Rhinoviruses

Robert L. Atmar

Rhinoviruses (RVs) cause approximately 30% to 50% of all acute respiratory illness. RVs are associated primarily with the common cold, but they also may be involved in bronchitis, sinusitis, pneumonia, and acute exacerbations of asthma.

The first recognized RV was isolated in 1954; now, at least 102 serotypes have been identified. The genus name was coined in 1963 to reflect the usual prominent nasal involvement. The RVs are one of four genera of human pathogens in the picornavirus family; the other genera include various enteroviruses (polioviruses, coxsackieviruses, echoviruses), parechoviruses, and hepatitis A (hepatovirus). Like the other picornaviruses, RVs are small (30 nm), nonenveloped (therefore, resistant to lipid solvents), and icosahedral (20-sided); they possess a single-stranded RNA genome. Several RVs have been sequenced completely, and they demonstrate much cross-homology among themselves and with the genomes of other picornaviruses. RVs differ from other picornaviruses in that they become noninfectious at a pH of less than 5. For all 102 RV serotypes, three binding sites exist where the virus attaches itself to the receptor on the cell: for 91 serotypes (the major receptor group), the binding site is intercellular adhesion molecule-1 (ICAM-1); for 10 serotypes (the minor receptor group), the binding site is a low-density lipoprotein receptor; and for serotype 87, the binding site is a third, as yet unidentified, molecule. After attaching to a cell, the RV probably is endocytosed within a vesicle, then the RNA is released into the cytoplasm. Viral replication occurs in the cytoplasm, and viruses are released by cell lysis.

RVs have been isolated from natural infections of cattle, chimpanzees, and humans. Human RVs infect only humans and chimpanzees; chimpanzee infections are subclinical.

EPIDEMIOLOGY

RV infections are found in varying degrees year round, but they show the greatest incidence in spring and early autumn. This seasonal pattern occurs worldwide. Several RV serotypes usually circulate simultaneously, frequently coincident with other respiratory viruses. However, all types generally are not equal in prevalence and do not spread to the same degree. In a University of Wisconsin student family study (Eagle Heights, Madison, WI) conducted between 1963 to 1965, 19 RV types were isolated, but only three types (43, 51, and 55) spread beyond the index family, and they spread widely. These dominant RV serotypes occur locally over relatively short periods but they do not extend over large geographic areas or for several years.

Unlike some major respiratory viruses [e.g., parainfluenza types 1 and 3 and respiratory syncytial virus (RSV)], individual RV serotypes seldom recur within a population from year to year. At Eagle Heights, ten RV types were found from 1963 to 1964 and nine types from 1964 to 1965, but only one type (RV15) occurred in both years. Also, RV43 and RV55 were predominant from 1963 to 1964, whereas RV51 prevailed from 1964 to 1965. Similar observations have been made in other epidemiologic studies.

The incidence of RV infections is highest in infants and lowest in older individuals. In Seattle, RV infection rates of 0.59 per person-year were found in family members with and without symptoms. Rates ranged from 0.8 to 1.2 for children aged 0 to 5 years to 0.2 to 0.3 for adults older than 20 years. Women of childbearing age, however, showed rates 1.2 to 1.5 times higher than those of men. The true incidence of RV infections probably is higher than reported, because RV diagnostic tests often are not very sensitive.

Prevalence studies show RV antibodies begin to appear early, increase throughout childhood and adolescence, peak in early adulthood, and then stabilize for years. In Charlottesville, Virginia, fewer than 10% of young children had serum antibodies to any of 56 RV types tested, whereas adults had antibodies to approximately 50%.

Transmission

Schoolchildren are the most important reservoir of RVs; home and school are locations of the highest rate of transmission. Dissemination within susceptible family members averages approximately 50% and, within a schoolroom, ranges from 0% to 50%. Long-term association with individuals with RV infections may be required for transmission to occur. Even then, transmission rates approach only 50%. Thus, short-term contacts such as occur when shopping, attending movies, visiting friends, and visiting a physician’s waiting room are reasonably risk-free. However, transmission rates may be higher among young children in day-care facilities, where all respiratory viruses may spread readily.

Because RVs often seem to disseminate slowly, the route of transmission is important. RVs can be transmitted either by large droplet aerosol or by direct contact; the relative importance of these two routes remains an open question. Blocking transmission in various habitats such as the schoolroom and home may be possible. Using an experimental epidemiologic model featuring natural RV16 transmission among human volunteers, researchers were able to stop RV16 transmission with virucidal facial tissues. More practically, careful handwashing, especially around young children, should be utilized as a means to interrupt transmission.

Only gold members can continue reading. Log In or Register to continue