tsetse fly
Tsetse flies are the primary transmitter of sleeping sickness Getty Images/Patrick Robert/Corbis

The parasites that spread sleeping sickness have been shown to stay under the skin of infected patients, not just in their blood, scientists have found out. This discovery could transform the way the disease is diagnosed, and influence how quickly it can be treated.

Sleeping sickness is prevalent in 36 sub-Saharan countries where tsetse flies – the primary transmitters of the disease – are often found. Just under 10,000 people have been diagnosed with the illness annually in recent years, by analysing blood samples for the presence of the Trypanosoma brucei gambiense parasite which is responsible for 98% of all cases (see box).

However, scientists have in the past identified cases of infections in which parasites remained undetected or were identified only in low concentrations in the blood.

They therefore hypothesised the existence of another anatomic reservoir of infection in the body – the skin.

Parasites hiding under the skin

To conduct the research, published in E-life, the scientists used mice and infected them with the sleeping sickness parasite, which they had genetically modified to be fluorescent – and therefore traceable in the bodies of the rodents. The researchers then tracked the dispersion of parasites in the mice after infection, and found that it was more present in the skin than in the blood.

Following this first experiment, they carried out tests in humans. They analysed human skin biopsies, where they identified parasites, even in samples of individuals who had previously not been diagnosed with blood tests. The scientists thus concluded that skin, both in animals and humans, represents a reservoir for the infection – one where the parasites hide.

This may have consequences for how the disease is diagnosed and subsequently treated in the future. "The identification of this novel reservoir requires a re-evaluation of current diagnostic methods and control policies," the authors write. Coming up with new tools that quickly identify Trypanosoma brucei gambiense under the skin, and testing both at the blood and the skin of people suspected to be sick may become crucial steps to fight off sleeping sickness.