Influenza is spread mainly from person to person by droplet infection or droplet nuclei created by sneezing, coughing or talking. The portal of entry of the virus is the respiratory tract.
Incubation Period The incubation period is about 18 to 72 hours.
Pathogenesis and Clinical Features The virus enters the respiratory tract and causes inflammation and necrosis of the superficial epithelium of the tracheal and bronchial mucosa, followed by secondary bacterial invasion. There is no viraemia. Both viruses cause the same symptoms – fever, chills, aches and pains, coughing and generalized weakness. Fever lasts from one to five days, averaging about three days in adults. The most dreaded complication is pneumonia, which should be suspected if fever persists beyond four or five days, or recurs abruptly after convalescence.
Anti–viral Drugs Due to limitations in the efficacy of influenza vaccines, anti–viral drugs have been tried for the prophylaxis and therapy of influenza type A infections. Controlled clinical trials have demonstrated the efficacy of both amantadine and rimantidine in the prophylaxis and therapy of influenza virus A infections. These drugs block penetration of the influenza A virus in the host cell and prevent virus replication. These compounds shorten the duration of fever, headache, cough, sore throat, general malaise and also reduce virus shedding. A dose of 100 mg of amantadine or rimantidine twice a day for about three to five days has been found effective for treatment, and a much longer period (throughout the period of exposure, to the virus) for prophylaxis as the person becomes fully susceptible when the drug is stopped. These drugs may also modify the severity of influenza – if started within 24 to 48 hours of the onset of illness. As for side–effects, they are much less in the case of rimantidine. The proper use of these drugs requires laboratory evidence of an outbreak of influenza A in the community, since these drugs are not effective against influenza B. However, these drugs have not been used as a public health measure for the widespread control and prevention of influenza A.