16.1 Respiratory, Hepatitis & Gastrointestinal Viruses + Viral Role in Disease

Respiratory, Hepatitis & Gastrointestinal Viruses

Quick Answer: The PA-CAT Microbiology content (Bulletin of Information, rev. 20240815, Table 7) groups remaining viruses into respiratory, hepatitis, and gastrointestinal categories plus a "viral role in disease" theme. Expect questions that pair a virus's structural category (enveloped vs. non-enveloped, RNA vs. DNA, segmented vs. monopartite) with its clinical syndrome and transmission route.

Respiratory Viruses

Influenza is an enveloped, negative-sense, single-stranded, segmented RNA virus of the Orthomyxoviridae family. Its genome has eight segments. The two surface glycoproteins — hemagglutinin (HA), which mediates host cell entry, and neuraminidase (NA), which cleaves sialic acid to release progeny virions — define subtypes (e.g., H1N1, H3N2). Antigenic drift is the accumulation of point mutations in HA/NA causing seasonal epidemics; antigenic shift is reassortment of segments between human, avian, and swine strains in a co-infected host, producing pandemic strains. Inactivated quadrivalent and high-dose vaccines are reformulated annually to track drift. Oseltamivir inhibits neuraminidase and shortens illness if started within 48 hours of symptom onset.

Respiratory syncytial virus (RSV) is an enveloped, negative-sense ssRNA paramyxovirus. Its F (fusion) protein forms the characteristic syncytia seen on histology. RSV is the most common cause of bronchiolitis and pneumonia in infants under 1 year. Palivizumab, a monoclonal antibody against the F protein, is passive prophylaxis for high-risk premature infants and children with hemodynamically significant congenital heart disease; it is given monthly during RSV season, not as treatment. A newer long-acting monoclonal, nirsevimab, and maternal vaccines have expanded prevention options.

Rhinoviruses are non-enveloped, positive-sense ssRNA picornaviruses and the dominant cause of the common cold. They replicate best at 33 °C (the nasal mucosa temperature), which explains their upper-respiratory tropism. Coronaviruses are enveloped, positive-sense ssRNA viruses with the largest RNA genomes; seasonal coronaviruses cause colds, while SARS-CoV-2 causes COVID-19, transmitted mainly via respiratory droplets and aerosols with an incubation period around 2–14 days.

Hepatitis Viruses

Hepatitis A and E are enteric, non-enveloped RNA viruses transmitted fecal-oral; both cause acute, self-limited hepatitis without chronic carriage. HAV is prevented by an effective inactivated vaccine. HEV is notable for high mortality in pregnant women (especially genotype 1).

Hepatitis B is a DNA virus transmitted via blood, sexual contact, and vertically at birth; chronic infection is common in infants but rare in adults. Serology matters: HBsAg indicates current infection; anti-HBs indicates immunity after vaccination or recovery. Hepatitis D (delta) is a defective RNA virus that requires HBsAg to assemble infectious particles, so it only co-infects or superinfects with HBV. Hepatitis C is a flavivirus-like RNA virus; most infections become chronic, but direct-acting antivirals (DAAs) now cure >95% of cases.

Gastrointestinal Viruses

Rotavirus is a non-enveloped, double-shelled capsid, double-stranded RNA virus of the Reoviridae family with 11 genome segments. It is the classic cause of severe watery, non-bloody diarrhea with vomiting and dehydration in unvaccinated infants — exactly the PA-CAT Bulletin sample item (8-month-old, unvaccinated, dehydrated). The live, attenuated oral rotavirus vaccines (Rotateq, Rotarix) are given beginning at 6–8 weeks of age and have markedly reduced severe disease.

Norovirus is a non-enveloped, positive-sense ssRNA calicivirus and the leading cause of non-bacterial gastroenteritis outbreaks in the U.S., spreading explosively in nursing homes, cruise ships, and schools via contaminated food, water, and surfaces. Adenoviruses are non-enveloped, dsDNA viruses; certain enteric serotypes (40/41) cause infantile diarrhea, while other serotypes cause pharyngitis, conjunctivitis, and pneumonia.

Viral Role in Disease

Viruses cause disease through cytopathic effects (direct cell killing or dysfunction), immune-mediated injury (e.g., hepatitis B liver damage largely from CD8+ T cell responses), and latency or persistence (herpesviruses, HIV). Pathogenesis depends on the virus's tropism (receptor expression on target tissue), incubation period, and shedding route. PA-CAT items often ask you to link a structural clue — segmented dsRNA, non-enveloped capsid, fecal-oral route — to a specific clinical picture; the rotavirus sample item is the template.

Differentiating Acute Viral Gastroenteritis

PA-CAT items commonly present a diarrhea outbreak or single case and ask you to identify the virus from clinical and epidemiologic clues. The table below separates the three major non-bacterial causes of acute viral gastroenteritis (AGE) by age, setting, season, and stool pattern — distinctions the Bulletin's rotavirus sample item illustrates.

Rotavirus peaks in cooler months and causes severe dehydration requiring IV fluids in infants; vaccination has cut U.S. cases dramatically but sporadic disease persists. Norovirus is the dominant U.S. AGE outbreak pathogen, with a short 12–48 hour incubation, explosive spread via food, water, and fomites, and illness lasting 1–3 days. Enteric adenovirus runs a longer 5–14 day course and should be suspected when daycare-age diarrhea persists beyond the typical viral window. When a PA-CAT vignette pairs an 8-month-old, winter, watery stool, and dehydration, rotavirus is the prototype; an adult cruise outbreak with projectile vomiting points to norovirus.