Sleeping sickness hidden reservoir found
Sleeping sickness, also known as African trypanosomiasis, is on the WHO's list of diseases that it wants to eliminate. Now scientists have discovered why previous efforts may have failed, by uncovering a formerly unknown reservoir for the parasite.
Sleeping sickness is a priority because over 60 million people are at risk from the parasite, which is spread by the bite of tsetse flies, in 30 countries across Africa.
Despite initially successful efforts to stamp out the infection, which first presents with fever, muscle and joint pain but progresses over months to years to a state of confusion, seizures and fatal coma when the brain is affected, sleeping sickness numbers periodically flare up again, although no one knew why.
Now new research just out in the journal eLife by Glasgow University parasitologist Annette Macleod suggests that the parasite might have a way to lurk below our surveillance radar.
In her lab she's modified trypanosome parasites, which resemble microscopic worms, so that they produce a fluorescent marker, making them glow red or green. This makes them much easier to spot in the tissues of mice that she infects experimentally to follow where the bugs go in the body.
A surprising finding is that, despite being regarded as a blood-borne infection, the parasites can hide in the skin, even in asymptomatic individuals, and can infect tsetse flies from this site.
"Tsetse flies are slash and suck biters," explains Macleod. "They have razor-edged mouthparts that cut through the skin in order to open blood vessels; they then drink the blood and the tissue fluid. But if the parasites are lurking in the skin, they can also be taken up and infect the fly, which passes them on to another individual."
The discovery is important because the existing tests being carried out for sleeping sickness look only at samples of a person's blood for the presence of the parasite. This means that there are may be large numbers of healthy carriers in the population; they have no symptoms, and the bug is kept in check by their immune system but they are capable of passing on the infection to others, possibly for extended periods of time.
"We've had one case of a man from West Africa who had lived healthily in the UK for 29 years. It was only when he received some immune suppressing treatment for an unrelated condition that he suddenly developed the symptoms of trypanosomiasis!"
Macleod warns that present therapies designed to eliminate sleeping sickness parasites need to be tested to see if they also rid the body of this skin reservoir.
"And we also need to bear this in mind when developing future therapies and screening tests too," she says. "Currently these all focus only on whether there are parasites in the blood."
Her solution has been to develop a scanning technique that will enable her to detect the trypanosome parasite non-invasively in human skin.
"We think we can use a process called Raman scattering to see through the skin to pick up parasites lurking there. We're about to launch a field trial to test the device early next year."