18.03.3 Drugs used to Treat the Flu

Viruses responsible for human flu (orthomyxoviruses) areRNA viruses (mainly influenzavirus A subtypes). They cause symptoms similar to a cold. However there are additional symptoms. With the flu, the fever is greater than with a cold. Also myalgia and prostration are symptoms of the flu not observed with a cold. Infection is spread via the respiratory system.

Each year in Australia, flu vaccines are developed against the influenza viruses that were most prevalent in the previous northern hemisphere winter. The vaccine prevents the viral infection.

Swine flu is also caused by an influenzavirus A subtype. The symptoms of swine flu in humans are similar to the common human flu. However, as it is a new virus, there is no natural immunity to swine flu. It takes a minimum of 6 months to develop a vaccine to a new type of flu.

Following replication and assembly of viral components, each virus particle exits the host cell, but remains attached until a viral specific enzyme (neuraminidase) releases it. The neuraminidase inhibitors prevent this release, and are used to treat the flu. Obviously, if the virus cannot leave the host cell, the further cell infection will be prevented. Zanamivir (Relenza) and oseltamivir (Tamiflu) are examples of neuraminidase inhibitors. Zanamivir was developed in Australia by a company (Biota) associated with Monash University. Zanamivir is administered as an aerosol, but for maximum benefit, must be taken within 48 hours of the onset of the flu symptoms. Oseltamivir is taken orally, also, as soon after the onset of flu symptoms, as possible.

Both zanamivir and oseltamivir give some relief of the symptoms of the flu, as well as shortening the time the symptoms last.. Neuraminidase inhibitors (mainly oseltamivir) were stockpiled as a treatment for bird flu. Fortunately the bird flu epidemic never happened. Neuraminidase inhibitors are probably effective in the swine flu, but this has not been fully evaluated.