Podcast: Rats, mice and the diseases they give us
A field vole
18 February 2014 by Richard Hollingham
This week in the Planet Earth podcast, Mike Begon, Andy Fenton and Federico Costa of the University of Liverpool describe the work they're doing to understand how infections like Weil's disease and bubonic plague move from rodents to people.
To assist those who find text-based content more accessible than audio, a transcript of this recording is available below.
Richard Hollingham: I'm Richard Hollingham and this time in the Planet Earth podcast - rats, mice and the diseases wild rodents spread. Well to discuss that I've come to the Wirral in North West England to an overgrown woodland just off the main road, a few miles north of Chester. It's a damp, overcast day, the trees are bare and the ground is covered in rotting leaves and dead wood, but this is an area favoured by scientists from the University of Liverpool to track rodents to monitor infections in wild populations, and I'm joined by Andy Fenton who studies UK rodent populations and also by Mike Begon who works on human diseases spread by rodents in Central Asia and South America and his visiting colleague, Federico Costa who works in Brazil. Now, Mike, let's talk about the sort of diseases rodents spread. Some of them are pretty nasty.
Mike Begon: Well some of them certainly are and the ones, I suppose, we've chosen to work on abroad are. We work on bubonic plague in Central Asia and although it is not as serious a problem as perhaps it was in the Middles Ages, as we know it, it wiped out large pars of the population of Europe, it's still with us and still a threat. With Federico we work on leptospirosis which is prevalent throughout the world and a really serious problem, morbidity and mortality as well.
Richard Hollingham: And, Andy, here you're looking more at diseases spread to animals?
Andy Fenton: That's right. We're interested in how diseases transmit amongst the animals themselves, so those rodents act as reservoir hosts for many human diseases, whereas in the UK we're interested in the spread of those diseases amongst the rodents, so how do they pass from individual to individual or from species to species.
Richard Hollingham: And this wood, it is not the most pleasant environment, particularly today, but this is your perfect habitat for the sorts of rodents you are after.
Andy Fenton: That's right. It's ideal for us and we're interested in wood mice and bank voles, so two very common species in Britain. Wood mice are found pretty much in any woodland, whereas the bank voles seem a little bit more fussy and this kind of woodland is perfect. It seems older, there's more vegetation, there's thistles and brambles all over the place, it's perfect for those kinds of rodents.
Richard Hollingham: What sort of diseases do they carry though?
Andy Fenton: Well we're particularly interested in some of the worms that they carry, they carry these parasitic worms just like many humans and other animals, so they carry these worms in their guts and we're interested in how those spread from animal to animal, but also how those worms interact and how they affect other diseases like viruses and bacteria that they also happen to be carrying.
Richard Hollingham: Now you trap rodents in these woods. We're not likely to see any today though are we?
Andy Fenton: No, that's right. The peak trapping is from May onwards and that's when the rodents really start to bred and that's when their population numbers increase. So there will be rodents out there but we would be very lucky if we managed to catch one today.
Richard Hollingham: Why wild rodents rather than using lab rats?
Andy Fenton: Well we're really interested in the variation that natural populations have. So individuals differ in terms of their condition, their food, their age and their parasitic infections, so obviously lab studies are perfect for understanding certain aspects of disease whereas we want to understand how the variability you get in natural populations affects the spread of diseases.
Richard Hollingham: Now we've mentioned this idea of them being reservoirs of disease and that's a major concern with them. It's no much that they get ill; it's that they spread disease to something else we might be concerned with such as livestock or, of course, humans.
Andy Fenton: Well that's right. I mean, again, in Britain rodents although they can infect humans with certain diseases it is really in other parts of the world where those major diseases are transmitted from the rodents and those are the diseases that Mike and Federico are studying.
Richard Hollingham: So let me just carefully step over the brambles to Federico. You work in Brazil - a rather different environment to this. What's that like, the place you work in?
Federico Costa: Well we work in the labs at favelas in Brazil and this is quite a different environment from the one that we are at today. Really it is an environment where we have a high risk of transmission of leptospirosis. This is a disease that is just made by the contact with [unintelligible] or mass contaminated with the urine of rats. So the indoors environment people live very crowded with very bad environmental conditions like near open sewage or without trash collection. More than 10,000 people in Brazil become infected every year and for those approximately 10% die.
Richard Hollingham: Mike, what are you trying to do there with studying the rodents? The rodents interactions with humans or what is the issue?
Mike Begon: Well I think the issue, as Federico has pointed out, it's already known at what level those risks factors are and its proximity to open sewage or to uncollected trash and those sorts of things, but we also know that the ultimate source of the infection is the wild rodents. And so I think the basis of our joint project is that if we better understood the way in which leptospirosis is spread within those rodent populations then we could better map the temporal and the spatial variation in the risks that they pose and so ultimately understanding the rodents will understand variations in risks to humans.
Richard Hollingham: This work here in this rather dingy wet woodland is connected to your work that you're doing in Brazil and Central Asia.
Mike Begon: Oh very much so. I think the work that Andy is doing here and I have to some extent done myself in the past, the government funded research here in the UK, is what is has allowed us to go into Central Asia and go into the favelas of Brazil with some kind of knowledge and understanding and work there as well, so I think there's an intimate connection and actually a very pleasing spin-off, if you like, from what the UK government is investing in here is not only taking us forward scientifically with longer terms implications but is having more immediate impact elsewhere in the world simply by the things that we have learnt here.
Richard Hollingham: So let's talk about trapping these rodents. First of all in the woodland here... again I'm getting tangled in the brambles! You say that there are plenty of rodents here. How do you trap them?
Andy Fenton: So we use metal boxes. These are specially designed traps that we place out. We bait them with grain and some vegetables. We leave them overnight and then we come back the next morning and we basically see what has gone into the traps and then we release them again to be caught again the following month.
Richard Hollingham: So you're taking blood samples from these... what are you doing?
Andy Fenton: We take a small drop of blood from the tips of their tails but we also collect poo samples, fecal samples, because as I say we're interested in their worms and that's the best way to see what worms they are carrying.
Richard Hollingham: It's one thing to do it in the woodland here, how do you do this, Federico, in the slums of Brazil?
Federico Costa: Well the process of trapping is similar to the wood but the rats are in very close contact to humans we put in the gardens or around the house, of course. So we've got the rats every morning and we took samples of [unintelligible] and urine and all their tissues because they are also reservoir for all the diseases, not just leptospirosis but also hantavirus and Bartonella, etcetera.
Richard Hollingham: How many rats do you get, Mike, when you go out there and were you shocked by the numbers?
Mike Begon: It's actually more difficult that you might imagine to trap rats. IU mean if you ask how many rats there are there are lots. If you ask how many rats we trap and it's never as many as we would like to. Rats are actually rather trap-shy and it's a problem. Andy can talk here about the mice and the voles more or less trapping themselves and it's almost magical they way that they appear in the traps the following morning. The rats are actually more difficult to entice into the traps. So that's part of a problem and it's one of the differences between working there and working here, unfortunately.
Richard Hollingham: Given that there are probably, what, tens of thousands of rats in these slums, all potential disease carriers, what sort of progress are you making?
Mike Begon: Well I think it's important to understand with all of so-called zoonotic diseases, that is to say diseases carried from wild life populations into humans. The thing that characterises them is that you're never going to get rid of those reservoirs. The idea that we can somehow ultimately control the rats and therefore control leptospirosis I think would be a fallacy. What we need to do is to better understand the interface between the rats and the humans and put up barriers and be very focused, that's the point really, being focused in our controls. So in terms of making progress I think it is in a sense scientific progress rather than feeling, well, you know, there were however many thousand rats when we started and there are almost as many when we're finished, that's not quite the point really. The rats of leptospirosis and all of these others zoonotic diseases will always be with us. What we can hope to do, I think, is just prevent that transmission from the reservoir into humans, or if not prevent it at least slow it down. I think that really is the point.
Richard Hollingham: Andy Fenton, Mike Begon and Federico Costa, thank you all very much. I'll take some pictures of our recording here and as ever we will put them on our Facebook page. You can also find us on Twitter - just search for Planet Earth online. And that's the Planet Earth podcast from the Natural Environment Research Council. I'm Richard Hollingham from a rather damp, grey woodland near Chester. Thanks for listening.