v21n11

DEPARTMENT OF PATHOLOGY
The Johns Hopkins Medical Institutions

Vol. 21, No. 11
THE JOHNS HOPKINS MICROBIOLOGY NEWSLETTER
Tuesday, March 26, 2002

Provided by Karen Fujii, Division of Outbreak Investigation, Maryland Department of Health and Mental Hygiene.

            8 outbreaks were reported to DHMH during MMWR Week 12 (March 17 - March 23):
            5 GE-type illness outbreaks
            3 GASTROENTERITIS outbreaks:
            * 2 at nursing homes (1 in Baltimore Co. and 1 in Montgomery Co.)
            * 1 at a school in Frederick Co.
            1 FOODBORNE GASTROENTERITIS outbreak associated with a restaurant in Anne Arundel Co.
            1 SHIGELLA outbreak at a daycare in Anne Arundel Co.
            1 respiratory illness outbreak
            1 PNEUMONIA (3 culture-positive Mycoplasma pneumoniae ) outbreak at a hospital in Washington Co.
            2 rash illness outbreaks
            1 SCABIES (2 culture-positive Sarcoptes scabiei ) outbreak at a nursing home in Anne Arundel Co.
            1 RASH ILLNESS outbreak at a school in Cecil Co.

B. The Johns Hopkins Hospital, Department of Pathology, Information provided by Jeffrey L. Seibel, MD, PhD, Department of Pathology

Patient Clinical Information: A 6-1/2-year-old male presented with a history of episodic fevers over the past three weeks. Temperatures ranged from 102 to 105 degrees F and were associated with vomiting for the first two weeks, and then fatigue, weakness, poor appetite, pallor and headache. There was no illness among the patient’s close contacts. He had immigrated to the U.S. from Nigeria 3 weeks ago. Immunization history was unavailable. The patient was afebrile and tachycardic. Examination of the head and neck, abdomen and extremities was unremarkable except for generalized pallor. The white blood cell count was 4,700 per cu mm with 39% neutrophils and 1% bands. Hematocrit was 19.7 and reticulocytes were 9%. The mean corpuscular hemoglobin was at the low end of the normal range. Quantitative glucose-6-phosphate dehydrogenase activity was normal. Platelets were 129,000 per cu mm. The metabolic panel was unremarkable. Urinalysis was negative. A peripheral blood smear showed Plasmodium falciparum organisms parasitizing 7% of red blood cells.

Plasmodium falciparum

Organism: The plasmodia are intracellular protozoa that have alternating sexual and asexual reproductive phases. The sexual phase occurs in the gut of the mosquito, while the asexual phase occurs in vertebrates. The four Plasmodium species that are known to cause disease in humans are P. falciparum, P. vivax, P. ovale and P. malariae. When the female Anopheles mosquito takes a blood meal from a plasmodium-infected vertebrate, it ingests gametocytes that are circulating in the peripheral blood. The gametocytes mature, fertilize and produce zygotes in the gut of the mosquito. Each zygote contains thousands of sporozoites. The zygote penetrates the stomach wall, ruptures and releases sporozoites into the body cavity. The mosquito is infectious when sporozoites are present in the salivary glands. This sexual cycle (sporogony) takes from 1-3 weeks. Schizogony, the asexual cycle, begins when the mosquito takes another blood meal and injects sporozoites into the peripheral blood of the new host. Sporozoites rapidly invade hepatocytes and produce thousands of merozoites. 1-2 weeks later, the hepatocytes rupture and release merozoites into the circulation. The merozoites enter red cells and can be seen initially as ring-shaped trophozoites that undergo multiple divisions to produce the multisegmented schizont form. Each segment represents a merozoite. Two to three days after the red blood cells are infected, they rupture and release the daughter merozoites into the circulation. This corresponds to the timing of clinical manifestations. Some merozoites infect new red cells, while some transform into male or female gametocytes.

Clinical manifestations: Plasmodium falciparum malaria affects more than 100 million people worldwide and kills more than 1 million people each year. Fever is the most common presenting symptom. Additional prodromal symptoms of headache, myalgia, malaise and fatigue can occur. Fever begins approximately 7-10 days after an infected mosquito bite. Rupture of erythrocytes with seeding of the blood with the merozoites causes temperature spikes whose periodicity depends on the species. P. falciparum, P. vivax, and P. ovale spikes occur every other day, while those for P. malariae occur every third day. Fever is preceded by peripheral vasoconstriction causing one hour of cyanotic lips and nailbeds with cold skin. The febrile episodes can be accompanied by headaches, chest and back pain, tachycardia and delirium. Splenic sequestration with splenomegaly can occur, therefore deep palpation can cause splenic rupture. P. falciparum is the species most associated with rapidly progressive disease and death. Its trophozoites can cause red cell membrane stickiness, which may cause small areas of cerebral infarction or hemorrhage. Sporozoites of P. vivax and P. ovale, but not P. falciparum, can remain dormant in hepatocytes and can be reactivated after the host's immune response has waned.

Identification: In the laboratory, thick and thin blood smears stained with Giemsa are examined. The erythrocytes are lysed in the thick smears and allow identification of the organisms in even mild parasitemia. Different characteristics of the infected red cells as well as gametocyte stages that are present allow identification of the particular species of plasmodia. Red blood cells infected with P. falciparum usually show multiple small ring-like trophozoites attached to the cell membrane. Often 2 or 3 parasites are found per cell. Additionally, intracytoplasmic granules called Maurer's dots may be present. Schizonts and merozoites are not present in the peripheral blood while banana-shaped gametocytes are infrequently found. Other species are distinguished by the shape and location of the trophozoites or presence of schizonts. Recently, non-microscopic assays have been developed to increase sensitivity, allow automation, and more accurately assess response to therapy. These include polymerase chain reaction (PCR) for plasmodial DNA and immunoassay detection of plasmodial proteins such as histidine-rich protein 2 (HRP-2) and parasite-specific lactate dehydrogenase (pLDH). Dipstick tests for HRP-2 and pLDH have been developed. With these methods, sensitivity compared to microscopy ranges from 80-90%, and specificity of over 99%. However, sensitivity may vary by species and some studies suggest a significant false positive rate with these methods. These tests are not FDA-approved or available in the U.S. Antigenic persistence after therapy limits the use of immunoassay techniques for follow-up. In addition, accurate speciation is still best achieved with microscopy.

Treatment and prophylaxis: Plasmodial forms in red cells are susceptible to chemotherapy. Sporozoites are resistant to all available drugs. Prophylaxis for travel to areas should begin about 1-2 weeks prior and extend for four weeks after return. Drug choice for prophylaxis or active infection depends on the presence of chloroquine resistant forms in the environment, which is a growing problem, particularly with P. falciparum. Progress towards anti-malarial vaccines is slow due to poor understanding of plasmodial biology and complex confounding clinical variables. SPf66, a synthetic polypeptide vaccine, has shown vastly different protection rates in different populations, perhaps due to pre-vaccination differences in incidence of parasitemia and immune status.

References:
1. Koneman et al. Color Atlas and Textbook of Diagnostic Microbiology, 5^th Edition, Lippincott-Raven, Philadelphia, 1997.

2. Iqbal J, Hira PR, Sher A, Al-Enezi AA. Diagnosis of imported malaria by Plasmodium lactate dehydrogenase (pLDH) and histidine-rich protein (PfHRP-2)-based immunocapture assays. Am J Trop Med Hyg (2001); 64:20-23.

3. Wolday D, Balcha F, Fessehaye G, Birku Y, Shepherd A. Field trial of the RTM dipstick method for the rapid diagnosis of malaria based on the detection of Plasmodium falciparum HRP-2 antigen in whole blood. Trop Doct (2001); 19-21.

4. Hanscheid T, Valadas E. Poor accuracy of rapid diagnostic tests and misdiagnosis of imported malaria: are PCR-based reference laboratories the answer? J Clin Microbiol. (2002);40:736-7.

5. Nacher M. Malaria vaccine trials in a wormy world. Trends in Parasitology (2001); 17:563-565.

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