yellow fever

[Guest guest]

Hi All,

My husband and I were watching an old time movie. In the movie people were

dropping like flies to yellow fever. So I went and did a Google search on

Yellow Fever.

I am always amazed at the WHO's ignorant thinking. I just thought I'd send

it your way.

First they say that even though they have a very effective vaccine, the

number of infected people has increased recently.

Second, they say that of all the people that are infected, only 15% have the

really bad symptoms. Then to top that off, only half of that 15% die.

But of course, you should be vaccinated. They admitted that there have been

some adverse reactions, including death from the vaccine. And yet they

strongly recommend that everyone be vaccinated from the all consuming

disease.

Is Yellow Fever vaccine used for children in the United States? I don't

think so, but since I haven't vaccinated my children, I didn't know if that

was included or not. And if it isn't, I'm sure that will be next.

Fact Sheet No. 100

Revised December 2001

YELLOW FEVER

Yellow fever is a viral disease that has caused large epidemics in Africa

and the Americas. It can be recognized from historic texts stretching back

400 years. Infection causes a wide spectrum of disease, from mild symptoms

to severe illness and death. The " yellow " in the name is explained by the

jaundice that affects some patients. Although an effective vaccine has been

available for 60 years, the number of people infected over the last two

decades has increased and yellow fever is now a serious public health issue

again.

Cause: The disease is caused by the yellow fever virus, which belongs to the

flavivirus group. In Africa there are two distinct genetic types (called

topotypes) associated with East and West Africa. South America has two

different types, but since 1974 only one has been identified as the cause of

disease outbreaks.

Symptoms: The virus remains silent in the body during an incubation period

of three to six days. There are then two disease phases. While some

infections have no symptoms whatsoever, the first, " acute " , phase is

normally characterized by fever, muscle pain (with prominent backache),

headache, shivers, loss of appetite, nausea and/or vomiting. Often, the high

fever is paradoxically associated with a slow pulse. After three to four

days most patients improve and their symptoms disappear.

However, 15% enter a " toxic phase " within 24 hours. Fever reappears 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 and/or stomach. Once this happens, blood appears in the vomit

and faeces. Kidney function deteriorates; this can range from abnormal

protein levels in the urine (albuminuria) to complete kidney failure with no

urine production (anuria). Half of the patients in the " toxic phase " die

within 10-14 days. The remainder recover without significant organ damage.

Yellow fever is difficult to recognize, especially during the early stages.

It can easily be confused with malaria, typhoid, rickettsial diseases,

haemorrhagic viral fevers (e.g. Lassa), arboviral infections (e.g. dengue),

leptospirosis, viral hepatitis and poisoning (e.g. carbon tetrachloride). A

laboratory analysis is required to confirm a suspect case. Blood tests

(serology assays) can detect yellow fever antibodies that are produced in

response to the infection. Several other techniques are used to identify the

virus itself in blood specimens or liver tissue collected after death. These

tests require highly trained laboratory staff using specialized equipment

and materials.

Regions affected: The virus is constantly present with low levels of

infection (i.e. endemic) in some tropical areas of Africa and the Americas.

This viral presence can amplify into regular epidemics. Until the start of

this century, yellow fever outbreaks also occurred in Europe, the Caribbean

islands and Central and North America. Even though the virus is not felt to

be present in these areas now, they must still be considered at risk for

yellow fever epidemics.

Thirty-three countries, with a combined population of 508 million, are at

risk in Africa. These lie within a band from 15°N to 10°S of the equator. In

the Americas, yellow fever is endemic in nine South American countries and

in several Caribbean islands. Bolivia, Brazil, Colombia, Ecuador and Peru

are considered at greatest risk.

There are 200,000 estimated cases of yellow fever (with 30,000 deaths) per

year. However, due to underreporting, only a small percentage of these cases

are identified. Small numbers of imported cases also occur in countries free

of yellow fever. Although yellow fever has never been reported from Asia,

this region is at risk because the appropriate primates and mosquitoes are

present.

Transmission: Humans and monkeys are the principal animals to be infected.

The virus is carried from one animal to another (horizontal transmission) by

a biting mosquito (the vector). The mosquito can also pass the virus via

infected eggs to its offspring (vertical transmission). The eggs produced

are resistant to drying and lie dormant through dry conditions, hatching

when the rainy season begins. Therefore, the mosquito is the true reservoir

of the virus, ensuring transmission from one year to the next.

Several different species of the Aedes and Haemogogus (S. America only)

mosquitoes transmit the yellow fever virus. These mosquitoes are either

domestic (i.e. they breed around houses), wild (they breed in the jungle) or

semi-domestic types (they display a mixture of habits). Any region populated

with these mosquitoes can potentially harbour the disease. Control

programmes successfully eradicated mosquito habitats in the past, especially

in South America. However, these programmes have lapsed over the last 30

years and mosquito populations have increased. This favours epidemics of

yellow fever.

Infection of humans: There are three types of transmission cycle for yellow

fever: sylvatic, intermediate and urban. All three cycles exist in Africa,

but in South America, only sylvatic and urban yellow fever occur.

Sylvatic (or jungle) yellow fever: In tropical rainforests, yellow fever

occurs in monkeys that are infected by wild mosquitoes. The infected monkeys

can then pass the virus onto other mosquitoes that feed on them. These

infected wild mosquitoes bite humans entering the forest resulting in

sporadic cases of yellow fever. The majority of cases are young men working

in the forest (logging, etc). On occasion, the virus spreads beyond the

affected individual.

Intermediate yellow fever: In humid or semi-humid savannahs of Africa,

small-scale epidemics occur. These behave differently from urban epidemics;

many separate villages in an area suffer cases simultaneously, but fewer

people die from infection. Semi-domestic mosquitoes infect both monkey and

human hosts. This area is often called the " zone of emergence " , where

increased contact between man and infected mosquito leads to disease. This

is the most common type of outbreak seen in recent decades in Africa. It can

shift to a more severe urban-type epidemic if the infection is carried into

a suitable environment (with the presence of domestic mosquitoes and

unvaccinated humans).

Urban yellow fever: Large epidemics can occur when migrants introduce the

virus into areas with high human population density. Domestic mosquitoes (of

one species, Aedes aegypti) carry the virus from person to person; no

monkeys are involved in transmission. These outbreaks tend to spread

outwards from one source to cover a wide area.

Treatment: There is no specific treatment for yellow fever. Dehydration and

fever can be corrected with oral rehydration salts and paracetamol. Any

superimposed bacterial infection should be treated with an appropriate

antibiotic. Intensive supportive care may improve the outcome for seriously

ill patients, but is rarely available in poorer, developing countries.

Prevention: Vaccination is the single most important measure for preventing

yellow fever. In populations where vaccination coverage is low, vigilant

surveillance is critical for prompt recognition and rapid control of

outbreaks. Mosquito control measures can be used to prevent virus

transmission until vaccination has taken effect.

Vaccination: Yellow fever vaccine is safe and highly effective. The

protective effect (immunity) occurs within one week in 95% of people

vaccinated. A single dose of vaccine provides protection for 10 years and

probably for life. Over 300 million doses have been given and serious side

effects are extremely rare. However, recently a few serious adverse outcomes

including deaths, have been reported in Brazil, Australia and the United

States. Scientists are investigating the cause of these adverse events, and

monitoring to ensure detection of any similar incidents.

The risk to life from yellow fever is far greater than the risk from the

vaccine, so those who may be exposed to yellow fever should be protected by

immunization. If there is no risk of exposure, for example, if a person will

not be visiting an endemic area, there is no necessity to receive the

vaccine. Since most of the other known side effects have occurred in

children less than six months old, vaccine is not administered to this age

group. The vaccine should only be given to pregnant women during vaccination

campaigns in the midst of an epidemic.

Vaccination can be part of a routine preventive immunization programme or

can be done in mass " catch-up " campaigns to increase vaccination coverage in

areas where it is low. The World Health Organization (WHO) strongly

recommends routine childhood vaccination. The vaccine can be administered at

age nine months, at the same time as the measles vaccine. Eighteen African

nations have agreed to incorporate yellow fever vaccine into their routine

national vaccination programmes. This is more cost effective and prevents

more cases (and deaths) than when emergency vaccination campaigns are

performed to control an epidemic.

Past experience shows the success of this strategy. Between 1939 and 1952

yellow fever cases almost vanished from French West Africa after intensive

vaccination campaigns. Similarly, Gambia instituted mass routine vaccination

after its 1979/1980 epidemic and later incorporated yellow fever vaccine

into its childhood immunization programme. Gambia reported 85% vaccine

coverage in 2000. No cases have been reported since 1980, yet the virus

remains present in the environment.

To prevent an epidemic in a country, at least 80% of the population must

have immunity to yellow fever. This can only be achieved through the

effective incorporation of yellow fever into childhood immunization

programmes and the implementation of mass catch-up campaigns. The latter is

the only way to ensure that coverage of all susceptible age groups is

achieved and will prevent outbreaks from spreading. Very few countries in

Africa have achieved this level to date.

Vaccination is highly recommended for travellers to high-risk areas. A

vaccination certificate is required for entry to many countries,

particularly for travellers arriving in Asia from Africa or South America.

Fatal cases in unvaccinated tourists have been reported.

Surveillance: Because vaccination coverage in many areas is not optimal,

prompt detection of yellow fever cases and rapid response (emergency

vaccination campaigns) are essential for controlling disease outbreaks.

Improvement in yellow fever surveillance is needed as evidenced by the gross

underreporting of cases (estimates as to the true number of cases vary

widely and have put the underreporting factor between three- and 250-fold).

A surveillance system must be sensitive enough to detect and appropriately

investigate suspect cases. This is facilitated by a standardized definition

of possible yellow fever cases, that is " acute fever followed by jaundice

within two weeks of onset of symptoms, or with bleeding symptoms or with

death within three weeks of onset " . Suspect cases are reported to health

authorities on a standardized case investigation form.

Ready access to laboratory testing is essential for confirming cases of

yellow fever, as many other diseases have similar symptoms. WHO has recently

recommended that every at-risk country have at least one national laboratory

where basic yellow fever blood tests can be performed. Training programmes

are being conducted and test materials are provided by WHO.

Given the likelihood that other cases have occurred (but have not been

detected), one confirmed case of yellow fever is considered to be an

outbreak. An investigation team should subsequently explore and define the

outbreak. This produces data for analysis, which guides the epidemic control

committee in preparing the appropriate outbreak response (e.g. emergency

vaccination programmes, mosquito control activities). This committee should

also plan for the long term by implementing or strengthening routine

childhood yellow fever vaccination.

Mosquito control: In general, eliminating potential mosquito breeding sites

is an important and effective means for controlling mosquito-transmitted

diseases. For prevention and control of yellow fever, priority is placed on

vaccination programmes. For example, mosquito control programmes against

wild mosquitoes in forested areas are not practical or cost-effective for

preventing sylvatic infections. Spraying to kill adult mosquitoes during

epidemics may have value by interrupting virus transmission. This " buys time

for immunity to develop after an emergency vaccination campaign.

In summary: Over the last 20 years the number of yellow fever epidemics has

risen and more countries are reporting cases. Mosquito numbers and habitats

are increasing. In both Africa and the Americas, there is a large

susceptible, unvaccinated population. Changes in the world's environment,

such as deforestation and urbanization, have increased contact with the

mosquito/virus. Widespread international travel could play a role in

spreading the disease. The priorities are vaccination of exposed populations

improved surveillance and epidemic preparedness.

In March 1998, WHO held a technical consensus meeting in Geneva to identify

obstacles to yellow fever prevention and control. Priorities identified

included: prevention through routine immunization and preventive mass

immunization campaigns; detection, reporting and investigation of suspect

cases; laboratory support; outbreak response; vaccine supply; and furthering

research. Guidelines for investigation and control of yellow fever outbreaks

and a background document reviewing topics of importance discussed at this

meeting have been published, and are available on the WHO web site at:

http://www.who.int/emc-documents/yellow_fever/whoepigen9809c.html

For further information, journalists can contact Office of the Spokesperson,

WHO, Geneva. Telephone (+41 22) 791 2599; Fax (+41 22) 791 4858; Email:

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