Influenza Essay Research Paper Clinical Features of

Influenza Essay, Research Paper

Clinical Features of Influenza

Influenza, commonly called “the flu,” is caused by viruses that infect the respiratory tract. Compared with most other viral respiratory infections, such as the common cold, influenza infection often causes a more severe illness. Typical clinical features of influenza include fever (usually 100oF to 103oF in adults and often even higher in children) and respiratory symptoms, such as cough, sore throat, runny or

stuffy nose, as well as headache, muscle aches, and often extreme fatigue. Although nausea, vomiting, and

diarrhea can sometimes accompany influenza infection, especially in children, gastrointestinal

symptoms are rarely prominent. The term “stomach flu” is a misnomer that is sometimes used to describe gastrointestinal illnesses caused by other microorganisms.

Most people who get the flu recover completely in 1 to 2 weeks, but some people develop

serious and potentially lifethreatening medical complications, such as pneumonia. In an

average year, influenza is associated with about 20,000 deaths nationwide and many more

hospitalizations. Flu related complications can occur at any age; however,the elderly and

people with chronic health problems are much more likely to develop serious complications

after influenza infection than are younger, healthier people.

The Influenza Viruses

Influenza viruses are divided into three types, designated A, B, and C. Influenza types A

and B are responsible for epidemics of respiratory illness that occur almost every winter

and are often associated with increased rates for hospitalization and death. Influenza type

C differs from types A and B in some important ways. Type C infection usually causes

either a very mild respiratory illness or no symptoms at all; it does not cause epidemics

and does not have the severe public health impact that influenza types A and B do. Efforts

to control the impact of influenza are aimed at types A and B, and the remainder of this

discussion will be devoted only to these two types.

Influenza viruses continually change over time, usually by mutation. This constant

changing enables the virus to evade the immune system of its host, so that people are

susceptible to influenza virus infection throughout life. This process works as follows: a

person infected with influenza virus develops antibody against that virus; as the virus

changes, the “older” antibody no longer recognizes the “newer” virus, and reinfection can

occur. The older antibody can, however, provide partial protection against reinfection.

Currently, three different influenza strains circulate worldwide: two type A viruses and one

type B. Type A viruses are divided into subtypes based on differences in two viral proteins

called the hemagglutinin (H) and the neuraminidase (N). The current subtypes of influenza

A are designated A(H1N1) and A(H3N2).

Influenza type A viruses undergo two kinds of changes. One is a series of mutations that

occur over time and cause a gradual evolution of the virus. This is called antigenic “drift.”

The other kind of change is an abrupt change in the hemagglutinin and/or the

neuraminidase proteins. This is called antigenic “shift.” In this case, a new subtype of the

virus suddenly emerges. Type A viruses undergo both kinds of changes; influenza type B

viruses change only by the more gradual process of antigenic drift.

Natural History of Human Influenza

Influenza A and B viruses continually undergo antigenic drift. This process accounts for

most of the changes that occur in the viruses from one influenza season to another.

Antigenic shift occurs only occasionally. When it does occur, large numbers of people,

and sometimes the entire population, have no antibody protection against the virus. This

results in a worldwide epidemic, called a pandemic. During this century, pandemics

occurred in 1918, 1957, and 1968, each of which resulted in large numbers of deaths, as

noted below.

Mortality associated with pandemics:

1918-19 “Spanish flu” A(H1N1) — Caused the highest known influenza-related

mortality: approximately 500,000 deaths occurred in the United States, 20 million

worldwide.

1957-58 “Asian flu” A(H2N2) — 70,000 deaths in the United States.

1968-69 “Hong-Kong flu” A(H3N2) — 34,000 deaths in the United States.

The emergence of the “Hong Kong flu” in 1968-69 marked the beginning of the type

A(H3N2) era. When this virus first emerged, it was associated with lower mortality than

that caused by the two previous pandemic viruses. Several possible reasons for this lower

mortality have been hypothesized. First, only the hemagglutinin changed from the “Asian”

strain [type A(H2N2)]; the neuraminidase (N2) stayed the same, and therefore existing

antibody could be expected to offer some protection. A second possibility is suggested by

evidence that a virus with a similar hemagglutinin may have circulated from the late 1890s

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