How is malaria diagnosed?
Since Charles Laveran (pictured at left) first visualized the malaria parasite in 18801 , the mainstay of malaria diagnosis has been the microscopic examination of the blood.
Fever and septic shock2 are commonly misdiagnosed as severe malaria in Africa, often leading to a failure to treat other life-threatening illnesses. In malaria-endemic areas, parasitemia does not ensure a diagnosis of severe malaria, because it – parasitemia – can be incidental to other concurrent disease. Recent investigations suggest that malaria retinopathy is better than any other clinical or laboratory feature in distinguishing malarial from non-malarial coma3 .
Although blood is the sample most frequently used to make a diagnosis, both saliva and urine have been investigated as alternative, less invasive specimens.
Symptomatic diagnosis
Areas that cannot afford even simple laboratory diagnostic testing will often use only a history of subjective fever as the indication to treat for malaria. Using Giemsa-stained blood smears from children in Malawi, one study showed that when clinical predictors (rectal temperature, nail bed pallor, and splenomegaly) were utilized as treatment indicators, rather than a history of subjective fevers, the percentage of correct diagnosis went from 21% to 41% of cases, and unnecessary treatment for malaria was decreased.
Microscopic examination of blood films
The most economic, preferred, and reliable diagnosis of malaria is microscopic examination of blood cells because each of the four (4) major parasite species has distinguishing characteristics. Two sorts of blood film are traditionally used. Thin films are similar to usual blood films and allow species identification because the parasite’s appearance is best preserved in this preparation. Thick films allow the microscopist to screen a larger volume of blood and are about eleven (11) times more sensitive than the thin film. The only drawback with using thick films is that the appearance of the parasite is much more distorted making distinguishing between different species more difficult.
One important thing to note is that P. malariae and P. knowlesi (which is the most common cause of malaria in South-east Asia) looks very similar under microscopic investigation. However, P. knowlesi parasitemia increases very quickly and causes a more severe disease than P. malariae , so it is important to identify and treat infections quickly.
Antigen tests
In areas where microscopy is not available, or where laboratory staff are not experienced at malaria diagnosis, there are commercial antigen detection tests that require only a drop of blood4 . Immunochromatographic tests (also known as “dipsticks” and pictured at right) have been developed, distributed, and field-tested. These tests utilize finger-stick or venous blood, the completed test takes a total of 15 – 20 minutes, and the results are read visually as the presence or absence of colored stripes on the dipstick making them suitable for use in the field. The threshold of detection by these rapid diagnostic test is in the range of 100 parasite/ μ of blood (commercial kits can range from about 0.002% to 0.1% parasitemia) compared to five (5) by thick film microscopy. One disadvantage is that the dipstick test are qualitative but not qualitative meaning they can determine if the parasite is present in the blood, but not how many.
Molecular methods
Molecular methods are available in some clinical laboratories and rapid real-time assays (for example, AT-NASBA based on the poymerase chain reaction) are being developed with the hope of being able to deploy them in endemic areas.
Is prevention possible?
Simply put, the answer is yes. Methods used to prevent the spread of the disease, or to protect individuals in endemic areas, include prophylactic drugs, mosquito eradication, and the prevention of mosquito bites. The continued existence of malaria in an area requires a combination of high human population density, high mosquito population density, and high rates of transmission from humans to mosquitoes and mosquitoes to humans. If any of these factors are lowered sufficiently, the parasite will sooner or later disappear from that area. Such was the case in North America, Europe, and much of the Middle East. It should be noted, however, that unless the parasite is eliminated from the entire world, it could become re-established if conditions were to revert to a combination that favors the parasite’s reproduction. Many countries are seeing an increasing number of imported malaria cases because of both extensive travel and migration.
Many researchers have argued that, in the long-run, prevention of malaria might be more cost-effective than treatment of the disease. The only downside to this is that capital cost required to do such prevented work is out of reach of many of the world’s poorest people
1Sutherland, C.J.; Hallett, R. (2009) “Detecting malaria parasite outside the blood“, Journal of Infectious Diseases, pp. 1561 – 1563
2Septic shock is a medical emergency resulting from decreased tissue perfusion and oxygen delivery as the result of severe infection and sepsis (blood poisoning).
Source: Wikipedia.com ( http://en.wikipedia.org/wiki/Septic_shock )
3Beare, N.A.; Taylor, T.E.; Harding, S.P.; et. al. (November 2006) “Malarial retinopathy: a newly established diagnostic sign in severe malaria“, American Journal of Tropical Medicine Hygiene, pp. 790 – 797
4Pattanasin, S; Prous, S,; et. al (2003) “Evaluation of a new Plasmodium lactate dehydrogenase assay for the detection of malaria“, Royal Society of Tropical Medicine , pp. 672 – 674