Ant Infection Control
This winter, we’ve seen very high levels of respiratory infections that have caused severe outbreaks when they’ve spread around care homes and hospitals in the UK. Transmission tends to occur in these environments because individuals are cared for at high density; and the way institutions control infections under these circumstances is by using strict policies that isolate and even exclude cases. That’s us humans, but there are other classes of animals that also live in high density and they’ve developed ingenious infection control strategies of their own. Speaking with Chris Smith, Chris Pull explains how he's been looking at what ants do…
Chris Pull - We know that social insects, so that's ants, bees, wasps, and termites have evolved collective disease defences to try and control epidemics in their colonies. But a lot of the work so far has looked at how they prevent infections. So, for example, they groom one another and they use anti-microbial disinfectants to prevent individuals which come into contact with pathogens from actually contracting an infection. But what we wanted to know is how they actually prevent successful infections from spreading. So in cases where these sort of first line defences fail to prevent disease what can a colony do to prevent the infection spreading into others.
Chris Smith - How do they know that they have an outbreak situation in the first place?
Chris Pull - So what we've been able to show through our research using chemical analyses, is that they can actually smell when another individual is sick. So we've shown that sick individuals when they have an infection and when you also inject them with an immune elicitor to increase cuticular hydrocarbons this attracts the attention of ants in the colony and triggers a response.
Chris Smith - So this is like "ant B.O." isn't it, these cuticular hydrocarbons that they can sniff on each other?
Chris Pull - Yes exactly. So they use them typically to tell if you're a member of the colony or not but we've been able to show now that they actually also change them in response to infection and that can actually tell others who's sick.
Chris Smith - So what is the situation when they pick up this chemical trace or chemical signature of diseases there. How do they respond?
Chris Pull - It's quite interesting. They, sort of, have this multi-component behaviour. So we were looking at infections in pupa and the pupae are these sort of developmental stage in between a larvae and an adult and they're going through metamorphosis and they're encased in these silk cocoons. What we found is that upon detecting an infection the ants will actually break open this silk cocoon and then they start biting the infected pupa and then they spray poison which is made up of formic acid and acetic acid from a gland at the end of their abdomen. And this ensures that the fungus or the pathogen grown inside the infected brood can't grow anymore. So, the acids actually seem to kill this fungus which is inside the body of the pupa and it seems like they do this because the poison itself can't actually penetrate into the body of the pupa unless the cocoon is removed and unless they make these holes in the body of the pupa itself
Chris Smith - And can you demonstrate that this really does mitigate or curtail the spread? So in other words, if you abolish this behaviour it would be curtains for the colony.
Chris Pull - Yes. So we've actually been able to show by mimicking a situation where they failed to detect and destroy these infections, so we simply kept ants with an infectious pupa. Forty percent of these small groups of ants actually contracted the infection and became infectious themselves. And you can imagine that in a full colony set up that can very quickly lead to a sort of huge mass break out of this disease, but by performing these behaviours we actually saw that there was zero disease transmission.
Chris Smith - Do other social insects that have similar problems deal with it the same way or do they have a different strategy?
Chris Pull - So we do see different strategies. So in honeybees, because they live in these hives and they forage on the wing, what they can do is to simply take disease brood out of the nest fly away a few hundred metres and just drop it somewhere in the vegetation because they forage on plants and they forage for wide distances around the colony. The chances that they encounter that infectious corpse are really low. The termites on the other hand, what they do is actually eat their dead so they are living encased in these sort of pieces of wood which are rotting away and for them it's hard to remove things from the wood because they really live inside it. So what they tend to do is to actually eat their diseased individuals but at the end of the day, they all use more or less a similar strategy so they're all trying to detect very early these infections and even remove, or to destroy, or to eat them before they have the chance to become infectious.
Chris Smith - That's quite an undertaking strategy isn't it, actually eating you're dead? But how do you think this evolved in the first place? Because it's quite a complicated behaviour isn't it? Involves the ability to do chemical detection and recognition and then to have evolved a strategy that is itself successful in mitigating the threat.
Chris Pull - Yeah so we think that these behaviours have evolved because a social insect colony is like a superorganism, so they behave and they reproduce like a single organism in itself, and in a way, they're very similar then to a multicellular body like a frog or a human being. And in the same way when a human has an infected cell in its body you have this immune reaction to remove that infected cell and we see then common processes in multicellular organisms and these sort of super organismal insect societies and we think that common evolutionary processes were at play during the evolution of both multicellular organisms and superorganisms, and been able to basically detect and remove elements which might harm the entire organism in itself were necessary, sort of, prerequisites or at least were necessary to evolve in order to ensure that, like, you have the survival of the whole over its parts.