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Objective. To evaluate the relationship between nasal quantity of respiratory syncytial virus (RSV) and disease severity in hospitalized infants without underlying cardiopulmonary disease or immunodeficiency.

Methods. Nasal aspirates were obtained from hospitalized infants <24 months of age with recently identified RSV infection and evaluated for RSV quantity by a standard plaque assay on HEp-2 cell monolayers. Subjects were classified as having "severe" disease if they required mechanical ventilation at the time of sample collection and as having "nonsevere" disease if they did not. Linear modeling was used to determine the relationship between nasal RSV quantity and various independent variables, including disease severity.

Results. Nasal aspirates from 39 patients were evaluated. Age, gender and mean duration of time from symptom onset to sample acquisition (5 days) were similar between the severe (n = 15) and nonsevere (n = 24) groups. Significantly more infants were born at <35 weeks gestation in the severe disease group (7 of 15 vs. 3 of 24, P = 0.017), and infants born at <35 weeks gestation were significantly more likely to be of non-Caucasian ethnicity than were infants born at >=35 weeks gestation (8 of 10 vs. 12 of 29, P = 0.035). The linear model found that higher nasal RSV quantities were associated with severe disease [mean /- SEM, 5.06 /- 0.34 log plaque-forming units (pfu)/ml vs. 3.91 /- 0.35 log pfu/ml, P = 0.022], gestational age >=35 weeks (5.44 /- 0.27 log pfu/ml vs. 3.52 /- 0.45 log pfu/ml, P = 0.002) and non-Caucasian ethnicity (5.16 /- 0.30 log pfu/ml vs. 3.80 /- 0.37 log pfu/ml, P = 0.006).

Conclusions. Nasal RSV quantity correlates with disease severity in hospitalized infants with recently identified RSV infection.

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