What is Human Influenza? Influenza A usually causes acute febrile respiratory tract infections in humans. Marked fever, cough, muscle/joint aches and absence of sneezing/runny nose aid in differentiating it from ‘common cold’ viruses. Peak incidence occurs in the winter months. Genetic mutations during viral replication lead to small variations in its structure year-to-year (‘antigenic drift’). Subsequent adaptation to infect humans and the level of protective immunity in the population determine the burden of disease for a given ‘influenza’ season. Most cases are self-limiting in healthy people, but marked morbidity and mortality can result in the elderly, neonates and those with chronic medical conditions (e.g. cardiovascular, respiratory illness, diabetes). Bacterial pneumonia frequently complicates the course of illness particularly in the elderly. Avian Influenza & Humans Disease Birds (especially water fowl) are the primary hosts for Influenza A virus. As with human viral strains, those affecting birds continuously undergo minor genetic change year-toyear, resulting in a varied ability to cause avian disease. Occasionally, large genetic rearrangements (‘antigenic shift’) occur resulting in a novel viral structure with pandemic potential. Historically, two previous human pandemics of influenza have resulted from novel strains emerging from their avian counterparts. This abrupt change with subsequent unfamiliar genetic components and lack of any previous immunity in humans resulted in a high disease burden globally. In 1997, a drastically altered viral strain (H5N1 strain) emerged from Hong Kong in poultry, and this potentially poses a risk of causing pandemic disease in humans. Currently, outbreaks in South East Asia, Russia & Eastern Europe are the result of crowded poultry farming, allowing the virus ample opportunity to interact with humans and human Influenza strains. Avian Influenza & The Current Situation Because of the high human population density and close association with infected poultry a small number of human cases have been reported in areas endemic with Avian Influenza. Unlikely seasonal influenza, high mortality rates have been reported in otherwise young healthy patients. What is different from previous bird to human transmissions is that the virus remains unaltered from the avian form suggesting direct jump to humans. Fortunately, efficient human-to-human transmission has not occurred suggesting a missing step in critical viral evolution. There is concern that full adaptation to humans will occur (either by a novel mutation in the Avian strain or genetic rearrangement with human strains) given its current prevalence and proximity to humans. Influenza & Management H5N1 Avian influenza has not been reported in Canada to date. However, late fall is when the usual human Influenza cases begin to surge. Hand washing and minimizing direct contact with people who have respiratory symptoms remain the best preventative measures for all Influenza strains (including Avian Influenza). Annual vaccination provides protective immunity to currently circulating human strains and reduces personto-person spread. It is particularly important for healthcare providers (HCP) to receive Influenza vaccinations annually. Studies have clearly demonstrated that vaccination reduce infections and absenteeism amongst HCP, and prevent mortality in their patients. People with symptoms of Influenza should stay home until the symptoms have resolved (usually 5 days) to avoid transmission. Oseltamivir (Tamifluļ) is used to lessen duration of symptoms in uncomplicated Influenza infections and to prevent outbreaks in ‘high risk’ populations. Its effectiveness against Avian Influenza remains uncertain. Ongoing global efforts to track and develop an effective vaccine for H5N1 remain a top priority. By William Ciccotelli, MD Infectious Diseases and Medical Microbiology Fellow, McMaster University