Influenza

Abstract

Influenza-A-viruses have a wide host range and occur with a wide spectrum of variants defined by 16 HA and 9 NA subtypes. All of these subtypes occur in birds, whereas only some of them have so far been observed in man, pig, horse, and a number of other mammals. In contrast, influenza-B and C-viruses occur only in man, and there are no subtypes of these viruses. Influenza-A-viruses occasionally can be transmitted from aquatic birds, their natural reservoir, to terrestrial birds and mammals. On rare occasions, they adapt to the new species and establish thus new virus lineages. Adaptation requires multiple mutations and it may involve gene reassortment after co-infection with another virus. By these mechanisms, viruses with new surface glycoproteins and therefore a distinct change in antigenicity are generated. If a new virus with such an antigenic shift occurs in man, it causes a pandemic. Antigenic drift, unlike antigenic shift, is characterized by slight changes in antigenicity resulting from successive mutations in HA and NA. Antigenic drift is responsible for the annual human epidemics. It occurs not only with influenza-A-viruses, but also with influenza-B viruses.

Influenza is a highly contagious disease that is transmitted by aerosols. Virus replication occurs in airway epithelia and reaches its peak 2–3 days after infection. Symptoms typically include high fever, chills, headache, sore throat, dry cough, myalgias, anorexia, and malaise. Complications include primary viral pneumonia, secondary bacterial pneumonia, or combined bacterial and viral pneumonia. Serious complications of influenza most often occur in people 65 years of age and older, in the very young, and in those of any age with underlying chronic cardiac, pulmonary, or metabolic disease. Vaccination is the most potent instrument for influenza control. Prime candidates for vaccination are persons at risk for complications and individuals who might transmit influenza to such persons. Inactivated vaccines obtained from infected chicken embryos are most commonly used. Neuraminidase inhibitors are the influenza antivirals of choice. Application is limited to a relatively small time window shortly before or after infection.