WORLD OF MICROBIOLOGY AND IMMUNOLOGY Yellow fever613•
when yeast systemic infection is involved is that they are not
detected by standard blood tests. However, laboratorial analy-
sis of collected samples of mucus and affected tissue may
detect yeast infection and identify the implicated species.
Another yeast infection, known as blastomycosis, is
caused by the species Blastomyces dermatitidis,a spherical
budding yeast. The main targets of this pathogen are the lung
alveoli (60%). Pulmonary blastomycosis is not easily diag-
nosed because its symptoms are also present in other lung
infections, such as cough, chest pain, hemoptysis, and weight
loss. Pulmonary lesions may include nodules, cavities, and
infiltration, with the severe cases presenting pleuritis.
Blastomycosis may also be disseminated to other organs, such
as liver, central nervous system, adrenal glands, pancreas,
bones, lymph nodes, and gastrointestinal and genitourinary
tracts. Osteomyelitis (bone infection) and arthritis may also be
caused by this yeast, and about 33% of the patients were diag-
nosed with skeletal blastomycosis as well. Although the cuta-
neous chronic infection is curable, the systemic form of the
disease has a poor prognosis.See also Food preservation; Food safety; Mycology;
Nosocomial infections; Parasites; Yeast artificial chromosome
(YAC); Yeast geneticsYYellow feverELLOW FEVER
Yellow fever is the name given to a disease that is caused by
the yellow fever virus. The virus is a member of the flavivirus
group. The name of the disease is derived from the appearance
of those infected, who usually present a jaundiced appearance
(yellow-tinted skin).
The agent of infection of yellow fever is the mosquito.
The agent was first identified in 1900 when the United States
Army Yellow Fever Commission (also referred to as the Reed
Commission after its leader, Walter Reed) proved that the
mosquito species Aedes aegyptiwas responsible for spread-
ing the disease. Until then, yellow fever was regarded as
requiring direct person-to-person contact or contact with a
contaminated object.
The disease has caused large outbreaks involving many
people in North America, South America, and Africa, stretch-
ing back at least to the 1700s. At that time the disease was
often fatal. The availability of a vaccinereduced the incidence
and mortality of the disease considerably in the latter part of
the twentieth century. However, since 1980 the number of
cases of the disease has begun to rise again.
There are now about 200,000 estimated cases of yellow
fever in the world each year. Of these, some 30,000 people die.
Most researchers and health officials regard these numbers as
underestimates, due to underreporting and because in the ini-
tial stages yellow fever can be misdiagnosed.
The yellow fever virus infects humans and monkeys—
no other hosts are known. Humans become infected when the
virus is transmitted from monkeys to humans by mosquitoes.
This is referred to as horizontal transmission. Several different
species of mosquito are capable of transmitting the virus.Mosquitoes can also pass the virus to their own offspring via
infected eggs. This form of transmission is called vertical
transmission. When the offspring hatch they are already
infected and can transmit the virus to humans when they have
a blood meal. Vertical transmission can be particularly insidi-
ous as the eggs are very hardy and can resist dry conditions,
hatching when the next rainy season occurs. Thus the infection
can be continued from one year to the next even when there is
no active infection occurring in a region.
The different habitats of the mosquitoes ensures a wide
distribution of the yellow fever virus. Some of the mosquito
species breed in urban areas while others are confined to rural
regions. The latter types were associated with the outbreak of
yellow fever that struck workers during the construction of the
Panama Canal in Central America in the nineteenth century. In
South America a concerted campaign to control mosquito
populations up until the 1970s greatly reduced the number of
cases of yellow fever. However, since that time the control
programs have lapsed and yellow fever has increased as the
mosquito populations have increased.
Infection with the yellow fever virus sometimes pro-
duced no symptoms whatsoever. However, in many people,
so-called acute (rapid-onset, intense) symptoms appear about
three to six days after infection. The symptoms include fever,
muscle pain (particularly in the back), headache, chills, nau-
sea, and vomiting. In this early stage the disease is easily con-
fused with a number of other diseases, including malaria,
typhoid fever, hemorrhagic feverssuch as Lassa fever, and
viral hepatitis. Diagnosis requires the detection of an antibody
to the virus in the blood. Such diagnosis is not always possi-
ble in underdeveloped regions or in rural areas that are distant
from medical facilities and trained laboratory personnel.
In many people the acute symptoms last only a few days
and recovery is complete. However, in about 15% of those
infected, the disease enters what is termed the toxic phase: a
fever reappears and several regions of the body become
infected as the virus disseminates from the point of the mos-
quito bite. Disruption of liver function produces jaundice.
Kidney function can also be damaged and even totally shut
down. Recovery from this more serious phase of the infection
can be complete; although half of those who are afflicted die.
Yellow fever appears in human populations in different
ways. One pattern of appearance is called sylvatic (or jungle)
yellow fever. As the name implies, this form is restricted to
regions that are largely uninhabited by humans. The virus
cycles between the indigenous monkey population and the
mosquitoes that bite them. Humans that enter the region, such
as loggers, can become infected.
Another cycle of infection is referred to as intermediate
yellow fever. This infection is found in semi-urban areas, such
as where villages are separated by intervening areas of farm-
land or more natural areas. Infections can spring up in several
areas simultaneously. Migration of people from the infected
areas to larger population centers can spread the infection.
This is the most common pattern of yellow fever occurring in
present day Africa.
The final pattern of yellow fever is that which occurs in
fully urban settings. The large population base can produce awomi_Y 5/7/03 9:15 AM Page 613