Poliomyelitis (polio) is an infectious disease caused by Polioviruses that affects the central nervous system and leads to paralysis.
Poliomyelitis is a viral infectious disease caused by the Polioviruses which affect the neurons of the gray matter, especially those located in the spinal cord. As a result, muscle weakness to full loss of muscle function in certain regions develop. Some individuals are able to regain muscle strength while others remain handicapped or may even die if the vital centers of the nervous system are affected. By the means of vaccination, the disease was eradicated in most countries worldwide, though it still remains endemic in three countries – Afghanistan, Nigeria, and Pakistan with occasional outbreaks occurring in Africa, South-East Asia, and Western Pacific region.
The disease is caused by the Poliviruses which belong to the Enterovirus family. There are three different serotypes (specific strains) of polioviruses. However, most cases of polio were caused by serotype 1.
The virus is transmitted via the fecal-oral route and by the aerosol droplets. Infection attaches to the mucosal membranes and starts to replicate, afterwards viral dissemination occurs. The virus tends to affect the motor neurons (the ones responsible for the movements) located in the spinal cord and brain stem (the posterior part of the brain, which includes the midbrain, the pons and medulla oblongata), less frequently – those located in the cerebellum and cerebral cortex. The severity of disease and grade of the accompanied disability depends on the distribution of the lesions and degree of the nervous system involvement.
The incubation period (the period from the contact with an infected person to the disease onset) for polio lasts on average for 9-12 days (though it may be as short as 5 days or as long as 35 days as well).
Asymptomatic illness occurs in about 90-95% of cases of Poliovirus infection.
Abortive poliomyelitis (or the minor illness) is characterized by fever, headache, sore throat, fatigue, vomiting, and abdominal pain. There are no characteristic polio symptoms so the disease can be suspected only when there is an epidemic.
Nonparalytic poliomyelitis (or aseptic meningitis) resembles other diseases that affect the meninges – membranes enveloping the spinal cord and brain, therefore, this variant of the infection is referred to as aseptic meningitis. In these cases, a person experiences fever and chills, headache, vomiting, light and sounds sensitivity.
Paralytic polio (the major illness) is considered very rare (about 0,1% of all poliovirus infections). The disease manifests with severe neck, back and muscle pain along with muscle weakness with preceding fever and other signs of meningitis such as neck stiffness.
- Spinal paralytic polio presents with muscular weakness from mild to paralysis affecting either one muscle or many of them (sometimes tetraplegia, the partial or total loss of function in all four extremities and torso, occurs). What’s especially typical for polio is that muscle involvement usually is not symmetrical, meaning that some muscles appear to be damaged while others remain functional. This phenomenon can be explained by selective damage to motor neurons. The muscle weakness typically progresses for 2-3 days and remains stable when the fever subsides.
- Bulbar paralytic polio develops when the neuron located in the brain stem are affected. These neurons innervate the tongue, soft palate, and pharynx, thus, their involvement is characterized by impaired swallowing (dysphagia), nasal speech and difficulties breathing.
The damaged neurons are able to restore and regain their function, however, if the motor neurons were destroyed paralysis will be unfortunately permanent. Usually, affected individuals bounce back and recover movements within month to years after the acute infection, though the most evident recovery is gained within 1 month.
Polioencephalitis may develop in infants, though it is an uncommon form of the disease. Confusion, seizures, and disorders of consciousness are caused by the inflammation of the brain called encephalitis.
Sometimes individuals who have recovered from polio may experience new episodes of progressive muscle weakness and atrophy, pain and fatigue years after the onset of acute polio infection (the average interval is as long as 36 years). Furthermore, the new onset of the disease seems to affect the same muscles as the initial infection with further involvement of the previously unaffected muscles. However, postpolio syndrome tends to have a milder course and usually is not associated with severe impairment except those cases when respiratory muscles are damaged. The exact cause of the postpolio syndrome, however, remains unknown.
Usually, the white blood cell count is elevated. Lumbar punction reveals the presence of lymphocytes (neutrophils may be present in the first days of the disease) and slightly elevated protein in the cerebrospinal fluid. The virus can be detected in the throat sectretions and in the feces (viral culture). A polymerase chain reaction is performed to verify the polio strain.
There is no treatment available for polio. Only supportive therapy can be provided to relieve the disturbing symptoms.
In 1988 the WHO, Rotary International, the US Centers for Disease Control and Prevention (CDC), the United Nations Children’s Fund (UNICEF) and the Bill & Melinda Gates Foundation in cooperation started the Global Polio Eradication Initiative. Initially the aim was to eradicate the infection by 2000 worldwide, though, to date 3 counties Afghanistan, Nigeria and Pakistan still remain endemic, outbreaks occasionally arise in 14 countries (Angola, Benin, Cameroon, Central African Republic, Democratic Republic of Congo, Ethiopia, Ghana, Niger, Mozambique, Somalia, Indonesia, Myanmar, China, Papua New Guinea, and Philippines) and 13 countries are considered to be at risk of polio returning (Chad, Equatoria Guinea, Guinea, Iran, Iraq, Kenya, Lao People’s Democratic Respublic, Liberia, Madagascar, Sierra Leone, South Sudan, Syrian Arab Republic, and Ukraine).
There are two types of vaccines available for polio prevention. Live-attenuated poliovirus vaccine contains waken virus and should be administered per os (orally). 4 doses schedule is recommended by the WHO with doses given at birth, 6, 10 and 14 weeks of life, an alternative schedule is – at birth, 2, 4 and 6 weeks.
Another one inactivated poliovirus vaccine is prepared by the inactivation of the virus. It is injected either intramuscularly or subcutaneously to kids at 2 months, 4 months, 6 to 18 months and 4 to 6 months of age.
Extremely rarely polio may develop following oral vaccination with an estimated incidence of 1 case per 2,5 million doses. Children with the impaired immune system are more likely to develop this complication of vaccination with oral vaccination – this vaccine contains live but weakened virus and those with impaired immunity may be unable to restrain the virus.