The ability to detect HRV in nasal wash samples was increased many times when a validated QPCR assay was used. However, in our study there was frequent detection of rhinovirus by QPCR in symptom-free individuals. This result raises a number of questions.
Was the QPCR accurately detecting HRV RNA? The assay was well validated in terms of specificity and sensitivity against reference strains. The curve generated by progressive dilution of HRV standards was linear. Nevertheless, the relative quantitation of each HRV strain may be limited by the use of a limited number of primers for all HRV serotypes. Designing specific primers for all rhinovirus serotypes seems a daunting task.
Can a QPCR cutoff that increases the correlation with illness be identified? As all positive tissue culture results were >150 copies, this could be considered a cutoff value for a positive QPCR result. However, the overlap of the quantity of virus present between individuals with and without symptoms was almost complete, and setting such a high cutoff raises the issue of how to interpret weakly positive samples. There is logic to a cutoff of 1 PFU equivalent, but the early literature suggested that human infectivity was a more sensitive measure than was tissue culture infectivity (5
). The wide overlap in QPCR values between well and sick individuals is also disconcerting. Any proposed assay needs a definitive correlation of QPCR level with clinical disease. We suggest that future studies must include tissue culture and QPCR determinations from healthy individuals in the population(s) being monitored in order to more accurately assess the relationship between HRV infection and clinical illness.
Are there unique aspects to the conduct of this trial that might have altered the findings or limited its generalizability? The studies were done outside of the winter respiratory viral season and were done with otherwise healthy individuals without a history of asthma. Each of these factors makes the study less general but does not change the validity of the comparisons between subjects with and without symptoms.
Do these observations give insight into the pathogenesis of HRV infection? The prolonged period of time that HRV can be detected is consistent with the high frequency of recovery of rhinovirus from tonsil and adenoid tissues (23
). At least two other reports have documented rhinovirus recovery in well children for up to 2 weeks after a respiratory illness (15
). A longitudinal study has shown an association of illness with rhinovirus recovery (28
). However, in that study only 62% of the illnesses ascribed to rhinovirus had a direct temporal association of virus identification with the onset of illness, and 20% of the rhinoviruses detected had no association with illness. Other studies have shown 4% of adults and 12 to 18% of well children with rhinovirus (16
). Certainly, from our study and those conducted by others, HRV are ubiquitous and occur in all ages, including early infancy, and appear to persist in the respiratory tract with or without illness for relatively long periods of time. Interpretation of our observations is confounded by the possibility of multiple strains in a single volunteer, affecting the duration of apparent shedding, and the possibility that detection of rhinovirus in asymptomatic individuals may be from a previous symptomatic infection. However, to be permitted entry into the trial, the volunteers could not have had a respiratory illness in the week before vaccine administration.
There is no question that rhinoviruses cause respiratory illness. Our data support that fact, with a statistically significantly higher identification of rhinovirus in subjects with illness than in those without. Experimental infections (5
) and the effectiveness of specific antivirals (11
) all make unequivocal arguments that HRV are causes of respiratory illness. Additionally, rhinovirus has been reported to be a cause of illness in immunologically compromised individuals (13
). However, given that HRV was detected in greater than 20% of well individuals, further research is necessary to convincingly show the extent to which the impact of HRV should be broadened beyond the traditional association with upper respiratory tract infections to having a major role in the triggering of asthma or as a causative agent in serious lower respiratory tract disease.