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Presentation of Quiz #35

A patient is a 55 year-old male with multiple medical problems who was seen in an emergency room complaining of weakness, cough, and fever. He had recently returned from a trip to Mexico and several areas in Central America. Although malaria prophylaxis was recommended, he failed to take the medication properly. On examination of both thick and thin blood films prepared from blood collected in EDTA tube (purple top), the following images were seen. Please comment on the identification of the organisms seen.

Both the thick and thin blood films were stained using Giemsa stain.

After the appropriate diagnosis was made, additional blood was drawn with the following images seen in the thick and thin blood films. Please comment on the objects seen below.

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Answer and Discussion of Quiz #35

Answers to Questions: The images presented after the first blood specimen was drawn represent Plasmodium vivax developing trophozoites in the thick blood film (left, at a lower magnification) and a developing trophozoite (ring) in the thin blood film (right). Note the presence of stippling and the enlarged RBC (typical of P. vivax).

After the second blood draw, the image on the left represents a mature schizont in a thick blood film, while the image on the right represents a mature schizont in the thin blood film (P. vivax). The number of merozoites in the mature schizont is always a clue to the species identification.

Although stippling (Schüffner's dots) is present, this morphologic characteristic may not always be found in EDTA-preserved blood, particularly if the blood is stored prior to blood film preparation and should not prevent the correct identification of Plasmodium vivax. Also, remember that stippling does not occur in the young rings (P. vivax), while they are seen in all stages in infections with P. ovale.

Key characteristics of P. vivax include the following:

  1. 48-hour cycle
  2. Tends to infect young cells
  3. Enlarged RBCs, RBC may be normal size until ring fills half of cell
  4. Schüffner's dots after 8-10 hours (providing storage, buffer pH criteria are met)
  5. Delicate ring, but heavy chromatin dot
  6. Very ameboid trophozoite
  7. Mature schizont contains 12-24 merozoites, usually around 16-1

Comments on the Patient: Since this patient had not taken proper prophylaxis during his trip to Mexico and Central America, his illness has been diagnosed as P. vivax malaria. With proper treatment, he recovered. Treatment included drugs for both the blood stages, as well as the liver stages (liver stages = hypnozoites, which could lead to a relapse at a later time without the use of the second drug).

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).  PARASITEMIA TABLE

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.


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  5. Wilcox, A., 1960. Manual for the Microscopical Diagnosis of Malaria in Man. U.S. Department of Health, Education, and Welfare, Washington, D.C.