The Picornaviridae form a family of nonenveloped viruses. Most genera in the family are unusually resistant to acids and alcohol, allowing them to survive in the gastrointestinal tract. The best-known viruses in the family include poliovirus and hepatitis A virus.
What are picornaviridae?
Picornaviridae or picornaviruses correspond to a family of viruses belonging to the order Picornavirales. The individual species are nonenveloped viruses with the genome of a single-stranded linear RNA of positive polarity. Viruses of the family Picornaviridae grow to a size of only 22 to 30 nm. This makes them the smallest viruses known to date. In relation to their size, they are also named “pico”, which literally means “very small”. Picornaviruses infect a wide variety of vertebrates, on which they can cause extremely diverse diseases. From a harmless cold to diarrheal diseases, mucosal inflammation, and central nervous system infections, a wide variety of symptoms can be attributed to the different species of the tiny viruses. The subspecies of the family are usually systematized subdivided into subtypes. They possess great surface variance and, concomitantly, are associated with antigenic variability. Approximately 370 subtypes of picornaviruses have now been classified. One of the most relevant representatives of the Picornaviridae for humans is the poliovirus. In addition, hepatitis A virus is among the Picornaviridae.
Occurrence, distribution, and characteristics
All Picornaviridae are equipped with a single- or, more rarely, double-stranded RNA chain composed of nucleic acid and located in a protein capsule called a capsid. Because of their lack of a lipid envelope, they are also referred to as non-enveloped viruses. They are not sensitive to ether or organic solvents because of the lack of an envelope. They are 30 nm or less in size and appear spherical in most cases. Their capsid is usually composed of four viral proteins, designated VP1 through VP4. In some species of the family, the capsid contains a low concentration of the precursor protein VP0, which actually becomes the proteins VP2 and VP4 during maturation by proteolytic cleavage processes. The four structural proteins of viruses form a capsomer. VP4 lines the inner capsid side and is associated with the virus RNA through positively charged amino acid residues. Approximately 60 capsomers assemble within a capsid to form a so-called icosahedron. The viral surface consists of the three proteins VP1 to 3, on which the antigenic property and the serotype classification of the individual viruses depend. Picornaviruses are extremely stable to all alcohols and milder detergents without a viral envelope. Genera such as enterovirus and hepatovirus are also stable against strong detergents and pH values of less than 3.0. Thus, they have high environmental resistance and are not rendered harmless by the acidic environment of the digestive tract. Particularly stable viruses of the family thus infect humans via the digestive tract and only reach target organs such as the central nervous system or the lungs from there. Less stable genera of the Picornaviridae are more likely to be transmitted by droplet and smear infection of the nasopharynx. Among some of the best-known diseases caused by Picornaviridae is poliomyelitis, which occurs after infection with poliovirus.
Diseases and symptoms
Poliovirus belongs to the enterovirus genus and is transmitted by smear infection. Thanks to vaccination, the incidence of disease is now almost zero. After up to three weeks of incubation, the virus causes diarrhea and respiratory symptoms. After that, meningitis or meningoencephalitis usually develops with the signs of meningismus (stiff neck). Flaccid paralysis sets in. In the spinal form of progression, the paralyses particularly affect the extremities and trunk. However, respiratory disorders also occur. Involvement of the spinal cord near the brain is enormously unfavorable in terms of prognosis and can cause central respiratory paralysis. Hepatitis A virus is also a disease associated with the Picornaviridae that leads to the onset of hepatitis A in humans. Hepatitis A virus infection is usually a fecal-oral infection; less commonly, the virus is transmitted parenterally.Raw or insufficiently cooked food or contaminated drinking water are the most common sources of infection. Hepatitis A is often asymptomatic. In a symptomatic course, a phase of nonspecific symptoms sets in after incubation periods of up to six weeks. In addition to fever, nausea and abdominal pain, myalgias (muscle pain) and arthralgias (joint pain) are usually present, which can initially be mistaken for an influenza infection. In the course of the disease, more or less severe liver symptoms develop, which can cause jaundice with discoloration of the stool and pressure pain on the liver. Not each of the above symptoms must necessarily be present. In fulminant hepatitis, additional symptoms occur and liver failure may develop. However, such a severe course occurs very rarely. Already one or two weeks before the onset of the disease, the patient can transmit the disease to other people. Picornaviridae do not exclusively infect humans, but also cause diseases in other vertebrates. This includes, for example, foot-and-mouth disease. For this disease, viral zoonosis exists, which means that transmission is possible regardless of species. The infection is transmitted to humans, for example, by cloven-hoofed animals such as cattle, pigs or sheep in the form of smear infection. Infected objects and contaminated dairy products are also a source of infection.
