Plasmodium testing basics

Malaria Parasite Lifecycle 

Malaria is caused by Plasmodium parasites, which are transmitted by infected Anopheles mosquitoes. The lifecycle begins when an infected mosquito bites a person to take a bloodmeal and, in doing so, injects sporozoites into that person’s bloodstream. These sporozoites quickly travel to the liver, where they invade hepatocytes and multiply silently. After about six or more days, thousands of merozoites are released back into the bloodstream, where they invade red blood cells (RBCs). Inside RBCs, the parasites progress through ring, trophozoite, and schizont stages, eventually rupturing the RBCs and releasing more merozoites. This asexual cycle in RBCs repeats every 48–72 hours, causing waves of fever, anemia, and other clinical symptoms. Some parasites differentiate into gametocytes, the sexual forms of the parasite. Gametocytes do not cause any direct illness in an infected person. Instead, when another mosquito feeds on the infected blood, it ingests gametocytes, which undergo sexual reproduction in the mosquito’s midgut. The resulting ookinetes develop into oocysts, releasing sporozoites that migrate to the mosquito’s salivary glands, ready to infect a new human host. This complex lifecycle allows Plasmodium parasites to persist and spread between humans and mosquitoes, making elimination challenging.

Diagnostic Tools for Plasmodium infections

• Giemsa-stained blood smears: Visualize parasites and perform species identification under a microscope (see UW Malaria Exam Thick & Thin Smears and Immunoassay); part of the standard approach to diagnose infections in returning travelers 

• Rapid Diagnostic Tests (RDTs): Detect parasite antigens using fast and simple test method, but less sensitive than other tools (see UW Malaria Exam Thick & Thin Smears and Immunoassay); part of the standard approach to diagnose infections in returning travelers 

• Molecular testing: The most sensitive testing option done to detect parasite nucleic acids. At the UW MMDL, we perform a reverse transcription PCR (RT-PCR) to detect parasite 18S rRNA (UW test code = "MAL18S"). This testing is extremely sensitive and can even identify low-level infections that can be below the level that would cause immediately apparent symptoms. This testing is usually reserved for clinical trials or for unique clinical indications.