelitis and infantile paralysis: changes in host and virus.

Death of motor neurones following invasion of the central nervous system by poliovirus may result in paralysis of specific muscles. Virulence may be tested by injection into monkeys by routes which bypass natural infection. Transmissibility is also very important, but cannot be measured, only inferred. An infection may lead to immunity or paralysis. In epidemics, the highest incidence among children 0-2 years was 2% and among those over 10 years was 25%: these figures fit a model of genetic susceptibility of homozygotes and heterozygotes with phenotypic susceptibility increasing with age. Hypogamma-globulinemics, some neonates and pregnant women are more susceptible than others. Intra-muscular injections may increase the risk of paralysis. Strenuous exercise and IM injections given when poliovirus has already reached the spinal cord can increase the severity of paralysis or convert a non-paralytic attack to paralysis. Although vaccines reduced polio in temperate countries, polio was thought to be no problem in the tropics. Since 1977 polio has been recognised as a massive problem in the third world: because it affects babies and very young children, it is properly infantile paralysis.