Emergence of rimantadine-resistant virus within 6 days of starting rimantadine prophylaxis with oseltamivir treatment of symptomatic cases

Abstract

Objectives: To report on the detection of rimantadine resistance within 6 days of starting rimantadine prophylaxis.

Design: Observational prospective study.

Setting: Fifty-bed nursing unit during the 2004/05 influenza season.

Participants: All residents.

Intervention: Clinical monitoring for new onset of respiratory illness followed by collection of nasopharyngeal swabs for Directigen AB testing and influenza culture. After outbreak identification, rimantadine was administered as prophylaxis, whereas oseltamivir was used to treat symptomatic cases. Laboratory monitoring for the emergence of rimantadine resistance was reinitiated on the fifth day of rimantadine prophylaxis.

Measurements: Tabulation of respiratory illnesses, rapid tests and cultures yielding influenza A, and rimantadine sensitivity determination in five index isolates.

Results: A total of 15 symptomatic cases were identified over 8 days. Amantadine sensitivity was determined in five cases. Three initial cases were sensitive to rimantadine, whereas two cases identified after 6 days of rimantadine prophylaxis were resistant to rimantadine.

Conclusion: The Centers for Disease Control and Prevention reported that 91% of isolates collected early the following season (2005/06) were resistant to rimantadine. Rimantadine treatment is no longer recommended. This experience anticipated the new recommendations. If reemergence of sensitivity follows discontinuation of rimantadine use, using rimantadine as prophylaxis and oseltamivir for treatment of symptomatic cases might be efficacious and economical on a national scale.