Adamantane-Resistant Influenza Infection During the 2004–05 Season

Adamantane-resistant influenza A is an emerging problem, but infections caused by resistant and susceptible viruses have not been compared. We identified adamantane resistance in 47% of 152 influenza A virus (H3N2) isolates collected during 2005. Resistant and susceptible viruses caused similar symptoms and illness duration. The prevalence of resistance was highest in children.

D uring the past 4 decades, antiviral drug therapy has been a useful strategy for both prophylaxis and treatment of infl uenza A (1,2). The most widely used drugs have been 2 adamantane derivatives (amantadine and rimantadine), which are effective for prophylaxis and reduce illness duration if started within 48 hours after symptom onset (3,4). These drugs block the M2 ion channel protein, preventing viral replication (5). From 1997 through 2004, few infl uenza A (H3N2) isolates were resistant to adamantanes, although resistance among isolates from Asia increased substantially in 2003 (6). In the United States, the proportion of infl uenza A (H3N2) isolates with adamantane resistance was 0.8%-2.2% during this period and 11% in the 2004-05 infl uenza seasons. However, in the 2005-06 season, >90% of infl uenza A (H3N2) isolates from patients in 26 states contained a mutation conferring resistance to adamantane drugs (7).
Ferret models suggest that virulence is not increased by adamantane resistance in infl uenza A (H3N2) infections (8), although experimental studies with recombinant infl uenza (H1N1) in mice have suggested that the mortality rate is increased by a double mutation conferring adamantane resistance (9). Little is known regarding clinical effects in humans infected with adamantane-resistant infl uenza viruses. We compared clinical and demographic characteristics of patients infected with either adamantane-susceptible or -resistant strains of infl uenza A during the 2004-05 season.

The Study
Study participants were derived from a study of infl uenza vaccine effectiveness conducted within a 14-Zip code region surrounding Marshfi eld, Wisconsin, during January-March 2005. Eligible participants included children 6-23 months of age, adults >65 years of age, and persons 24 months-64 years of age with a high-risk medical condition. Research coordinators recruited eligible patients and obtained samples for infl uenza culture during inpatient and outpatient encounters for acute respiratory illness. Each participant (or parent) completed a short interview form to assess symptoms and onset date, and culture-confi rmed patients were contacted again to determine the illness recovery date. The study was approved by the Marshfi eld Clinic institutional review board, and all participants provided written informed consent. Infl uenza virus was isolated by inoculation and incubation in monolayered rhesus monkey kidney cells. Cultures were examined for cytopathic effect, and cultures showing no cytopathic effect were tested twice for hemagglutination titers. Infl uenza A and B were confi rmed by immunofl uorescence (Chemicon International, Inc., Temecula, CA, USA). All infl uenza A isolates were tested for adamantane resistance at the Centers for Disease Control and Prevention (CDC). Procedures for RNA extraction and pyrosequencing have been previously described (7). A medical record review was performed to assess any use of antiviral drugs by infl uenza A patients from 60 days before through 14 days after the enrollment date.
Univariate comparisons were performed, and crude and adjusted prevalence ratios were computed to evaluate the association between adamantane resistance and age, gender, vaccination status, date of clinical encounter, and presence of a high-risk medical condition. Enrollment dates were grouped into 4 consecutive 3-week periods beginning January 3, 2005. The referent period was the second 3week period when the number of infl uenza cases peaked.
All analyses were conducted with SAS version 9.1 (SAS Institute Inc., Cary, NC, USA).
Number of symptoms and duration of illness were similar for patients with adamantane-resistant and -susceptible infl uenza isolates ( Table 1). The proportion of patients hospitalized was nearly the same in each group, and the distribution of individual symptoms did not differ signifi cantly. Forty-fi ve patients received antiviral therapy (37 adamantanes, 8 oseltamivir), but only 1 was treated with amantadine before infl uenza culture was obtained. Clinical features and duration of illness were similar whether or not patients received adamantane therapy. In a multivariable model that included patients of all ages, adamantane resistance was not associated with gender, time of clinical encounter, or infl uenza vaccination status ( Table 2). Children 6-23 months old without a high-risk medical condition were signifi cantly more likely to be infected with adamantane-resistant infl uenza A than were adults >65 years of age who had a high-risk condition. In a secondary analysis, no association was found between adamantane resistance and number of symptoms, duration of illness, or hospitalization.

Conclusions
This study provides epidemiologic evidence that the point mutation (S31N) conferring adamantane resistance has not altered infl uenza A (H3N2) virulence as measured by clinical symptoms and duration of illness. Hospitalization rates were also similar, although the power to detect a difference was low. These results are consistent with prior research in ferrets, which demonstrated that virulence was similar for adamantane-resistant and -susceptible infl uenza A (H3N2) isolates (8). The source population for this study included persons who were eligible for infl uenza vaccination based on age group or the presence of a high-risk medical condition. Consequently, participants were more likely to experience infl uenza complications relative to the general population. The absence of any difference in clinical severity in this higher risk population provides some reassurance that adamantane resistance will not affect overall infl uenza morbidity.
Factors contributing to rapid emergence of adamantane-resistant infl uenza viruses are not fully known. Resistance has been observed in a variety of outbreak and institutional settings for many years (10-12) but did not spread into the general population until recently (6,7). The proportion of human infl uenza A (H3N2) isolates with adamantane resistance increased dramatically in People's Republic of China and Taiwan in 2003 (6). However, the degree to which the spread of resistant viruses from Asia contributed to the rapid escalation of adamantane resistance in the United States is uncertain.
The 2004-05 infl uenza season provided an opportunity to learn more about the epidemiology of adamantane resistance during a period when drug resistance was increasing in Wisconsin but was not yet universal. Children had the highest risk of acquiring an adamantane-resistant infl uenza infection and are an important reservoir for transmission of infl uenza to adult household members and the community (13). Still, we cannot explain why children were preferentially infected with adamantine-resistant strains in 2004-05. Only 1 patient was treated with amantadine before enrollment. The higher prevalence of resistance in children was therefore not attributable to individual adamantane use. There may have been multiple introductions of adamantane-resistant infl uenza viruses from other geographic areas during the 2004-05 season. This phenomenon was described in a recent phylogenetic analysis of infl uenza A (H3N2) isolates from New York state over a 9-year period (14). The analysis suggested that viral evolution within epidemic seasons is dominated by random importation of distinct viral strains from other geographic areas. CDC continues to recommend that adamantane drugs not be used for infl uenza treatment in the United States. Given the recent emergence of oseltamivir-resistant infl uenza A and B infections (15,16), ongoing monitoring of infl uenza virus susceptibility to adamantanes and neuraminidase inhibitors is essential.