Controlling cases of asymptomatic malaria is fundamental, says study

A study conducted at USP reveals that two-thirds of people infected with malaria at an agricultural settlement in the Amazon did not have symptoms but could spread the disease.

By Karina Toledo

Agência FAPESP – If Brazil fails to invest in diagnosing and treating asymptomatic infections caused by the malaria parasite, especially in agricultural settlements in the Amazon region, the country’s success in fighting the disease will always be only partial. This is the opinion of Marcelo Urbano Ferreira, a professor at Universidade de São Paulo’s Institute of Biomedical Sciences who has coordinated research projects on malaria financed by FAPESP for the last 10 years.

According to Ferreira, the Brazilian strategy to control malaria is based on early diagnosis and the treatment of infections confirmed in laboratories. This strategy is because when an Anopheles mosquito bites a carrier of the disease, it is contaminated by the protozoa that causes malaria – Plasmodium – and transmits the disease to its next victim.

“When the infected individual begins treatment, he or she no longer produces the gametocytes, which are forms of the parasite that can infect mosquitos. The existing gametocytes continue to circulate in the organism for some time. But the earlier the treatment, the less time the individual remains infected,” he explains.

To meet this objective, the National Malaria Prevention and Control Program conducted by the Health Ministry has a broad network of health clinics offering free treatment and agents that go door-to-door looking for people with symptoms of the disease. Through these actions, the number of cases was cut in half from 1995-2011.

Still, in 2011, there were roughly 300,000 cases in the country – 99.9% in the Amazon Basin. One of the factors driving this high number, according to Ferreira, is the asymptomatic infections by Plasmodium going unnoticed by the control system.

From March 2010 to April 2011, the team coordinated by the scientist conducted four transversal inquiries with 396 volunteers at the rural Remansinho agricultural settlement on the border of Amazonas State with Acre and Rondônia. The objective was to determine the prevalence of asymptomatic infections and the percentage of infected individuals carrying gametocytes.

To analyze the blood samples, the researchers utilized a technique known as PCR (polymerase chain reaction), which is capable of detecting even low concentrations of the parasite that would be imperceptible with standard microscopic examinations.

The work in the field was largely conducted by biologist Amanda Begosso Gozze, a FAPESP scholarship holder, and resulted in her master’s dissertation. In the first investigation, 19 cases were discovered with microscopy, whereas PCR uncovered 46 infected people. Approximately 48.8% of these cases were asymptomatic. In the second investigation, 16 samples were positive with microscopy and 43 were positive with PCR. The rate of asymptomatic cases was 70%.

The third investigation revealed 11 infected people with microscopy and 17 with PCR, with an asymptomatic rate of 72%. In the last investigation, microscopy revealed only three infections compared with 14 discovered using the molecular technique. Almost 80% of the cases were asymptomatic.

To evaluate the prevalence of infectious individuals, the researchers selected 44 volunteers who tested positive for the presence of Plasmodium and utilized a reverse transcription technique followed with PCR in real time to determine whether the volunteers had the pvs25 gene, which is only found in mature gametocytes.  

Among the 44 infected people, 42 had circulating gametocytes. Only 21 had the parasites identified with the routine microscopic examination. These results culminated in the master’s dissertation of biologist Nathália Ferreira Lima, a Capes fellow. “The preliminary conclusion is that even individuals who are asymptomatic or have low concentrations of parasites are a potential infection reservoir,” explains Ferreira.

The researcher stressed that according to Health Ministry rules, only cases confirmed with microscopic examination can receive treatment. “The others can only monitor their condition to see if they will develop the disease and how long they will carry gametocytes. We want to know how long these people are invisible to the system,” she said.

One of the main factors that explain why people who are infected by Plasmodium do not manifest symptoms is a previous exposure to malaria. “After five to eight years living in endemic regions, the number of clinical episodes diminishes because the individual acquires immunity to the parasite. However, the number of infections is not necessarily lower,” says Ferreira.

For this reason, river-dwelling populations and agricultural settlements – targets of the research project – have the most asymptomatic cases. “We studied the river-dwelling population in Jaú National Park. For every symptomatic infection, there were five asymptomatic cases. However, when we think about the problem of malaria transmission as a whole, the contributions of river-dwelling people are small,” she noted.

The agricultural settlements, according to Ferreira, are currently the major centers of disease transmission in Brazil and should be the focus of the active pursuit of asymptomatic cases. The other important gap in the Brazilian strategy, according to the researcher, is controlling the transmitting mosquito. “Cyclical spraying of homes with insecticides has been abandoned over the last decade, and few studies have sought alternatives to improve control of the vector,” she says.