By Jonas Waldenström
Dogs do it, mice do it, and probably a bunch of other mammals do it too! Do what? Use their noses to sniff the disease status of conspecifics. Chances are, you do it too – certain diseases give an odor taint, a recognizable miasma, suggesting us to keep our distance. For a dog, the world is made up of fragrances, and a sniff in the but is a social call and a way of catching up on the latest developments (e.g. what you ate, which reproductive state you currently are at, and your disease status). However (unfortunately?), humans very rarely smell each other’s bottoms – it is simply not socially acceptable, and we are not very bendy animals. Even though our noses are better than we give them credit for.
But what about ducks? How and what do they smell? In the rear end? After infection? These questions are about to finally get an answer through the very recent publication of Dr Kimball and his USDA and Chicago State University colleagues in the journal PLOS One! Read it, it has already gone viral on the internet – some studies are just so unexpected that they flutter into the limelight for a while. But similar to IgNoble prizewinning studies, it does tell you some things worth remembering.
So lets tease this study apart and look at the rationales and the results. First of all, influenza A virus infection in Mallards is a gastrointestinal infection, with viruses primarily replicating in the cells of the smaller intestine. Virus progeny is released in huge amounts with feces out into the environment. These well-known facts make the rear end a good starting point for studies on the possible olfactory difference between infected and uninfected birds. Mallards are fairly gregarious, although not extremely social birds, and the viruses released from the behind of one individual need to find its way to the front of the next individual. Mallards spend a large part of their lives dabbling – a behavior where surface water is taken into the mouth, the beak closes and the tongue presses upward, forcing the water through thin lamellae on the side of the beak. The yummy stuff, and perhaps viruses too, are stuck on the lamellae and is swallowed down. This behavior is thought to be one reason why Mallards (and other dabbling ducks) are especially frequent influenza A virus hosts.
Given this, it would actually be beneficial for a duck to be able to tell if the duck ‘over there’ is excreting viruses or not. Kimball et al. took this to heart and infected six domestic Mallards with an H5N2 low-pathogenic avian influenza virus. The researchers collected Mallard feces (the fancy word for bird shit) before experimental infection, and up till 10 days post infection. These fecal samples were then used to train mice – yes, you read it right: MICE – to differentiate between duck feces from infected and uninfected individuals. In the training sessions, mice were rewarded if they went to the right fecal sample (i.e. the infected one), and this was later tested in a double-blind test where both the mice and the operator had no clue where the different samples went in.
The tests showed that mice were actually quite good at learning to differentiate which birds that had been positive. The remainder of the paper examines which volatile compounds that were present in the feces and that may have given the results. For those of you that are keen chemists, I suggest you read the original article. For the rest of us, let’s just settle with that there were differences in the chemical spectra of the two groups. Particularly, acetoin (3-hydroxy-2-butanone) was more prevalent in the fecal samples from the infected birds.
Let us pause here and summarize. Mice can be trained to distinguish the smell of fecal samples from an influenza-infected duck. End of story. But a rather fun story. And not too far-fetched. The volatiles associated with infections are coming more and more in medicine, all the way from dogs trained at sniffing out cancers, to breath test to diagnose Helicobacter pylori infections. Smell is the future. However, the results of the current publication are not strong enough to say much on the use of olfactory cues among Mallards. Unfortunately, the paper is very thin in the material and methods section and the scant information on the infection protocol, the methods used for detecting influenza A virus in the samples, and for how long individual birds where shedding virus aren’t very helpful. If we are to believe the results we also need to be able to read all relevant information. For instance, the sex of each bird, whether control birds (if they indeed where controls) were housed together with the others, or separately, whether the diet and husbandry was the same for all birds etc. Many small questions, but where answers are important for interpretation.
And the bigger question, of course, is whether ducks themselves can distinguish the fragrance of infection. And whether that translates into a modified behavior. My personal feeling is that a duck with its bill spooning up water with muck and filth like a boss probably doesn’t take the time to smell the roses.
Citation: Kimball BA, Yamazaki K, Kohler D, Bowen RA, Muth JP, et al. (2013) Avian Influenza Infection Alters Fecal Odor in Mallards. PLoS ONE 8(10): e75411. doi:10.1371/journal.pone.0075411