risk factors for the disease. Unfortunately, unless a careful history is obtained, the diagnosis of malaria can be missed or delayed. Tests to exclude malaria should be considered for patients who work or live near an international airport and who present with an acute febrile illness. The potential danger of disseminating the mosquito vectors of malaria via aircraft is well recognized; however, modern disinfection procedures have not yet eliminated the risk of vector transportation. Not only can insects survive nonpressurized air travel, but also they may be transported further by car or other means after arriving at the airport.
We usually associate malaria with patients having a history of travel within an area where malaria is endemic. However, other situations that may result in infection involve the receipt of blood transfusions, use of hypodermic needles contaminated by prior use (for example, by drug addicts), possibly congenital infection, and transmission within the United States by indigenous mosquitoes that acquired the parasites from imported infections.
Malaria is usually associated with patients having a history of travel within an area where malaria is endemic, although other routes of infection are well documented. Frequently, for a number of different reasons, organism recovery and subsequent identification are more difficult than the textbooks imply. It is very important that this fact be recognized, particularly when one is dealing with a possibly fatal infection with P. falciparum. It is important to ensure that clinicians are familiar with the following issues.
Automated Instrumentation
Potential diagnostic problems with the use of automated differential instruments have been reported. Some cases of malaria, as well as Babesia infection, have been completely missed by these methods. The number of fields scanned by a technologist on instrument-read smears is quite small; thus, failure to detect a low parasitemia is almost guaranteed. In cases of malaria and Babesia infection, after diagnosis had been made on the basis of smears submitted to the parasitology division of the laboratory, all previous smears examined by the automated system were reviewed and found to be positive for parasites. Failure to make the diagnosis resulted in delayed therapy. These instruments are not designed to detect intracellular blood parasites, and the inability of the automated systems to discriminate between uninfected erythrocytes and those infected with parasites may pose serious diagnostic problems in situations where the parasitemia is ≤0.5%.
Patient Information
When requests for malarial smears are received in the laboratory, some patient history information should be made available to the laboratorian. This history can be obtained by asking the ordering physician important questions such as the following:
1. Where has the patient been, and what was the date of return to the United States? (Where do you live and where do you work? [“airport malaria”])
2. Has malaria ever been diagnosed in the patient before? If so, which species was identified?
3. What medication (prophylaxis or otherwise) has the patient received, and how often? When was the last dose taken?
4. Has the patient ever received a blood transfusion? Is there a possibility of other needle transmission (drug user)?
5. When was the blood specimen drawn, and was the patient symptomatic at the time?
6. Is there any evidence of a fever periodicity?
Answers to such questions may help eliminate the possibility of infection with P. falciparum or P. vivax, usually the only two species that can cause severe disease and, in the case of P. falciparum, can rapidly lead to death.
Conventional Microscopy
Often, when the diagnosis of malaria is considered, only a single blood specimen is submitted to the laboratory for examination; however, single films or specimens cannot be relied upon to exclude the diagnosis, especially when partial prophylactic medication or therapy is used. Partial use of antimalarial agents may be responsible for reducing the numbers of organisms in the peripheral blood and lead to a blood smear that contains few organisms and a conclusion that reflects a low parasitemia when in fact serious disease is present. Patients with a relapsing case or an early primary case can also have few organisms in the blood smear. It is recommended that both thick and thin blood films be prepared immediately, and at least 300 oil immersion fields should be examined on both films before a negative report is issued. Since one set of negative smears does not rule out malaria, additional blood specimens should be examined over a 36-h period. Although Giemsa stain has been recommended for all parasitic blood work, the organisms can also be seen if other blood stains, such as Wright’s stain or any of the rapid blood stains, are used. Blood collected with the use of EDTA anticoagulant is preferred over heparin; however, if the blood remains in the tube for approximately an hour or more, true stippling might not be visible within the infected erythrocytes (e.g., those infected with P. vivax). Using EDTA, if blood is held for more than 2 h prior to blood film preparation, several artifacts may be seen; after 4 to 6 h, some of the parasites will be lost. During the time when the parasites are in the tube of blood, they continue to grow and change according to the life cycle for that species. Also, when using anticoagulants, it is important to remember that the proper ratio between blood and anticoagulant is necessary for good organism morphology—fill the tube with blood. Both thick and thin blood films should be prepared immediately after receipt of the blood. If the specimen is sent to a reference laboratory, both the thick and thin blood films, as well as the tube of blood (room temperature), should be sent. Since this test is always considered a STAT request, it is also important to know what turnaround times are available from the reference laboratory.
All requests for malaria diagnosis are considered STAT requests, and specimens should be collected, processed, examined, and reported accordingly. Although other diagnostic tests can be ordered, any request for examination of blood films should include a possible diagnosis of malaria; thus, these requests are always considered STAT. Not only should the blood collection be considered STAT, but also the processing and examination of both thick and thin blood films should be performed immediately on receipt of the blood. Often immunologically naive individuals with no prior exposure to malaria can present to the emergency room or clinic with symptoms such as fever and malaise and a relevant travel history to an area of the world where malaria is endemic. These patients can have very vague symptoms, but they have the potential to become very ill with malaria, even with a low parasitemia (0.0005% to 0.1%).
SECTION 2 Parasite Classification and Relevant Body Sites
Although common names are often used to describe parasites and parasitic infections, these names may refer to different parasites in different parts of the world. To eliminate these problems, a binomial system of nomenclature is used in which the scientific name consists of the genus and species.
Based on life cycles and organism morphology, classification systems have been developed to indicate the relationship among the various parasite species. Closely related species are placed in the same genus, related genera are placed in the same family, related families are placed in the same order, related orders are placed in the same class, and related classes are placed in the same phylum, one of the major categories in the animal kingdom. As one moves up the classification schema, each category becomes more broad; however, each category still has characteristics in common.
Parasites of humans are classified in six major divisions (Table
