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 (Tamiflu) 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