Picornavirus

This family includes several genera, including Enterovirus, Rhinovirus, and Hepatovirus. The clinically important genera discussed here are Enterovirus and Rhinovirus.

• Members of the Enterovirus genus are found in the gastrointestinal tract and include Poliovirus, Coxsackievirus, Echovirus, and numbered Enteroviruses.
• Rhinoviruses are found in the nose.

Poliovirus

Three antigenic types (1, 2, and 3) are determined based on outer capsid proteins.

• C antigen is associated with the empty virion.
• D antigen is associated with the whole virion, and antibody against it is protective.

PVR (human poliovirus receptor) is a cell surface protein that plays an important role in cell-mediated immune response. 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 reaches the brain and spinal cord, where it multiplies in neurons. It can also spread retrogradely 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 is present in stools.
2. Minor illness: Fever, sore throat, viremia, maybe paralysis.
3. Major illness: More severe, with onset of headache, neck stiffness, and aseptic meningitis without paralysis.
4. Paralytic poliomyelitis: Presents as flaccid paralysis with lower motor neuron signs and lost reflexes. A post-polio syndrome can occur 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. A rise in antibody titres shown by neutralization test and CFT can be used.

Coxsackie viruses

There are two types: Coxsackie A and Coxsackie B. Both groups cause different diseases.

• Group A has a predilection for skin and mucosa.
• Group B 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 Coxsackie viruses. Aseptic meningitis with fever, chills, headache, irritability, etc. is seen with both types (more with B). Encephalitis, flaccid polio-like paralysis, and Guillain Barre Syndrome (type A) can occur. Myopericarditis with cardiomegaly, friction rub, cardiac failure, and dyspnea can be seen.

Hand foot mouth disease (HFMD) : Seen in children. It starts with fever and sore throat, followed by blisters that later become 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 the 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 a 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

Many Enteroviruses are named numerically (e.g., Enterovirus 68, Enterovirus 70, and Enterovirus 71). 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

Note: Hepatitis A virus was historically designated Enterovirus 72, but under the current classification it belongs to the genus Hepatovirus within the family Picornaviridae and is no longer classified as an Enterovirus.

Rhinoviruses

Rhinovirus is the most common cause of the common cold. It has more than 100 serotypes. It replicates preferentially at lower temperatures of 33 degrees C, which relates to the common cold (the nose is cooler than the rest of the body). It is readily killed by gastric acid.

ICAM 1 is the cell surface receptor for Rhinovirus.

Common cold presents as rhinorrhea, headache, and sore throat. Fever is typically absent unless there is a secondary bacterial infection like sinusitis or otitis media.

Laboratory diagnosis is made by isolation in tissue culture. Serological tests are not useful due to the occurrence of multiple serotypes.