Picornaviruses

            The Picornaviridae family comprises of a large number of very small RNA (pico:small, rna;RNA) viruses with a size of 27-30 nm. They are nonenveloped viruses, resistant to ether and other lipid solvents. Two groups of picornaviruses are of medical importance,  the enteroviruses that parasitise the enteric tract and the rhinoviruses that infect the nasal mucosa. Enteroviruses include Polioviruses type 1, 2, 3, Coxsackie viruses A & B, Enterovirus type 68-72.

Polioviruses causes poliomyelitis, a very ancient disease.

Morphology : The virion is a spherical particle, about 27nm in diameter, composed of 60 subunits, each with 4 viral subproteins- VP1, VP2, VP3, VP4, arranged in an icosahedral symmetry. The genome is a single strand of positive sense RNA, ie.,it can be directly translated by host ribosomes to form a polyprotein.

Resistance : Resistant to ether, chloroform, bile, proteolytic enzymes of the intestinal contents and detergents. It is stable at pH 3 in faeces it can survive for months at 4⁰C and years at –20⁰C. It is readily inactivated by heat,ie., 55⁰C for 30 minutes. Chlorination destroy the virus in water but organic matter delays inactivation. Milk and icecream provides protection from heat inactivation.

Antigenicity, Host range & Cultivation : Two antigens-C & D. D antigen is the dense antigen associated with the whole virion and is type specific. The C antigen is less specific and associated with non infectious viruses. Natural infection only in humans, experimentally in monkeys, chimpanzees. The virus grows readily in tissue cultures of primate origin. Primary monkey kidney cultures are used for diagnostic cultures and vaccine production. The infected cells round up, eosinophilic intranuclear inclusion bodies may be demonstrated in stained preparations.

Pathogenicity :  The virus is transmitted by the oral-faecal route through ingestion. The virus usually multiplies in the epithelial cells of the alimentary canal, tonsils and the lymphatic tissues. It then spreads to the regional lymph nodes and enters the blood stream. After further multiplication in tne reticuloendothelial system, the virus enters the blood stream again and is carried to the brain and spinal cord. In the cental nervous system virus multiplies in the neurons and destroys them. Lesions  are mostly in the anterior of the spinal cord and causes paralysis. The nature and degree of paralysis depend on which neurons in the spinal cord and brain are infected and how severely they are damaged.

            The incubation period is about 10 days. The earliest symptoms include fever, headache, sore throat, malaise which lasts for 1-5 days. This is called minor illness. If the infection progresses, the minor illness is followed 3-4 days later by major illness,ie., the fever comes on again, along with headache, stiff neck and other features of meningitis. This marks the stage of viral invasion of the central nervous system. Mortality range from 5-10% and is mainly due to respiratory failure. Recovery of the paralyzed muscles takes place in the next 4-8 weeks and is usually complete after 6 months, leaving behind varying degrees of residual paralysis. Malnutrition, physical exhaustion, corticosteroids, radiation and pregnancy can increase the severity of the disease.

            Neural spread may occur in children with inapparent infection at the time of tonsillectomy. Poliovirus present in the oropharynx may enter nerve fibres exposed during surgery and spread to brain. There are 4 types of poliovirus infection: Inapparent infection – patient does not have any symptom but the virus may be isolated from stool or throat, seen in 90-95% individuals. Minor illness – patient develop mild, transient influenza -like illness, seen in 4-8% cases. Non-paralytic poliomyelitis – in addition to minor illness patient also develops headache, neck stiffness and back pain, illness lasts for 2-10 days with rapid and complete recovery, seen in 1-2% individuals. Paralytic poliomyelitis – patient develops paralysis during the course of illness, occur in 0.1-2% infections.

Laboratory Diagnosis : is made by isolating the virus from pharyngeal swabs or faeces, culturing it and noting its cytopathic effects. Methods such as complement fixation tests are available for identifying antibodies to the virus in the serum.

Immunity : in poliomyelitis is type specific. Humoral immunity provided by circulating and secretory antibody is responsible for protection against poliomyelitis. IgM antibody appears within a week of infection and lasts for about 6 months. IgG antibody persists for life. Secretory IgA in the gastrointestinal tract provides mucosal immunity preventing intestinal infection and virus shedding. Breast milk containing IgA antibody protects infants from infection.

Prophylaxis : Live polio vaccine is administered orally and is ∴ known as oral polio vaccine(OPV/Sabin), was developed by Albert Sabin in 1962. it contains live attenuated strains of the three serotypes of poliovirus grown in cultures of monkey kidney cells or human diploid cells. The virus in the vaccine is unable to multiply in the CNS and therefore lacks neurovirulence. This oral vaccine stimulates both local secretory IgA antibodies in the pharynx and alimentary tract and humoral IgG antibodies. Virus is excreted in the faeces for several weeks. Precautions should be taken to ensure freedom from extraneous agents like SV 40. a single dose should be sufficient to establish infection and immunity, but in practice 3 doses are given at 4-8 week intervals, to ensure that all 3 types of the vaccine virus multiply in the intestine, overcoming interference among themselves and with other enteric viruses. Killed vaccine or Inactivated Polio Vaccine (IPV/Salk) is given by deep subcutaneous or intramuscular injection. It was developed by Jonas Salk in 1956. Three doses given 4-6 weeks apart, followed by a booster 6 months later. IPV produces long lasting immunity to all 3 poliovirus types. It stimulates the production of IgG antibodies in the serum.  Immunisation provides a tool for eradicating the disease altogether.

Coxsackie viruses are typical enteroviruses. It is necessary to employ young mice for the isolation of the virus. Adult mices are not susceptible. Some types grow well in monkey kidney tissue cultures, HeLa cells. The viruses have an affinity for the pericardium and myocardium. They can cause meningoencephalitis, diarrhoea, rashes, pharyngitis, liver diseases. Epidemic muscle pain, diabetes and inflammation of the pancreas, heart muscle and pericardium are associated with Coxsackie B virus. The virus is highly infectious, spread among family members and institutions. Infections occur by oral-faecal route. The infections during pregnancy causes congenital defects.

Laboratory Diagnosis : Virus isolation from the lesions or from faeces made by inoculation into suckling mice. Identification by studying the tissues of the mice and neutralisation tests.

Prophylaxis : Vaccination is not practicable as there are several serotypes and immunity is type specific.

Rhinoviruses  resembles the size and structure of other picornaviruses. They are more acid labile, but heat stable. They are inactivated below pH 6, relatively stable at 20-37⁰C. Apart from humans they produce infection in chimpanzees only. They can be grown in tissue cultures of human/simian origin with cytopathic changes.

Pathogenicity: Rhinoviruses are the most common cause of common colds or coryza. The virus attaches to receptors on nasal ciliated epithelial cells, enters and replicates within them, spreading to other cells. The cilia and cells are damaged and the epithelium is subjected to secondary bacterial infection. Since they grow best at 33-34⁰C, replicate in the epithelium of the upper respiratory tract, where air movements lower the tissue temperature. Local inflammation and cytokines may be responsible for the symptoms of common cold. Interferon production occurs early and specific antibody appears in nasal secretions.

            Cold viruses spread often by fomites, blowing the nose and handling used tissues contaminates the fingers so that anything touched becomes contaminated. Using tissues once, discarding them immediately in covered containers and thoroughly washing hands after each nose wipe can significantly reduce the spread of rhinoviruses.

Laboratory Diagnosis & Prophylaxis: Isolation of the virus obtained from nasal or throat swabs collected early in the infection, in human cell cultures. Cytopathic effects take 2 weeks to appear. Experiments with certain kinds of human interferon blocks or limits rhinovirus infections.

ECHOviruses – Enteric Cytopathogenic Human Orphan viruses(ECHO viruses), since they could not be associated with any particular clinical disease, they are called orphans. These viruses infects only human beings naturally. Most of the infections are asymptomatic. Fever with rash and aseptic meningitis. Occasional cases of paralysis and hepatic necrosis have also been reported. Faeces, throat swabs, CSF inoculated into monkey kidney tissue cultures and virus growth detected by cytopathic changes. They inhabit the alimentary tract primarily and are spread by the faecal-oral route.