A section of skeletal muscle (cyan) from a mouse whose cells are infected with Trypanosoma cruzi (red), surrounded by immune cells (green and magenta). Image credit: Fernando Sanchez Valdez (CCBY 4.0)
Chagas disease is a dangerous tropical illness caused by single-cell parasites known as Trypanosoma cruzi. In most cases, if not treated immediately, the infection becomes chronic: the immune system of the host greatly reduces the number of parasites present in the body yet fails to fully eradicate them. Current diagnostic approaches often fail to detect these low numbers of parasites. More broadly, without a reliable way to measure Trypanosoma cruzi levels, researchers and clinicians struggle to test new treatments as well as determine whether patients carrying more parasites tend to develop more severe disease.
In response, White et al. aimed to develop a new way of measuring parasitic loads. They based their approach on standard PCR (polymerase chain reaction), an experimental method that amplifies specific DNA sequences in proportion to their initial levels in a sample. This allows researchers to not only pinpoint the presence of the parasites, but also to assess their relative number However, the approach has limited sensitivity: if the amount of target DNA initially collected is too low, it may not be detected.
To bypass this limitation, White et al. adopted a ‘deep-sampling PCR’ approach and made several crucial changes. First, they collected several samples from the same patient, therefore increasing the overall sampling volume. Second, they fragmented the sample DNA before the amplification step in order to disperse the target DNA, thus increasing the chances of its detection in each PCR reaction. Third, they performed as many as 400 PCRs per sample. These modifications greatly improved sensitivity compared to usual approaches.
White et al. used their newly improved method to examine Trypanosomas cruzi levels in infected macaques over long periods. The results show that parasitic burden can be stable over a year, but vastly differs between individuals (with some having a million times more parasites than others). Similar findings were also observed in humans and dogs.
The method developed by White et al. is likely to be too labour intensive to be routinely used in diagnostic laboratories; however, it represents an important and immediate advance for researchers testing new compounds to treat Chagas disease.
