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Silent infections

Daniel BuenoBoasting a population of 2.5 million, Belo Horizonte, the capital of Minas Gerais State, is one of the largest Brazilian cities where visceral leishmaniasis is endemic, with 1,255 cases reported from 2001 to 2011. This disease, which can be fatal if left untreated, is caused by a single-cell parasite – the protozoan Leishmania infantum or chagasi – that lodges in the body’s defense cells and compromises the spleen, liver, and bone marrow. In order to chart the epidemiological profile of leishmaniasis in major Brazilian cities, a team headed by Mariângela Carneiro of the Federal University of Minas Gerais (UFMG) analyzed the blood of 1,875 healthy children living in Belo Horizonte. The researchers found that 16.9% of these youngsters had had contact with the parasite, even though they presented no signs of the disease. One year later, the researchers repeated the tests on 199 of the infected children and confirmed that they remained disease-free (PlOS NTD, December 2012). “These children displayed a low parasite load, averaging 56 protozoa per milliliter of blood, almost 40 times lower than what is observed in clinical cases,” says Carneiro. “For some reason we don’t understand yet, their organisms kept the infection under control.” According to the researcher, this is the first study involving a large cohort to show that people residing in endemic regions can be asymptomatic carriers. In 2000, Carneiro and her team had noted that in Sabará, a town next to Belo Horizonte, 8% of the population presented the parasite but had not developed the disease. Although human beings are carriers, they do not serve as reservoirs for the parasite, which also infects dogs in urban areas and is transmitted to people by the bite of sand flies.