WORLD OF MICROBIOLOGY AND IMMUNOLOGY Fluorescence in situ hybridization (FISH)221•
of this particular influenza outbreak, people were infected
around the world.
The pandemic killed more people, some 20 to 40 mil-
lion, than had been killed in the just-ending Great War (now
known as World War I). Indeed, the pandemic is still the most
devastating microbiological event in the recorded history of
the world. At the height of the epidemic, fully one-fifth of the
world’s population was infected with the virus.
The disease first arose in the fall of 1918, as World War
I was nearing its end. The genesis of the disease caused by the
strain of influenza virus may have been the deplorable condi-
tions experienced by soldiers in the trenches that were dug at
battlegrounds throughout Europe. The horrible conditions ren-
dered many soldiers weak and immunologically impaired. As
solders returned to their home countries, such as the United
States, the disease began to spread. As the disease spread,
however, even healthy people fell victim to the infection. The
reason why so many apparently healthy people would sud-
denly become ill and even die was unknown at the time.
Indeed, the viral cause of disease had yet to be discovered.
Recent research has demonstrated that the particular
strain of virus was one that even an efficiently functioning
immune systemwas not well equipped to cope with. A muta-
tion produced a surface protein on the virus that was not
immediately recognized by the immune system, and which
contributed to the ability of the virus to cause an infection.
The influenza outbreak has also been called the
“Spanish Flu” or “La Grippe.” The moniker came from the
some 8 million influenza deaths that occurred in Spain in one
month at the height of the outbreak. Ironically, more recent
research has demonstrated that the strain of influenza that rav-
aged Spain was different from that which spread influenza
around the world.
The influenza swept across Europe and elsewhere
around the globe. In the United States, some 675,000
Americans perished from the infection, which was brought to
the continent by returning war veterans. The outbreaks in the
United States began in military camps. Unfortunately, the sig-
nificance of the illness was not recognized by authorities and
few steps were taken to curtail the illnesses, which soon
spread to the general population.
The resulting carnage in the United States reduced the
statistical average life span of an American by 10 years. In the
age range of 15 to 34 years, the death rate in 1918 due to pneu-
moniaand influenza was 20 times higher than the normal rate.
The large number of deaths in many of the young generation
had an economic effect for decades to come. South America,
Asia, and the South Pacific were also devastated by the infec-
tion.
In the United States the influenza outbreak greatly
affected daily life. Gatherings of people, such as at funerals,
parades, or even sales at commercial establishments were
either banned or were of very short duration. The medical sys-
tem was taxed tremendously.
The influenza outbreak of 1918 was characterized by a
high mortality rate. Previous influenza outbreaks had displayed
a mortality rate of far less than 1%. However, the 1918 pan-
demic had a much higher mortality rate of 2.5%. Also, the ill-ness progressed very quickly once the symptoms of infections
appeared. In many cases, an individual went from a healthy
state to serious illness or death with 24 hours.
At the time of the outbreak, the case of the illness was
not known. Speculations as to the source of the illness
included an unknown weapon of war unleashed by the
German army. Only later was the viral origin of the disease
determined. In the 1970s, a study that involved a genetic char-
acterization of viral material recovered from the time of the
pandemic indicated that the strain of the influenza virus likely
arose in China, and represented a substantial genetic alteration
from hitherto known viral types.
In November of 1919, the influenza outbreak began to
disappear as rapidly as it had appeared. With the hindsight of
present day knowledge of viral epidemics, it is clear that the
number of susceptible hosts for the virus became exhausted.
The result was the rapid end to the epidemic.See alsoEpidemics, viral; History of public healthFLUORESCENCEIN SITUHYBRIDIZATION
(FISH)Fluorescence in situhybridization (FISH)
Fluorescent in situhybridization (FISH) is a technique in
which single-stranded nucleic acids (usually DNA, but RNA
may also be used) are permitted to interact so that complexes,
or hybrids, are formed by molecules with sufficiently similar,
complementary sequences. Through nucleic acid hybridiza-
tion, the degree of sequence identity can be determined, and
specific sequences can be detected and located on a given
chromosome. It is a powerful technique for detecting RNA or
DNA sequences in cells, tissues, and tumors. FISH provides a
unique link among the studies of cell biology, cytogenetics,
and molecular genetics.
The method is comprised of three basic steps: fixation
of a specimen on a microscopeslide, hybridization of labeled
probe to homologous fragments of genomic DNA, and enzy-
matic detection of the tagged probe-target hybrids. While
probe sequences were initially detected with isotopic reagents,
nonisotopic hybridization has become increasingly popular,
with fluorescent hybridization now a common choice.
Protocols involving nonisotopic probes are considerably
faster, with greater signal resolution, and provide options to
visualize different targets simultaneously by combining vari-
ous detection methods.
The detection of sequences on the target chromosomes
is performed indirectly, commonly with biotinylated or digox-
igenin-labeled probes detected via a fluorochrome-conjugated
detection reagent, such as an antibodyconjugated with fluo-
rescein. As a result, the direct visualization of the relative posi-
tion of the probes is possible. Increasingly, nucleic acid probes
labeled directly with fluorochromes are used for the detection
of large target sequences. This method takes less time and
results in lower background; however, lower signal intensity is
generated. Higher sensitivity can be obtained by building lay-
ers of detection reagents, resulting in amplification of the sig-womi_F 5/6/03 2:15 PM Page 221