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Yellow Fever:
Causes and Symptoms
Yellow fever is a viral disease of humans spread by infected mosquitoes. The infectious agent is
a flavivirus. Yellow fever is primarily a disease of tropical and pantropical areas of South
America and sub-Saharan Africa. The absence of yellow fever from Asia is curious but may be
explained by the lack of suitable reservoirs. Two types of yellow fever are recognized, sylvan
yellow fever and urban yellow fever.
Both types are highly communicable, but neither is directly transmitted via personal contact
between humans. A human contracts the disease through the bite of an infected mosquito that, in
turn, has taken a blood meal from an infected monkey or other human. The viral incubation
period within the mosquito is nine to twelve days. The virus then appears in the saliva and can be
transmitted during the mosquito's next blood meal. Thereafter, the mosquito carries the virus
throughout its life.
Sylvan yellow fever is also known as jungle yellow fever. Forest monkeys are its primary
reservoir, but marmosets and possibly marsupials may also harbor the disease. Sylvan yellow
fever is spread from monkey to monkey by the bite of infected Sabethes or Haemagogus
mosquitoes, which breed in water-filled tree holes.
People who visit or work in rain-forest environs, such as miners, engineers, wildlife biologists,
and foresters, are at risk of contracting the disease. Sylvan yellow fever is rare or nonexistent
outside tropical rain forests, but cases are reported every year in areas where it is endemic.
Urban yellow fever is spread by the Aedes aegypti mosquito, which is the main reservoir of the
disease, along with humans. Aedes commonly lives and breeds in trash dumps and waste places
around human habitations. Urban yellow fever often results in thousands of cases and high
fatality rates.
In humans, the first symptoms of yellow fever typically appear from three to six days following
the bite of an infected mosquito. Symptoms vary widely; the mildest cases may pass unnoticed,
but in more severe cases the patient shows a number of flulike symptoms, such as muscle aches,
backaches, headache, high fever, chills, nausea, and vomiting. Liver damage may occur as early
as the fourth or fifth day of the illness, leading to progressive and extensive jaundice. In the most
severe cases, periods of nausea, vomiting, renal failure, and extensive bleeding (hemorrhage)
may prove fatal. The fatality rate may range from as low as 5 percent in indigenous populations
to 50 percent or more during epidemics. Higher fatality rates are typically seen in nonindigenous
people.
Clinical diagnosis of yellow fever is aided by isolation of the virus in mice, by microscopic
examination of liver tissue for necrotic lesions that characterize this disease, by detection of the
yellow fever viral antigen in blood or liver tissue fluid of the infected patient, or by detection of
the viral genome in liver tissue. Diagnosis can also be determined serologically through the
detection of specific antibodies.
Treatment and Therapy
There is no specific antiviral drug for yellow fever, so treatment goals include providing
symptom relief, such as oxygen support, medications to reduce fever and pain, and the
administration of vitamin K and replacement fluids. Dialysis may be required for patients
suffering from kidney failure. Complete recovery may take several weeks, after which the
individual has a lifelong immunity to yellow fever.
Perspective and Prospects
Yellow fever has long been a dreaded tropical and subtropical disease of humans. Throughout
the centuries of exploration, yellow fever epidemics occurred with alarming regularity, and high
fatality rates were seen among nonindigenous visitors and settlers in regions where the disease is
endemic. The presence of yellow fever on a ship was indicated by flying the fever flag or yellow
jack.
Despite modern medical advances, yellow fever remains a problem in many areas. Small
numbers of cases of sylvan yellow fever occur every year in certain areas of Amazonia and
northern South American countries, including Peru and Bolivia. In central Africa, the yellow
fever belt extends from Ethiopia westward through Senegal, Sudan, and Ghana. Both occasional
cases and epidemics occur irregularly. Between 1986 and 1988, for example, thirty thousand
cases of yellow fever were reported in Nigeria, and ten thousand people died.
Dr. Walter Reed was the first to describe the epidemiology of yellow fever and to recognize the
importance of the mosquito vector in its spread. His research elucidated control and eradication
measures that provide containment during epidemics. It was largely through his insights that
yellow fever was eliminated from urban areas and was one of the main reasons that the Panama
Canal could be completed by the United States after the failure of the French effort led by
Ferdinand de Lesseps, the architect of the Suez Canal.
The traditional and still most effective method of preventing urban yellow fever is the
eradication of the Aedes aegypti mosquito vector. Control measures involve a combination of
insecticide spraying and the cleanup or removal of potential breeding sites. The prevention of
yellow fever can also be achieved through the vaccination of all people within the endemic
regions. A single vaccination with an attenuated strain of virus grown in chick embryos has been
shown to be effective for thirty or more years, but revaccination is recommended every ten years.
The elimination of sylvan yellow fever is probably impossible because of the difficulty of
eradicating jungle populations of Sabethes and Haemagogus mosquitoes. Prevention must
therefore be achieved through defensive measures for people who work or visit jungle areas,
including immunization and the use of protective clothing, netting, and mosquito repellents.
-- Dwight G. Smith, Ph.D.
"Yellow fever." Magill's Medical Guide, 4th Rev. ed.. 2008. eLibrary Science. Web. 08 Dec
2011.