RNA Viruses: Structure, Classification, and Human Diseases

RNA Viruses

Overview of RNA Viruses

RNA viruses are unique infective 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 (+ssRNA viruses that convert their genome to DNA)

• Negative single-stranded RNA (-ssRNA)

• Double-stranded RNA (dsRNA)

Positive-sense RNA acts directly as mRNA and can be translated by host ribosomes, while negative-sense RNA must first be transcribed into mRNA before translation can occur.

Naked, Positive ssRNA Viruses

Picornaviridae and Related Families

Picornaviridae are the smallest animal viruses and include several genera that cause human disease, such as Rhinovirus, Enterovirus, and Hepatovirus. Other families like Caliciviridae, Astroviridae, and Hepeviridae are larger and primarily cause gastrointestinal diseases.

Common Colds Caused by Rhinoviruses

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 can develop against specific serotypes, and the frequency of infection decreases with age. Handwashing is the most effective preventive measure.

Diseases of the Enteroviruses

Enteroviruses are transmitted via the fecal-oral route, often through contaminated food, water, or fomites. They initially infect the pharynx and intestine, then spread via the bloodstream to various organs. The main enteroviruses are polioviruses, coxsackieviruses, and echoviruses.

Poliomyelitis

• Caused by one of three serotypes of poliovirus

• Four clinical forms: asymptomatic, minor polio, nonparalytic polio, and paralytic polio (which can include bulbar poliomyelitis)

• Postpolio syndrome involves long-term muscle deterioration

• Polio has been nearly eliminated due to vaccines: Inactivated Polio Vaccine (IPV) and Oral Polio Vaccine (OPV)

Other Enterovirus Diseases

• Coxsackie A viruses: Cause lesions, fever, herpangina, and hand-foot-and-mouth disease

• Coxsackie B viruses: Associated with myocarditis, pericardial infections, and can cross the placenta

• Echoviruses: Cause meningitis and colds, acquired intestinally

Diagnosis, Treatment, and Prevention

• Most enterovirus infections are mild and not diagnosed unless severe

• No effective antiviral therapy

• Prevention relies on good hygiene and sanitation

• Vaccines are available for hepatitis A and polio

Hepatitis A

Hepatitis A virus is resistant to common disinfectants and is transmitted via the fecal-oral route. Symptoms are due to the immune response, and chronic liver disease does not occur. Most patients recover completely.

Comparison of Hepatitis Viruses

The following table compares the main features of hepatitis viruses A-E:

Feature	Hepatitis A	Hepatitis B	Hepatitis C	Hepatitis D	Hepatitis E
Common names of disease	Infectious hepatitis	Serum hepatitis	Non-A, non-B hepatitis	Delta agent hepatitis	Hepatitis E, enteric hepatitis
Virus family	Picornaviridae	Hepadnaviridae	Flaviviridae	Arenaviridae	Hepeviridae
Genome	+ssRNA	dsDNA	+ssRNA	-ssRNA	+ssRNA
Envelope	Naked	Enveloped	Enveloped	Naked	Naked
Transmission	Fecal-oral	Needles, sex	Needles, sex	Needles, sex	Fecal-oral
Severity/risk	Mild (<0.5% fatal)	Occasionally severe (1–2% fatal)	Usually subclinical (<1% fatal)	Requires coinfection with hepatitis B; may be severe (high)	20% fatal in pregnant women
Chronic carrier state?	No	Yes	Yes	—	No
Other diseases associated	—	Hepatic cancer	Cirrhosis	—	—

Acute Gastroenteritis

Caliciviruses and astroviruses cause acute gastroenteritis, often resulting in outbreaks in group settings. Caliciviruses cause diarrhea, nausea, and vomiting, while astroviruses cause diarrhea without vomiting. Treatment is supportive, focusing on fluid and electrolyte replacement.

Hepatitis E

Hepatitis E virus, formerly classified as a calicivirus, causes enteric hepatitis. It is particularly fatal in pregnant women (up to 20%). There is no specific treatment, and prevention relies on interrupting fecal-oral transmission.