Influenza epidemics have plagued man for centuries. In fact, influenza was probably the disease described by Hippocrates in 412 B.C. Today, influenza it remains a major cause of morbidity and mortality worldwide, with large segments of the human population affected every year.
Many animal species can be infected by influenza viruses, often with disastrous consequences. A continuing threat is the possibility of a pandemic similar to that experienced in 1918, estimated to have been responsible for 50 million deaths worldwide.
Influenza viruses belong to the family of Orthomyxoviridae; viruses with segmented RNA genomes that are negative sense and single-stranded.
There is some evidence of development of drug resistance by influenza viruses. Due to the limitations in the respective studies and their methodologies, the information should be considered very carefully. In vitro studies demonstrate cross-resistance among neuraminidase inhibitors. However, adamantane derivative susceptibility does not seem to be affected.
Antiviral therapy should be considered in the setting of suspected influenza infection within 48 hours of illness onset in individuals older than 1 year who are at high-risk for influenza infection. Treatment should be continued for five days.
During community influenza outbreaks and influenza outbreaks in other settings, consider chemoprophylaxis if unvaccinated high-risk persons aged 1 year or greater.
Consider treatment or chemoprophylaxis of high-risk or healthy persons for: (a) seriously ill patients admitted to the hospital for influenza treatment; (b) for chemoprophylaxis of family members of high-risk individuals during outbreaks; and (c) if adequate supplies available, for treatment of persons without high-risk factors who are infected with influenza.
Neuraminidase inhibitors: These drugs have activity against influenza A and B viruses. Zanamivir is an orally inhaled powder approved for treatment of influenza in individuals older than 7 years. The drug, however, is not approved for chemoprophylaxis. Oseltamivir is an oral capsule or suspension that is approved for treatment of influenza in persons older than 1 year. It is also approved for chemoprophylaxis in individuals older than 13 years. Neuraminidase inhibitors cause viral aggregation at the host cell surface and a decrease in the number of viral particles released from the infected cell by blocking the active site of the viral enzyme neuraminidase. This enzyme is common to both influenza A and B.
Adamantane derivatives: These drugs are orally administered for treatment and chemoprophylaxis of influenza A. This class of drugs have no effect on influenza B virus. Amantidine is approved for the treatment of influenza A in persons older than 1 year. Rimantidine is approved for treatment of influenza A in adults. Both rimantidine and amantidine are approved for chemoprophylaxis of influenza A in persons older than 1 year. Adamantane derivatives are thought to interfere with viral M2 protein, a membrane ion channel protein, and inhibit viral uncoating, thereby inhibiting viral replication and resulting in decreased viral shedding.
Notes & References
 Ratliff J. Drug therapy for influenza infection. Current Topics from the Drug Information Center 2004;34(2):1-3.
 Antiviral agents for influenza: Background information for clinicians. Available online at: http://www.cdc.gov/flu/professionals/antiviralback.htm. Last accessed on June 2, 2008.
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