Yellow
fever
Definition
Yellow fever is a severe infectious disease, caused by a virus called
a "flavivirus." This flavivirus can cause outbreaks of epidemic proportions
throughout Africa and tropical
America. The first
written evidence of such an epidemic occurred in the Yucatan in 1648. Since that
time, much has been learned about the interesting transmission patterns of this
devastating illness.
Description
In order to
understand how yellow fever is passed, several terms need to be defined. The
word "host" refers to an animal that can be infected with a particular disease.
The term "vector" refers to an organism which can carry a particular
disease-causing agent (such as a virus or bacteria) without actually developing
the disease. The vector can then pass the virus or bacteria on to a new host.
Many of the common illnesses in the
United States
(including the common cold, many viral causes of diarrhea, and influenza or
"flu") are spread via direct passage of the causative virus between human
beings. Yellow fever, however, cannot be passed directly from one infected human
being to another. Instead, the virus responsible for yellow fever requires an
intermediate vector, a mosquito, which carries the virus from one host to
another.
The hosts of
yellow fever include both humans and monkeys. The cycle of yellow fever
transmission occurs as follows: an infected monkey is bitten by a tree-hole
breeding mosquito. This mosquito acquires the virus, and can pass the virus on
to any number of other monkeys that it may bite. When a human is bitten by such
a mosquito, the human may acquire the virus. In the case of South American
yellow fever, the infected human may return to the city, where an urban mosquito
(Aedes aegypti) serves as a viral vector, spreading the infection rapidly
by biting humans.
Symptoms
Once a mosquito
has passed the yellow fever virus to a human, the chance of disease developing
is about 5-20%. Infection may be fought off by the host's immune system, or may
be so mild that it is never identified.
In human hosts
who develop the disease yellow fever, there are five distinct stages through
which the infection evolves. These have been termed the periods of incubation,
invasion, remission, intoxication, and convalescence.
Yellow fever's
incubation period (the amount of time between the introduction of the virus into
the host and the development of symptoms) is three to six days. During this
time, there are generally no symptoms identifiable to the host.
The period of invasion lasts two to five days, and begins with an
abrupt onset of symptoms, including fever and chills, intense headache and lower
backache, muscle aches, nausea, and extreme exhaustion. The patient's tongue
shows a characteristic white, furry coating in the center, surrounded by a
swollen, reddened margin. While most other infections that cause a high fever
also cause an increased heart rate, yellow fever results in an unusual finding,
called Faget's sign. This is the simultaneous occurrence of a high fever with a
slowed heart rate. Throughout the period of invasion, there are still live
viruses circulating in the patient's blood stream. Therefore, a mosquito can
bite the ill patient, acquire the virus, and continue passing it on to others.
The next phase
is called the period of remission. The fever falls, and symptoms decrease in
severity for several hours to several days. In some patients, this signals the
end of the disease; in other patients, this proves only to be the calm before
the storm.
The period of intoxication represents the most severe and potentially
fatal phase of the illness. During this time, lasting three to nine days, a type
of degeneration of the internal organs (specifically the kidneys, liver, and
heart) occurs. This fatty degeneration results in what is considered the classic
triad of yellow fever symptoms: jaundice, black vomit, and the dumping of
protein into the urine. Jaundice causes the whites of the patient's eyes and the
patient's skin to take on a distinctive yellow color. This is due to liver
damage, and the accumulation of a substance called bilirubin, which is normally
processed by a healthy liver. The liver damage also results in a tendency toward
bleeding; the patient's vomit appears black due to the presence of blood.
Protein, which is normally kept out of the urine by healthy, intact kidneys,
appears in the urine due to disruption of the kidney's healthy functioning.
Patients who
survive the period of intoxication enter into a relatively short period of
convalescence. They recover with no long term effects related to the yellow
fever infection. Further, infection with the yellow fever virus results in
lifelong immunity against repeated infection with the virus.
Diagnosis
Diagnosis of
yellow fever depends on the examination of blood by various techniques in order
to demonstrate either yellow fever viral antigens (the part of the virus that
stimulates the patient's immune system to respond) or specific antibodies
(specific cells produced by the patient's immune system which are directed
against the yellow fever virus). The diagnosis can be strongly suspected when
Faget's sign is present. When the classic triad of symptoms is noted yellow
fever is strongly suspected.
Treatment
There are no current anti-viral treatments available to combat the
yellow fever virus. The only treatment of yellow fever involves attempts to
relieve its symptoms. Fevers and pain should be relieved with acetaminophen, not
aspirin or ibuprofen, both of which could increase the already-present risk of
bleeding. Dehydration (due to fluid loss both from fever and bleeding) needs to
be carefully avoided. This can be accomplished by increasing fluids. The risk of
bleeding into the stomach can be decreased through the administration of
antacids and other medications. Hemorrhage may require blood transfusions.
Kidney failure may require dialysis (a process that allows the work of the
kidneys in clearing the blood of potentially toxic substances to be taken over
by a machine, outside of the body).
Prognosis
Five to ten percent of all diagnosed cases of yellow fever are fatal.
Jaundice occurring during a yellow fever infection is an extremely grave
predictor. Twenty to fifty percent of these patients die of the infection. Death
may occur due to massive bleeding (hemorrhage), often following a lapse into a
comatose state.
Prevention
A very safe,
very effective yellow fever vaccine exists. About 95% of vaccine recipients
acquire long-term immunity to the yellow fever virus. Careful measures to
decrease mosquito populations in both urban areas and jungle areas in which
humans are working, along with programs to vaccinate all people living in such
areas, are necessary to avoid massive yellow fever outbreaks.
Key Terms
Epidemic
A situation in which a particular
disease spreads rapidly through a population of people in a relatively short
period of time.
Faget's sign
The simultaneous occurrence of a
high fever with a slowed heart rate.
Host
The organism (such as a monkey or
human) in which another organism (such as a virus or bacteria) is living.
Vector
A carrier organism (such as a fly
or mosquito) which serves to deliver a virus (or other agent of infection) to a
host.
