Positive Sense RNA Viruses: Picornaviruses, Caliciviruses and Hepevirus

These viruses carry positive-sense, single-stranded RNA genomes that can be translated directly by host ribosomes immediately after entry, without a prior transcription step. All are nonenveloped, which contributes to the environmental stability, detergent resistance, and faecal-oral or waterborne transmission pattern shared by essentially every organism covered here. Rhinovirus is the principal transmission-route exception: it spreads respiratorily rather than enterically despite sharing the same nonenveloped picornavirus structure.

Picornaviruses are a large family of small, nonenveloped, positive-sense RNA viruses, the name reflecting "pico," small, and "RNA." The family encompasses several medically important and clinically distinct genera.

Poliovirus

Genome: positive-sense ssRNA. Envelope: absent.

Transmission is faecal-oral. Initial replication occurs in oropharyngeal and intestinal lymphoid tissue, with a minority of infections progressing to viraemia and subsequent invasion of the anterior horn motor neurons of the spinal cord, and less commonly, brainstem motor nuclei, a tropism directly responsible for the disease's characteristic clinical pattern.

The great majority of infections are asymptomatic or produce only a mild, nonspecific febrile illness. A small minority progress to paralytic poliomyelitis, an acute, asymmetric flaccid paralysis reflecting destruction of anterior horn motor neurons, a pure lower motor neuron pattern with no sensory involvement, since the virus specifically targets motor rather than sensory neurons, a key distinguishing clinical feature.

Global eradication efforts, based on effective vaccination using both inactivated and live-attenuated oral formulations, have brought wild-type poliovirus close to eradication in most of the world, though not yet fully complete.

Diagnosis relies on viral culture or PCR from stool, throat, or cerebrospinal fluid. Distinguishing wild-type from vaccine-derived strains is a specific public health surveillance consideration in the eradication era.

Coxsackievirus and echovirus

Genome: positive-sense ssRNA. Envelope: absent.

Transmission is faecal-oral, and for some syndromes, respiratory droplet spread. Tropism is broad and varies by specific serotype, encompassing cardiac, meningeal, and mucocutaneous tissue among others.

This is a genuinely diverse group of syndromes depending on specific serotype. Hand-foot-and-mouth disease produces vesicular lesions on the hands, feet, and oral mucosa, classically in young children, most commonly caused by coxsackievirus A16 and enterovirus 71.

Herpangina produces painful vesicular and ulcerative posterior oropharyngeal lesions, distinct in distribution from herpes gingivostomatitis, an occasionally confused differential given the similar name and appearance. Enteroviruses, including coxsackievirus and echovirus, are collectively the most common identifiable cause of viral, aseptic, meningitis.

Coxsackievirus B is the classic cause of viral myopericarditis, myocarditis and pericarditis, and of pleurodynia, severe pleuritic chest and abdominal wall pain sometimes called devil's grip.

Diagnosis is by PCR from the relevant specimen: cerebrospinal fluid for meningitis, stool or throat for general surveillance, vesicle fluid for hand-foot-and-mouth disease.

Rhinovirus

Genome: positive-sense ssRNA. Envelope: absent.

Transmission is via respiratory droplets and, given the organism's nonenveloped structural stability, fomites, contributing to its efficient spread. Tropism is specific to upper respiratory tract epithelium, with a preference for the cooler temperature of the nasal mucosa over the warmer lower respiratory tract, directly explaining its predominantly upper-tract disease pattern.

Rhinovirus is the single most frequent cause of the common cold across all age groups, generally mild and self-limited in immunocompetent hosts, though capable of triggering exacerbations of underlying asthma or chronic obstructive pulmonary disease.

Diagnosis is clinical in routine practice, with PCR-based respiratory panels increasingly identifying rhinovirus specifically when broader respiratory viral testing is performed for other clinical reasons.

Hepatitis A virus

Genome: positive-sense ssRNA. Envelope: absent.

Transmission is faecal-oral, via contaminated food or water, or direct person-to-person contact in settings of poor sanitation. Shellfish harvested from contaminated water is a classically associated food vehicle. Tropism is hepatocytes, reached via the bloodstream following initial intestinal replication.

Hepatitis A causes acute hepatitis, typically self-limited, with no chronic carrier state, a defining and frequently tested distinguishing feature from hepatitis B and hepatitis C. Severity generally increases with age at infection, often subclinical or mild in young children, and more likely to produce symptomatic, occasionally severe, though still self-limited, disease in adults.

Diagnosis relies on IgM anti-HAV, indicating acute or recent infection, and IgG anti-HAV, indicating past infection or vaccination-induced immunity.

Norovirus

Genome: positive-sense ssRNA. Envelope: absent.

Transmission is faecal-oral, with an extremely low infectious dose and marked environmental and detergent resistance, making norovirus exceptionally efficiently transmitted via contaminated food, water, fomites, and direct person-to-person contact, and the classic cause of explosive outbreaks in closed settings such as cruise ships, hospitals, care homes, and schools. Tropism is small intestinal epithelium.

Norovirus is the leading cause of acute gastroenteritis outbreaks across all age groups in many high-income settings, producing acute-onset vomiting and watery diarrhoea, generally self-limited over one to three days, though dehydration risk is significant, particularly at the extremes of age.

Diagnosis is by PCR from stool, increasingly the standard approach given superior sensitivity to older antigen-based tests. A specific infection control point bears emphasis: alcohol-based hand sanitiser is substantially less effective against norovirus than against enveloped viruses, making soap-and-water hand hygiene and environmental chlorine-based disinfection central to outbreak control.

Hepatitis E virus

Genome: positive-sense ssRNA. Envelope: absent.

Transmission is faecal-oral, predominantly via contaminated water in many endemic regions, a transmission pattern paralleling hepatitis A, with an additional recognised zoonotic transmission route in some settings via undercooked pork or other animal meat, a distinguishing feature from hepatitis A, which lacks a significant animal reservoir. Tropism is hepatocytes.

Hepatitis E causes acute hepatitis, generally self-limited and clinically similar to hepatitis A in most patients, but with a specific and critical exception.

Hepatitis E carries a markedly elevated risk of fulminant hepatic failure and mortality in pregnant women, particularly in the third trimester. This is a distinctive and frequently tested organism-specific obstetric risk not shared by hepatitis A.

Chronic infection can occur, unusually for this virus family, specifically in immunocompromised hosts, notably solid organ transplant recipients. This is a further point of distinction from the uniformly self-limited pattern of hepatitis A.

Diagnosis relies on IgM and IgG anti-HEV serology and PCR.