Parasites: Getting under the skin

Now our skin is an important barrier against infections, but there are also lots of microbes that make their home there. While most of this is harmless or useful, sometimes something unexpected can hide away there. Sleeping sickness is a disease that affects sixty million people in rural parts of East, West and Central Africa. It’s caused by parasites that get into your skin and blood system. If left untreated, Sleeping Sickness severely damages the nervous system and can be fatal. Efforts to eliminate it haven't been going as well as hoped, but could it be because we’ve been ignoring the skin’s important role in the transmission? Izzie Clarke spoke to Annette Macleod from the University of Glasgow who’s working to improve the diagnosis technique.

Annette - The disease is transmitted by the tsetse fly. When the tsetse fly comes to take a blood meal it bites your skin, they make a bit of a mess of the skin. They suck on the pool of blood and lymph that collect at the bite site, and that’s when they inject the parasites, and that’s how you get infected.

Izzie - How is this normally diagnosed?

Annette - Up until very recently it wa always thought that sleeping sickness was a blood disease, and you would diagnose that by looking in people’s blood. But what we have discovered is that the parasites are also in the skin and, in fact, some people don’t have any parasites in their blood, or very few, so you cannot detect them, but they can have quite a lot of parasites in their skin. So these individuals are not diagnosed, but the could contribute to transmission when a tsetse fly comes to bite you.

Izzie - If these parasites are residing in the skin so a blood test may not actually be the best way forward - is that right?

Beverley - That’s right. There’s a significant proportion of people that have antibodies to the parasites, but we couldn’t find any parasites in their blood. These individuals were not treated and kind of ignored. But we believe now that these individuals have got an infection and they are important in terms of disease transmission. They could actually be hindering the WHOs programme to eliminate the disease because if you have people that are transmitting the disease but seem to be relatively healthy, they can keep spreading the disease and it’s very difficult then to eliminate it. What we want to do is identify these individuals so we can treat them and then, hopefully, eliminate the disease that way.

Izzie - If we can’t use blood tests to find out who’s been infected by these parasites, how can we get around that problem?

Beverley - We’ve been trying to develop with our collaborators from Strathclyde University, Duncan Graham, to develop a non-invasive handheld tool that will detect the trypanosomes under the skin. This is based on raman technology so, basically, you shine a laser on somebody's skin and you can hopefully detect a parasite that are under there. You can see a difference in the raman signal that you get from infected skin versus uninfected skin, and trypanosomes have a unique signal so we can focus in on that signal and say oh yes, this person’s infected.

So that’s the idea and that’s what we’re trying to develop, and we tested a prototype of this in New Guinea a couple of months ago, and it’s by no means perfect but it’s very promising. I think we can develop it further.

Izzie - Is there any way that once you’ve found out that there is this infection that it can be treated?

Beverley - Yes. You can treat the disease with drugs that are free from the WHO, but you have to be hospitalised for about two weeks, and that’s part of the reason why people with latent infection are not generally treated. You only treat people that you have seen parasites in the blood - that is the current WHO standard.

Izzie - Why is it important to investigate this so thoroughly?

Beverley - It’s holding up the elimination programme. What’s happened previously is that the number of cases of sleeping sickness had come right down in the 1960s, and then we took our eye of the ball and then the disease re-emerged. We got lots of cases, up to 300,000 cases in the 1980s. So now we’re back getting the disease under control but we have to understand how the disease re-emerges. We think it’s these individuals that don’t have symptoms but they’re carrying the disease and they are seeding the next epidemic, so we need to stamp that out.

Izzie - How long can this last in the system?

Beverley - We were involved in this very interesting case of somebody who’s from Africa, from Sierra Leone, and he came to this country 29 years ago, and then came down with sleeping sickness. So he was fine for 29 years, and then he was treated with immunosuppressants for an unrelated disease and he came down with sleeping sickness. He was able to control the disease for all that time.

Izzie - If you are able to improve this method, can it be used for anything else?

Beverley - Yes. This technology has been used to identify skin cancer, for example. Even to detect fake whiskey in whisky bottles, so it’s got lots of applications. When you go to the airport and they scan your bag with a little cloth and then put it in the machine, that’s raman that they’re detecting to see if you’ve got cocaine or explosives. But we can also use this technology to look at other vector-borne diseases that have the same component like Onchocerciasis, for example, where we know the parasites are in the skin, so I think it’s got lots of potential.