Asymptomatic Malaria: Natural History Classification Study

Malaria detection in persons who do not have symptoms is critical to public health initiatives to better control this tropical disease in areas where the mosquito-borne parasite is common. After first testing negative, asymptomatic people harboring the parasite can still spread the disease or become ill later.
Because of the pathogen’s dynamic lifetime, parasite concentrations might rapidly fall below detection levels, especially when older, less sensitive tests are utilized. Such changes can make determining whether a seemingly healthy person is infected challenging when testing at a particular moment in time.

Malaria can cause severe chills, sweating fevers, headaches, nausea, and other symptoms. Despite this, many infected people appear to be healthy.

A recent asymptomatic detection research was undertaken in Uganda’s Katawki District, which has a high malaria incidence.

“We found that parasite dynamics and parasite species present were highly variable among patients with low-level, asymptomatic infections,” said Dr. Sean C. Murphy of UW Medicine, one of the study’s main experts. He observed that sampling every other or third day was sufficient to detect a proportion of infections comparable to daily sampling. However, even with sophisticated diagnostics, testing once a week or less frequently could misclassify the genuine infection status of up to one-third of the people.

According to Murphy, this finding is significant for enhancing studies on the prevalence of malaria infection and, by extension, clinical trials of malaria vaccines and medicines. To establish the infection status of asymptomatic patients, the majority of these trials use single-timepoint tests or recurrent but infrequent tests. If the parasite densities of the participants fall below the limit of the test, this method is likely to overlook infections.

Murphy is a physician-scientist who is also the chief of pathology and laboratory medicine at Seattle Children’s Hospital and a professor of laboratory medicine, pathology, and microbiology at the University of Washington School of Medicine.

The Murphy lab collaborated with Dr. Thomas Egwang and his research team, which included Tonny Owalla of Med Biotech Laboratories in Kampala, Uganda, and Dr. Jennifer E. Balkus, professor of epidemiology at the University of Washington School of Public Health. The work was directed by Dr. Dianna E. B. Hergott, a graduate student mentored by Murphy and Balkus, and Med Biotech’s Owalla.

The Uganda-based team carried out the study’s community-based components. Participants were healthy, nonpregnant adults aged 18 to 59 who were not on antimalarial medication, as well as older children aged 8 to 17.

“We instructed participants in how to collect one dried blood spot at home every day for up to 29 days,” Owalla wrote in a statement. Participants would visit the study clinic once a week to turn in their blood spots from the previous week, acquire new blood spot cards, and have traditional blood draws.

The dried blood spots were tested for the presence, categorization, and densities of Plasmodium ribosomal RNA, which aids in the production of parasite proteins. The testing approach also included the “pooling” of dried blood spots. This method allowed the researchers to examine more samples at a lower cost, similar to the tactics used during the COVID-19 epidemic.

The researchers aimed to discover a sample frequency equal to testing every day to consistently identify asymptomatic cases, but less demanding, by evaluating the generated data. The study team, on the other hand, intended to prevent an infrequent schedule that would result in missed infections.

The scientists classified each participant’s infection trajectory as follows: no infection, newly detected infection, cleared infection, persistent infection, or unable to determine. They evaluated how many infections would have been found if the sampling frequency had been reduced based on the daily data.

A Plasmodium infection was identified in approximately 60% of all subjects at some time during the month-long trial. At the start of the trial, less than half of the participants had an infection. During the study period, the lowest daily report revealed a prevalence of 30%.

The findings have been published in The Lancet Microbe.

Several previous research had called into doubt the precision of a single measurement for determining infection status. Asymptomatic illnesses that go undetected may have an unintended impact on research outcomes.

“Serial testing,” the paper’s authors said, “should be considered when attempting to determine an individual’s true infection status.”

According to the authors, one of the drawbacks of their own study was that participants were not required to collect their dried blood samples at the same time each day. They discovered that parasite populations could fluctuate up to 100-fold in six hours.

Owalla is presently a graduate student in pathobiology at the University of Washington School of Public Health. He intends to use his education to further develop sophisticated malaria treatments in Africa, the continent most impacted by the disease.

For more information: Assessing the daily natural history of Plasmodium infection in adults and older children in Katakwi Uganda: a longitudinal cohort study, The Lancet Microbe (2024). https://doi.org/10.1016/S2666-5247(23)00262-8