Picornaviruses

Genera of the family Picornaviridae

• Enterovirus

• Parechovirus

• Rhinovirus

• Hepatovirus

• Aphtovirus

• Cardiovirus

Structure of virion• Ss +RNA.• Virion is icosahedral 22-30nm

in diameter.• Capsid has 60 copies of 4

proteins: – VP1, VP2, and VP3 are exposed

on the virion surface, – VP4 lies buried in close

association with the RNA core.  

• Immunogenic sites are located on the external parts of the capsid.

Reproduction Cycle of Picornaviruses

Unique properties of human Picornaviruses

• Small nonenveloped viruses.

• Because they contain no essential lipids, they are ether, acidic pH, and detergents resistant.

• Rhinoviruses are labile at acidic pH.

• They replicate in the cytoplasm.

• Most viruses are cytolytic.

• Picornaviruses do not have a common group-specific antigen. Each serotype has a type-specific antigen, which is identifiable by neutralization tests

GENUS ENTEROVIRUSESThe enterovirus genus is so-called because these viruses generally replicate in the intestine.

History• Poliovirus - first identified by Karl Landsteiner in

1908 by inoculation of specimens into monkeys. The virus was first grown in cell culture in 1949 which became the basis for vaccines.

• Coxsackieviruses - were identified in 1948 by inoculation into newborn mice from two children with paralytic disease. These agents were named coxsackieviruses after the town in New York State.

• Echoviruses - were identified in 1951 which produced cytopathic changes in cell culture and was nonpathogenic for newborn mice.

• More recently, new enterovirus types have been allocated sequential numbers (68 - 71).

Pathogenesis of enteroviruses infections• Routes of transmission:

– fecal-oral, – via salivary and respiratory droplets, – via conjunctival secretions and skin lesion exudates,

• Portal of entry: nasopharynx and intestinal mucosa.

• Replication: in the nasopharyngeal and intestinal epithelium and regional lymph nodes.

• Spread: by the bloodstream in the reticuloendothelial system and to target organs such as the spinal cord, brain meninges, heart, liver and skin.

• From the CNS the virus travel via neural pathways to skeletal and heart muscles.

Pathogenesis of enterovirus infections

Pathogenesis of enteroviruses infections Cox = Coxsackie virus A or B, Hep A = hepatitis A virus,

Echo = echovirus, Polio = poliovirus

Poliovirus3 serotypes of poliovirus (1, 2, and 3) but not common antigen.

Humans are the only susceptible host

Transmission electron micrograph of poliovirus

Polio endemic countries (where polio is prevalent)

EPIDEMIOLOGY• Source — human with clinical and subclinical

infection.• Transmission: primarily via the fecal-oral, by

ingesting contaminated food or water .

• Incubation period - 3 to 35 days, on average — 7-14 days.

• Polio is most infectious between 7 and 10 days before and after the appearance of symptoms, but transmission is possible as long as the virus remains in the saliva (2 weeks) or feces (3-5 months).

Pathogenesis of polio viruses infection• The alimentary phase: Primary replication in

the oropharyngeal and intestinal mucosa. • The lymphatic phase: Virus spreads to the

tonsils and Peyer's patches. • The viremic phase: The virus is carried by the

bloodstream to various internal organs and regional lymph nodes.

• The neurologic phase: The virus spreads hematogenously to the spinal cord or brain stem or to both. Damage results in paralysis of the muscles innervated by the affected motor nerves. Paralysis is usually irreversible, and residual paralysis remains for life.

Pathogenesis of polio viruses infection

Poliovirus can survive and multiply within the blood and lymphatics for 17 weeks

• Poliomyelitis - infammation of the gray (polios) anterior matter of the spinal cord (myelos)

• Death of motor neurons

• Sensory neurons unaffected

The location of motor neurons in the anterior horn cells of the spinal column

.• Diagram of healthy spinal cord section with nerve cells (motor nerve cells) branching to muscles

• Neuron structure

Degeneration of Nerve cells (Neurons)

during Acute Polio • The nerve cell is invaded

by poliovirus

• One of the nerve cells has been infected by the poliovirus, while its neighbor has not

• Destruction of the infected nerve cells results a lack of nerve supply to the muscles

• If this process occurs on a large enough scale, it can result in loss of muscular function, and can cause weakness or paralysis

Recovery From Acute Paralytic Polio • Immediately following

paralytic polio, surviving motor nerve cells in the brain stem and spinal cord extend new branches to re-connect the nerve cell to the muscle. These are called sprouts.

• The new sprouts are now capable of triggering contraction in the muscles and muscle function can be partially or fully regained

• Thus, many motor nerve cells end up supplying several times the number of muscle fibers they would normally supply.

• Recovery is usually complete in 6 to 8 months.

A single motor neuron that once controlled 200 muscle cells might control 800 to 1000 cells.

Mechanism of Post-Polio Syndrome • These new sprouts are not

indefinitely stable.

• After many years these motor neurons begin to break down, causing new muscle weakness.

Clinical syndromes associated with polio viruses

• Inapparent (subclinical) infection.• Abortive infections (a minor influenza-like illness).

• Aseptic meningitis.• Encephalitis.• Paralytic poliomyelitis:

– spinal poliomyelitis - complete flaccid paralysis of one or more limbs;– bulbar poliomyelitis - paralysis of neck and respiratory muscles;– bulbospinal poliomyelitis.

• Post-polio syndrome: the virus is no longer present. It may occur many years after the infection and involves further loss of function in affected muscles as a result of further neuron loss.

Outcomes of poliovirus infectionOutcome Proportion of cases

Asymptomatic 90–95%

Minor illness 4–8%

Aseptic meningitis 1–2%

Paralytic poliomyelitis 0.1–0.5%

Spinal polio 79% of paralytic cases

Bulbospinal polio 19% of paralytic cases

Bulbar polio 2% of paralytic cases

Symptoms of poliomyelitis• Early symptoms:

– high fever, headache, – stiffness in the back and neck, – asymmetrical weakness of muscles, sensitivity to touch, – difficulty swallowing, – muscle pain, – paresthesia (pins and needles), – constipation, or difficulty urinating.

• Paralysis generally develops 1 to 10 days after early symptoms begin, progresses for 2 to 3 days, and is usually complete by the time the fever breaks.

Egyptian stele from the 18th dynasty showing a victim of polio with a withered leg  

Children with polio sequelae 

Muscle paralysis can sometimes result in skeletal deformities, tightening of the joints and movement disability.

IMMUNITY• Active immunity:• Life-long, type-specific• Mechanism:

– Local: IgA are present in the tonsils and gastrointestinal tract, and are able to block virus replication;

– Humoral: IgG and IgM can prevent the spread of the virus to motor neurons of the CNS.

• Passive immunity: Maternal antibodies cross the placenta, and protect the infant from polio infection during the first 2-3 months of life.

• Jonas Edward Salk (1914-1995) was an American medical researcher and virologist, developed the first inactivated polio vaccine in 1952. 

• Albert Sabin (1906-1993), an American medical researcher, developed an oral attenuated live polio vaccine in 1955 and perfected and tested widely in 1961

Prevention

• Intramuscular Poliovirus Vaccine (IPV)– consists of formalin inactivated virus of all 3 serotypes. – Produces serum antibodies only: does not induce local

immunity and thus will not prevent local infection of the gut. – However, it prevent paralytic poliomyelitis since viraemia is

essential for the pathogenesis of the disease.– No risk of vaccine-related disease.

• Oral Poliovirus Vaccine (OPV)– Consists of live attenuated virus of all 3 serotypes. – Produces local immunity through the induction of an IgA

response as well as systemic immunity.– Lifelong immunity.– Rarely causes paralytic poliomyelitis, around 1 in 2 million

doses.

Serum and secretory antibody response to intramuscular inoculation of IPV and to live attenuated OPV

Inactivated polio vaccine Live polio vaccine

Schedule of immunization

• New guidelines suggest administration of the IPV for the first two doses to eliminate the risk of vaccine-related paralytic polio disease followed by two doses of the OPV on a schedule of – 2 months of age (inactivated), – 4 months (inactivated), – 12-18 months (oral), – 4-6 years (oral). 

• In children with varies immune deficiencies the live vaccine is not used!

Clinical syndromes associated with non-polio Enteroviruses

• Aseptic meningitis, encephalitis and ascending paralysis.

• Common cold syndrome.• Coxsackie A

– Herpangina (sore throat, fever, vesicles).– Hand-foot-and mouth disease.

• Coxsackie B– Myocarditis, pericarditis.– Pleurodynia (Bornholm’s disease or Devil’s Grippe).

• ECHO viruses: exanthematous disease (rash).• Enterovirus 70: acute hemorrhagic conjunctivitis.• Enterovirus 71: severe CNS disease.

Diseases associated with Enteroviruses

Syndrome Polio Cox A Cox B EchoAsymptomatic infection + + + +Paralytic disease + + + +Meningitis-encephalitis + + + +Respiratory infection + + + +Hand-foot-mouth disease +Herpangina +Pleurodynia +Carditis + + +Rash disease + + +Hemorr. conjunctivitis +

Aseptic meningitis and encephalitis

• Both Coxsackie virus A and B can cause aseptic meningitis which is so-called because it is not of bacterial origin.

• Headache, stiff neck, fever, avoidance of light, and eye pain are characteristic. Drowsiness may be prominent, and other symptoms include sore throat, cough, muscle pain, and rash.

• Occasionally, not only the meninges, the covering around the brain and spinal cord is infected, but also brain tissue itself, producing encephalitis.

• The illness resolves after about a week or so, and permanent damage is unusual.

• Enteroviruses can also produce the Guillian-Barré syndrome, which involves weakness and paralysis of the extremities and even the muscles of respiration.

• The disease is most prevalent in the summer and fall.

Hand-foot-and-mouth disease This is an exanthem (a rash) caused by Coxsackie type A16.

Symptoms include fever and blisters on the hands, palate and

feet, which lead to ulcers. Symptoms generally resolve within a

week.

Herpangina Hemorrhagic

conjunctivitis •Coxsackie viruses A can cause a fever with painful ulcers on the palate and tongue leading to problems swallowing and vomiting. Symptoms can persist for several weeks.

EV70 have been linked as the causative agents of acute hemorrhagic conjunctivitis

Pleurodynia, epidemic myalgia, devil's grip and Bornholm disease

- Onset is usually abrupt, with fever, headache, and stabbing pain in muscles of the chest and/or upper abdomen. Pain occurs as acute episodes, lasting 30 minutes or so and is intensified by respiration and movement and may persist for a few weeks.

- The disease is self-limiting, but relapses, with recurrences of fever and other symptoms, are common.

- Occasional complications include pleuritis and orchitis.

Myocarditis and/or pericarditis involves infection of the heart muscle (myocardium) and the covering around the heart (pericardium). The disease usually begins as an upper respiratory tract infection with cough, shortness of breath, and fever. Chest pain, increasing shortness of breath, irregularities of cardiac rhythm, and heart failure sometimes develop.

Neonatal myocarditis is accompanied by involvement of various organs, especially the CNS and liver.

Onset may be abrupt, with lethargy, feeding difficulties, fever and often signs of cardiac or respiratory distress. The infant may die within days or may recover over the next few weeks.

These infections are acquired from infected mothers or during a nursery outbreak.

The echoviruses

• (enteric, cytopathic, human, and orphan viruses) are grouped together because they produce cytopathogenic effects in cell cultures but generally are not pathogenic for mice, and they differ antigenically from the polioviruses.

• They were first named "orphan viruses" because their relationship with disease was obscure.

Maculopapular Rash. Echovirus 19

Echoviruses cause diseases: aseptic meningitis, encephalitis, paralysis, febrile rash,minor respiratory illness, diarrhea.

Laboratory diagnosis of enteroviruses infection• 1. Culture and isolation of viruses from feces, pharyngeal swabs,

saliva, nasal aspirates, skin lesions, conjunctiva, cerebrospinal fluid, spinal cord, brain, heart, and blood.

Choice of Virus Isolation System

Virus Tissue culture Suckling mice

Polioviruses + -

Coxsackieviruses group A - +

Coxsackieviruses group B + +

Echoviruses + -

Identification: NtT, CFT, ELISA, HAIT.

CPE of enteroviruses in monkey kidney cellsNormal cell culture

• Coxsackieviruses are differentiated by their pathogenicity for suckling mice and by classification of their antigenicity.

– Group A viruses produce a diffuse myositis with acute inflammation and necrosis of fibers of voluntary muscles.

– Group B viruses produce focal degeneration in the brain and spinal cord, spastic paralysis, and necrosis of fat pads between the shoulders.

Laboratory diagnosis of enteroviruses infection (2)

• 2. Serology (four-fold or greater rise in neutralizing antibody titer between paired sera or high IgM titers to a single serotype):

– NT;

– CFT;

– IF;

– ELISA;

– HAIT.

Enterovirus 71 infection• Transmission:

– fecal-oral,– by respiratory droplets, – through contact with the fluid in the blisters.

• The virus can be in the faeces for at least 2 months after symptoms develop. A person is infectious for as long as the virus is present in the faeces.

• Incubation period: 3-5 days. • Infection may be asymptomatic, mild or severe neurological

disease. After fever develops it usually takes 1-3 days before meningitis occurs, but paralysis can occur within 10-30 hours after symptoms begin.

• Treatment usually focuses on the person's symptoms. • Prevention: • Good hygiene is the single most effective way of preventing the

spread of the disease. Hands should be washed thoroughly in soap and water for at least 15 seconds and then dried thoroughly.

Entervirus 71Symptoms:• rashes, • diarrhea, • colds, • hand-foot-mouth disease, • myocarditis, • paralytic poliomyelitis-like disease, • bulbar and brain encephalitis, meningitis,• rapidly fatal pulmonary edema and

hemmorrhage.

Hand-foot-mouth disease caused by the

Enterovirus 71 • It is spread from person to person by direct contact with the

nasal discharge, saliva, and fluid from the rash of an infected

person. • There is no specific treatment for the infection. Symptomatic

treatment is given to provide relief from fever, aches and pain.

The throat and tonsils develop small ulcers while the hands, feet, and diaper area are affected by a rash with characteristic vesicles (very small blisters). This is usually a mild illness with the rash healing in 5 to 7 days.

Pathogenesis of severe

EV71

infections

Parechoviruses

• Six types have been identified: – human parechovirus 1 (formerly ECHO-virus 22), – human parechovirus 2 (formerly ECHO-virus 23), – human parechoviruses 3, 4, 5 and 6.

• More than 95% of humans are infected by human parechoviruses early in life, within 2 to 5 years of age.

• Cause:– mild, gastrointestinal or respiratory illness, – myocarditis and encephalitis.

Rhinoviruses

• In this scanning electron micrograph rhinoviruses are seen infecting the surface of a nasal epithelial cell (nasopharynx region).

Rhinovirus is made up of a cubical protein capsid with a genome of one strand of RNA

117 serotypes are responsible for almost 35 % of colds

Rhinoviruses infection

• Transmission:– mainly by respiratory droplets;– hand to hand transmission (via nasal and ocular secretions).

• Incubation period – 1-5 days, sometimes – few hours.• Symptoms:

– Sneezing, nasal clogging, runny nose, sore throat – ~7 days acute illness, 2-3 weeks cough.

• Complications:– Chronic bronchitis.– Sinusitis.– Otitis media.

• Immunity: 1-2 years, type-specific.

• Therapy: interferon.

Laboratory diagnosis

• Virus Isolation from nasopharyngeal secretions in cell culture

–Identification: NT, IF.

• Serology: NT.