Sporozoa: Plasmodium and Babesia

PLASMODIUM SPECIES
As noted, there are five species of Plasmodium
known to be of concern regarding transmission
to humans. General information, including a historical
perspective and generic description of the
six most commonly encountered morphologic
forms, is followed by a discussion of each of
these species in detail.
Historical Perspective
Historical accounts of events leading to the formation
of the United States have cited malaria
as being of significance on several occasions.
Malaria-infected individuals from southwest
England apparently transported Plasmodium
spp. parasites as they migrated to the New World.
Malaria quickly spread throughout the colonies,
resulting in a shortage of healthy workers. The
demand for replacement workers played a significant
role in the emergence of the African slave
trade. Interestingly, many of those from West
Africa who were brought to America to work did
not contract malaria. It has since been determined
that they were genetically protected from
select species of malaria-causing Plasmodium
spp. parasites.
Malaria was considered endemic in many of
the colonies during the Revolutionary War, particularly
in areas with significant water sources
in which mosquito vectors thrived and soldiers
often camped. A group of British soldiers, under
the direction of General Charles Cornwallis, was
no exception. After several exposures in areas
in which malaria was rampant, almost all the
British soldiers contracted malaria and were
unable to continue fighting. Ultimately, General
Cornwallis surrendered, resulting in a successful
end to the war.
By some accounts, malaria was first described
by a French army doctor, Charles Louis Alphonse
Laveran, in 1880. In 1907, he received the Nobel
Prize for Physiology or Medicine for his work on
malaria. Since then, numerous physicians and
scientists have studied the diseases caused by
members of the genera Plasmodium and have
made great strides toward our understanding of
these diseases. Today, malaria lethally affects
almost 2.5 million people worldwide. Although
North America was declared malaria free in
1970, travel and immigration bring it back to the
continent regularly.
Morphology and Life Cycle Notes
Ring Forms (Early Trophozoites). The ring
form, as the name implies, refers to a ringlike
appearance of the malarial parasite following
invasion into a previously healthy RBC. The
typical ring, when stained with Giemsa stain,
consists of a blue cytoplasmic circle connected
with or to, depending on the species, a red chromatin
dot, also referred to in some texts as a
nucleus. The space inside the ring is known as a
vacuole.
Developing Trophozoites. The appearance of
the developing trophozoite varies among the
Plasmodium species. There are numerous growing
stages in this category for each organism.
However, remnants of the cytoplasmic circle and
chromatin dot, which are in some cases both still
intact until late in development, are present in
the developing trophozoite form. Pigment, primarily
brown in color, is often visible. In general,
because the parasite is actively growing during
this stage, the amount of RBC space invaded is
significantly more than that of the ring form. A
representative developing trophozoite will be
CHAPTER 6 Select Sporozoa: Plasmodium and Babesia 133
described in more detail under the discussion of
each malarial parasite.
Immature Schizonts. Although still unorganized,
evidence of active chromatin replication is
seen in the typical immature schizont. Visible
cytoplasmic material surrounds the growing
chromatin. Pigment granules, often brown in
color, are also commonly seen. As the parasite
continues to multiply, it expands and occupies
more space within the RBC.
Mature Schizonts. Mature schizonts are characterized
by the emergence of the fully developed
stage of the asexual sporozoa trophozoite known
as merozoites. The number and arrangement of
these merozoites vary and are described in detail
under the discussion of each malarial species.
With the exception of Plasmodium vivax, cytoplasmic
material is not visible and is presumed
to be absent.
Microgametocytes. With the exception of P.
falciparum, which is crescent-shaped, the typical
microgametocyte is roundish in shape. This morphologic
form consists of a large diffuse chromatin
mass that stains pink to purple and is
surrounded by a colorless to pale halo. Pigment
is usually visible; its distribution and color vary
by species.
Macrogametocytes. Macrogametocytes range
in shape from round to oval, with the exception
of P. falciparum, which is crescent-shaped. The
compact chromatin mass is partially to completely
surrounded by cytoplasmic material.
Pigment is also present, and its color and distribution
in this morphologic form vary by individual
Plasmodium species. Specific details are
described under the discussion of each species.
Life Cycle Notes. Members of the mosquito
genus Anopheles are responsible for the transmission
of malaria to humans via a blood meal.
This vector transfers the infective stage of the
parasite known as sporozoites from its salivary
gland into the human bite wound. Following
entrance into the body, the sporozoites are carried
through the peripheral blood to the parenchymal
cells of the liver. It is here that schizogony
(asexual multiplication) occurs. This exoerythrocytic
cycle, which literally means reproduction
outside of red blood cells (in this case in human
liver cells), of growth and reproduction lasts
from 8 to 25 days, depending on the specific
Plasmodium species involved. The infected liver
cells eventually rupture and introduce merozoites
into the circulating blood. These migrating merozoites
target age- and size-specific RBCs to invade
and thereby initiate the phase of reproduction
involving red blood cells known as the erythrocytic
cycle of growth. These RBC specifics vary
among each species and are described under the
life cycle notes of each species. It is in this asexual
phase that the plasmodia feed on hemoglobin
and pass through the numerous stages of growth,
including their six morphologic forms.
On formation of the merozoites, one of three
paths may be taken. Some of the RBCs infected
with merozoites rupture, releasing these forms to
target and infect new RBCs, and this part of the
cycle repeats itself. A number of erythrocytic
cycles may occur. However, other infected RBCs
containing merozoites develop into microgametocytes
and macrogametocytes, and still others
are destroyed by the immune system of an otherwise
healthy individual. Although never demonstrated
in human infection, it is presumed that
hypnozoites (dormant Plasmodium-infected liver
cells) may form during infection with P. vivax or
P. ovale. These forms, also known as sleeping
forms, may be dormant for months to years after
the initial infection. The mechanism behind the
reactivation of such cells was not well described
in any of the references used to prepare this
chapter. However, once stimulated, the hypnozoites
rupture and introduce merozoites into the
circulating blood, thus initiating the erythrocytic
cycle and a relapse infection, or recrudescence.
Transmission of the parasite back into the
vector occurs when the mosquito ingests mature
male (micro) and female (macro) sex cells called
gametocytes during a blood meal, thus initiating
the sexual cycle of growth. Male and female
gametocytes unite in the mosquito’s stomach and
form a fertilized cell called a zygote (also known
as an ookinete). The zygote becomes encysted
and matures into an oocyst. On complete
maturation, the oocyst ruptures and releases
134 CHAPTER 6 Select Sporozoa: Plasmodium and Babesia
numerous sporozoites, which migrate into the
salivary gland of the mosquito and are ready to
infect another unsuspecting human. Thus, the
cycle repeats itself.
In addition to contracting malaria via an
Anopheles mosquito bite, there are several other
modes of transmission for Plasmodium species.
Transfusion malaria, as the name implies, occurs
when uninfected patients receive blood tainted
with malaria collected from an infected donor.
Malaria may also be spread through the sharing
of needles and syringes, a common practice
among intravenous drug users; this type of infection
is referred to as mainline malaria. Although
rarely documented, congenital malaria, which is
the passing of the parasite from mother to child,
may also occur.
Careful and thorough screening of all smears is
crucial to ensure the correct identification, reporting,
and ultimately the proper treatment of all
Plasmodium organisms present.
The timing of blood collection for the study
of malaria is crucial to success in retrieving the
malarial parasites. The various morphologic
forms of parasites visible at any given time
depend on the stage of organism development at
the time of specimen collection. For example,
when the infected RBCs rupture, merozoites are
present in the circulating blood. This stage, when
found, is difficult to serve as a species identifier.
However, gametocytes may be present at this
point in time and they are readily discernible.
The greatest number of parasites is present in the
blood in between characteristic bouts of fever
and chills resulting from the release of merozoites
and toxic waste products from infected RBCs,
known as paroxysms. Thus, this is the optimal
time to collect peripheral blood samples to determine
the presence of Plasmodium spp. parasites
(Table 6-1).
It is important to note that multiple sets of
blood films, which, as noted, consist of thick and
thin smears, are necessary to rule out malarial
infections. It is recommended that blood be collected
every 6 to 12 hours for up to 48 hours
before considering a patient to be free of Plasmodium
spp. parasites.
In addition to blood films, serologic tests and
polymerase chain reaction (PCR) techniques for
malaria are available. These tests are not that
helpful in regard to the actual treatment of
malarial infections. However, one benefit of
TABLE 6-1 Occurrence of Cyclic
Paroxysms in Common
Plasmodium Species
Plasmodium Species
Timing of Cyclic
Paroxysms
P. vivax Every 48 hr
P. ovale Every 48 hr
P. malariae Every 72 hr
P. falciparum Every 36-48 hr
Quick Quiz! 6-1
The infective stage of Plasmodium is (are) the: (Objective
6-6)
A. Merozoites
B. Oocyst
C. Sporozoites
D. Gametocytes
Laboratory Diagnosis
Giemsa-stained peripheral blood films are the
specimens of choice for the laboratory diagnosis
of malaria. Wright’s stain may also be used and
will result in an accurate diagnosis. However,
because Giemsa is the recommended stain for all
blood films submitted for parasite study, the specific
morphologic discussion of each Plasmodium
species is based on the use of this stain. Both
thick and thin blood films should be made and
examined. Thick blood smears serve as screening
slides, whereas thin blood smears are used in
differentiating the Plasmodium species. All blood
films should be studied under oil immersion. It
is important to note that mixed Plasmodium
infections may occur, with the most frequently
encountered being P. vivax and P. falciparum.
CHAPTER 6 Select Sporozoa: Plasmodium and Babesia 135
serologic testing is that this methodology does
appear to help rule out malaria in patients suffering
from a fever of unknown origin, and PCR
techniques can confirm the malarial speciation,
but is usually not necessary. Representative laboratory
diagnostic methodologies are presented in
Chapter 2, as well as in each individual parasite
discussion, as appropriate.
months to years after the initial infection, as
is often the case with P. vivax and P. ovale
infections.
Additional malarial symptoms may include
headache, lethargy, anorexia, ischemia (insufficient
blood supply in other body tissues caused
by blockage of the capillaries and blood sinuses),
nausea, vomiting, and diarrhea. Anemia, central
nervous system (CNS) involvement, and nephrotic
syndrome may occur in all Plasmodium infections.
It is interesting to note that malaria may
mimic a number of other diseases, including
meningitis, pneumonia, gastroenteritis, encephalitis,
or hepatitis. Specific clinical symptoms are
described under the discussion of each individual
organism.
Furthermore, persons exhibiting erythrocyte
structural abnormalities such as heterozygous
(GdA-/GdB) glucose-6-phosphate dehydrogenase
(G6PD) deficiency and certain hemoglobinopathies
(S, C, E, and thalassemia) tend to have a
greater resistance to malarial infections than
those who do not possess the abnormalities.
Similarly, those individuals who are Duffy blood
group–negative also tend to show a greater resistance
than those who are positive for the antigens
on their red blood cells.
Quick Quiz! 6-2
The best time to collect blood for Plasmodium parasites
is: (Objective 6-10)
A. Between paroxysms
B. During paroxysms
C. Morning
D. Evening
Pathogenesis and Clinical Symptoms
The typical patient remains asymptomatic following
the initial mosquito bite and exoerythrocytic
cycle of malarial infection. However, once
the erythrocytic phase is initiated and large
numbers of rupturing RBCs simultaneously
occur, the resulting merozoites and toxic waste
byproducts in the blood system produce the first
clinical symptom, a paroxysm. Considered in
part as an allergic response of the body to the
development of the schizonts and to the circulating
parasitic antigens following the release of
merozoites, a paroxysm is characterized by chills
(also known as rigor), typically lasting for 10 to
15 minutes or longer, followed by 2 to 6 hours
or more of a fever. As the fever subsides and
returns to normal, the patient experiences profuse
sweating and extreme fatigue. The periodicity of
paroxysms varies and is defined under the discussion
of each Plasmodium species; periodicity
often accounts for one of the common names
associated with each Plasmodium species as well.
Patients may experience these clinical symptoms
as a result of having a recrudescence. A recurrence,
or true relapse, occurs when patients
become reinfected with rupturing hypnozoites
Quick Quiz! 6-3
A paroxysm is: (Objective 6-1)
A. An allergic reaction
B. A periodic episode characterized by fever, chills,
sweats, and fatigue
C. Both A and B are correct.
D. None of the above
Classification
Malaria belongs to the phylum Apicomplexa,
class Aconoidasida, order Haemosporida, family
Plasmodiidae, genus Plasmodium. All five of the
Plasmodium species discussed in this chapter are
found in the blood, as indicated in Figure 6-1.