15.1 Respiratory, Vesicle, Meningitis, and GI Viruses

15.1 Respiratory, Vesicle, Meningitis, and GI Viruses

Quick Answer: Match the syndrome to the specimen first, then to the short list of M-tested viruses. Nasopharyngeal swab in universal/viral transport medium for influenza, RSV, parainfluenza, and SARS-CoV-2; vesicle-base swab for HSV and VZV; CSF for HSV and enterovirus; stool for norovirus, rotavirus, and enteric adenovirus. Transmission is droplet or contact for respiratory and vesicular disease and fecal-oral for enterovirus and viral gastroenteritis. Temperate-zone seasonality still matters: influenza and RSV peak in winter, enterovirus meningitis in summer–fall, and norovirus in winter outbreaks.

Clinical virology on the M(ASCP) exam is syndrome-driven. The 2025-09-25 content guideline tests whether you can connect a disease state to the correct source, the major pathogen, and the usual route of spread. Do not start with an exotic virus name. Start with the anatomic site and the specimen the laboratory actually receives.

Syndrome, source, and major pathogens

A mismatched source fails the test even when the virus name is correct. Influenza PCR on stool, norovirus testing on CSF, or HSV PCR on a dry lesion swab that sat without medium does not represent the infection the outline is asking about.

Respiratory viruses

Influenza is an enveloped, segmented, negative-sense RNA orthomyxovirus. Types A and B cause seasonal human disease; type A is subtyped by hemagglutinin and neuraminidase (H1N1, H3N2). Antigenic drift produces yearly epidemic change in A and B. Antigenic shift is possible only in influenza A, because segmented genomes can reassort in animal reservoirs and yield a pandemic strain. In temperate climates influenza concentrates in winter. Transmission is large-droplet and contact; collect a nasopharyngeal swab and place it immediately into UTM. Disease ranges from self-limited febrile myalgia to viral pneumonia and bacterial superinfection.

Respiratory syncytial virus (RSV) is an enveloped RNA pneumovirus and the leading viral cause of bronchiolitis and pneumonia in infants. Older adults and immunocompromised patients also develop severe lower-airway disease. RSV is a winter pathogen in temperate regions and spreads by droplet plus contaminated hands or fomites; it persists on surfaces, which is why contact precautions matter on pediatric units. Cell-to-cell fusion produces the syncytia that give the virus its name.

Parainfluenza viruses (HPIV-1 through HPIV-4) are paramyxoviruses. HPIV-1 and HPIV-2 classically cause laryngotracheobronchitis (croup) in toddlers, often in fall. HPIV-3 is more associated with bronchiolitis and pneumonia and may circulate in spring. Transmission is droplet and contact. The exam expects you to link croup to parainfluenza, infant bronchiolitis to RSV, and winter influenza-like illness in adults to influenza or SARS-CoV-2—not to treat every “respiratory virus” as interchangeable.

SARS-CoV-2 is an enveloped positive-sense RNA coronavirus. It causes COVID-19, from asymptomatic infection through pneumonia and ARDS. Transmission is droplet and aerosol. Unlike classical influenza, it has not settled into a single winter peak; waves can occur year-round. NAAT on a nasopharyngeal or other validated upper-airway specimen is the diagnostic standard. Antigen tests exist but are less sensitive than NAAT, a point expanded in section 15.3.

Human metapneumovirus and rhinovirus appear on multiplex panels and cause real disease, but the named respiratory list for M is influenza, RSV, parainfluenza, and SARS-CoV-2. Know those four cold.

Vesicles and cutaneous lesions

Herpes simplex virus types 1 and 2 are enveloped DNA herpesviruses that establish latency in sensory ganglia. HSV-1 more often causes orolabial vesicles; HSV-2 more often causes genital vesicles; either type can infect either site. Primary infection may be systemic; recurrences are usually localized clusters of painful vesicles on an erythematous base. Transmission is direct contact with lesions or infected secretions, including asymptomatic shedding. Unroof the vesicle, swab the base firmly, and place the swab in VTM. PCR has replaced Tzanck smears and most viral culture for lesion testing.

Varicella-zoster virus (VZV) is the herpesvirus of varicella (chickenpox) and herpes zoster (shingles). Primary varicella is a generalized pruritic vesicular rash at different stages of evolution and spreads by airborne droplets plus contact with vesicle fluid. After latency in dorsal-root ganglia, reactivation produces a dermatomal vesicular eruption. Zoster lesions remain infectious by contact until crusted. Laboratory detection uses the same vesicle-base swab strategy as HSV; VZV PCR distinguishes zosteriform HSV from true zoster.

Viral meningitis

Enteroviruses (Coxsackie A/B, echovirus, and numbered enteroviruses) are naked positive-sense RNA picornaviruses and the most common cause of community viral (aseptic) meningitis in children and young adults. Cases peak in summer and early fall. Spread is fecal-oral, with a respiratory contribution. CSF is the specimen; stool or a throat swab can support an enterovirus diagnosis when CSF volume is limited, but the exam association is CSF PCR.

HSV, particularly HSV-2, causes recurrent lymphocytic meningitis, including Mollaret meningitis. HSV-1 is the classic cause of focal necrotizing encephalitis (temporal lobe), which is not the same syndrome as meningitis—do not collapse the two. Neonatal HSV can present as skin-eye-mouth disease, encephalitis, or disseminated infection; surface swabs plus blood and CSF PCR are used clinically. For M, remember: vesicular lesions → HSV/VZV; meningitis CSF → enterovirus first, HSV next.

Viral gastroenteritis

Norovirus (Caliciviridae, naked positive-sense RNA) is the leading cause of outbreak gastroenteritis in healthcare facilities, restaurants, and cruise ships. The infectious dose is extremely low. Fecal-oral spread, vomitus aerosols, and environmental persistence drive explosive attack rates. Seasonality is winter-predominant. The virus is not cultivated in routine laboratories; diagnosis is stool NAAT or antigen immunoassay.

Rotavirus (Reoviridae, segmented double-stranded RNA) historically caused severe dehydrating diarrhea in infants each winter. Live oral vaccines have reduced incidence, but pediatric cases still occur, especially in unvaccinated children. Transmission is fecal-oral. Stool antigen EIA or NAAT is used.

Adenovirus, a naked DNA virus, includes enteric types 40 and 41 that cause pediatric gastroenteritis. Other adenovirus types cause pharyngoconjunctival fever and pneumonia; those are respiratory, not stool, problems. Enteric adenovirus is detected in stool by antigen or NAAT and circulates more year-round than classical rotavirus.

Seasonality and what is not M-tested

Winter clustering of influenza, RSV, norovirus, and classical rotavirus is still a high-yield exam pattern. Enterovirus meningitis clusters in summer–fall. HSV and VZV are year-round because they reactivate from latency. SARS-CoV-2 and adenovirus are not reliable winter-only markers.

The M outline does not test HTLV, West Nile virus, Zika, MERS, dengue, or Ebola as “other viruses.” Those agents are SM-only. If a stem describes winter bronchiolitis in a 3-month-old, the answer is RSV, not an arbovirus.