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Chapter 9 - Phylum Apicomplexa: Malaria
Taxonomy
P. Apicomplexa
C. Coccidia
O. Haemosporida
G. Plasmodium
Introduction
Malaria is one of the most prevalent and debilitating diseases afflicting humans
Worldwide prevalence is at approximately 400-500 million cases, making malaria the
most prevalent human parasitic disease, with an annual death toll of about 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
General Life Cycle
The life cycle of Plasmodium that infects humans is spent in 2 hosts: 1) the human host
and 2) the insect vector, a female mosquito belonging to the genus Anopheles
Only female mosquitoes can serve as vectors; Males feed solely on plant juices, while
females can also feed on blood, which is usually required for oviposition
During feeding, the mosquito secretes sporozoite-bearing saliva beneath the epidermis of
the human victim, thus inoculating the sporozoites into the blood stream
After approximately one hour, the sporozoite disappears from the circulation, reappearing
24-48 hr later in the parenchymal cells in the liver
There is a protein that covers the surface of the sporozoite bears a ligand molecule that
specifically binds to receptors on the cell surface of the hepatocyte cell membrane
And this is why sporozoites enter liver cells and not other cells of the body
Secretions from the rhopteries that allow the sporozoites to enter the cells
The entery of sporozoites into hepatocytes initiates the exoerythrocytic shizogonic cycle
or pre-erthrythroocytoc cycle
Inside the liver cell, the sporozoite develops into a active feeding trophozoite
They eventually undergo merogony producing thousands of merozoites
These rupture from the host cell, enter the blood circulation, and invade RBCs, initiating
the erythrocytic shizogonic cycle
Some sporozoites may become dormant hypnozoites (more later)
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The immune system can eventually recognize and attack malarial parasites in the liver
But the attack takes time – enough time for the parasites to produce merozoites that burst
from the liver cells and seek out RBCs
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
Merozoite penetration requires presence of at least 1 of 2 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
P. ovale and P. falciparum malaria are not influenced by Duffy antigens, thus accounting
for their prevalence in West Africa
Once in the RBC, the merozoite assumes an early trophozoite shape consisting of a ring
of cytoplasm and a dotlike nucleus - signet ring stage
These early trophozoites feed on host hemoglobin, grow to the mature trophozoite
stage, and then undergo merogony, producing a characteristic number of merozoites in
each infected RBC
Each merozoite is capable of infecting a new RBC and one of 2 fates will follow: 1) It
can become another signet ring trophozoite and begin merogony anew 2) Later in the
cycle some merozoites invade cells and become a male microgametocyte or a female
macrogametocyte
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
Male gametogony - the microgametocytes undergo a maturation process known as
exflagellation
Female gametogony - 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 after
12-24 hr, elongates into a motile wormlike ookinete
This 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
Following a period of growth, the oocyst bulges on the homocoel side of the gut
Growth of the oocyst is, in part, due to the proliferation of haploid cells called
sporoblasts, within the oocyst
Sporoblast nuclei undergo numerous divisions (sporogony), producing thousands of
sporozoites enclosed within the sporoblast membranes
As membranes rupture, sporozoites enter the cavity of the oocyst
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Within 10-24 days after the mosquito ingests the gametocytes, the sporozoite-filled
oocysts themselves rupture, releasing the sporozites in the hemocoel
The sporozoites are carried to the salivary gland ducts of the insct and are ready to be
injected into the next victim when another blood meal is taken
Life Cycle Variations
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Epidemiology
Endemicity of human malaria is usually determined by the geographic distribution of its
mosquito vector
Local environmental factors determine which particular species of mosquito transmits
malaria in a given area
Water dependency for breeding varies greatly
Some species of Anopheles favor small bodies of water, while other require large bodies
of water such as ponds and even lakes
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.
Under the auspices of the World Health Organization (WHO), malaria was controlled or
drastically reduced in many parts of the world by the 1960’s
However, there has been a marked resurgence in the disease since the 1970s
Several factors have been responsible for this, most important of which are the
development of widespread resistance on the part of mosquitoes to insecticides
Also, P. falciparum has become resistant to various antimalarial drugs
Relapse and Recrudescence
It has long been known that victims of vivax or ovale malaria may suffer a relapse
It has been suggested that there are 2 different populations of sporozoites
Short prepatent sporozoites (SSPs) - upon entering the human host, undergo the usual
exoerythrocytic and erythrocytic phases of development and cause malaria
Long prepatent sporozoites (LPPs) 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
The ratio of LPPs to SPPs in P. vivax infections in a given human population appears to
vary according to strain
Recurrence of malaria among victims infected by P. malariae is thought to be due to
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
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The situation may persist for as long as 53 years before something (e.g., splenic
dysfunction) triggers a parasite population explosion with accompanying disease
manifestations - recrudesence
Symptomatology and Diagnosis (P. falciparum)
Pathology in human malaria 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 hemozin and parasite metabolic wastes
These ruptures are accompanied by fever paroxysms that are usually synchronous except
during the primary attack (correlated with merozoites rupturing from the 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, since the infection may arise from several populations of liver
merozoites at different stages of development, synchrony may not be evident
How the parasites’ development gradually assumes a synchronous pattern remains
unknown
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, the more mature stages of the
organism, develop electron dense “knobs” by which they adhere to the endothelium of
capillaries in visceral organs
Engorged with hordes of infected RBCs, the capillaries become obstructed, causing the
affected organs to become anoxic
In terminal cases, blocked capillaries in the brain cause it to become swollen and
congested
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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 a20-50% mortality rate, usually due to renal failure
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 (e.g., quinine)
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 destroys the schizogonic stages of malaria, but it has little or no effect on the
exoerythrocytic stages or gametocytes
The synthetic drug Atabrine dihydrochloride 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
The folic acid cycle provides a suitable metabolic pathway for chemotherapeutic
management of malaria
This cycle happens to be vital to the parasite for synthesizing bases in nucleic acid
formation
Synthetic drugs (pyrimethamine used in combination with sulfadoxine) inhibits portions
of the cycle and is therefore lethal to the parasites
Lariam (mefloquine hydrochloride) the popular malaria-prevention drug, prescribed to
thousands of U.S. travelers and military personnel, has been linked to serious psychiatric
side effects, including suicide.
An alarming phenomenon in the treatment of malaria is the increasing resistence of the
parasites to chemotherapy, probably resulting from mutagenic changes
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Immunity
In addition to chemotherapy research, development of a protective vaccine against
malaria is being pursued
Since many of the developmental stages of malarial parasites in the vertebrate host are
intracellular and therefore protected from the host’s immune response, the extracellular
forms become the targets for vaccine
One line of research has attempted to inject a sequence of DNA into muscle tissue of
potential hosts
The DNA makes its way into the muscle cells and starts making the same protein made
by the malaria and displayed by liver cells
In theory, the muscle cells should carry this vaccine protein to their own surface, and
killer T cells that come across it will be able to fight off an actual infection when it comes
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 low-level
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
Several genetic conditions are known to affect the malarial organism
1. Susceptibility conferred by the presence of Duffy antigens
e.g., vivax merozoite penetration of RBCs requires 1 of 2 Duffy antigens
2. Genetic deficiency in G6PDH activity in RBCs (favism) creates and inhospitable
environment for the parasites.
3. Humans heterozygous for sickle cell anemia possess a selective advantage over
individuals with normal hemoglobin in regions where P. falciparum is endemic