Transcript Chapter 9 - Phylum Apicomplexa: Malaria

Chapter 9 - Phylum
Apicomplexa: Malaria
Taxonomy
P. Apicomplexa
C. Coccidia
O. Haemosporida
G. Plasmodium
Overview
• Malaria is one of the most prevalent and debilitating diseases
afflicting humans
• Worldwide prevalence is at approximately 489 million cases,
making malaria the most prevalent human parasitic disease, with an
annual death toll of 2 million
• There are more than 50 species of Plasmodium, but only 4
commonly cause malaria in humans - P. vivax, P. falciparum, P.
malariae, and P. ovale
Life Cycle Overview
• The life cycle of Plasmodium that
infect humans includes 2 hosts:
•1) the human host and 2) the insect
vector, a female mosquito belonging to
the genus Anopheles
Anopheles sp.
•Like other apicomlexa, a significant feature of the life cycle is the
alternation of sexual and asexual phases in the 2 hosts
• The asexual cycles, termed merogony, occur in the human
• The sexual cycle, termed gamogony occurs mainly in the mosquito
• Subsequent to the sexual stage, another asexual phase of reproduction
occurs in the mosquito, termed sporogony
• The infective form in humans is
the slender, elongated sporozoite
Plasmodium sp. sporozoites
Life Cycle (Detail)
• During feeding, the mosquito secretes sporozoite-bearing saliva beneath
the epidermis of the human victim, thus inoculating the sporozoites into
the blood stream
• About 24-48 hr
later, sporozoites
appear in the
parenchymal cells in
the liver, initiating
the exoerythrocytic
shizogonic cycle or
pre-erthrythrocytic
cycle
Exoerythrocytic Shizogonic Cycle
• Inside the liver cell, the sporozoite develops into a trophozoite,
feeding on host cytoplasm with its functional micropore
• After 1-2 weeks, the nucleus of the trophozoite undergoes
multiple fission, producing thousands of merozoites
• These rupture
from the host cell,
enter the blood
circulation, and
invade RBCs,
initiating the
erythrocytic
shizogonic cycle
• Some sporozoites
become dormant
hypnozoites
Note:
• Studies of P. vivax show that the membrane receptor site
for the engulfment phenomenon is determined by the type
of antigen present on the surface of the RBC - e.g.,
merozoite penetration requires the presence of at least one
of two Duffy antigens (Fya+ or Fy b+ )
• People that lack the Duffy antigens (almost all West
Africans and about 70% of American blacks) are resistent
to vivax malaria
• However, P. ovale and P. falciparum malarias are not
influenced by Duffy antigens, thus accounting for their
prevalence in West Africa
Erythrocytic Shizogonic Cycle
• Electron microscopy has confirmed that merozoites interact with the RBC
plasma membrane and actively invade the cell
• During this process, rhoptries and micronemes are believed to secrete surface
active molecules that cause the host RBC membrane to expand and invaginate
to form a parasitophorous vacuole which envelops the parasite
Merozoite entering erythrocyte
Erythrocyctic trophozoite
Erythrocytic Shizogonic Cycle cont.
• Once in the RBC, the merozoite assumes an early trophozoite shape
consisting of a ring of cytoplasm and a dot-like nucleus - the signet ring
stage
• These early
trophozoites feed on
host hemoglobin,
grow to the mature
trophozoite stage,
and then undergo
multiple fission as
schizonts, producing
a characterisitc
number of merozoites
in each infected RBC
Erythrocytic Shizogonic Cycle cont.
• Merozoites eventually rupture
RBCs and each merozoite is
capable of infecting a new RBC
Scanning electron micrograph of
Plasmodium-infected red blood cells
Erythrocytic Shizogonic Cycle cont.
One of 2 fates await
these merozoites:
1. Become signet ring
trophozoites and
begin shizogony
anew
2. Differentiate into
sexual stages,
becoming male
microgametocytes or
female
macrogametocytes
Life Cycle cont.
• The sexual phase occurs in the female Anopheles mosquito and
begins when the mosquito takes a blood meal that contains
microgametocytes and macrogametocytes
• Once the
surrounding RBC
material is lysed, the
gametocytes are
released into the
lumen of the
stomach
• The
microgametocytes
undergo a
maturation process
known as
exflagellation
Exflagellation
• The nucleus undergoes 3 mitotic
divisions, producing 6-8 nuclei that
migrate to the periphery of the
gametocyte
• Accompanying the nuclear divisions
are centriolar divisions, following
which one portion joins each nuclear
segment to become a basal body,
providing the center from which the
axoneme subsequently arises
Life Cycle cont.
• The nucleus with the axoneme and a small amount of cytoplasm
form a microgamete, which detaches from the mass and swims to the
macrogametocyte
• During this period the
macrogametocytes
have developed into
macrogametes which
become penetrated by
the microgamete
• The fusion of male
and female pronuclei
(syngamy) produces a
diploid zygote that
elongates into a motile
wormlike ookinete
Life Cycle cont.
•The ookinete penetrates the gut wall of the mosquito to the
area between the epithelium and the basal lamina, where it
develops into a rounded oocyst
• Growth of the
oocyst is, in part,
due to the
proliferation of
haploid cells called
sporoblasts, within
the oocyst
Life Cycle cont.
• Sporoblast nuclei undergo numerous divisions, producing
thousands of sporozoites enclosed within the sporoblast membranes
• As membranes rupture, sporozoites enter the cavity of the oocyst
• The sporozoitefilled oocysts
themselves rupture,
releasing the
sporozites in the
hemocoel
• The sporozoites are
carried to the
salivary gland ducts
of the insect and are
ready to be injected
into the next victim
when another blood
meal is taken
Sporozoites isolated from the
salivary glands of a mosquito
Longitudinal section of mosquito
intestine showing numerous
oocysts
Plasmodium vivax (benign tertian malaria)
• Less than 1% of the
total RBC population is
parasitized
• Predilection for
immature RBCs
(reticulocytes)
• Schuffner’s dots
usually stains pink to
red when subjected to
stains
• Hemozoin granules,
by-products of
hemoglobin
degradation by the
parasite, are prominent
•The cytoplasm of the
trophozoite stages is
very irregular and
displays an active
ameboid movement
P. ovale (mild tertian malaria)
•Less than 1% of the
total RBC population
is parasitized
• Predilection for
immature RBCs
(reticulocytes)
• Schuffner’s dots
usually stains pink to
red when subjected to
stains
• Hemozoin granules,
by-products of
hemoglobin
degradation by the
parasite, are
prominent
•The cytoplasm of the
trophozoite stages is
very irregular and
displays an active
ameboid movement
Plasmodium malariae (quartan malaria)
• Parasitizes about
0.2% of older RBCs
• Trophozoites
accumulate pink
staining Ziemann’s
dots
• Hemozoin granules
appear in the center
or periphery of the
shizont
• Trophozoite often
appear as a band
across the cell
• Mature trophozoites
resemble
macrogametocytes
• Recrudescensces as
long as 52 years after
initial infection
Plasmodium falciparum (Malignant tertian malaria)
• Only ring trophozoites and
gametocytes seen in
peripheral circulation; later
stages trapped in capillaries of
muscle and visceral organs
• Plasma membranes of
infected RBCs undergo
alteration causing them to
adhere to the walls of
capillaries
• Infects RBCs of any age;
about 10% of the total RBCs
• Multiple infections of single
RBCs are common
• Gametocytes are crescent
shaped cells
• Hemozoin as well as
Maurer’s dots (precipitates
in the cytoplasm of RBCs
infected to P. falciparum),
tend to aggregate around the
nuclear region of gametocytes
Epidemiology
• Endemicity of human malaria is usually determined by the geographic
distribution of its anophelene mosquito; areas where the vector is not present are
free of the disease
• Local environmental factors determine which particular species of mosquito
transmits malaria in a given area; local epidemiological surveys can be used to
assay the prevalent vectors
• Precipitin tests of ingested blood from infected mosquitoes reveal whether the
vectors have zoophilic or anthrophilic feeding preferences
• Water dependency for breeding varies greatly
• The control of malaria depends on a variety of factors, such as availability of
antimalarial drugs, use of screens on houses to keep out mosquitoes, proper use of
insecticides, elimination of mosquito breeding sites, etc.
Relapse of Infection
• Victims of vivax or ovale malaria may suffer a relapse
• Originally, the relapse was thought only to be due to populations of cryptozoites
(pre-erythrocyte shizont) being maintained in the exoerythrocytic cycle
• While one population progressed to the usual erythrocytic phase, underwent
shizogony and released merozoites into the circulating blood stream causing
malaria, the other population maintained an ongoing exoerythocytic cycle known
as a para-erythrocytic cycle
• Parasites in the hepatic stages of the cycle were thought to be protected from the
host antibodies until activated by some physiological change within the host that
allowed them to erupt from the hepatocytes, precipitating another bout of malaria
• A more recent view also recognizes the existence of 2 different populations of
sporozoites
• Short prepatent sporozoites - upon entering the human host, undergo the usual
exoerythrocytic and erythrocytic phases of development and cause malaria
• Long prepatent sporozoites or hypnozoites - remain dormant in the
hepatocytes for an indefinite period
• Some kind of physiological fluctuation activates them into exoerythrocytic and
erythrocytic cycles and a relapse occurs
Recrudesence
• Recurrence of malaria among victims infected by P. malariae
many years after apparent cure fostered the idea that this species
produced relapses like those produced by P. vivax and P. ovale
• But, it has been shown that the periodic increase in numbers of
parasites results from a residual population persisting at very low
levels in the blood after inadequate or incomplete treatment of the
initial infection
• The situation may persist for as long as 53 years before
something triggers a parasite population explosion with
accompanying disease manifestations •This phenomenon is referred to as recrudesence
Symptomatology and Diagnosis
• Pathology in human malaria (P. falciparum) is generally manifested in 2 basic
forms: host inflammatory reactions and anemia
• Host inflammatory reactions are initiated by the periodic rupture of infected
RBCs, which release malarial pigment such as hemozoin and parasite metabolic
wastes
• These ruptures are accompanied by fever paroxysms that are usually
synchronous except during the primary attack (correlated with the merozoites
rupturing from RBCs)
• During cell rupturing, toxins are released which in turn cause macrophage cells
to release tumor necrosis factor (TNF); it’s TNF that actually induces the fever
• During the primary attack synchrony may not be evident, since the infection
may arise from several populations of liver merozoites at different stages of
development
Symptomatology and Diagnosis cont.
• Macrophages, particularly those in the liver, bone marrow, and spleen, phagocytose
released pigment
• In extreme cases the amount of pigment is so great that it imparts a dark green,
reddish brown hue to the visceral organs such as the liver, spleen and brain
• With increased RBC destruction, accompanied by the body’s inability to recycle
iron bound in the insoluable hemozoin, anemia develops
• TNF toxicity may also induce splenic removal of unparasitized RBCs and inhibit
bone marrow production of new RBCs
• One pathological element unique to P. falciparum is vascular obstruction
• Plasma membranes of RBCs infected with schizonts develop electron dense
“knobs” by which they adhere to the endothelium of capillaries in visceral organs
• As a consequence, the capillaries
become obstructed, causing the
affected organs to become anoxic
• In terminal cases, blocked capillaries
in the brain (=cerebral malaria) cause
it to become swollen and congested
Infected RBC
showing
surface knobs
Black Water Fever
• A condition known as black water fever often accompanies
falciprum malaria infections
• It is characterized by massive lysis of RBCs and it produces
abnormally high levels of hemoglobin in urine and blood
• Fever, vomiting with blood, and jaundice also occur
• There is between 20-50% mortality rate, usually due to renal
failure; probably due to renal anoxia
• The exact cause of this condition is uncertain
• It may be a reaction to quinine, or it may result from an
autoimmune phenomenon in which hemolytic antibodies are
produced in response to chemotherapy
Chemotherapy
• Malaria control requires effective treatment of the disease in humans and
continuous efforts to control mosquito populations
• The first known antimalarial drug was quinine
• The drug primarily destroys the schizogonic stages of malaria
• The synthetic drug Atabrine dihydrochloride (circa 1936-36) proved useful
against erythrocytic stages and in suppressing clinical stages
• Since WWII several synthetic drugs have been used: chloroquine, amodiaquin,
and primaquine
• Chloroquine is a weak base and it increases the pH of the food vacuole which in
turn prevents the digestion of hemoglobin
•Pyrimethamine used in combination with sulfadoxine have been effective in
inhibiting the folic acid cycle of malarial parasites
Immunity
• In addition to chemotherapy research, development of a protective vaccine
against malaria is being pursued
• Interestingly, the surface coat of the sporozoite acts as a renewable “decoy” to the
vertebrate host’s immune system, stimulating the production of antibodies
• When the sporozoite is attacked and its “decoy” coat sloughs off, a replacement
coat is synthesized and its “decoy” effect continues
• This system provides ideal protection for the sporozoite which only spends a brief
amount of time in the blood before it penetrates a liver cell as is protected from
circulating antibodies
• In endemic areas, premunition is the basis for protective immunity as long as lowlevel infection persists; however, with complete cure, the victim regains
susceptibility
• Also, while nursing infants in endemic areas are protected through antibodies in
their mother’s milk, they are at risk at the time of weaning
• Also, P. falciparum can cross the placenta and cause infection on the fetus
Genetics and Malaria Infections
Several genetic conditions are known to affect the malarial
organism:
• Susceptibility is conferred by the presence of Duffy antigens
e.g., vivax merozoite penetration of RBCs requires 1 of 2 Duffy
antigens
•Genetic deficiency in G6PDH activity in RBCs (favism) creates
and inhospitable environment for the parasites
• Humans heterozygous for sickle cell anemia possess a selective
advantage over individuals with normal hemoglobin in regions
where P. falciparum is endemic