Polio is an acute and very contagious infectious disease, caused by a virus (poliovirus) that attacks the nervous system by affecting the neural cells and inducing paralysis that becomes total in the most serious cases.
The virus affects only the human race and infection occurs via the faeco-oral route, through the ingestion of water or contaminated foods, or through saliva and droplets emitted with coughing and sneezing from sick subjects or healthy carriers.
Symptoms and complications
Regardless of the mode of transmission, the poliovirus penetrates the body through the mouth and multiplies at the level of the pharynx and the gastrointestinal tract. The virus can reach the central nervous system, where it affects motor neurons, leading to muscle weakness and paralysis. Once the infection has occurred, the virus has an incubation period lasting from 6 to 20 days. The pathology can present in four forms:
- Asymptomatic: about 90-95% of poliovirus infections start asymptomatically. Even if people do not show any symptoms, they pass the virus through the faeces.
- Aspecific: about 4-8% of infections present non-specific symptoms (sore throat, fever, nausea, vomiting, abdominal pain) and generally resolve within a week. These flu-like forms are difficult to distinguish from other viral infections.
- Non-paralytic aseptic meningitis: in about 3% of cases, the virus enters the central nervous system and most of these patients develop non-paralytic aseptic meningitis which presents with headache, pain in the neck, back, abdomen and extremities, as well as fever, vomiting, lethargy and irritability.
- Flaccid paralysis: less than 1% of all infections lead to flaccid paralysis. Symptoms start 1 to 10 days after the initial symptoms and last for 2-3 days. There are three forms of paralytic poliomyelitis: spinal is the most common form and is characterised by an asymmetric paralysis which mainly affects the legs. The bulbar form causes weakness of those muscles that are innervated by the cranial nerves. The bulbospinal form is a combination of the first two.
Patients with asymptomatic or non-specific forms recover completely. In those who develop only aseptic meningitis, symptoms may persist for 2 to 10 days, followed by complete recovery. In the paralytic forms of poliomyelitis, muscle weakness can be permanent. This can lead to skeletal deformities, tightening of the joints and difficulty in moving: for example, children can develop a malformation of the foot (equine foot) that prevents them from walking normally.
Mortality from the paralytic form of poliomyelitis is generally 2-5% for children and 15-30% for adults.
Impact on the population
The extensive worldwide vaccination programme developed by the World Health Organization (WHO) for the eradication of poliomyelitis has enabled the change from a situation of about 500,000 cases of paralytic disease each year to the current situation in which it is absent in the Americas, the Western Pacific Region and in Europe. Currently, polio is endemic in Afghanistan, Nigeria and Pakistan. The rest of the world is free of the disease, although there is still the risk of importation from countries where viruses are still endemic.
Sources / Bibliography
- Modlin, JF. Poliovirus. In: Mandell, Douglas, and Bennett's Principles and Practice of Infectious Diseases, 6th ed, Mandell, GL, Bennett, JE, Dolin, R (Eds), Elsevier, Philadelphia 2005. p.2141
- DeBiasi, RL, Solbrig, MV, Tyler, KL. Infections of the nervous system: viral infections. In: Neurology in Clinical Practice, 4th ed, Bradley, WG, Daroff, RB, Fenichel, GM, Jankovic, J, (Eds), Butterworth Heineman, Philadelphia 2004. p.1515
- Mueller S, Wimmer E, Cello J. Poliovirus and poliomyelitis: a tale of guts, brains, and an accidental event. Virus Res 2005; 111:175
- Cohen, JL. Enteroviruses and retroviruses. In: Harrison's Principles of Internal Medicine, 16th ed, Kasper, DL, Brunwald, E, Fauci, AS, et al (Eds), McGraw-Hill, New York 2005. p.1143