Polio (also called poliomyelitis) is a contagious, historically devastating disease that was virtually eliminated from the Western hemisphere in the second half of the 20th century. Although polio has plagued humans since ancient times, its most extensive outbreak occurred in the first half of the 1900s before the vaccination, created by Jonas Salk, became widely available in 1955.
At the height of the polio epidemic in 1952, nearly 60,000 cases with more than 3,000 deaths were reported in the United States alone. However, with widespread vaccination, wild-type polio, or polio occurring through natural infection, was eliminated from the United States by 1979 and the Western hemisphere by 1991.
At the height of the polio epidemic in 1952, nearly 60,000 cases with more than 3,000 deaths were reported in the United States alone. However, with widespread vaccination, wild-type polio, or polio occurring through natural infection, was eliminated from the United States by 1979 and the Western hemisphere by 1991.
Causes
Several viruses are transmitted to humans through animals. But the poliovirus resides only in humans and enters the environment in the feces of someone who's infected. Poliovirus spreads primarily through the fecal-oral route, especially in areas where sanitation is inadequate.
Poliovirus can be transmitted through contaminated water and food — there's some evidence that flies may spread the virus to food — or through direct contact with someone infected with the virus or who has recently received an oral polio vaccine (OPV), which contains live virus. Polio is so contagious that anyone living with a recently infected person is likely to become infected too. Although people carrying the poliovirus are most contagious seven to 10 days before and after signs and symptoms appear, they can spread the virus for weeks in their feces.
Once poliovirus invades your body, it multiplies in your throat and intestinal tract and then travels to your central nervous system through your blood and lymph. As it moves along your nerve fibers, poliovirus damages or destroys the motor neurons that carry messages between your brain and your muscles.
In polio, to compensate for the missing neurons, the remaining nerve cells sprout new branching fibers (axons). Depending on how many neurons have been damaged, this regeneration may allow you to regain some or all of your muscle function, but it also places added stress on the nerve cell body, which has to nourish the additional fibers. Over time, this stress may lead to the gradual deterioration of the new fibers and eventually to the neuron itself. Researchers have theorized that this may cause the recurrence of signs and symptoms of post-polio syndrome.
Poliovirus can be transmitted through contaminated water and food — there's some evidence that flies may spread the virus to food — or through direct contact with someone infected with the virus or who has recently received an oral polio vaccine (OPV), which contains live virus. Polio is so contagious that anyone living with a recently infected person is likely to become infected too. Although people carrying the poliovirus are most contagious seven to 10 days before and after signs and symptoms appear, they can spread the virus for weeks in their feces.
Once poliovirus invades your body, it multiplies in your throat and intestinal tract and then travels to your central nervous system through your blood and lymph. As it moves along your nerve fibers, poliovirus damages or destroys the motor neurons that carry messages between your brain and your muscles.
In polio, to compensate for the missing neurons, the remaining nerve cells sprout new branching fibers (axons). Depending on how many neurons have been damaged, this regeneration may allow you to regain some or all of your muscle function, but it also places added stress on the nerve cell body, which has to nourish the additional fibers. Over time, this stress may lead to the gradual deterioration of the new fibers and eventually to the neuron itself. Researchers have theorized that this may cause the recurrence of signs and symptoms of post-polio syndrome.
Signs, Symptoms and Complications
- Incubation period commonly 6–20 days with a range of 3–35 days.
- poliovirus infection is highly variable and categorised on the basis of the severity of clinical presentation.
- Up to 95% of all polio infections are asymptomatic. Infected persons without symptoms shed virus in the stool.
- Approximately 4%–8% of polio infections consist of a minor, nonspecific illness This clinical presentation is known as abortive poliomyelitis, and complete recovery occurs in less than a week.
- Three syndromes observed with this form of poliovirus infection are upper respiratory tract infection (sore throat and fever), gastrointestinal disturbances (nausea, vomiting, abdominal pain, constipation or, rarely, diarrhoea), and influenza-like illness. These syndromes are indistinguishable from other viral illnesses.
- Nonparalytic aseptic meningitis (symptoms of stiffness of the neck, back, and/or legs), usually following several days after a prodrome similar to that of minor illness, occurs in 1%–2% of polio infections. Increased or abnormal sensations can also occur. Typically these symptoms will last from 2 to 10 days, followed by complete recovery.
- Less than 1% of all polio infections result in flaccid paralysis. The frequency of paralytic disease increases with age. Paralytic symptoms generally begin 1 to 10 days after prodromal symptoms and progress for 2 to 3 days.
- Generally, no further paralysis occurs after the temperature returns to normal. The prodrome may be biphasic, especially in children, with initial minor symptoms separated by a 1- to 7-day period from more major symptoms.
- Additional prodromal signs and symptoms can include a loss of superficial reflexes, initially increased deep tendon reflexes and severe muscle aches and spasms in the limbs or back. The illness progresses to flaccid paralysis with diminished deep tendon reflexes, reaches a plateau without change for days to weeks, and is usually asymmetrical. Strength then begins to return.
- Patients do not experience sensory losses or changes in cognition.
- Many persons with paralytic poliomyelitis recover completely and some degree of muscle function usually returns.
- Weakness or paralysis still present 12 months after onset is usually permanent.
- The post polio syndrome occurs 30-40 years after poliomyelitis. Patients experience muscle pain and exacerbation of existing muscle weakness.
Treatments and drugs
Because no cure for polio exists, the focus is on increasing comfort, speeding recovery and preventing complications. Supportive treatments include:
- Antibiotics for secondary infections (none for poliovirus)
- Analgesics for pain
- Portable ventilators to assist breathing
- Moderate exercise
- A nutritious diet
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