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 longterm
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.