42. Picornaviridae. Genus Enterovirus – the causative agent of polyomyelitis, coxackie infections and ECHO infections. Genus Rhinovirus – the causative agents of infectious rhinitis. Genus Aphtovirus – the causative agent of foot-and-mouth disease.

I. General Characteristics of Picornaviridae

• Family Picornaviridae = pico (small) + RNA.
• Small, non-enveloped, icosahedral viruses.
• Positive-sense single-stranded RNA genome.
• Replicate in the cytoplasm of host cells.
• Stable in acidic environments → resistant to gastric acid → fecal–oral transmission.
• Divided into several genera; clinically important ones include:
• Enterovirus (Poliovirus, Coxsackievirus, Echovirus, Enterovirus D68)
• Rhinovirus (common cold — NOT acid-stable)
• Hepatovirus (Hepatitis A virus)

III. Genus Enterovirus

• Transmission: Fecal–oral (except Enterovirus D68 — respiratory).
• Tropism: Initially infects gut-associated lymphoid tissue, then spreads hematogenously.
• Asymptomatic infection is common, but viruses can target CNS, heart, skin, conjunctiva, etc.

IV. Poliovirus (Virus poliomyelitidis)

A. Overview

• 3 serotypes: PV1, PV2, PV3 (PV1 most common in outbreaks).
• Causes poliomyelitis — highly infectious neurotropic virus.
• Human is the only reservoir.

B. Pathogenesis

• Enters via oropharynx or intestine → replicates in Peyer’s patches.
• Spreads via blood (viremia) → CNS.
• Targets anterior horn cells of spinal cord → motor neuron death → flaccid paralysis.

C. Clinical Manifestations

• Asymptomatic (90%).
• Abortive poliomyelitis (minor illness): sore throat, fever, headache.
• Non-paralytic poliomyelitis: aseptic meningitis.
• Paralytic poliomyelitis:
• Flaccid asymmetric paralysis.
• No sensory loss.
• Bulbar poliomyelitis → respiratory failure.