Chapter 117 - Avian and swine influenza viruses

Abstract

Influenza viruses are associated with high morbidity and mortality and are responsible for an average of 389,000 deaths and more than 9,000,000 hospitalizations worldwide each year. Influenza viruses are segmented, negative-sense, single-stranded RNA viruses that belong to the Orthomyxoviridae family of viruses. They contain a ribonucleoprotein that lacks proofreading functionality, resulting in frequent nucleotide and amino acid mutations. These mutations may allow the virus to evade host immune responses and cause an epidemic by a process known as antigenic drift. In addition, the segmented nature of the influenza virus presents opportunities for genetic reassortment to occur, where segments of one virus mixes with segments from other viruses leading to greater genetic diversity. This process can create a novel hemagglutinin–neuraminidase subtype virus that crosses the species barrier, often from swine or avian species, and enters a population that is immunologically naïve, termed antigenic shift, which may cause a pandemic. In the past century, four human influenza pandemics have been documented, causing up to an estimated 100 million deaths. Thus it is imperative to understand the nature of influenza infections, how the viruses cross species barriers, and strategies for the prevention and control of influenza outbreaks. This chapter begins with a brief overview on the classification of influenza viruses, the major pandemics over the past century, and the zoonosis and transmission of influenza viruses. We then examine the pathogenesis of influenza-associated illnesses and clinical management. Finally, we discuss strategies for the prevention and control of seasonal influenza outbreaks, cross-species infections, and future influenza pandemics.