Definition of Yellow Fever
According to WHO,
yellow fever is an acute viral disease usually associated with fever, chills, loss
of appetite, nausea, muscle pains particularly in the back, and headaches
transmitted by infected mosquitoes. The “yellow” in the name refers
to the jaundice that affects some patients.
yellow fever is an acute viral disease usually associated with fever, chills, loss
of appetite, nausea, muscle pains particularly in the back, and headaches
transmitted by infected mosquitoes. The “yellow” in the name refers
to the jaundice that affects some patients.
Signs and symptoms
Once contracted, the virus incubates in the body
for 3 to 6 days, followed by infection that can occur in one or two phases. The
first, “acute”, phase usually causes fever, muscle pain with
prominent backache, headache, shivers, loss of appetite, and nausea or
vomiting. Most patients improve and their symptoms disappear after 3 to 4 days.
for 3 to 6 days, followed by infection that can occur in one or two phases. The
first, “acute”, phase usually causes fever, muscle pain with
prominent backache, headache, shivers, loss of appetite, and nausea or
vomiting. Most patients improve and their symptoms disappear after 3 to 4 days.
However, 15% of patients enter a second, more toxic
phase within 24 hours of the initial remission. High fever returns and several
body systems are affected. The patient rapidly develops jaundice and complains
of abdominal pain with vomiting. Bleeding can occur from the mouth, nose, eyes
or stomach. Once this happens, blood appears in the vomit and faeces. Kidney
function deteriorates. Half of the patients who enter the toxic phase die
within 10 to 14 days, the rest recover without significant organ damage.
phase within 24 hours of the initial remission. High fever returns and several
body systems are affected. The patient rapidly develops jaundice and complains
of abdominal pain with vomiting. Bleeding can occur from the mouth, nose, eyes
or stomach. Once this happens, blood appears in the vomit and faeces. Kidney
function deteriorates. Half of the patients who enter the toxic phase die
within 10 to 14 days, the rest recover without significant organ damage.
Populations at Risk
Forty-four endemic countries in Africa and Latin America, with a
combined population of over 900 million, are at risk. In Africa, an estimated
508 million people live in 31 countries at risk. The remaining population at
risk is in 13 countries in Latin America, with Bolivia, Brazil, Colombia, Ecuador
and Peru at greatest risk.
combined population of over 900 million, are at risk. In Africa, an estimated
508 million people live in 31 countries at risk. The remaining population at
risk is in 13 countries in Latin America, with Bolivia, Brazil, Colombia, Ecuador
and Peru at greatest risk.
According to WHO estimates from the early 1990s, 200 000 cases of yellow
fever, with 30 000 deaths, are expected globally each year, with 90% occurring
in Africa. A recent analysis of African data sources due to be published later
this year, estimates similar figures, but a slightly lower burden of 84,000 –
170,000 severe cases and
29 000 – 60 000 deaths due to yellow fever in Africa for the year 2013. Without
vaccination, the burden figures would be much higher.
fever, with 30 000 deaths, are expected globally each year, with 90% occurring
in Africa. A recent analysis of African data sources due to be published later
this year, estimates similar figures, but a slightly lower burden of 84,000 –
170,000 severe cases and
29 000 – 60 000 deaths due to yellow fever in Africa for the year 2013. Without
vaccination, the burden figures would be much higher.
Small numbers of imported cases occur in countries free of yellow fever.
Although the disease has never been reported in Asia, the region is at risk
because the conditions required for transmission are present there. In the past
centuries, outbreaks of yellow fever were reported in North America (Charleston,
New Orleans, New York, Philadelphia, etc) and Europe (England, France, Ireland,
Italy, Portugal and Spain).
Although the disease has never been reported in Asia, the region is at risk
because the conditions required for transmission are present there. In the past
centuries, outbreaks of yellow fever were reported in North America (Charleston,
New Orleans, New York, Philadelphia, etc) and Europe (England, France, Ireland,
Italy, Portugal and Spain).
Transmission
The yellow fever virus is an arbovirus of the flavivirus
genus, and the mosquito is the primary vector. It carries the virus from one
host to another, primarily between monkeys, from monkeys to humans, and from
person to person.
genus, and the mosquito is the primary vector. It carries the virus from one
host to another, primarily between monkeys, from monkeys to humans, and from
person to person.
Several different species of the Aedes and Haemogogus
mosquitoes transmit the virus. The mosquitoes either breed around houses
(domestic), in the jungle (wild) or in both habitats (semi-domestic). There are
three types of transmission cycles.
mosquitoes transmit the virus. The mosquitoes either breed around houses
(domestic), in the jungle (wild) or in both habitats (semi-domestic). There are
three types of transmission cycles.
- Sylvatic (or jungle) yellow fever: In
tropical rainforests, yellow fever occurs in monkeys that are infected by
wild mosquitoes. The infected monkeys then pass the virus to other
mosquitoes that feed on them. The infected mosquitoes bite humans entering
the forest, resulting in occasional cases of yellow fever. The majority of
infections occur in young men working in the forest (e.g. for logging). - Intermediate yellow fever: In
humid or semi-humid parts of Africa, small-scale epidemics occur.
Semi-domestic mosquitoes (that breed in the wild and around households)
infect both monkeys and humans. Increased contact between people and
infected mosquitoes leads to transmission. Many separate villages in an
area can suffer cases simultaneously. This is the most common type of
outbreak in Africa. An outbreak can become a more severe epidemic if the
infection is carried into an area populated with both domestic mosquitoes
and unvaccinated people. - Urban yellow fever:
Large epidemics occur when infected people introduce the virus into
densely populated areas with a high number of non-immune people and Aedes
mosquitoes. Infected mosquitoes transmit the virus from person to person.
Treatment
There is no specific treatment for yellow fever, only supportive care to
treat dehydration, respiratory failure and fever. Associated bacterial
infections can be treated with antibiotics. Supportive care may improve
outcomes for seriously ill patients, but it is rarely available in poorer
areas.
treat dehydration, respiratory failure and fever. Associated bacterial
infections can be treated with antibiotics. Supportive care may improve
outcomes for seriously ill patients, but it is rarely available in poorer
areas.
Prevention
1.
Vaccination
Vaccination is the single most important measure for preventing yellow
fever. In high risk areas where vaccination coverage is low, prompt recognition
and control of outbreaks through immunization is critical to prevent epidemics.
To prevent outbreaks throughout affected regions, vaccination coverage must
reach at least 60% to 80% of a population at risk. Few endemic countries that
recently benefited from a preventive mass vaccination campaign in Africa
currently have this level of coverage.
fever. In high risk areas where vaccination coverage is low, prompt recognition
and control of outbreaks through immunization is critical to prevent epidemics.
To prevent outbreaks throughout affected regions, vaccination coverage must
reach at least 60% to 80% of a population at risk. Few endemic countries that
recently benefited from a preventive mass vaccination campaign in Africa
currently have this level of coverage.
Preventive vaccination can be offered through routine infant
immunization and one-time mass campaigns to increase vaccination coverage in
countries at risk, as well as for travelers to yellow fever endemic area. WHO
strongly recommends routine yellow fever vaccination for children in areas at
risk for the disease.
immunization and one-time mass campaigns to increase vaccination coverage in
countries at risk, as well as for travelers to yellow fever endemic area. WHO
strongly recommends routine yellow fever vaccination for children in areas at
risk for the disease.
The yellow fever vaccine is safe and affordable, providing effective
immunity against yellow fever within 10 days for 80–100% of people and 99%
immunity within 30 days. A single dose of yellow fever vaccine is sufficient to
confer sustained immunity and life-long protection against yellow fever disease
and a booster dose of yellow fever vaccine is not needed. Serious side effects
are extremely rare. Serious adverse events have been reported rarely following
immunization in a few endemic areas and among vaccinated travelers (e.g. in
Australia, Brazil, Peru, Togo and the United States of America). Scientists are
investigating the causes.
immunity against yellow fever within 10 days for 80–100% of people and 99%
immunity within 30 days. A single dose of yellow fever vaccine is sufficient to
confer sustained immunity and life-long protection against yellow fever disease
and a booster dose of yellow fever vaccine is not needed. Serious side effects
are extremely rare. Serious adverse events have been reported rarely following
immunization in a few endemic areas and among vaccinated travelers (e.g. in
Australia, Brazil, Peru, Togo and the United States of America). Scientists are
investigating the causes.
In regard to the use of yellow fever vaccine in people over 60 years of
age, it is noted that while the risk of yellow fever vaccine-associated
viscerotropic disease in persons ≥60 years of age is higher than in younger
ages, the overall risk remains low. Vaccination should be administrated after
careful risk-benefit assessment, comparing the risk of acquiring yellow fever
disease versus the risk of a potential serious adverse event following
immunization for persons ≥60 years of age who have not been previously
vaccinated and for whom the vaccine is recommended.
age, it is noted that while the risk of yellow fever vaccine-associated
viscerotropic disease in persons ≥60 years of age is higher than in younger
ages, the overall risk remains low. Vaccination should be administrated after
careful risk-benefit assessment, comparing the risk of acquiring yellow fever
disease versus the risk of a potential serious adverse event following
immunization for persons ≥60 years of age who have not been previously
vaccinated and for whom the vaccine is recommended.
The risk of death from yellow fever disease is far
greater than the risks related to the vaccine. People who should not be recommended
to be vaccinated include:
greater than the risks related to the vaccine. People who should not be recommended
to be vaccinated include:
- children aged less than 9 months (or between
6–9 months during an epidemic, where the risk of disease is higher than an
adverse event of the vaccine); - pregnant women – except during a yellow
fever outbreak when the risk of infection is high; - people with severe allergies to egg protein;
and - people with severe immunodeficiency due to
symptomatic HIV/AIDS or other causes, or in the presence of a thymus
disorder.
Travelers, particularly those arriving to Asia from Africa or Latin
America must have a certificate of yellow fever vaccination. If there are
medical grounds for not getting vaccinated, International Health Regulations
state that this must be certified by the appropriate authorities.
America must have a certificate of yellow fever vaccination. If there are
medical grounds for not getting vaccinated, International Health Regulations
state that this must be certified by the appropriate authorities.
2.
Mosquito Control
In some situations, mosquito control is vital until vaccination takes
effect. The risk of yellow fever transmission in urban areas can be reduced by
eliminating potential mosquito breeding sites and applying insecticides to
water where they develop in their earliest stages. Application of spray
insecticides to kill adult mosquitoes during urban epidemics, combined with
emergency vaccination campaigns, can reduce or halt yellow fever transmission,
“buying time” for vaccinated populations to build immunity.
effect. The risk of yellow fever transmission in urban areas can be reduced by
eliminating potential mosquito breeding sites and applying insecticides to
water where they develop in their earliest stages. Application of spray
insecticides to kill adult mosquitoes during urban epidemics, combined with
emergency vaccination campaigns, can reduce or halt yellow fever transmission,
“buying time” for vaccinated populations to build immunity.
Historically, mosquito control campaigns successfully eliminated Aedes
aegypti, the urban yellow fever vector, from most mainland countries of
Central and South America. However, this mosquito species has re-colonized
urban areas in the region and poses a renewed risk of urban yellow fever.
aegypti, the urban yellow fever vector, from most mainland countries of
Central and South America. However, this mosquito species has re-colonized
urban areas in the region and poses a renewed risk of urban yellow fever.
Mosquito control programmes targeting wild mosquitoes in forested areas
are not practical for preventing jungle (or sylvatic) yellow fever
transmission.
are not practical for preventing jungle (or sylvatic) yellow fever
transmission.
3.
Epidemic Preparedness and Response
Prompt detection of yellow fever and rapid response through emergency
vaccination campaigns are essential for controlling outbreaks. However,
underreporting is a concern – the true number of cases is estimated to be 10 to
250 times what is now being reported.
vaccination campaigns are essential for controlling outbreaks. However,
underreporting is a concern – the true number of cases is estimated to be 10 to
250 times what is now being reported.
WHO recommends that every at-risk country have at least one national
laboratory where basic yellow fever blood tests can be performed. One
laboratory confirmed case of yellow fever in an unvaccinated population could
be considered an outbreak, and a confirmed case in any context must be fully
investigated, particularly in any area where most of the population has been
vaccinated. Investigation teams must assess and respond to the outbreak with
both emergency measures and longer-term immunization plans.
laboratory where basic yellow fever blood tests can be performed. One
laboratory confirmed case of yellow fever in an unvaccinated population could
be considered an outbreak, and a confirmed case in any context must be fully
investigated, particularly in any area where most of the population has been
vaccinated. Investigation teams must assess and respond to the outbreak with
both emergency measures and longer-term immunization plans.
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