Poliovirus is an RNA virus that belongs to the Picornaviridae family and the enterovirus group. The virus is the causative agent of poliomyelitis (polio).
What is the poliovirus
The disease poliomyelitis is also known as polio or poliomyelitis. The causative agent of the disease is the poliovirus. Polioviruses belong to the order Piconavirales. There are three different serotypes of the poliovirus species. Serotype 1 is the most common and often causes severe disease. Serotype 2 tends to cause mild courses. Type 3 is rather rare, but causes an extremely serious course of disease. The disease polio has been known for a long time, but it was not until the beginning of the 20th century that it was recognized that polio is an infectious disease spread by contact. In 1908, Karl Landsteiner and Erwin Popper were able to prove that the poliovirus was the trigger of the dreaded polio. The poliovirus has a very simple structure. It has a diameter of 28 to 30 nanometers and is uncoated. Each of the round virus particles contains a copy of a single-stranded RNA. This is encased in an icosahedral capsid composed of copies of the four capsid proteins. In one region, the viral RNA contains a so-called internal ribosomal entry site (IRES). Translation of the viral RNA into the host cell occurs via this entry site. To enter the host cell, the virus requires the CD155 protein as a receptor. Poliovirus can then replicate in the host cell fluid.
Occurrence, distribution, and characteristics
Before the introduction of polio vaccination, the viruses were distributed worldwide. Because the virus was also ubiquitous throughout Europe before that, initial contact with the virus usually occurred in childhood. That is why poliomyelitis is still called polio today. Today, more than 80% of people worldwide live in a polio-free area. America, the Western Pacific region, Europe and Southeast Asia are polio-free, according to the World Health Organization (WHO). The last case of the disease in Germany was registered in 1990. Endemic polio cases are now found only in Pakistan and Afghanistan. Individual cases are also known in Russia, Turkmenistan and Kazakhstan. The only known pathogen reservoir for polioviruses is humans. The virus can also replicate exclusively in the cells of humans and in the cells of some other primates. Transmission is mainly by the fecal-oral route through smear infections. Polioviruses preferentially infect intestinal cells. Shortly after infection, the intestinal epithelia of the diseased produce many virus cells. Within a short time, the sick excrete 10⁶-10⁹ infectious viruses per gram of stool. The virus also multiplies in the epithelial cells of the pharyngeal skin. As a result, the virus can also be transmitted aerogenically via droplet infection immediately after infection. The worse the hygienic conditions are, the better polio infections can spread. In the body, the virus infects the lymph nodes and then travels via the blood and lymphatic pathways to the nerve cells of the anterior horn in the spinal cord. These ß-motoneurons control striated muscle. In response to infection of the nerve cells, defense cells called leukocytes migrate into the spinal cord. This causes inflammation, in which the nerve cells of the spinal cord are severely damaged or even destroyed. In addition to the spinal cord, the inflammation often affects the brain. Especially in the medulla oblongata, bridge, and cerebellum, inflammatory infiltrates and neuronal cell deaths are found.
Diseases and symptoms
The incubation period ranges from 3 to 35 days. More than 90 percent of all infected persons show no symptoms. The disease is asymptomatic, but neutralizing antibodies are formed. This process is called silent feiication. In abortive poliomyelitis, a three-day infection develops after one to two weeks and is accompanied by fever, lassitude, vomiting and diarrhea. This attenuated form of poliomyelitis heals without consequences or complications in most cases. The nerve cells of the central nervous system are not affected. The central nervous system is involved in 5 to 10 percent of all symptomatic patients. The prodromal stage resembles abortive poliomyelitis. Affected individuals have fever, feel tired, and suffer from diarrhea or vomiting.This is followed by a one-week fever-free period without symptoms. Then patients usually develop a non-suppurative inflammation of the meninges (aseptic meningitis). However, the characteristic paralysis of poliomyelitis is absent. This is a nonparalytic poliomyelitis. Meningitis is accompanied by fever, headache, and neck stiffness. An increased cell count and protein concentration can be found in the cerebrospinal fluid of affected individuals. Only one percent of infected individuals develop paralytic poliomyelitis. Paralytic poliomyelitis is the most severe form of the disease and the “classic polio” in the true sense of the word. After a fever- and symptom-free phase of two to ten days after the meningitis, the characteristic morning paralysis occurs. The child was still healthy the evening before, but the next morning flaccid paralyses appear. The paralyses are symmetrically distributed and preferentially affect the thigh muscles. The affected regions are often very painful. In the bulbar form of paralytic poliomyelitis, the areas of origin of the cranial nerves are affected by the virus. Patients suffer from high fever and breathing difficulties. Circulatory regulation is also disturbed. Normally, the symptoms of poliomyelitis resolve completely within a year. In some cases, paralysis, circulatory disorders or joint damage remain. After years or decades, post-polio syndrome can occur as a late consequence. It manifests as extreme fatigue, muscle wasting, and muscle pain.
