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Text VIII. Influenza

Epidemiology. Influenza A virus is the most frequent cause of clinical influenza. Spread is by person-to-person contact and airborne droplet spray. Infection produces sporadic respiratory illness every year. Acute epidemics usually occur about every 3 years, generally nationwide during late fall or early winter. A major shift in the prevalent antigenic type of influenza A virus occurs about once in a decade and results in an acute major epidemic. Persons of all ages are afflicted, but prevalence is highest in school children, and severity is greatest in the very young, aged, or infirm. Epidemic illness occurs in two waves – the first in students and active family members, the second mostly in shut-ins and persons in semi-closed institutions.

Influenza B causes epidemics about every 5 years and is much less often associated with pandemics. Influenza C is an endemic virus which sporadically causes mild respiratory disease.

Symptoms and Signs. The incubation period is about 48 hours. Transient viremia may occur before infection localizes in the lower respiratory tract. Influenza A or B is sudden in onset, with chills and fever up to 39 to 39.5ºC (102 to 103ºF) developing over 24 hours. Prostration and generalized aches and pains (most pronounced in the back and legs) appear early. Headache is prominent. Respiratory tract symptoms may be mild at first, with sore throat, substernal burning, nonproductive cough, and sometimes coryza, but later become dominant. Cough may become severe and productive. The skin, especially on the face, is warm and flushed. The soft palate, posterior hard palate, tonsillar pillars, and posterior pharyngeal wall may be reddened but there is no exudate. The eyes water easily and the conjunctiva may be mildly inflamed. Usually, after 2 to 3 days acute symptoms subside rapidly and fever ends. Weakness, sweating and fatigue may persist for several days or occasionally for weeks.

Treatment. Treatment is symptomatic. The patient should remain in bed or rest adequately and avoid exertion during the acute stage and for 24 to 48 hours after the temperature becomes normal. If situational symptoms of acute uncomplicated influenza are severe, antipyretics and analgesics are helpful. To relieve nasal obstruction, 1 or 2 drops of 0.25% phenylephrine may be instilled into the nose periodically. Gargles of warm isotonic saline are useful for sore throat. Steam inhalation may alleviate respiratory symptoms somewhat and also prevent drying of secretions.

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