Biomedical Engineering Reference
In-Depth Information
the intracellular form of the organism. The only disease organism in Class
Trypanistomatida that does not have an amastigote phase is
Trypanosoma
brucei
, the cause of African trypanosomiasis. This organism has no intra-
cellular phase and is found as a swimming (flagellate) form in blood and
body fluids.
4.
Organisms have novel methods to control or avoid the host immune
response. For example, Leishmania organisms that pass from insect vector
(female sandfly; see Glossary item, Sandfly) to human host are quickly
phagocytosed by neutrophils. The neutrophils serve as Trojan horses for
the leishmania. Normally, after a neutrophil engulfs foreign organisms, the
neutrophil soon dies, along with the contained organisms. Leishmania
actively stabilizes neutrophils for several days, until macrophages arrive to
eat the neutrophil and the leishmania within. The leishmanial organisms
live as intracellular organisms within macrophages.
The two infectious genera in Class Trypanistomida are Leishmania and
Trypanosoma. Leishmania species cause leishmaniasis, a disease that infects
about 12 million people worldwide. Each year, about 60 000 people die from
the visceral form of the disease. It is a tropical disease that occurs most often
in India, Africa, and Brazil (in the order of decreasing incidence).
Many different Leishmania species infect a wide range of animals, with
about 21 different species infecting humans. In most cases, the species that
are infective in humans have a non-human animal reservoir.
The target organs, and the subsequent clinical syndrome, vary with the
species of Leishmania. All infective species are transmitted by the bite of a
female sandfly. There are many species of sandfly, but all of the species that
transmit leishmaniasis seem to belong to the Phlebotominae family.
The clinical forms of disease, which are dependent on the infective spe-
cies of Leishmania, are: cutaneous (localized skin lesion), diffuse cutaneous,
mucocutaneous, and visceral (also known as kala-azar). The visceral form of
the disease is the most severe form and often results in death, if not treated.
Trypanosoma brucei
is the cause of African trypanosomiasis (sleeping
sickness). Two subspecies cause infections in humans:
Trypanosoma brucei
gambiense
and
Trypanosoma brucei rhodesiense
. The disease occurs almost
exclusively in sub-Saharan Africa. The reported numbers of cases are consid-
ered to be unreliable, but it has been estimated that infection with
Trypanosoma
brucei
accounts for about 50 000 deaths each year. The intermediate host and
vector for the disease is the tsetse fly (Genus Glossina), and the reservoir for the
tsetse fly is infected humans or animals. As
Trypanosoma brucei
does not have
an amastigote stage (capable of intracellular growth), it is found exclusively
in extracellular fluids; principally, lymph, blood, and cerebrospinal fluid.
Symptoms begin with swollen lymph nodes in the back of the neck, headache,
and other clinical features of generalized infection. When the organism crosses
the blood
brain barrier, the neurologic phase begins. Symptoms may include
