The images presented
after the first blood specimen was drawn represent Plasmodium malariae
developing schizonts in the thick blood film (left) and a mature schizont
(containing merozoites) in the thin blood film (right). Note the small
size of the infected RBC (typical of P. malariae). The number
of merozoites in the mature schizont is always a clue to the species
identification; in this case there appear to be about 8 merozoites.
After the second
blood draw, the images represent developing trophozoites, which are
spread across the infected RBC as "band" forms (again P.
malariae). This configuration is not always seen, but is very suggestive
of P. malariae.
Key
characteristics of P. malariae include the following:
1. 72-hour cycle
2. Tends to infect old cells
3. Small to normal sized RBCs
4. No Schüffner's dots present; small dots (Ziemann's dots) may
be present, but they are not considered true stippling.
5. Thick ring, and heavy chromatin dot
6. "Band" form trophozoites are often present.
7. Mature schizont contains about 8 merozoites, often arranged around
the remaining clump of malarial pigment. This configuration of the merozoites
is often called the "daisy" form.
COMMENTS
ON THE PATIENT:
Since
this patient had received several blood transfusions, it is logical
to assume one of the transfusions was the source of the infection. Individuals
can harbor P. malariae in the blood for many years at very low
levels of parasitemia; these individuals are asymptomatic. Often, even
with a complete blood donor history, the possibility of malaria infection
with this organism is not suspected. Individuals have been known to
harbor this parasite up to 42 years. In spite of severe splenomegaly
and splenectomy, after therapy with chloroquine, the patient was considered
cured and remained asymptomatic.
COMMENTS
ON
DIAGNOSTIC METHODS:
It is
very important to realize that a single set of thick and thin blood films
can be negative, although the patient may be positive. In this case, both
thick and thin blood films were positive. A second draw was taken to examine
the thick and thin blood films for additional stages and/or evidence of
a mixed infection. Venipunctures were performed for both blood draws,
with the recommended EDTA anticoagulant in the lavender (purple) top tube.
It is important that the slides be prepared as quickly as possible after
the blood draw, in order to prevent organism distortion and possible loss
that can occur if the blood is allowed to stand for a period of hours
prior to slide preparation. Remember, every request for malaria blood
films should always be considered a STAT request and the laboratory coverage
should be 24 hours/day, 7 days/week.
Examination
of the thin blood film is relatively simple when the parasitemia is high,
as in this slide. However, a returning traveler with his or her first
malaria infection may experience the typical clinical symptoms of high
fever, chills, myalgia, and headache with a much lower parasitemia. Also,
these patients may present to the emergency room with vague symptoms that
do not represent the typical textbook description; they may have malaise,
a steady low-grade fever, and may even have diarrhea. These low levels
of parasitemia are often impossible to detect using thin blood film examination
only. For this reason, the key to successful detection of malaria parasites
in the peripheral blood is the examination of both thick and thin blood
films from every patient suspected of having malaria (or any patient from
whom blood is submitted to the laboratory for blood film examination).
Thick
films allow a larger amount of blood to be examined, which increases the
possibility of detecting light infections. Species identification from
the thick film examination, particularly in the case of malaria, may be
difficult for those with little experience examining thick blood films.
The morphological characteristics of blood parasites are best seen in
thin films, particularly the relationship between the size of the infected
RBC and those that are uninfected. However, in cases with a low parasitemia,
the identification to the species level may have to be accomplished by
thick film examination.
The
accurate examination of thick and thin blood films and identification
of parasites depends on the use of absolutely clean, grease-free slides
for preparation of all blood films. Old (unscratched) slides should be
cleaned first in detergent and then with 70% ethyl alcohol; new slides
should also be cleaned with alcohol before use.
Blood
films are usually prepared when the patient is admitted; in instances
in which malaria is a possible diagnosis, after the first set of negative
smears, samples should be taken at intervals of 6 to 8 h for at least
3 successive days, particularly if P. falciparum has not been excluded
as a diagnosis.
REFERENCES
Garcia,
L. S. 2001. Diagnostic Medical Parasitology, 4th Ed., ASM Press,
Washington, D.C.
Isenberg,
H. D. (ed.). 2004. Clinical Microbiology Procedures
Handbook, 2nd Ed, vol. 1, 2 and 3. ASM Press, Washington, D.C.
Isenberg, H. D. (ed.).
1995. Essential Procedures for Clinical Microbiology, ASM Press, Washington,
D.C.
National
Committee for Clinical Laboratory Standards. 2000. Laboratory Diagnosis
of Blood-borne Parasitic Diseases. Approved Guideline, M15?A. National
Committee for Clinical Laboratory Standards, Villanova, Pa.
Wilcox,
A., 1960. Manual for the Microscopical Diagnosis of Malaria in Man.
U.S. Department of Health, Education, and Welfare, Washington, D.C.
