Poliovirus is transmitted by the faecal–oral route or by ingestion of pharyngeal secretions. The incubation period for poliomyelitis is commonly 7 to 14 days for paralytic disease, with a reported range of 3 to 35 days. The risk of transmission of infection is greatest shortly before to shortly after the onset of symptoms. The virus persists in the pharynx for approximately one week, and in the faeces for three to six weeks or longer, particularly in immunosuppressed individuals, where cases have been reported shedding for many years.
The virus is highly neurotropic and its primary effect occurs in the neurones of the spinal anterior horn or the motor ganglia of the brain stem. Infection is clinically inapparent in up to 95 percent of infections, and ranges in severity from a non-paralytic fever to viral meningitis and ﬂaccid paralysis.
Symptoms include fever, headache, gastrointestinal disturbances, malaise, stiffness of the neck and back, and pain in the limbs, back and neck, with or without paralysis. In children who develop paralysis, the illness may be biphasic, the initial phase of one to three days’ duration being indistinguishable from that of other viral infections. The patient appears to recover, only to be struck down abruptly two to five days later with meningism, followed by paralysis. In adults and adolescents the illness usually presents with a gradual onset of paralysis and pain without the early symptoms.
Asymptomatic people with the infection will shed the virus in their stool and may spread the infection to others. Infection rates may be as high as 100 percent in households where there are non-immune young children, although paralysis may occur in only 0.1–2 percent of infected individuals. Paralysis is more common in adults, occurring in up to 1 in 75 cases of infection.
Case fatalities from paralytic polio vary from 2–5 percent among children and up to 15–30 percent for adults, increasing to 25–75 percent with bulbar involvement.
The post-polio syndrome may occur some 30 to 40 years after poliomyelitis. The cause is not known, but is probably related to the ageing or death of nerves and muscles that were compensating for the original damage. Patients experience muscle pain and exacerbation of existing muscle weakness. The risk of developing post-polio syndrome is greater in women than in men, and the risk increases with time from the episode of acute polio.2