eLife Episode 39: Spotlight on tropical diseases
In this special episode of the eLife Podcast, we discuss diseases common in tropical countries including tuberculosis, Zika, malaria and schistosomiasis.
In this episode
00:22 - Why should we fund research on neglected tropical diseases?
Why should we fund research on neglected tropical diseases?
with Prabhat Jha, University of Toronto
This month, the eLife Podcast is focusing on tropical diseases in this edition of the programme. But why are they neglected, and why shouldn’t they be? Chris Smith speaks to eLife Senior editor and the University of Toronto's Professor of Global Health, Prabhat Jha...
Prabhat - When we talk about neglected tropical diseases, they represent a grab bag. It’s actually quite different parasitic or vector diseases which actually are quite different in different settings – African trypanosomiasis, schistosomiasis, leishmaniasis, leprosy, dengue, rabies, things like hookworm disease. What is common to them, they actually don’t kill in very large numbers but they cause a substantial amount of disability. The second thing that unites these diseases, they are among the most heavily concentrated in the poorest parts of the world. Now, that’s important because getting rid of them has double challenges. You're working in difficult places of the world. The upside is control of these diseases. It’s probably one of the best pro-poor things the world could do in poor countries.
Chris - Really, the spinoff of what you're saying is, if we intervene in these countries, a) the money goes further because they're poorer countries and things turn a bit more cheaper – notwithstanding the challenges but b) there's a huge economic benefit because of the way these diseases cause chronic disability.
Prabhat - Absolutely. Most of these conditions – take leprosy, which is still quite common in India is a big cause of poverty of the household. Visceral leishmaniasis which again is in many parts of India causes lifelong disability in those affected. These are eminently treatable and we have had successes. We have been able to get rid of Guinea worm in Africa and the odds is on its way to being eradicated. There are targets. The World Health Organisation has set targets for getting rid of leprosy, trachoma, and a few of these other conditions. It’s going to be difficult because as we know from the experience in eliminating polio, when you start out in a campaign to get rid of these diseases. The first few cases you detect and treat are generally easy to find and cheaper but as you go along, they become harder to detect and therefore, they need more resources and they need newer science as well.
Chris - It’s also a moving target though, because with polio and things like that, we have seen cultural barriers to eradication. We’ve seen walls so we’ve seen a lot of human factors intervening which actually have thrown a huge spanner in the works and we would probably be a lot further along. It wasn’t the science. It wasn’t actually the funding that held the process up. It was other people.
Prabhat - Absolutely and those are part of the challenges of working in these poor countries. But the good news I think is the world made progress on polio because it put money into it and it invested in moving from oral polio to injected polio. There is good scientific thinking behind that because the injected polio was less likely to cause disease in a way that oral polio would. We know that even earlier on from the smallpox eradication that it required a new technology which was basically using ring vaccination, finding a case of smallpox and vaccinating everyone around that. That scientific breakthrough of using ring vaccination was exactly what the world has tested in the latest Ebola vaccine. So the key there is that as these targets get met, it will need a lot more money and the last mile is always the more expensive.
Chris - Does it not come down to human population? We’re invading more virgin territory where these diseases are endemic in nature and it’s more likely we’re going to see spill over into humans. So until we get to the bottom of the population problem, we’re really ignoring the elephant in the room.
Prabhat - Well, I think they're related but not causally related. But the positive news is that if you look at areas where there is an increasing urban concentration and you’ve got urban infrastructure, you can actually have clinics then it is in some ways easier to control these.
Chris - Ironically, people have said that with Ebola, it was people coming across the border to come to these towns where there were albeit – not terribly good – but there were medical facilities and this was actually helping to spread the outbreak.
Prabhat - That’s right and the other factors which is important in Ebola was the burial practices where you had to hug the body goodbye. Their response to it was late but very crude isolation strategies that were brought in were effective.
Chris - So we need better resources. We need more science thrown at this in order to be more crafty and clever as time goes on. But will people pay for it because the harsh reality is, Ebola, the vaccines we now have something like 7 different vaccines in trials at various stages of development or more for Ebola? Now they're not rocket science. Those vaccines have been made using fairly mundane techniques actually that we’ve had for decades in some cases. Yet, no one wanted to make them and deploy them because they didn’t think Ebola was a country’s problem that had the capacity to make those vaccines.
Prabhat - In the short or medium term, it’s in everybody’s interest to stamp out epidemics where they occur because it spreads. But I would argue that public funding of science which has really decreased since the 1970s really needs to be back on the same level because that science creates products and tools that you can use to fight epidemics. The private sector won't take care of these epidemics. They have no incentive to.
06:48 - Shortlisting the Zika suspects
Shortlisting the Zika suspects
with Michelle Evans and Courtney Murdock, University of Georgia
One of the newest kids on the biological block is Zika virus, which we know to be spread mainly by the Aedes family of mosquitoes. But that doesn’t mean that other species can’t transmit it too. But which ones should we worry about? Chris Smith hears from Michelle Evans and Courtney Murdock, both at the University of Georgia, who have taken a leaf out of the way popular music gets sold and marketed...
Courtney - This was an attempt to try to be proactive. And so, we developed models that allowed us to make a list of potential mosquito species that could be important transmitters of viruses like Zika for a given area – for the United States for example. As an empiricist, it’s really nice to have a list of species as well as their potential rank order of importance so that we can streamline empirical efforts to look more closely at these species and their relative importance for transmission.
Chris - Michelle, how did you approach that because there are many, many different species of mosquito and they have many, many different geographies in which they can operate? So, how did you do this project?
Michelle - That’s definitely correct. There's over 3,000 species of mosquitoes in the world and it would just be impossible to empirically test them all. Even to create a model that could consider all of them would be hard. So we focused on a subset of the mosquito species that transmit a certain group of viruses that are very closely related to the Zika virus. We created a model, kind of like a recommendation system that Spotify or Pandora might use to recommend a new musical artist. And so, the way these systems work is based on the traits of artist you like, their algorithms will recommend new music based on a ranking system. Our model does the same thing except in place of an artist, genre, or the rhythm of the music, we used common mosquito and virus traits such as their geographic range, or disease symptoms, or severity. This allowed us to get a list of mosquito species that could potentially transmit Zika.
Chris - Now, what geographies were you considering or were you looking everywhere?
Michelle - The way we use that trait was more about the presence of a species in a certain area of the world. We had a couple of different traits – one was how widespread is the species and another is if it was specifically on a certain continent.
Chris - Right and you marry that with the likelihood that there is a virus a bit like Zika being circulated in that area and how much.
Michelle - Yeah and then also, the presence of other mosquito species there that might carry the virus. So I was able to draw on these connections.
Chris - Courtney, when you do this sort of analysis, what did you actually see?
Courtney - What was produced was a list of potential mosquito species in the US that could be potential transmitters of Zika virus and other viruses similar to Zika and this was rank ordered. So obviously, we had the top two candidates were species that people would already expect like Aedes aegypti which is a yellow fever mosquito which is the primary vector for dengue and chikungunya, as well as Zika, and then Aedes albopictus which is the Asian tiger mosquito which has much wider distribution. But then there were some other candidate species that were slightly surprising. Culex quinquefasciatus: a species that does live in close proximity with humans but has been more involved in transmission of other viruses like West Nile virus – came up as a potential candidate, and we had some other Aedes species come up as potential candidates.
Chris - Michelle, these data obviously are predictions as Courtney says about what mosquitoes might transmit Zika. But at the moment presumably, we don’t know. We just know that they might because they help to transmit viral relatives of Zika therefore, they might be involved. So is the next step then is to actually go and test some of the leading candidates?
Michelle - Yes, that’s exactly right. In our paper, what we really strongly recommend is that this provides a list for empirical tests to be done and that as these tests are being done, existing ideas about the biological realism of these mosquitoes has also brought into to play. So for example, some mosquitoes that we predict might transmit Zika, might not really be in close proximity to humans or they might not prefer to bite humans. And so, those kinds of species can be discounted from the list relative to ones that we know are more likely to bite people.
Chris - Could you Courtney, translate this to other geographies? You focused on the Americas at this stage, but would it work elsewhere?
Courtney - Yes. I think our approach would work for any given geography.
Chris - Michelle, bottom line of this study – if you had to summarise in one sentence and say to politicians this is what we found, what would you say?
Michelle - I think the important kind of takeaway from our study is that this is a method that can be used especially in future disease outbreaks or even before these outbreaks happen to help get researchers on the right track as far as which species they should be considering or even vaccine development: which are the viruses we should be targeting? And it just helps us prioritise where to spend our limited time and resources.
12:40 - Malaria parasites on the move
Malaria parasites on the move
with Dennis Klug, University of Heidelberg
In numbers terms, malaria is probably the world’s most important tropical disease. But its lifecycle - moving from a mosquito’s blood meal into the insect’s salivary glands and then back into human blood via a liver cell - makes studying some aspects of malaria biology very difficult. Dennis Klug has been studying what goes on mid-way along the mosquito’s intestine where it forms a structure called an oocyst where the infecting parasites multiply. Speaking with Chris Smith, he’s found a critical gene that releases the progeny into the mosquito’s blood...
Dennis - The parasite has to colonise first the midgut of the mosquito and develops into a stage that is called the oocyst where it multiplies. From this oocyst, parasites are released into the blood of the mosquito. These parasites are called sporozoites and these sporozoites then invade the salivary glands and can then be transmitted with the next bite to a new host. We are interested how the sporozoites are able to do that. It egress from the oocysts and to invade the salivary glands. So what we do is we take infected mosquitoes and then we extract the salivary glands and then we can extract these sporozoites and then we just imaged some. These sporozoites are highly motile cells and make continuous circles on this microscopic slide which makes some very easy to image.
Chris - What about the oocyst though, because that’s a different beast entirely, the one that’s sitting in the midgut of mosquito and needs to get into the blood of the mosquito? It can't be very easy to see in there.
Dennis - What we are really interested in is the motility of these sporozoites. We know that there are certain surface proteins called adhesins that play a crucial role in this interaction of the sporozoite with its environment. We found a new protein that was not previously investigated. By making a gene deletion of this protein, we found that the sporozoites are no longer able to egress from this oocyst. Probably because of a motility defect that makes it unable to break free from this rigid membrane.
Chris - What's the protein and how did you find it in the first place?
Dennis - We believe the protein is a surface protein and we found this protein because the adhesins that I described already in the literature, they have a very specific main organisation and we looked out for proteins that have a similar domain organisation and we found the specific one, and then we investigated this one by different genetic approaches – taking the protein to fluorescent tag or by gene deletion.
Chris - So you knock it out and the sporozoites can no longer escape from this oocyst. Have you done the experiment to put it back in and prove just by discretely putting that protein back in, you can restore the ability of the sporozoites to get where they need to go? Thus, proving it must be that protein that’s responsible.
Dennis - Yeah, exactly. We did two different gene knockouts and the next step, we rescued these by putting the gene back and once the gene is back in place, sporozoites are again able to egress from the oocysts and also to invade the salivary glands.
Chris - Do you have a feel for what these proteins do yet or do you know that they're only involved in helping the sporozoites to move?
Dennis - It’s not known exactly what these proteins are actually doing. There are some speculations that some of these adhesins have adhesive properties, parasites needs them to adhere to substrates. It could also be that some of these proteins play a role as a sensor. The parasites need them to interact with the environment and to sense where it is or if it is in close proximity to other parasites.
Chris - Are they used exclusively in the escape from the oocyst or does the malaria parasite use these same genes or the same protein at any other point in its life cycle because it’s got to go through a lot of transitions between different tissues and different cells in switching between the different hosts it infects, hasn’t it?
Dennis - This specific protein that I investigated in the study was also important for the invasion of salivary glands because we found that in some mosquitoes, some of these sporozoites are able to egress from the oocyst probably because of mechanical stress and these sporozoites are then also not able to invade the salivary glands. It is known that other adhesins play a role in salivary gland invasion for example in motility but also in migration through the skin once these sporozoites are deposited in the skin of a bitten human for example.
17:40 - Safety in numbers: how TB infects
Safety in numbers: how TB infects
with Alex Sigal, Africa Health Research Institute
It was in 1882 that Robert Koch announced to the world that he’d discovered the cause of the disease tuberculosis; he did it, in part, by looking down a microscope. But although Koch linked the TB bacterium to the disease, despite more than a century of study since, we still don’t really know how the infection unfolds in the average person. Now, speaking with Chris Smith, Africa Health Research Institute scientist Alex Sigal explains how he has also been looking down the microscope and has got a new piece to add to the TB puzzle…
Alex - TB exposure is very common. One-third of individuals throughout the world are exposed mostly in developing countries, 10 million active infections, and about 2 million deaths a year. It’s quite a healthcare burden and it actually is a burden in countries which can least afford to deal with it.
Chris - The causative organism, it’s a bacterium, a mycobacterium.
Alex - Yeah. So it’s Mycobacterium tuberculosis and it’s been studied for 100 years or more.
Chris - What's our present understanding of how it causes the diseases that it does and how it spreads?
Alex - It’s mostly a disease of the lung. It spreads by aerosol – so people cough and somebody actually breathes in the droplets, inside the droplets are the microorganisms.
Chris - Once they land in your lungs, what do they do?
Alex - They're taken up by a cell called a macrophage. At some point, that cell may attract additional cells and this whole conglomeration of cells becomes a structure called a granuloma where you have some infected macrophages at the core and outer cuff of fibres and other immune cells.
Chris - The mycobacterium is actually inside the infected macrophages – not killed. It’s in there and viable.
Alex - It’s in there. In most cases, it’s quiescent and the person’s immune system is able to control it.
Chris - What about that model were you uncomfortable with which provoked you to start looking in the study that you're presently publishing?
Alex - The transition to really, the active disease is not that clear. I mean, it’s known that the immune system controls it. But let’s say now the immune system stops controlling it, what are the steps that need to happen for the infection to actually grow?
Chris - In other words, in the vast majority of people, the infection gets into the lung but then the immune system holds it back and doesn’t go anywhere, but in a number of people and for various reasons, it suddenly becomes productive and it starts to grow in the infected cells, in the infected person, but we don’t really understand what the triggers are or what the immune processes are that allow that escape.
Alex - Yeah, that allow not only the escape but also the continuous growth of the bacteria.
Chris - So, how did you manage to study it, given that people have been looking at TB for a century and they haven't yet been able to find this?
Alex - The tools that we use are fairly recent, so it’s time-lapse microscopy. We figured that the best way to understand it is just to see it. We just filmed the infection. So we used a very simplified model, so we just infected macrophages and we looked at what happens – how the infection actually grows or does not grow.
Chris - What did you find?
Alex - We’re actually surprised. First, that these cells are tough as nails. The person’s cells which contain these mycobacteria, they can contain the infection when they're in fact with low numbers. Up to about 50 bacteria, they're okay, but above that, they start to die. But what surprised us more is that the bacteria don’t die with the host cell and they grow very nicely in the dead cell where before, in the live cell, they were actually controlled even at high numbers.
Chris - So basically, by killing the cell, they’ve now released a whole bunch of nutrients and food stuffs that those microbes can then assimilate and used to fuel the further amplification of numbers.
Alex - Yes, so it seems. So, the cell before was contained then in a compartment called the phagosome. Once the cell is dead, it’s dead. It’s like a sack of nutrients – whatever is left of it.
Chris - And then what happens?
Alex - Well, macrophages are phagocytes so they pick up all the junk and bacteria, and so on that surround the lung. But it will now then go for another live cell but what they will do is engulf a dead cell even if it is infected with a lot of bacteria. So, you have the first cell which died because of a high bacteria load. It’s going to be engulfed by another cell. Now, that cell is faced with an equal or greater problem because now, the bacteria had some chance to grow. Now, once that cell dies, the bacteria have extra nutrients and the next cell is going to come along, and pick that cell up. So basically, cells, at that stage of infection have two functions – one is they provide fuel for the infection to grow and second, they're bait for other cells to come in and engulf them and die.
Chris - So in a nutshell, we have a 3-phase lifecycle going on where you have an initial infection. If the number of bacteria in the initial infected cell increases to beyond a threshold of about 50, the cell is compromised. In the course of dying, it then becomes bait to pull in more potential victims and so, you get this sort of positive feedback going on where your amplifying the bacteria, feeding them in the process, and attracting more food as they grow.
Alex - Right. The only thing you need for it to start this process is you need enough bacteria in one place. So the bacteria need to be concentrated in a clump; this is in fact what a TB naturally do.
23:38 - Schistosomiasis: what happened next?
Schistosomiasis: what happened next?
with Jim Collins, University of Texas Southwestern Medical Center
Last year the Naked Scientists heard from Jim Collins at the University of Texas Southwestern Medical Center. He’s been studying schistosomes, and when we last spoke he’d just discovered one of the ways that these parasites, which multiply inside freshwater snails and then infect humans when they enter the water, fend off our immune response. Chris Smith caught up with him to hear how the work’s progressed…
Jim - Schistosomes which are these parasitic flatworms infect hundreds and millions of people have stem cells. Once they're inside their human host, they're capable of surviving there for decades. We’ve postulated that the stem cells are playing an important role in keeping their tissues young and healthy throughout this really long lifespan that they have.
Chris - Because one of the big challenges is, these parasites have to live up-close and personal with the immune system, and they're therefore potentially being attacked and assailed all the time, aren't they? So they may well have tissues that need replacing.
Jim - Exactly. So they live in the blood which is basically the frontlines of the immune system. And so, what we found is that these schistosomes – the so-called blood flukes – they have these stem cells but it seems that the main job of these stem cells in the parasite is to constantly rejuvenate the parasite’s skin which is the structure called tegument. And so, it suggests that stem cells are playing a role in perhaps allowing the parasite to maybe even to survive in the blood, but maybe also as a means that they were able to invade the host immunity.
Chris - Now that’s sort of where we left off when we spoke last time, but how have you taken the story forward and actually, challenged that notion to say, “Well, if that’s true, we’d better prove it.”
Jim - So it turns out that that’s a very challenging experiment to do – to directly test the model that the stem cells were involved in modulating how the parasite interphases with the host. And so, what we’ve been focusing on is the developmental aspects of how the stem cells are rejuvenating this tegumental structure.
Chris - Presumably, if you were to disable this system then you would render these animals much more vulnerable to things like immune attack. And therefore, the likelihood of them being cleared from the body would go up enormously.
Jim - Exactly. So, if we could figure out a way to block the stem cells’ ability to make these tegumental cells, then that absolutely could be a way to think about trying to target these parasites. If you blunt their ability to make new tegumental cells, our hypothesis is that then the parasites would no longer be able to survive the hostile environment within the immune system and those are experiments that we're actively trying to get to work in the lab. It turns out it’s not such an easy experiment because what it actually requires is a viable immune system which we have really no means of replicating inside of a dish where we, in cell culture where we do a lot of these experiments to the parasites, what's it going to require is taking parasites out of a host such as a mouse, manipulating them to deplete their stem cells and then transplanting them surgically back into an animal that’s capable of evoking immune response against the parasites. And so, we’ve developed a procedure based on Pplusic literature to be able to do this in the lab. We successfully done that and so now, we’re working to basically kill the stem cells and then put the parasites back in and see what happens.
Chris - Is there no way you could wire some kind of genetic vulnerability into the cells so that you could for instance administer a drug and then kill them off that way?
Jim - Yeah. It’s one of the challenges of studying – when we collected tropical disease is that in many ways, the organisms that we use had experimental neglected and we don’t necessarily have the tools that you have to study things like fruit flies or nematodes. And so, one kind of pie in the sky idea that a number of people in the community are trying to develop is genome editing techniques where you basically use this system called CRISPR/Cas9 to make changes of the genome in the organism and then you can exploit those to try to understand the biology of the animal. Unfortunately, given the way that parasites live, they have a very complicated life cycle. It makes it really challenging to try to develop ways to implement this kind of new genome editing technologies to manipulate these parasites.