April 30, 2007
Yellow Fever: Ongoing
Emergence in Africa and South America
Yellow fever is a flavivirus that can
cause a wide spectrum of clinical manifestations ranging from mild symptoms to
fever and jaundice (hence its name) to severe illness, bleeding, hepatitis, and
death. It is a vector-borne disease that is transmitted to humans
primarily via domestic mosquitoes, most notably the same species that can also
transmit dengue virus, namely Aedes
aegypti.
There are three types of yellow fever
transmission cycles: sylvatic
(occurring in the jungle), intermediate, and urban (see the table and diagram
below). All three cycles exist in Africa, but in South America, only
sylvatic and urban yellow fever occur.
|
Type of Spread |
Pattern of Spread |
Occurrence location |
Who is affected |
Outcome |
|
|
Sylvatic (Jungle) |
Infected monkeysà
mosquitoesà humans |
Tropical rainforests |
Young men working in the forest |
Sporadic cases |
|
|
Intermediate |
Semi-domestic mosquito infect monkeys
and humans |
Humid or semi-humid savannahs of
Africa |
Habitants of rural villages |
Small-scale epidemics due to
increased contact between humans and infected mosquitoes |
|
|
Urban |
Domestic mosquitoes (Aedes aegypti)
carry virus from person to person |
Areas with high human population
density in both Africa and South America |
Both travelers and habitants of these
areas |
Large explosive epidemics spreading
outward from the initial source |
The disease is endemic to regions of
Africa and the Americas, and it is once again making resurgence in
parts of these continents. Although there is an effective vaccine
available, there has been an increase in the number of infected people over the
last two decades. Thus, yellow fever has reemerged as a serious public
health concern. Yellow fever causes epidemics that can affect up to 20%
of the population. When epidemics occur in unvaccinated populations,
case-fatality rates may exceed 50%. No antiviral treatment for yellow fever
virus. As cited on WHO and ProMed websites, from September of 2006 until
the present, there have been either documented or suspected cases in the
following countries: Togo, Bolivia,
Peru, and Angola.
WHO recommends
vaccination as the most effective preventative strategy. In regions of current
outbreaks, mass vaccination campaigns have been underway as an attempt to
control the situation.
The vaccine is part of the global emergency yellow fever vaccine stockpile.
In the past, such mass vaccination campaigns have been an effective method
of yellow fever control. This combined with efforts to control the
disease vector—through spraying as well as behavioral modifications—had led to
significant decreases in occurrence of the disease. However, such
campaigns must be met with sustained effort in order to ensure continued
success.
Alicia Chou,
Pete Harlan, Jordan Kanter, and Stephanie Hrycaj, Georgetown
University, M.S. Graduate Program in Biohazardous Threat Agents and Emerging
Infectious Diseases “Emerging Infectious Diseases: The Past as Prologue
(MICB-524)”. Course Instructor: Daniel R. Lucey, MD, MPH. EROne Institute,
Department of Emergency Medicine, Washington Hospital Center, Washington, DC.
Website: www.BePast.org
References:
Yellow Fever: http://www.who.int/csr/disease/yellowfev/en/
http://www.who.int/csr/disease/yellowfev/global_partnership/en/index.html#control