2. DEFINITION
Yellow fever is a zoonotic disease caused by an
arbovirus
It affects principally monkeys and other vertebrates
in tropical America and Africa and is transmitted to
man by certain culicine mosquitoes.
It shares clinical features with other viral
haemorrhagic fevers (e.g., dengue HF, Lassa fever)
but is characterized by more severe hepatic and
renal involvement.
3. EPIDEMIOLOGICAL DETERMINANTS
Agent factors
(a) AGENT : The causative agent, FlaviiJirus
fibricus formerly classified as a group B arbovirus, is
a member of the togavirus family..
(b) RESERVOIR OF INFECTION :
In forest areas, the reservoir of infection is mainly
monkeys and forest mosquitoes.
In urban areas, the reservoir is man (subclinical and
clinical cases) besides Aedes aegypti mosquitoes.
4. EPIDEMIOLOGICAL DETERMINANTS
(c) PERIOD OF COMMUNICABILITY :
(i) MAN : Blood of patients is infective during the
first 3 to 4 days of illness.
(ii) MOSQUITOES: After an "extrinsic incubation
period" of 8 to 12 days, the mosquito becomes
infective. The virus multiplies in the insect vector.
After becoming infective, the mosquito remains
so for life.
5. HOST FACTORS
(a) AGE AND SEX : All ages and both sexes are
susceptible to yellow fever in the absence of
immunity.
(b) OCCUPATION : Persons whose occupation
brings them in contact with forests (wood cutters,
hunters) where yellow fever is endemic are
exposed to the risk of infection.
(c) IMMUNITY : One attack of yellow fever gives
lifelong immunity; second attacks are unknown.
Infants born of immune mothers have antibodies up
to 6 months of life.
6. ENVIROMENTAL FACTORS
(a) CLIMATE : A temperature of 24 deg.C or over is
required for the multiplication of the virus in the
mosquito.
It should be accompanied by a relative humidity of
over 60 per cent for the mosquitoes to live long
enough to convey the disease.
(b) SOCIAL FACTORS : In Africa, urbanization is
leading to the extension of yellow fever. In addition,
the expanding population is encroaching on areas
that were previously sparsely populated, thereby
bringing man closer to· the jungle cycles of yellow
fever..
7. MODES OF TRANSMISSION
There are three known cycles of transmission, the jungle,
intermediate and the urban cycles .
1. 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)
8. CONT..
2. 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. This is the most
common type of outbreak in Africa.
9. CONT..
3. 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
10. TREATMENT
There is no specific treatment for yellow fever, only
supportive care to treat dehydration and fever.
Associated bacterial infections can be treated with
antibiotics. Supportive care may improve outcomes for
seriously ill patients.
INCUBATION PERIOD:
3 to 6 days (6 days recognized under International Health
Regulations).
11. CONTROL OF YELLOW FEVER
1. Jungle yellow fever :
Jungle yellow fever
continues to be an
uncontrollable disease.
The virus maintains itself
in the animal kingdom.
Mosquito control is difficult
and can be considered only
in restricted areas.
Vaccination of humans with
17D vaccine is the only
control measure
12. CONTROL OF YELLOW FEVER
2. Urban yellow fever :-
1.VACCINATION:Rapid immunization of the population at
risk is the most effective control strategy for yellow fever.
For international use, the approved vaccine is the 17D
vaccine.
It is a live attenuated vaccine prepared from a non-
virulent strain (17D strain).
It has to be stored between +5 and -30 deg.C, preferably
below zero deg.C until reconstituted with the sterile, cold
physiological saline diluent provided
13. CONTROL OF YELLOW FEVER
Reconstituted vaccine should be kept on ice, away from
sunlight, and discarded if not used within half an hour.
The vaccine is administered subcutaneously at the
insertion of deltoid in a single dose of 0.5 ml irrespective
of age.
Immunity begins to appear on the 7th day and lasts for
more than 35 years, and possibly for life .
However, WHO recommends revaccination after 10
years for international travel.
14. CONT...
The risk of death from yellow fever is much higher than
the risks related to the vaccine. People who should not be
vaccinated include
(a) children aged under 9 months for routine
immunization (or under 6 months during an epidemic)
(b) pregnant women - except during a yellow fever
outbreak when the risk of infection is high.
(c) people with severe allergies to egg protein.
(d) people with severe immunodeficiency caused
by symptomatic HIV/AIDS or other causes, or in the
presence of thymus disorder.
15. CONTROL OF YELLOW FEVER
2. VECTOR CONTROL :- The other principal method of
preventing yellow fever is through intensive vector control.
vigorous anti-adult and anti-larval measures.
The long-term policy should be based on organized
"source reduction" methods (e.g., elimination of breeding
places) supported by health education aimed at securing
community participation.
Personal protection against contact with insects is of major
importance in integrated vector control.
Such protection may include the use of repellents,
mosquito nets, mosquito coils and fumigation mats
16. CONTROL OF YELLOW FEVER
3.SURVEILLANCE :-
For the surveillance of Aedes mosquitoes, the WHO
uses an index known as Aedes aegypti index. This
is a house index and is defined as "the percentage
of houses and their premises, in a limited well-
defined area, showing actual breeding of Aedes
aegypti larvae".
This index should not be more than 1 per cent in
towns and seaports in endemic areas to ensure
freedom from yellow fever.
17. INTERNATIONAL MEASURES
The virus of yellow fever could get imported into
India in two ways:
1 :Through infected travellers (clinical and subclinical
cases), and
2 :Through infected mosquitoes. Measures designed
to restrict the spread of yellow fever are specified in
the "International Health Regulations" of WHO .
18. INTERNATIONAL MEASURES
Broadly these comprise :
(i) TRAVELLERS :
All travellers (including infants) exposed to the risk of
yellow fever or passing through endemic zones of yellow
fever must possess a valid international certificate of
vaccination against yellow fever before they are allowed to
enter yellow fever "receptive" areas.
If no such certificate is available, the traveller is placed on
quarantine, in a mosquito-proof ward, for 6 days from the
date of leaving an infected area.
If the traveller arrives before the certificate becomes
"valid", he is isolated till the certificate becomes valid.
19. CONT..
(ii) MOSQUITOES :
The aircraft and ships arriving from endemic areas are
subjected to aerosol spraying with prescribed insecticides
on arrival for destruction of insect vectors.
Further, airports and seaports are kept free from the
breeding of insect vectors over an area extending at least
400 metres around their perimeters.
20. INTERNATIONAL CERTIFICATE OF VACCINATION
India and most other countries require a valid certificate
of vaccination against yellow fever from travellers
coming from infected areas.
The validity of the certificate begins 10 days after the
date of vaccination and extends up to 10 years.
Revaccination performed before the end of the validity of
the certificate renders the certificate valid for a further
period of 10 years starting on the day of revaccination.
