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Laboratory Diagnosis of Malaria in the United States

Image from a blood smear from a 48-year old woman who developed Plasmodium falciparum malaria after travel to the Dominican Republic. Examination under the microscope at 1000-fold magnification shows four of the red blood cells infected by parasites (“P”). (CDC photo)

The Importance of Laboratory Diagnosis

Each year in the United States over a thousand persons come down with malaria. Most are travelers who became infected while visiting a malaria risk area without taking the necessary precautions. Because malaria can be a severe, even fatal disease, these patients must receive a prompt and accurate diagnosis, permitting a timely treatment. The clinical diagnosis, where malaria is suspected based on the history, symptoms and clinical findings, must always be confirmed by a laboratory diagnosis.

Microscopy

Microscopy is the established method for laboratory diagnosis of malaria. A drop of the patient’s blood is collected by fingerprick, or from a larger venous blood specimen. It is then spread on a glass slide (“blood smear”), dipped in a reagent that stains the malaria parasites (Giemsa stain), and examined under a microscope at a 1000-fold magnification. Malaria parasites are recognizable by their physical features and by the appearance of the red blood cells that they have infected. These characteristics often allow the laboratorians to identify the type (species) of parasite causing the infection, a finding that will guide the treatment.

The laboratorian can also assess the percentage of red blood cells that are infected, a measure of severity of the infection.

A laboratorian at the Georgia Public Health Laboratory performs PCR, a back-up diagnostic test for malaria; CDC has assisted state public health laboratories in introducing PCR techniques for malaria and other parasitic diseases. (CDC photo)

Other Laboratory Tests

In addition to microscopy, other methods have been developed recently for detection of malaria parasites. Parasite antigens and other products can be detected by rapid “dipstick” tests, and parasite DNA can be detected by polymerase chain reaction (PCR). PCR is currently the most accurate test and can identify low levels of infection not detectable by other methods.

Serology detects antibodies against malaria parasites. Such antibodies are produced by the immune response of the infected person and can persist in the blood for several months after the infection is over. Thus, serology measures a person’s past experience with malaria, but does not necessarily detect current infections.

While microscopy is a routine test, PCR and serology can be performed only in specialized reference laboratories, and the kits for “dipstick” tests are not yet approved by the Food and Drug Administration (FDA) for use in the United States.

Microscopy is the established method for laboratory confirmation of malaria; here, a laboratorian at CDC’s reference diagnostic laboratory for parasitic diseases. (CDC photo)

Laboratories Working Together

The microscopic diagnosis of malaria is initially made in hospitals or commercial laboratories. When unsure about the diagnosis, these laboratories can send their specimens to the Department of Health of their state for further review. If needed, the state laboratories can request additional diagnostic assistance from the national reference laboratory at CDC’s Division of Parasitic Diseases. Every year CDC performs malaria laboratory tests (microscopy, PCR, serology) on approximately 300 specimens sent by state health departments or other laboratories.

This close collaboration between laboratories is valuable because malaria is rarely seen in the United States. Consequently, laboratorians are too often unfamiliar with malaria parasites and may fail to detect malaria parasites or to identify correctly the infecting species of parasites. Being able to send specimens to a more specialized laboratory allows laboratories to provide more accurate diagnoses and to improve their diagnostic skills.

Participants at a training workshop on bloodborne diseases in Albuquerque, New Mexico (August 2005) practice their skills at blood smears. (CDC photo)

Strengthening the Laboratory Diagnosis of Malaria in the United States

In addition to its reference functions, CDC offers training workshops on diagnosis of bloodborne diseases (malaria and babesiosis) for laboratory personnel throughout the United States. CDC also provides Wed-based training through DPDx, a Web site for laboratory identification of parasites of public health concern. The Web site contains information about diagnosis of malaria as well as other parasites, and sends out diagnostic quizzes at monthly intervals to a list of over 1,300 subscribers. DPDx also allows telediagnosis, where outside laboratories can send by email to CDC digital images of their microscopy findings, and receive feedback from CDC staff within minutes or hours. Services offered by DPDx are free of charge; for more information contact dpdx@cdc.gov.