Researchers at the U.S. Food and Drug Administration (FDA) have developed a novel blood test that can identify asymptomatic carriers of malaria who might otherwise be missed by even the most sensitive method available today.

Malaria is a mosquito-borne disease caused by an intraerythrocytic parasite (parasite that infects red blood cells) called Plasmodium falciparum. The disease can cause a variety of symptoms, such as fever, chills, and severe anemia, and can be fatal if not treated. Mosquitoes that bite infected individuals ingest the parasite and spread it when they bite other humans. The disease threatens about half the population of the world and causes over 400,000 deaths each year.

The use of blood tests that measure nucleic acid (building blocks of genetic material) and proteins produced by malaria parasites has significantly increased the ability of public health workers to identify asymptomatic individuals, i.e., people who are infected but have no symptoms of the disease. These types of molecular tests can detect very low-level asymptomatic infections that would have been otherwise missed by microscopy; low-level infections can still transmit malaria when bitten by a mosquito.

The FDA scientists based their improved laboratory-based test on a gene that encodes for the Plasmodium protein called Pgf17, which the parasite makes during its stage of development known as a gametocyte. Gametocytes are male and female forms of the parasite that the mosquito ingests after it draws blood from a human. The male and female gametocytes then combine to produce the stage of malaria cycle that the mosquito transmits through its saliva when it bites humans.

Molecular tests for identifying individuals carrying gametocyte carriers use a technique called polymerase chain reaction (PCR) to greatly amplify the genetic material that codes for a protein called Pfs25, which gametocytes make in humans. Such molecules are called biomarkers, since they indicate the presence of a specific disease or physiological condition.

The FDA scientists used a technique called microarray to identity 56 novel potential biomarkers for the gametocyte stage of a human malaria parasite. (The microarray technique enables researchers to determine which genes from an organism are actively making the product they code for, in this case the genetic biomarkers made by gametophytes.) Based on their studies of these biomarkers, they found that one of them, Pfg17, was significantly more effective than a currently used biomarker in detecting gametocytes in blood samples that had been “spiked” with these parasites. The Pfg17-based PCR assay could detect as few as 10 gametocytes per milliliter (about ¼ of a teaspoon), while Pfs25-based tests required about 25 per milliliter.

The FDA scientists then performed a real-world comparison of Pfs17 and Pfs25 assays using blood samples from both children and adults living in areas of the African country of Ghana where malaria is common. The Pfg17 test identified as gametocyte positive 19.1% more samples among Ghanaian children who tested negative for the parasite by a Pfs25-based test, and 40% more samples from asymptomatic adults.

The Pfs25 test and the FDA Pfg17 test for malaria detection are laboratory-based and not commercially available.

The findings suggest that tests based on Pfg17 would help public health officials determine the rate of asymptomatic malaria carriers in areas of countries where the disease is present. Additionally, Pfg17-based assays may be more effective than Pfs25-based tests in measuring the efficacy of anti-gametocyte drugs and vaccines in endemic areas.

Title

“A Novel Gametocyte Biomarker for Superior Molecular Detection of the Plasmodium falciparum Infectious Reservoirs”

The Journal of Infectious Diseases https://doi.org/10.1093/infdis/jix442
Published 24 August 2017

Authors

Edward Essuman³, Bryan Grabias³, Nitin Verma³, Joanna K. Chorazeczewski⁴, Abhai K. Tripathi⁵, Godfree Mlambo⁵, Ebenezer A. Addison⁶, Albert G.B. Amoah⁶, Isabella⁴ Quakyi⁶, Miranda Oakley⁴, and Sanjai Kumar³

³Laboratory of Emerging Pathogens, Division of Emerging and Transfusion Transmitted Diseases, Center for Biologics Evaluation and Research, Food and Drug Administration, Silver 8 Spring, MD 20993, United States; ⁴Division of Bacterial, Parasitic, and Allergenic Products, Center for Biologics Evaluation and Research, Food and Drug Administration, Silver Spring, Maryland, United States; ⁵The Johns Hopkins Malaria Research Institute, Bloomberg School of Public Health, The Johns Hopkins University, Baltimore, MD 21205, United States; Department of Molecular Microbiology and Immunology, Bloomberg School of Public Health, The Johns Hopkins University, Baltimore, MD 21205, United States; ⁶School of Public Health, College of Health Sciences, University of Ghana, Legon, Ghana. EE and BG are co-first authors.