Influenza - Expect the unexpected.

Influenza is an acute self-limited febrile illness caused by infection with influenza type A or B viruses, and has been causing cyclical epidemics of febrile respiratory disease for centuries. It occurs in outbreaks in almost every winter season in northern and southern hemispheres. The attack rates vary during these outbreak cycles but have been reported as high as 20% to 40% during the peak period of influenza activity (1). Influenza continues to be associated with significant morbidity in the general population, with the elderly, the very young and patients with comorbid illnesses being particularly susceptible. Significant increases in mortality are often seen during influenza epidemics and the excess mortality is not only a direct result of pneumonitis but also of other cardiopulmonary diseases that may be exacerbated by the influenza virus infection. It has been reported that more than 20,000 influenzaassociated deaths occurred during each of nine different epidemics between 1972 and 1992 (2), although this figure has been adjusted to an average of approximately 36,000 deaths/year due to influenza in the United States during 19901999 based on different modelling (3). In the past, Health Canada has reported that, on average, 500 to 1500 deaths per year are due to influenza or pneumonia occurring as a complication of influenza. It is acknowledged that many more deaths may occur in people with underlying medical conditions complicated by influenza. After using new modelling, however, Health Canada estimates that the figure may actually be from 700 to 2500 per annum (4). Given the early and somewhat unexpected onset to this year’s influenza season, and the reappearance of avian influenza in Asia, it is timely to briefly review the current epidemiology of influenza, particularly with respect to the influenza A Fujian strain which is predominant this season. Influenza A and influenza B viruses are the two types of influenza viruses which cause human epidemic disease. Influenza A viruses are found in many different animals, including ducks, chickens, pigs, whales, horses and seals. Influenza B viruses circulate widely only among humans. Influenza A viruses are divided into subtypes based on two antigens on the surface of the virus: the hemagglutinin (H) and the neuraminidase (N). There are 15 different H subtypes and nine different N subtypes identified to date, all of which have been found among influenza A viruses in wild birds. The H antigen acts as a site of attachment of the virus to host cells to initiate infection and also to erythrocytes from which its name originally was derived (5). The H antigen contains common and strain-specific antigens and demonstrates antigenic variation. The N antigen contains subtype-specific antigens and also demonstrates antigenic variation between subtypes. The N antigen (also known as sialidase antigen) is a surface glycoprotein possessing enzymatic activity essential for viral replication in both influenza A and B viruses. The N antigen enables the release of newly produced virions from infected host cells, prevents the formation of viral aggregates after release from the host cells, and prevents viral inactivation by respiratory mucous (6,7). It is thought that this enzyme may also promote viral penetration into respiratory epithelial cells and may contribute to the pathogenicity of the virus by promoting production of pro-inflammatory cytokines such as interleukin-1 and tumour necrosis factor from macrophages (8-10). Wild birds are the primary natural reservoir for all subtypes of influenza A viruses and are thought to be the source of influenza A viruses in other animals. Most influenza viruses cause asymptomatic or mild infection in birds; however, the range of symptoms in birds varies greatly depending on the