Picornaviruses (Table 1) are small (from the Italian word piccolo, meaning small), nonenveloped, single stranded RNA viruses. They are among the simplest of the RNA viruses, with a highly structured capsid that has limited surface elaboration. This family of viruses includes the enteroviruses, rhinoviruses, and HAV. Enterovirus infections are among the most common human viral infections (Table2), and although these infections often are mild, the viruses also can cause serious disease. Enteroviruses are responsible for a variety of diseases and conditions, including aseptic meningitis, paralytic polio myelitis, and encephalitis, in addition to respiratory illness, myocarditis, and pericarditis. Enteroviruses are the most common cause of aseptic meningitis, an inflammation of the brain parenchyma, and have been isolated from more than 40% of patients with this disease.

Table1. Picornaviruses

Table2. Enterovirus Infections

Before the development of the polio vaccine, the polio enterovirus was responsible for paralytic poliomyelitis around the world. Polio virus infections were identified as early as the 1800s, when cases involving paralysis with fever were noted. During the polio outbreaks of the first half of the twentieth century, thousands of people, especially children, developed an acute, flaccid (relaxed, “rag doll”) paralysis that affected their ability to breathe. To assist these patients with breathing during viral infection, the “iron lung,” or tank respirator, was invented. The iron lung was an airtight chamber that encased the patient and created negative air pressure around the thoracic cavity, causing air to rush into the lungs. Control of polio through a vaccine began in 1955 with the Salk inactivated polio vaccine, which was administered by intramuscular injection. In 1961 the Sabin oral live attenuated vaccine was licensed in the United States. This drug frequently was administered as a sugar cube coated with the vaccine. In the later 1980s, WHO began a massive campaign to eradicate polio from the world population. By 2006 the number of countries where polio was still endemic had been reduced to four: Afghanistan, India, Nigeria, and Pakistan. A global effort to eradicate this disease through continued surveillance and vaccination programs continues.

The early studies of poliovirus are landmarks in the discipline of the virology and the understanding of the pathogenesis, treatment. and control of enteroviruses. Investigation of this virus started in the early twentieth century. From the evidence they collected, scientists were able to prove the communicable nature of the disease and the importance of asymptomatic infection in the transmission of the disease. These studies also provided breakthrough observations related to the propagation of viruses and cell culture.

The enteroviruses originally were divided into poliovirus, coxsackie virus, and echovirus groups based on similarity of characteristics in cell culture and disease in humans. Classification based on these criteria resulted in the definition of 67 serogroups of enterovirus. Genetic diversity among these viruses, recognized through the application of modern molecular techniques, dictates that newly characterized strains be given enterovirus-type designations rather than serotype status in one of the three original groups. The molecular and serotype designations provide an improved classification system because of the previously poor disease- and phenotype-based classification system. Human enteroviruses have now been reclassified into five species, human enteroviruses A to D, and poliovirus.

The virus is transmitted by the respiratory and fecal oral routes. Therefore, the primary site for enterovirus infection is the respiratory epithelium or the gastrointestinal tract. Specimens of choice for detecting enterovirus are (in order of preference) stool specimens or rectal swabs, throat swabs or washings, and CSF. For cases of acute hyperemia conjunctivitis caused by enterovirus 70, conjunctival swabs or tears can be used. Several enterovirus species can be readily grown in cell culture and produce a characteristic CPE of visible cell rounding and shrinking, as well as refractility and cell degeneration. However, no one cell line supports the growth of all types of enteroviruses. A variety of primate and human cell lines may be used for virus isolation. CPE can be observed within 24 hours if the inoculum contains substantial infectious particles. CPE appears rapidly and often destroys the entire monolayer of cells within hours.

An enterovirus diagnosis is confirmed using a panenterovirus IFA, and specific confirmation of enterovirus identification is completed using cell culture neutralization and type-specific antisera. These confirmatory tests often are available only in specialty laboratories. Molecular testing is fast replacing traditional cell culture for confirmation of an enterovirus diagnosis, especially in CSF from patients showing symptoms of meningitis. Some molecular procedures are also capable of further characterizing the enterovirus into the specific type using genomic sequencing. The major advantages of using nucleic acid testing for enterovirus are faster detection of the virus, increased sensitivity, and the ability to detect enterovirus types incapable of growth in cell culture. Serologic testing for the presence of IgM antibody with ELISA can be used for suspect cases of enterovirus infection, and has been used as an epidemiologic tool in enterovirus outbreaks.

Rhinovirus is the cause of the “common cold.” Its name reflects the fact that the primary infection and replication site is the epithelium cells in the nose. Rhinoviruses are responsible for more than 50% of viral colds and cause more upper respiratory viral infections than any other virus. Although frequently mild, rhinovirus infections can cause complications such as otitis media and sinusitis and can exacerbate previously existing conditions such as asthma, chronic obstructive pulmonary disease (COPD), and cystic fibrosis, in which the risk of severe lower respiratory disease is significantly increased and morbidity can result. Considerably more cases of lower respiratory tract disease in adults are caused by rhinovirus than was previously known. Infection occurs by person-to-person transmission of infected respiratory secretions. Infection usually occurs through self-inoculation through the eyes or nose and also occurs through contact with infectious aerosols. Symptoms usually begin 2 to 3 days after exposure. The clinical presentation includes a profuse, watery nasal discharge frequently accompanied by symptoms of headache, malaise, sneezing, nasal congestion, sore throat, and cough. Illness generally lasts 10 days to 2 weeks.

Neutralization studies have defined more than 100 serotypes of rhinovirus, making development of antigen detection tests difficult. As a result of the numerous serotypes, infections continue to occur year after year. It is also important to note that a previous infection with one serotype of rhinovirus does not confer immunity to a subsequent infection with a different serotype. Confirmation of rhinovirus infection is infrequently required for clinical reasons, because the infection typically is self-limiting. However, the specimen of choice for diagnosis is nasal secretions. Culture can be performed for rhino virus using human cell lines such as MRC-5. CPE usually occurs 1 to 4 days after inoculation. CPE appears as large and small round refractile cells in the fibroblast cell line. The rhinoviruses grow best or exclusively at lower temperatures (30°C); therefore, detection in clinical virology laboratories often is unlikely because typical incubation temperatures for viral cell culture are 35° to 37°C. If required, cell culture conditions should resemble the physiologic environment in the nasal passages, including a pH of 7 and a temperature of 33° to 35°C. An acid treatment of the clinical sample before culture inoculation may be used to distinguish rhinovirus growth from acid-stable enteroviruses. However, this test is not readily available in clinical laboratories because of its complexity and long turnaround time. IFA cannot be used to confirm rhinovirus in cell culture, because no monoclonal anti bodies or antigen detection assays are available. The use of PCR to detect rhinoviruses has expanded understanding of the range of diseases caused by this group. PCR frequently is used for detection of rhinovirus because of its faster detection time and increased sensitivity.

Hepatitis A virus, another member of the picornaviruses, causes an infectious nonchronic hepatitis. HAV is usually transmitted through contaminated food or water or household contact with an infected person. Other transmission routes include sharing of contaminated needles (illicit drug use), travel to endemic countries, and homosexual male intercourse. The virus is significantly different from the other picornavirus based on the liver tissue tropism, high thermal stability, and viral assembly. This is the only hepatitis group of viruses capable of growth in cell culture. However, currently diagnosis is still completed using a serologic assay to identify the IgM antibody (Figure 1). A vaccine against this virus for adults and for children older than 2 years of age became available during the 1990s.

Fig1. Time course of disease and immune response to hepatitis A virus. (Modified from Murray PR, Kobayashi GS, Pfaller MA et al,  editors: Medical microbiology, ed 2, St Louis, 1994, Mosby.)

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مواضيع ذات صلة

Polyomaviruses

Parvoviruses

Paramyxoviruses

Properties of Paramyxoviruses

Influenza Virus Replication

Antigenic Drift and Antigenic Shift

Structure and Function of Neuraminidase

Complications of COVID-19

Clinical Features of SARS-CoV-2 and COVID-19

Modes of Transmission SARS-CoV-2 and COVID-19

Papillomaviruses

Arenaviruses