Influenza Virus (Flu Virus)

Classification

Orthomyxoviridae

Structure

seg., lin., ssRNA-, helical, env.

Diseases

Sites and Sources

Diagnostic Factors

Virulence Factors

Treatment and Prevention

Commentary

Influenza virus was responsible for the worst epidemic to strike the world since the plague epidemics of the middle ages. The influenza pandemic of 1918-1919 caused the deaths of 20 million people, more than died in all the battles of World War I. The virus initially infects epithelial cells in the upper respiratory tract. This has the two-fold effect of causing desquamation of the ciliated epithelium and loss of the protective muco-ciliary escalator; it also results in a non-productive cough. In addition, viral replication causes interferon induction and this causes the array of symptoms that define flu-like symptoms, i.e., fever, headache, chills, malaise and myalgia. Influenza characteristically has an abrupt onset. The upper respiratory illness lasts for approximately a week unless a secondary bacterial infection occurs leading to bronchitis or pneumonia or the influenza virus itself invades the lower respiratory tract. The denuded ciliated epithelium allows the easy entry of bacteria into the lower respiratory tract and it is this or infection of the lungs by the influenza virus causing influenza pneumonia that is the cause of so many of the deaths. A vaccine exists to stimulate IgG that will prevent influenza pneumonia, but due to the antigenic variation caused by the genetic instability of the virus frequent revaccination is necessary. The current (injected) vaccine does not provide complete protection against the upper respiratory tract illness. An experimental nose-spray vaccine that stimulates IgA and would prevent the upper respiratory infection (flu) as well as the pneumonia is being tested. The virus has two mechanisms of antigenic change, genetic shift and genetic drift. Genetic shift is caused by reassortment of individual segments of the genome while genetic drift is caused by point mutation. Genetic shifts occur when animals, presumably pigs, are co-infected by different strains of the virus. Even if the strains are from different species, the viral gene segments will randomly reassociate and create hybrid viruses. Reassortment of virus segments from birds, pigs, and perhaps other animals is thought to be the source of the new strains that infect humans. One way this could happen is as follows -- the virus is shed in the bird feces which gets into the pigs' drinking water. Humans may then handle and/or cough on the pigs. New viruses are then made containing segments from humans, birds and pigs. Because of the close proximity of humans, pigs, and ducks in China it has been long considered that China is the breeding ground for new influenzas strains - hence the names Asain flu, Hong Kong flu, etc. Influenza is also likely the origin of the motherly advice to drink lots of fluids and stay warm and in bed when respiratory infection strikes. The fluids keep the epithelial cells hydrated and keep the mucociliary escalator functioning for as long as possible. The bed rest keeps deep breathing at a minimum and the warmth minimizes the replication of the virus which replicates best at 34 degrees. For more information and some fun cases, see http://www.medinfo.ufl.edu/cme/flu/flu.html