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The rapid and accurate detection and species identification of Plasmodium
Detection of Babesia, trypanosomes, and some species of microfilariae
Malaria is a major tropical disease infecting approximately 500 million people and causing 1.5 to 2.7 million deaths annually. Ninety percent of the deaths occur in sub-Saharan Africa and most of these occur in children <5 years old; it is the leading cause of mortality in this age group. This disease is also widespread in Central and South America, Hispaniola, the Indian subcontinent, the Middle East, Oceania, and Southeast Asia. In the United States, individuals at risk include travelers to, and visitors from endemic areas.
Malaria is caused primarily by 4 species of the protozoa Plasmodium: Plasmodium falciparum, Plasmodium vivax, Plasmodium malariae, and Plasmodium ovale. A fifth Plasmodium species, Plasmodium knowlesi, is a similar parasite that may be an important source of human infection in some regions of Southeast Asia. Differentiating Plasmodium falciparum and Plasmodium knowlesi from other species is important since both can cause life-threatening infections. In addition, Plasmodium falciparum is typically resistant to many commonly used antimalarial agents such as chloroquine.
Babesiosis is an emergent zoonosis caused by an intraerythrocytic protozoan in the genus Babesia. Babesia microti is responsible for the vast majority of human cases in the United States, with "hot spots" of disease along the Northeast Coast (eg, Martha's Vineyard, Long Island, and Nantucket) and Midwest states, although the distribution of disease is spreading. In addition, a small number of cases of Babesia duncani and Babesia duncani-like human infection (WA and CA strains) have been reported along Pacific Coast states from Washington to northern California, and Babesia divergens/Babesia divergens-like strains have been isolated from humans in Missouri (MO-1 strain), Kentucky, and Washington. At this time, only Babesia microti is a nationally notifiable disease.
Babesia microti shares a tick vector with Borrelia burgdorferi and Anaplasma phagocytophilum, the causative agents of Lyme disease and human granulocytic anaplasmosis (HGA), respectively. Recent studies suggest that exposure to Babesia microti is quite common in areas endemic for Lyme disease and anaplasmosis, so it is prudent to consider testing for all 3 diseases concurrently. Less commonly, babesiosis may be acquired through blood transfusion, and therefore donor units are tested for this parasite in some endemic areas.
Most patients with babesiosis have a mild illness or are asymptomatic, but some develop a severe illness that may result in death. Patient symptoms may include fever, chills, extreme fatigue, and severe anemia. The most severe cases occur in asplenic individuals and those over 50 years of age. Rare cases of chronic parasitemia, usually in immunocompromised patients, have been described.
Microscopy of Giemsa-stained thick and thin blood films is the standard laboratory method for diagnosis and differentiation of Plasmodium and Babesia species. Under optimal conditions, the sensitivity of the thick film microscopy is estimated to be 10 to 30 parasites per microliter of blood. This test can also detect trypanosomes that cause Chagas disease (Trypanosoma cruzi) and African sleeping sickness (Trypanosoma brucei), as well as some species of filariae. If filarial infection is suspected, FIL / Filaria, Blood is recommended since it is more sensitive than the traditional blood smear examination.
Examination of the thin film allows for calculation of malaria percent parasitemia, which can be used to predict prognosis and monitor response to treatment for patients with malaria and babesiosis. The percentage parasitemia represents the percentage of infected red blood cells. This is calculated from representative microscopic fields on the thin blood film. Malarial gametocytes are not included in the calculation since they are not infectious to humans and are not killed by most antimalaria drugs.
If positive, organism identified and percent parasitemia calculated, if applicable.
A positive smear indicates infection with the identified species of Plasmodium or with Babesia.
Species identification can indicate the appropriate antimalarial therapy.
For most sensitive detection of Plasmodium, thick smears must be examined.
Any exam that does not include a thick smear cannot be considered adequate.
Since the degree of parasitemia may change rapidly due to natural parasite replication and administration of antimalarial therapies, it is most useful to calculate the percentage of infected cells immediately after the blood is drawn and malaria parasites are detected. A percent parasitemia that is calculated more than 8 hours after the blood is drawn will not accurately reflect the patient’s current state of parasitemia.
Calculation of the percent parasitemia may not be possible if the malaria parasites are degraded or have altered morphology due to age of the specimen or suboptimal transportation conditions.
Hoffman SL: Diagnosis, treatment, and prevention of malaria. Med Clin North Am 1992;76:1327-1355