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Infectious Disease and Microbiology – Poliomyelitis
Poliomyelitis is an acute viral infection caused by a small RNA enterovirus known as poliovirus. Most infections are asymptomatic or inapparent. However, in approximately 1% of infected individuals, the virus invades the central nervous system and causes varying degrees of paralysis. Poliomyelitis was once a major global cause of disability and death, but widespread vaccination has dramatically reduced disease incidence worldwide.
No cases of wild-type poliovirus infection have been reported in the United States since 1979. Before eradication efforts intensified, vaccine-derived polioviruses were reported at an average rate of 8–10 cases annually until 1998. Humans are the only known reservoir, and there are no chronic carriers or animal reservoirs. Wild-type polioviruses, vaccine strains from oral poliovirus vaccine (OPV), and vaccine-derived polioviruses may circulate in susceptible populations. By 2005, indigenous transmission of wild-type poliovirus types 1 and 3 had been eliminated from all but four countries: Afghanistan, India, Nigeria, and Pakistan. Wild-type poliovirus type 2 transmission has been globally interrupted. Despite major eradication efforts, outbreaks continue to occur after importation of wild-type virus into previously polio-free countries.
Risk factors for poliovirus infection include overcrowding and poor sanitation, both of which facilitate fecal–oral transmission. Factors associated with increased risk of paralytic disease include B-cell immunodeficiency, strenuous exercise during the early phase of illness, intramuscular injections into affected limbs, and recent tonsillectomy.
Two vaccines are available for prevention: the inactivated poliomyelitis vaccine (IPV) and the live attenuated oral poliomyelitis vaccine (OPV). IPV is administered parenterally and contains all three poliovirus serotypes. It produces excellent systemic immunity, with nearly 100% seroconversion after the third dose, but induces limited intestinal immunity, allowing asymptomatic infection and viral shedding. OPV, used mainly in developing countries, is inexpensive and easy to administer. It induces strong intestinal immunity and enhances herd immunity because vaccinated individuals shed attenuated virus in stool. However, OPV carries a rare risk of vaccine-derived paralytic poliomyelitis, especially in immunocompromised individuals. For this reason, all-IPV schedules are now recommended in the United States and Europe. The routine childhood IPV schedule includes four doses administered at 2 months, 4 months, 6–18 months, and 4–6 years of age.
Humans are the only known reservoir for poliovirus. In developing countries, the fecal–oral route is the principal mode of transmission, while respiratory droplet spread is relatively more important in developed countries. After entering the body, the virus initially infects intestinal epithelial cells and replicates within local lymphatic tissue, causing minor viremia. Infection may terminate at this stage or progress to major viremia involving reticuloendothelial tissues. Central nervous system invasion occurs by retrograde axonal transport from muscle to nerve and spinal cord. Poliovirus preferentially destroys anterior horn cells and motor nuclei within the spinal cord, pons, and medulla, leading to flaccid paralysis affecting the axial muscles, limbs, cranial nerves, or brainstem.
Poliovirus belongs to the family Picornaviridae and genus Enterovirus. Three serotypes exist: types 1, 2, and 3. Infection with one serotype confers immunity only to that serotype, with little cross-protection against the others. Prior to widespread vaccination, serotype 1 caused most paralytic cases.
The incubation period from exposure to the onset of minor illness is approximately 9–12 days, while paralysis generally develops after 11–17 days. Approximately 95% of infections are asymptomatic and detectable only by serology or viral isolation. Abortive poliomyelitis, also known as minor illness, occurs in 4–8% of infected individuals and presents as a nonspecific upper respiratory tract infection with sore throat, low-grade fever, abdominal pain, and diarrhea. Recovery is rapid and complete.
Nonparalytic poliomyelitis occurs in about 1% of infections and is characterized by signs of aseptic meningitis, including neck and back stiffness and pain, following a nonspecific febrile illness. Symptoms usually resolve within 2–10 days.
Spinal paralytic poliomyelitis represents the major form of disease and occurs in approximately 0.1% of infections. Patients develop localized muscle pain and fasciculations several days after the minor illness stage. Fever returns, followed by the rapid onset of asymmetric flaccid paralysis, predominantly involving proximal muscles. Lower extremities are more commonly affected than upper extremities. Sensory loss is typically absent, but deep tendon reflexes are lost. Severe cases may progress to quadriplegia and bulbar involvement.
Bulbar paralytic poliomyelitis involves cranial nerve nuclei, particularly cranial nerves IX and X, leading to dysphagia, nasal speech, and respiratory compromise. Bulbar involvement occurs in 5–35% of paralytic cases and may progress rapidly over several days or even hours. Mixed spinal and bulbar involvement is common. A rare form, polioencephalitis, occurs primarily in children and may present with seizures.
Laboratory evaluation commonly demonstrates cerebrospinal fluid pleocytosis similar to other causes of aseptic meningitis. Poliovirus can be isolated from pharyngeal secretions during the first week of illness and from stool specimens for several weeks. Isolation from cerebrospinal fluid is uncommon. Serologic testing with acute and convalescent sera can confirm infection through neutralizing antibody responses against all three serotypes. Molecular genotyping is important in distinguishing wild-type from vaccine-derived strains.
Neuroimaging studies such as CT or MRI are mainly used to exclude alternative causes of paralysis. MRI may reveal T2-weighted hyperintensities involving the bilateral anterior horn cells. Pathological examination demonstrates neuronal destruction and inflammatory lesions affecting the gray matter of the anterior horns and motor nuclei of the brainstem.
The differential diagnosis includes other enteroviral infections, botulism, West Nile virus infection, transverse myelitis, epidural spinal lesions, tick paralysis, Guillain-Barré syndrome, neuropathies, and stroke. Guillain-Barré syndrome is distinguished by its symmetric ascending paralysis.
There is no specific antiviral therapy for poliomyelitis. Management is primarily supportive. During the acute phase, hospitalization is required for monitoring and supportive care. Strict bed rest is essential to minimize progression of paralysis. Positive pressure ventilation may be necessary when respiratory muscle weakness causes reduced vital capacity. Physical therapy should begin promptly once progression of paralysis has stopped in order to preserve muscle function and prevent contractures. Patients should consume a diet rich in fiber because constipation is common.
The prognosis varies according to disease severity. Bulbar poliomyelitis carries the highest mortality rate, approaching 60%. Approximately two-thirds of patients with paralytic disease are left with some degree of permanent weakness. Complete recovery is uncommon in patients with severe paralysis. Survivors of bulbar paralysis often show substantial improvement within 10 days, and long-term outcome can usually be estimated after one month.
Complications of paralytic poliomyelitis include respiratory failure, gastric dilatation, and paralytic ileus. Years after recovery, some patients develop post-poliomyelitis syndrome, characterized by gradual onset of muscle weakness, pain, fatigue, and muscle atrophy affecting previously involved muscles. This syndrome typically appears 25–35 years after the initial infection.
Poliomyelitis is an acute viral infection caused by a small RNA enterovirus known as poliovirus. Most infections are asymptomatic or inapparent. However, in approximately 1% of infected individuals, the virus invades the central nervous system and causes varying degrees of paralysis. Poliomyelitis was once a major global cause of disability and death, but widespread vaccination has dramatically reduced disease incidence worldwide.
No cases of wild-type poliovirus infection have been reported in the United States since 1979. Before eradication efforts intensified, vaccine-derived polioviruses were reported at an average rate of 8–10 cases annually until 1998. Humans are the only known reservoir, and there are no chronic carriers or animal reservoirs. Wild-type polioviruses, vaccine strains from oral poliovirus vaccine (OPV), and vaccine-derived polioviruses may circulate in susceptible populations. By 2005, indigenous transmission of wild-type poliovirus types 1 and 3 had been eliminated from all but four countries: Afghanistan, India, Nigeria, and Pakistan. Wild-type poliovirus type 2 transmission has been globally interrupted. Despite major eradication efforts, outbreaks continue to occur after importation of wild-type virus into previously polio-free countries.
Risk factors for poliovirus infection include overcrowding and poor sanitation, both of which facilitate fecal–oral transmission. Factors associated with increased risk of paralytic disease include B-cell immunodeficiency, strenuous exercise during the early phase of illness, intramuscular injections into affected limbs, and recent tonsillectomy.
Two vaccines are available for prevention: the inactivated poliomyelitis vaccine (IPV) and the live attenuated oral poliomyelitis vaccine (OPV). IPV is administered parenterally and contains all three poliovirus serotypes. It produces excellent systemic immunity, with nearly 100% seroconversion after the third dose, but induces limited intestinal immunity, allowing asymptomatic infection and viral shedding. OPV, used mainly in developing countries, is inexpensive and easy to administer. It induces strong intestinal immunity and enhances herd immunity because vaccinated individuals shed attenuated virus in stool. However, OPV carries a rare risk of vaccine-derived paralytic poliomyelitis, especially in immunocompromised individuals. For this reason, all-IPV schedules are now recommended in the United States and Europe. The routine childhood IPV schedule includes four doses administered at 2 months, 4 months, 6–18 months, and 4–6 years of age.
Humans are the only known reservoir for poliovirus. In developing countries, the fecal–oral route is the principal mode of transmission, while respiratory droplet spread is relatively more important in developed countries. After entering the body, the virus initially infects intestinal epithelial cells and replicates within local lymphatic tissue, causing minor viremia. Infection may terminate at this stage or progress to major viremia involving reticuloendothelial tissues. Central nervous system invasion occurs by retrograde axonal transport from muscle to nerve and spinal cord. Poliovirus preferentially destroys anterior horn cells and motor nuclei within the spinal cord, pons, and medulla, leading to flaccid paralysis affecting the axial muscles, limbs, cranial nerves, or brainstem.
Poliovirus belongs to the family Picornaviridae and genus Enterovirus. Three serotypes exist: types 1, 2, and 3. Infection with one serotype confers immunity only to that serotype, with little cross-protection against the others. Prior to widespread vaccination, serotype 1 caused most paralytic cases.
The incubation period from exposure to the onset of minor illness is approximately 9–12 days, while paralysis generally develops after 11–17 days. Approximately 95% of infections are asymptomatic and detectable only by serology or viral isolation. Abortive poliomyelitis, also known as minor illness, occurs in 4–8% of infected individuals and presents as a nonspecific upper respiratory tract infection with sore throat, low-grade fever, abdominal pain, and diarrhea. Recovery is rapid and complete.
Nonparalytic poliomyelitis occurs in about 1% of infections and is characterized by signs of aseptic meningitis, including neck and back stiffness and pain, following a nonspecific febrile illness. Symptoms usually resolve within 2–10 days.
Spinal paralytic poliomyelitis represents the major form of disease and occurs in approximately 0.1% of infections. Patients develop localized muscle pain and fasciculations several days after the minor illness stage. Fever returns, followed by the rapid onset of asymmetric flaccid paralysis, predominantly involving proximal muscles. Lower extremities are more commonly affected than upper extremities. Sensory loss is typically absent, but deep tendon reflexes are lost. Severe cases may progress to quadriplegia and bulbar involvement.
Bulbar paralytic poliomyelitis involves cranial nerve nuclei, particularly cranial nerves IX and X, leading to dysphagia, nasal speech, and respiratory compromise. Bulbar involvement occurs in 5–35% of paralytic cases and may progress rapidly over several days or even hours. Mixed spinal and bulbar involvement is common. A rare form, polioencephalitis, occurs primarily in children and may present with seizures.
Laboratory evaluation commonly demonstrates cerebrospinal fluid pleocytosis similar to other causes of aseptic meningitis. Poliovirus can be isolated from pharyngeal secretions during the first week of illness and from stool specimens for several weeks. Isolation from cerebrospinal fluid is uncommon. Serologic testing with acute and convalescent sera can confirm infection through neutralizing antibody responses against all three serotypes. Molecular genotyping is important in distinguishing wild-type from vaccine-derived strains.
Neuroimaging studies such as CT or MRI are mainly used to exclude alternative causes of paralysis. MRI may reveal T2-weighted hyperintensities involving the bilateral anterior horn cells. Pathological examination demonstrates neuronal destruction and inflammatory lesions affecting the gray matter of the anterior horns and motor nuclei of the brainstem.
The differential diagnosis includes other enteroviral infections, botulism, West Nile virus infection, transverse myelitis, epidural spinal lesions, tick paralysis, Guillain-Barré syndrome, neuropathies, and stroke. Guillain-Barré syndrome is distinguished by its symmetric ascending paralysis.
There is no specific antiviral therapy for poliomyelitis. Management is primarily supportive. During the acute phase, hospitalization is required for monitoring and supportive care. Strict bed rest is essential to minimize progression of paralysis. Positive pressure ventilation may be necessary when respiratory muscle weakness causes reduced vital capacity. Physical therapy should begin promptly once progression of paralysis has stopped in order to preserve muscle function and prevent contractures. Patients should consume a diet rich in fiber because constipation is common.
The prognosis varies according to disease severity. Bulbar poliomyelitis carries the highest mortality rate, approaching 60%. Approximately two-thirds of patients with paralytic disease are left with some degree of permanent weakness. Complete recovery is uncommon in patients with severe paralysis. Survivors of bulbar paralysis often show substantial improvement within 10 days, and long-term outcome can usually be estimated after one month.
Complications of paralytic poliomyelitis include respiratory failure, gastric dilatation, and paralytic ileus. Years after recovery, some patients develop post-poliomyelitis syndrome, characterized by gradual onset of muscle weakness, pain, fatigue, and muscle atrophy affecting previously involved muscles. This syndrome typically appears 25–35 years after the initial infection.
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