Ant-y Infection Control
This winter in the northern hemisphere hospitals and care homes have faced one of their worst flu seasons ever in terms of numbers. Transmission of respiratory illnesses tends to occur more readily in these sorts of places because there’s a high density of people so it’s easy for them to pass infections amongst themselves. But there are other classes of animals that also live in very high density and yet they’ve developed ingenious infection control strategies to ensure that this doesn’t happen to them. Chris Smith spoke to Chris Pull from Royal Holloway University who has been looking at what ants do.
Chris P - 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 another and they use antimicrobial 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 to others?
Chris S - How do they know that they have an outbreak situation in the first place?
Chris P - What we’ve been able to show through our research using chemical analysis is that they can actually smell when another individual is sick. We’ve shown that sick individuals when they have an infection, and when you also inject them with an immune elicitor, increased cuticular hydrocarbons and this attracts the attention of ants in the colony and triggers a response.
Chris S - So this is like ant BO isn’t it, these cuticular hydrocarbons, that they can sniff on each other?
Chris P - Yes, exactly. They use them typically to tell if you are a member of the colony or not. We’ve been able to show now that they also change the immune response to infection and that can tell others who are sick.
Chris S - What is the situation when they pick up that this chemical trace or chemical signature of disease is there, how do they respond?
Chris P - It’s quite interesting that they have this multi-component behaviour. We were looking at infections in pupa, and the pupae are the developmental stage in between a larvae and and adult ant. They’re going through metamorphosis and they’re encased in these silk cocoons and what we found is that upon detecting an infection, the ants will 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. This ensures that the fungus or pathogen growing inside the infected brood can’t grow anymore, so the acid seems to kill this fungus which is inside the body of the pupa. It seems like they do all this because the poison itself can’t 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 S - Can you demonstrate that this really does mitigate or curtail the spread? So, in other words, if you were to abolish this behaviour it would be curtains for the colony?
Chris P - Yeah. We’ve actually been able to show by mimicking a situation where they fail to detect and destroy these infections. We simply kept ants with an infectious pupa; 40% of these groups of ants contracted the infection and became infectious themselves. You can imagine that in a full colony setup, that could very quickly lead to a huge mass breakout of this disease. But by performing these behaviours we saw that there was zero disease transmission.
Chris S - Do other social insects that have similar problems deal with it the same way or do they have a different strategy?
Chris P - We do see different strategies. In honey bees, because they live in these hives then they forage on the wing, what they can do is simply take the diseased 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 re-encounter those infectious corpse are really low.
The termites on the other hand, what they do is eat their dead. They live 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 live inside it, so what they tend to do is to 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 either remove or to destroy or to eat them before they have the chance to become infectious.
Chris S - That’s quite an undertaking strategy isn’t it, actually eating your dead? But how do you think this evolved in the first place because it’s quite a complicated behaviour isn’t it? 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 P - Yeah. We think that these behaviours have evolved because social insect colonies are like a superorganism, so they behave and the reproduce like a single organism in itself. In a way then, they’re very similar 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. We see then common processes in multicellular organisms and these super-organismal insect societies. And we think that common evolutionary processes were at play during the evolution of both multicellular organisms and superorganisms and be able to detect and remove elements, which might harm the entire organism in itself were necessary prerequisites, or at least were necessary to evolve in order to ensure that you have the survival of the whole over its parts.