The picornavirus is categorized among the largest and most important family of viruses this class includes viruses such as enteroviruses, poliovirus and, parechoviruses. As the name suggests picornavirus is a small size RNA containing virus, pico refers to the small size of the virus which is about 27nm, and RNA in the name suggests that the nucleic material of this virus is RNA. This article deals with the understanding of the features of picornavirus, its structure, picornavirus replication, and its pathogenicity in the human body. This article also covers picornavirus classification and discussion of medically important genera of the virus.
Picornavirus is the class of the virus that constitutes many genera of the virus, it is among one of the largest groups of the viral classification. Picornavirus is also known as Picornaviridae according to the taxonomic classification, the Picornaviridae is classified as a family of the virus. This family of the virus contains nine genera of the viruses out of which six are medically important, these six genera of the Picornaviridae family are as follows,
All these six genus of viruses are medically important as they play an important part in the pathogenicity in humans. They are responsible for causing hepatitis, polio, mouth, and foot disease in the human host.
Enterovirus- The enterovirus genus includes the poliovirus, coxsackievirus and, human and animal enteroviruses.
Parechovirus- The parechovirus genus of the virus includes the species of echovirus 22
Hepatovirus- The hepatovirus genus of the virus includes the hepatitis A virus, which causes hepatitis in humans.
Rhinovirus- The rhinovirus genus includes the viral species that contains RNA as genetic material or in other words, the species of virus that belongs to the retrovirus category are classified under the rhinovirus genus.
Apthovirus - The genus apthovirus contains viruses that are responsible for causing foot and mouth disease, a common example of the virus which belongs to the genus includes Equine rhinovirus type I.
Cardiovirus- The cardiovirus genus of the virus includes the following, Encephalomyocarditis, and Theilers murine encephalomyelitis virus. This genera of the virus generally infect rodents.
Characteristics of Picornavirus
There are certain characteristics according to which the picornavirus classification is performed, the picornavirus classification is based on the following, morphology, physicochemical and biologic properties, antigenic structures, genomic sequence, and mode of replication. This table lists some of those characteristics.
Some of the common sites for the isolation of such viruses include the nose, throat, and lower intestine. Some of the isolated viruses specifically infect rodents generally newborn mice. A common example of such viruses is Cardiovirus.
Structure of Picornavirus
The picornavirus is icosahedral in shape, the diameter of the virus of this family is about 28-30 nm. They have a cubic symmetry. They are generally nonenveloped. The virus capsid (the outer layer of the virus) is made up of 60 subunits of the protein. The capsid also contains the site of antibody binding, these are more commonly called as antibody binding sites. The capsid of the virus contains VP 1 - VP 4. The VP 1 and VP 4 are the main antibody binding proteins whereas the VP 3 acts as an integral protein. The capsid of the virus encloses the nucleic material in this case RNA of the virus. There are approximately 7,500 nucleotides in the RNA strand. A non-capsid viral protein commonly known as VPg is bound covalently to the RNA at the 5’ end and the 3’ polyadenylated tail. It is important to note that the ether resistant property of the virus can be attributed to the fact that they do not have any essential lipid.
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The replication in picornavirus takes place in the cytoplasm of the host. The virus contains the single stranded RNA of (molecular weight, approximately 2 × 106 to 3 × 106. The replication of the virus can only occur when the virus has entered the cell of the ghost, the virus uses the replication machinery of the host to complete its replication. The viral genome serves as an mRNA thus prompting the translation of the protein using the components of the host. The replication starts with the attachment of the virion to the specific cell receptor. The site of the attachment is in the plasma membrane. It is suggested that the hydrophobic N terminal of the integral protein of virion is inserted into the plasm membrane of the host. After the insertion, the endosome engulfs the protein and endosomal translocation. This results in reaching the viral nucleic acid at the cytoplasm. It is followed by the uncoating of the viral genetic material, the VPg protein that is associated with the viral RNA is removed by the cellular enzymes of the host. The uncoated RNA of the virus acts as viral mRNA thus initiating the translation. The ribosome binds to the internal ribosomal entry site (IRES) within the 5′ UTR. it folds into a clover leaf structure allowing the binding of the host protein.
It is important to note that the viral genome does not contain the internal translational stop codons. This results in the synthesis of the single long polyprotein chain. This protein then undergoes translational modifications to ensure the production of the viral protein. The viral protein called proteinases performs the cleavage of the encoded protein to obtain the desired end proteins.
The minus RNA strand is also known as antisense RNA strand, its synthesis is initiated at the 3′ end of the virion RNA, it uses the VPg as a primer. As the synthesis of the antisense RNA is complete the antisense RNA strand acts as a template for the synthesis of the sense RNA strand, which is also known as the plus strand. This sense RNA and the viral capsid proteins assemble to form the encasing into the viral head. Thus it can be concluded that the virion replication is performed via two different replicative intermediates (RI).
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Pathogenesis and Host Defense Mechanism
Picornavirus enters the body generally through the nose and mouth. The picornavirus generally replicates into the nasopharyngeal epithelium, another site of the virus replication in regional lymphoid tissues. The replication leads to asymptomatic infections or respiratory illnesses. It is seen that some of the viruses of this family can infect the lower intestine it can be attributed to fact that some of the strains can grow in the intestine as they are resistant to intestinal and bile acidic content. The viral strain is then carried into the bloodstream to target organs such as the spinal cord, brain meninges, heart, liver, and skin. The virus also reaches the skeletal system through the CNS. The picornavirus is associated with the following disease.
Undifferentiated febrile illness
Respiratory tract infections
Mucous membrane disintegration
Lesions in mucous membranes and skin
Generalize neonatal infections
The host response to the virus can be seen as an increase in the concentration of IgA antibodies in the mucous. This is the case in respiratory tract infections. increased IgA concentration can also be seen in the gut of the human body. The first antibody produced by the body in response to the virus is the IgM antibody. After some time there is class switching, this results in the increased production of IgG antibody in the human body. The concentration of IgG antibody peeks for about 2-3 weeks then comes to the stationary phase, after which the concentration decreases. It is also important to note that clearance of antibody and antigen complex takes place by phagocytosis, digestion, and excretion.