Nurseries are notorious hotbeds of disease and contagion. Place one snotty child in a room with 10 healthy kids and you're on the cusp of an epidemic. Now imagine this nursery multiplied by hundreds and you'll have an idea of life inside an ant colony, where sick ants can easily transmit pathogens to their sisters. To reduce this risk, ants use different strategies to detect and disinfect diseased nestmates, effectively acting as nurses, executioners or morticians. But what if the nurse herself is infected? Will she fall victim to disease or can she act to reduce her risks of further decline? The answer, as shown in an exciting new paper in PNAS by Matthias Konrad and his colleagues from the Institute of Science and Technology in Austria, is that it depends on what her patient is infected with.
Konrad and his colleagues began by exposing ants to one of two different fungal pathogens. The first, called Metarhizium, kills ants rapidly, while the second, called Beauveria, causes only minimal mortality. When ants infected with one pathogen were subsequently co-infected with the other, known as a heterologous infection, things got much worse and mortality rates shot up. This, in itself, is not surprising. Co-infection often elevates mortality because it forces animals to simultaneously solve distinct immunological problems. But what was surprising is that the singly infected ants behaved as if they were aware of the added dangers of a heterologous infection – and acted to reduce it.
To examine the possibility that singly infected ants are risk adverse, the team confronted them with a nestmate carrying the other pathogen. What they found was both amusing and sensible. In short, singly infected ants are cranky, just like any other sick animal. In contrast to their relatively docile uninfected counterparts, singly infected ants bite, grab and drag their nestmates about. Worse still, they spray their infected nestmates with an antimicrobial poison, especially if their nestmates carry heterologous infections. Both types of response are sophisticated and effective.
Konrad and his team argue that becoming ornery is part of a suite of adaptive responses called ‘sickness behaviors’ that involve things like crankiness, lethargy and a diminished appetite. By reducing activity and keeping others at bay, ‘sickness behaviors’ are thought to help animals fight infection. At the same time, acid spraying serves two crucial functions. First, ants that spray have fewer fungal spores transmitted to them. This lowers their risk of heterologous co-infection more than twofold, thereby forestalling a premature death. Second, because sprayed ants are disinfected, they become less dangerous to the rest of the colony.
People have a natural aversion to sickness. Watch the horror in passengers' eyes when someone sneezes on the subway without covering their nose and mouth! But we generally don't distinguish between specific types of contagion, simply because we don't know which of the myriad of diseases our fellow travelers are carrying. Did the sneeze indicate flu, the common cold or something as benign as a snuffle of dust? We can't tell. But remarkably, infected ants can tell, and moreover they then use this recognition to dictate their behavior. At present, the authors don't understand the mechanisms that regulate this complex feedback, but finding out promises to be an exciting journey.