Pathogenic RNA Viruses: Naked, Positive ssRNA Viruses (Picornaviridae, Caliciviridae, Astroviridae, Hepeviridae)

Pathogenic RNA Viruses

Overview of RNA Viruses

RNA viruses are unique infectious agents that store their genetic information in RNA molecules. They are classified based on their genomic structure, the presence or absence of an envelope, and the size and shape of their capsid. There are four main types of RNA viruses:

• Positive single-stranded RNA (+ssRNA)

• Retroviruses (a subset of +ssRNA viruses that convert their genome to DNA)

• Negative single-stranded RNA (−ssRNA)

• Double-stranded RNA (dsRNA)

Positive-sense RNA can be directly used by ribosomes to synthesize proteins, while negative-sense RNA must first be transcribed into mRNA before translation can occur.

Naked, Positive ssRNA Viruses: Picornaviridae, Caliciviridae, Astroviridae, and Hepeviridae

General Characteristics

These families of viruses are non-enveloped and possess a positive-sense single-stranded RNA genome. They are responsible for a variety of human diseases, including respiratory, gastrointestinal, and hepatic illnesses.

• Picornaviridae: The smallest animal viruses, containing many human pathogens.

• Caliciviridae, Astroviridae, Hepeviridae: Larger than picornaviruses and primarily cause gastrointestinal diseases.

Common Colds Caused by Rhinoviruses (Picornaviridae)

Rhinoviruses are the most common cause of the common cold, with infections typically limited to the upper respiratory tract. Transmission occurs via aerosols, fomites, or direct contact, with person-to-person contact being the most common route. Immunity develops against specific serotypes, leading to decreased infections with age.

• Diagnosis: Based on clinical manifestations.

• Treatment: Pleconaril may reduce severity and duration if taken early; symptomatic relief is common.

• Prevention: Handwashing is the most effective preventive measure.

Diseases of Enteroviruses (Picornaviridae)

Enteroviruses are transmitted via the fecal-oral route, infecting the pharynx and intestine before spreading through the bloodstream. They are cytolytic, killing host cells. The main enteroviruses include polioviruses, coxsackieviruses, and echoviruses.

• Poliomyelitis: Caused by one of three serotypes of poliovirus. Four clinical forms exist: asymptomatic, minor polio, nonparalytic polio, and paralytic polio (which can result in bulbar poliomyelitis).

• Postpolio syndrome: Progressive muscle deterioration in previously affected muscles.

• Vaccines: Inactivated polio vaccine (IPV, Salk) and oral polio vaccine (OPV, Sabin) have nearly eradicated polio.

Comparison of Polio Vaccines

Vaccine	Advantages	Disadvantages
Salk Vaccine (IPV)	Effective, inexpensive, stable, no risk of vaccine-related disease	Requires boosters, injected (painful), higher community immunization rate needed
Sabin Vaccine (OPV)	Induces secretory antibody response, easy to administer, herd immunity	Requires boosters, more expensive, less stable, risk of reversion to virulence, not for immunocompromised

Other Diseases of Enteroviruses

• Coxsackie A viruses: Cause herpangina, hand-foot-and-mouth disease, acute hemorrhagic conjunctivitis, and some colds.

• Coxsackie B viruses: Associated with myocarditis, pericarditis, pleurodynia, and can cross the placenta.

• Echoviruses: Cause meningitis and colds, acquired via the intestine.

Epidemiology and Prevention of Enteroviruses

Enteroviruses are globally distributed, with higher incidence in areas lacking adequate sewage treatment. Transmission occurs via ingestion of contaminated food or water, or contact with infected hands or fomites. The greatest risk is to fetuses and newborns. Prevention relies on good hygiene and sanitation; polio can be prevented by vaccination.

Hepatitis A (Picornaviridae)

Hepatitis A virus is transmitted via the fecal-oral route and causes symptoms due to the immune response, including fever, fatigue, nausea, anorexia, and jaundice. The infection is usually self-limiting and does not cause chronic liver disease. The virus is resistant to common disinfectants, and vaccines are available.

Acute Gastroenteritis (Caliciviridae and Astroviridae)

Caliciviruses and astroviruses are major causes of acute gastroenteritis, especially in group settings such as schools and cruise ships. Caliciviruses (notably noroviruses) cause diarrhea, nausea, and vomiting, while astroviruses cause diarrhea without vomiting. Treatment is supportive, focusing on fluid and electrolyte replacement. Prevention includes proper sanitation and hand hygiene.

Hepatitis E (Hepeviridae)

Hepatitis E, also known as enteric hepatitis, is transmitted via contaminated food, drink, or blood transfusion. The disease is self-limiting but can be fatal in pregnant women. Prevention focuses on interrupting the fecal-oral route.

Comparison of Hepatitis Viruses

Feature	Hepatitis A	Hepatitis B	Hepatitis C	Hepatitis D	Hepatitis E
Common Names	Infectious hepatitis	Serum hepatitis	Non-A, non-B hepatitis; chronic hepatitis	Delta agent hepatitis	Hepatitis E, enteric hepatitis
Virus Family (genus)	Picornaviridae (Hepatovirus)	Hepadnaviridae (Orthohepadnavirus)	Flaviviridae (Hepacivirus)	Arenaviridae (Deltavirus)	Hepeviridae (Orthohepevirus)
Genome	+ssRNA	dsDNA	+ssRNA	−ssRNA	+ssRNA
Envelope?	Naked	Enveloped	Enveloped	Enveloped	Naked
Transmission	Fecal-oral	Needles; sex	Needles; sex	Needles; sex	Fecal-oral
Severity (mortality rate)	Mild (<0.5%)	Occasionally severe (1–2%)	Usually subclinical (0.5–4%)	May be severe (high, with HBV coinfection)	Mild (1–2%), except in pregnant women (20%)
Chronic Carrier State?	No	Yes	Yes	No	No
Other Disease Associations	—	Hepatic cancer	Hepatic cancer	Liver damage; hepatic cancer (with HBV)	—

Summary Table: Manifestations of Respiratory Infections

Ailment	Manifestations
Common Cold (viral)	Sneezing, rhinorrhea, congestion, sore throat, headache, malaise, cough
Influenza (viral)	Fever, rhinorrhea, headache, body aches, fatigue, dry cough, pharyngitis, congestion
“Strep” Throat (bacterial)	Fever, red and sore throat, swollen lymph nodes in neck
Viral Pneumonia	Fever, chills, mucus-producing cough, headache, body aches, fatigue
Bacterial Pneumonia	Fever, chills, congestion, cough, chest pain, rapid breathing, possible nausea and vomiting
Bronchitis (viral or bacterial)	Mucus-producing cough, wheezing
Inhalation Anthrax (bacterial)	Fever, malaise, cough, chest discomfort, vomiting
Severe Acute Respiratory Syndrome (SARS)	High fever (>38°C), chills, shaking, headache, malaise, myalgia
Middle East Respiratory Syndrome (MERS)	Fever, cough, shortness of breath

Key Concepts and Additional Information

• Why are there no vaccines for rhinoviruses? There are too many serotypes, making vaccine development impractical.

• Best prevention for Hepatitis A: Immunization and interruption of the fecal-oral route.