The Australian Immunisation Handbook 10th Edition 2013

4.23 YELLOW FEVER
4.23.1 Virology
Yellow fever is a viral haemorrhagic fever caused by an RNA flavivirus that
is transmitted by mosquitoes. Aedes aegypti, a highly domesticated mosquito
found throughout the tropics, is the vector responsible for person-to-person
transmission of the yellow fever virus in urban and inhabited rural areas in
endemic countries.
4.23.2 Clinical features
The clinical spectrum of yellow fever varies from a non-specific febrile illness to
fatal haemorrhagic fever. 1 After an incubation period of 3 to 6 days, the disease
begins abruptly with fever, prostration, myalgia and headache. The patient
appears acutely ill with congestion of the conjunctivae; there is an intense
viraemia during this ‘period of infection’, which lasts 3 to 4 days. 1 This may be
followed by the ‘period of remission’, in which the fever and symptoms settle
over 24 to 48 hours, during which the virus is cleared by immune responses. 1
Approximately 15 to 25% of patients may then relapse with a high fever,
vomiting, epigastric pain, jaundice, renal failure and haemorrhage: ‘the period
of intoxication’. 1 These complications can be severe, and reflect the viscerotropic
nature of the yellow fever virus (its ability to infect the liver, heart and
kidneys). The case-fatality rate varies widely, but can be more than 20% in local
populations. 2
4.23.3 Epidemiology
Yellow fever occurs in tropical regions of Africa and Central and South America.
In both regions the virus is enzootic in rainforest monkeys and canopy mosquito
species; sporadic human cases occur when people venture into these forests
(‘sylvatic or jungle yellow fever’). 1
In moist savannah regions in Africa, especially those adjacent to rainforests, tree
hole-breeding Aedes mosquito species are able to transfer yellow fever virus from
monkeys to people and then between people, leading to small-scale outbreaks
(‘intermediate yellow fever’).
Ae. aegypti occurs in both heavily urbanised areas and settled rural areas in
tropical Africa and the Americas. 1 Epidemics of ‘urban yellow fever’ occur when
a viraemic individual (with yellow fever) infects local populations of Ae. aegypti;
such epidemics can be large and very difficult to control. Although Ae. aegypti
also occurs throughout much of tropical Asia and Oceania (including north
Queensland), yellow fever has never been reported from these regions.
Although yellow fever is undoubtedly markedly under-reported, it is clear that
there has been a considerable increase in the reported number of outbreaks, and
therefore cases, of yellow fever in past decades. 3 Most of this increase was in
PART 4 VACCINE-PREVENTABLE DISEASES 439
4.23 YELLOW FEVER