Picornaviridae
Picornaviridae is a family of small, single-stranded RNA viruses that includes well-known genera such as Enterovirus, Rhinovirus, and Hepatovirus. These viruses are significant human pathogens, with enteroviruses being responsible for a wide range of infections, including poliovirus and various respiratory tract infections. Over 300 types of enteroviruses are recognized, and they primarily spread through fecal contamination, respiratory droplets, or direct contact with infected individuals or contaminated surfaces.
Infections often target the gastrointestinal tract but can also affect multiple organs, leading to conditions ranging from mild illnesses to severe complications, particularly in infants, children, and pregnant women. The replication cycle of these viruses is rapid, occurring typically within eight hours, which contributes to their widespread nature.
While there is a vaccine for poliovirus that has significantly reduced its incidence, no vaccines are available for other enteroviruses, making ongoing research crucial for developing effective preventive measures. Antiviral therapies face challenges due to the rapid viral replication, underscoring the need for continued efforts in understanding the genetics and pathogenicity of these viruses. Overall, Picornaviridae represents a critical area of study due to its impact on public health and the potential for serious health outcomes.
Picornaviridae
Definition
Picornaviruses, specifically rhinoviruses and other enteroviruses, are major causes of infections. More than one hundred types of enteroviruses, including three different polioviruses, can cause disease in humans. Common examples of the genera affecting humans include Enterovirus (coxsackievirus, echovirus, poliovirus, rhinovirus), Hepatovirus (hepatitis A virus), and Parechovirus (respiratory tract viruses).
Natural Habitat and Features
The Picornaviridae family comprises small, sensitive, single-stranded, ribonucleic acid (RNA) viruses whose genomes are surrounded by sixty copies of each of the four structural proteins. Enterovirus is the most important genus of the picornaviruses. According to the International Committee on Taxonomy of Viruses, more than 300 virus types genetically characterized as members of the species of the genus Enterovirus. For the poliovirus, humans are the only likely hosts.
Enteroviruses are acquired usually through fecal contamination; occasionally through respiratory droplets or other secretions such as sputum, saliva, or nasal mucus; and primarily through direct contact with an infected person or through indirect contact with contaminated objects (fomites) or surfaces such as telephones, cell phones, and drinking glasses.
Pathogenicity and Clinical Significance
Replication of Picornaviridae pathogens occurs entirely in the cytoplasm, and the cycle is rapid—between five and ten hours, with eight hours as the norm. The pathogens feature different cellular receptors (such as poliovirus CD155 and rhinovirus ICAM-1), and the RNA enters the cell through the membrane at the center of the penton; this occurs after one of the viral proteins has enclosed itself in the cell’s membrane. A secondary viremia may occur when symptoms are present.
The spread of the virus goes into the gastrointestinal (GI) tract, whereby the secondary viremia starts ten days or so after the initial infection. This action leads to a cell-mediated and humoral immune response. It rapidly limits the replication of the virus in all tissues except the GI tract. A picornavirus induces disease depending on its viral genetics and how it adapts to the host cells.
Enteroviruses can damage many organs and systems in the body, including the heart, liver, kidney, pancreas, lungs, muscles, skin, and central nervous system (CNS). The damage is caused by local necrosis and the inflammatory response of the host. CNS infections are connected many times with mononuclear pleocytosis (increased cell count) of the cerebrospinal fluid, consisting of macrophages and activated T lymphocytes; this increase causes a meningeal inflammatory response.
Every person is at risk for contracting a picornavirus infection. Infants, children, and teenagers are at a higher risk because they are unlikely to have developed an adequate immune response to infection. Among adults, pregnant women have a higher risk of serious illness, especially if they do not have antibodies from earlier exposures. The non-polio enteroviruses, which usually occur in the United States in the fall and summer months, are common and second in prevalence to the common cold viruses. The rhinoviruses are the most common viruses among humans. Enteroviruses cause an estimated 10 to 15 million infections annually in the United States alone.
Drug Susceptibility
Viral replication is a major problem that limits the effectiveness of antiviral therapy for a Picornaviridae-caused infection. Pleconaril, a drug which inhibits viral growth by blocking the viral uncoating and viral attachment to the host cell, showed potent, broad-spectrum activity against the rhinovirus. However, in later studies it showed a lack of efficacy and presented some safety concerns.
Vaccines are a major preventive measure against viruses. Highly effective vaccines have almost eradicated poliomyelitis worldwide. No vaccines exist, however, for the coxsackievirus and other enteroviruses. These viruses are not considered life-threatening, although older or immune-compromised persons may acquire serious infections that can be life-threatening. Continued research for new vaccines is ongoing because the genetics of virulence phenotypes of picornaviruses needs further study to be understood.
Bibliography
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