Picornavirus

This family includes two groups - Enteroviruses and Rhinoviruses. Enteroviruses are located in the gastrointestinal tract and include Poliovirus, Coxsackievirus, Echovirus and Hepatitis A virus. The important members of this group are Poliovirus, ECHO Virus, Coxsackie Virus and Enteroviruses. Rhinoviruses are located in the nose.

Poliovirus

Three antigenic types 1,2 and 3 are determined on the basis of outer capsid proteins. C antigen is associated with empty virion. D antigen is associated with the whole virion and its antibody is protective. PVR also called human Poliovirus receptor is a cell surface protein which plays an important role in cell mediated immune response. The virus progeny are released after lysis of the host cell.

Pathogenesis: Once it enters the human body, it multiplies in the epithelial and lymphoid tissues of the gastrointestinal tract. It causes a viraemia during which it multiplies in cells of the reticuloendothelial system and reach the brain and spinal cord where they multiply in the neurons. It can also spread in a retrograde fashion to the CNS along nerve axons. It preferentially multiplies in the anterior horn cell motor neurons of the spinal cord. Lysis of cells during release of progeny virions causes loss of motor neurons and consequent flaccid paralysis. [Quick question - What kind of paralysis is seen in UMN lesions? ]

Clinical features: Poliovirus can cause four types of infections as follows.

1. Inapparent infection: Asymptomatic but virus present in stools.
2. Minor Illness: Fever, sore throat, viremia, maybe paralysis.
3. Major Illness: More severe with onset of headache, neck stiffness, aseptic meningitis without paralysis.
4. Paralytic Poliomyelitis: It presents as flaccid paralysis with lower motor neuron signs, lost reflexes. There can be a post polio syndrome occurring years later with deteriorating muscle strength and atrophy. In the Bulbar variant, death can occur from diaphragmatic palsy, respiratory failure, dysphagia and vasomotor imbalances.

Laboratory diagnosis of Polio: Virus can be isolated from feces and throat early in the infection. Electron microscopy can demonstrate virus in samples. Cell culture shows rounding up of cells and eosinophilic, intranuclear inclusions. Rise in antibody titres shown by neutralization test and CFT can be used.

Coxsackie Viruses

It has two types Coxsackie A and B. Both groups cause different diseases. A group has a predilection for skin and mucosa while B group mainly involves internal organs like the heart, pleura, pancreas and liver. Insulin dependent diabetes mellitus has been correlated with Coxsackie B infections.

Clinical features: Several clinical syndromes are caused by the Coxsackie viruses. Aseptic meningitis with fever, chills, headache, irritability etc is seen with both types, more with B. Encephalitis, flaccid polio like paralysis, Guillain Barre Syndrome (type A) can occur. Myopericarditis with cardiomegaly, friction rub, cardiac failure, dyspnea can be seen.

Hand foot mouth disease (HFMD) : Seen in children, starting with fever, sore throat, blisters later becoming painful ulcers on the hands, feet, legs and buttocks. The rash involves the palms and feet. Seen in Coxsackie A.

Herpangina: Caused by Coxsackie A. Sore ulcers and blisters are seen on the posterior oropharynx with fever, sore throat, odynophagia and dysphagia. Differentiate from HFMD by location of blisters.

Epidemic pleurodynia with chest muscle pain and spasms (type B) and epidemic hemorrhagic conjunctivitis are other diseases caused. [Do you remember the other viral cause of hemorrhagic conjunctivitis?].

Laboratory diagnosis of Coxsackie virus infections: Diagnosis is made by isolation of virus in cell culture or suckling mice, PCR for viral RNA in CSF and rise in antibody titre by neutralization tests.

Echoviruses

ECHO stands for enteric cytopathic human orphan virus. They are transmitted by the feco-oral route. There are 34 serotypes. They primarily infect the gastrointestinal tract. They cause common cold, aseptic meningitis, encephalitis, gastroenteritis, hemorrhagic conjunctivitis, fever and rash. Laboratory diagnosis is made by culture and antibody detection by neutralization tests.

Other Enteroviruses

All Enteroviruses discovered after 1969 are named numerically e.g. Enterovirus 70 and so on. The specific diseases caused by them are as follows.

1. Enterovirus 68 - pneumonia
2. Enterovirus 70 - acute hemorrhagic conjunctivitis
3. Enterovirus 71 - meningitis, encephalitis, flaccid paralysis, diarrhea, HFMD, herpangina and pulmonary hemorrhages
4. Enterovirus 72 - hepatitis A

Rhinoviruses

It is the most common cause of the common cold. It has more than 100 serotypes. They replicate preferentially at lower temperatures of 33 degrees C hence relation with common cold (nose is cooler than the rest of the body). They are readily killed by gastric acid. ICAM 1 is the cell surface receptor for Rhinovirus. Common cold presents as rhinorrhea, headache, sore throat. Fever is typically absent unless there is secondary bacterial infection like sinusitis, otitis media. Laboratory diagnosis is made by isolation in tissue culture. Serological tests are not useful due to the occurrence of multiple serotypes.