Disease is a property of the individual

Ecologists are obsessed with variation, in any form, the more bizarre, the better. We really love it! But why?

The textbook explanation is that variation among individuals, if heritable, work as a template for selection and thus drives evolution. Without variation, little can change. Evolutionary important variation relates to genetic traits that make the organism better adapted to its environment, a better competitor, more disease resistant, or relates to traits that make him/her more attractive to the other sex, thereby increasing the likelihood of siring offspring.


And additional explanation, and sometimes equally important, is that it is fun with variation: an animal may be short or long, have a peculiar nostril shape, vary in the curvature of antlers, or have striking plumage colors. Simply, humans like variation, and the diversity in itself therefore drives curiosity-driven researchers.

This said, when it comes to disease in animals most researchers tend to neglect variation. Disease is commonly treated as a constant; the animal is either infected with parasite X or is not. However, in reality what the researcher denotes as parasite X may actually be a plethora of different pathogen genotypes, all seemingly dressed in the same costume (the phenotype), or sometimes even consist of cryptic species. This is dangerous, as things that look the same in the microscope (or in a conserved gene used for molecular screening) may have fundamental differences in traits that are relevant for infection processes, such as pathogenicity, transmission and virulence. Simply, we may run the risk of not seeing patterns that are there, or jump to the wrong conclusion based on simplified assumptions.

Further, surprisingly often wildlife diseases are treated at the level of the population (especially abundant in veterinary medicine), and not at the level of the individual animal. For instance, prevalence, the proportion of individuals carrying a particular disease at a given time, is much more frequently used than estimates of incidence, which relates to the risk of acquiring infection. In the former you can adhere to a ‘hit and run’ sampling approach, in the latter you need to monitor individuals across time and take repeated samples.

For a long time, actually since 2002, we have studied influenza A virus in a migratory population of Mallards in SE Sweden. We also started at the level of population, describing temporal variation in influenza A virus prevalence in the duck population, and describing differences in prevalence among ages and sexes. And yes, we treated the virus as pathogen X, not at the level of subtype (which there are many of in flu). But with time we have moved to assessing what is happening at the individual level, and how differences among individuals in susceptibility drive disease dynamics, and how disease histories and immunity patterns in turn drive evolution in the virus.

These efforts are starting to pay, and in a paper published this week (http://www.plosone.org/article/info%3Adoi%2F10.1371%2Fjournal.pone.0061201) we address the issue of individual variation among Mallards in influenza A virus infection risk. The question we asked is how individuals with the same background, in a shared environment with similar exposure to influenza, differ in disease histories and immune responses.

In our monitoring program we use a large duck trap to catch wild ducks. By providing grain we give the birds an incentive to visit the trap, and as additional attraction we have a compartment with lure ducks, that are supposed to get the wild ducks to enter. In this study, we used the lure ducks as a natural infection experiment. Ten immunologically naïve, juvenile Mallards from a farm were placed in the trap and were then followed throughout an autumn season, and then for the next spring, summer and autumn. Fecal samples were collected daily and blood samples approximately every second week. A lot of samples, and collected with a precision that allowed us to give very detailed infection histories for each individual.


In turned out that our study ducks varied tremendously in disease patterns, despite being of the same age, raised in the same farm, sharing the same little experimental enclosure and being exposed to the same environmental variation. All ducks became infected with flu within the first five days of being placed in the trap, but the number of infection days varied tremendously. And so did the number of retrieved virus subtypes, thus different individuals were infected with varying number of virus variants, in this case equal to different infection events.

Furthermore, we got really nice long-term patterns. After the initial primary infections early on in the first autumn, and a number of secondary infections later the same autumn, we recorded only a single infection day the next spring and summer. It wasn’t until the second autumn, when migration of wild ducks started in earnest again, that new infections were seen in the lure ducks. And in this case, no infection was of a subtype the individual had experienced the year before, suggesting very strong and long-lasting homosubtypic immunity.

Individuals also varied profoundly in their immune responses. We measured the humoral immune response, manifested as anti-influenza-antibodies (raised against the conserved nucleoprotein of the virus), across time. Have a look at the figure below; it really shows variation both on a temporal scale, but also at the individual scale, both in patterns and in height of response.


So what does it tell us? To start with, there is a large difference between individuals in resistance/susceptibility to influenza A virus infection in Mallards. This difference is not only manifested in different infection histories, but also as very variable immune responses. Second, these differences are very likely determined by genetic differences, meaning that there are heritable differences, and thus traits that could be selected for by natural selection. Not all ducks are equal – and this important for our ability to model disease dynamics in this system. Is it really the mean that is important for assessing the transmission probabilities along migration? Perhaps it is the outliers that are driving the processes?

This study is a first step to adress individual variation, and there are already a couple of follow-up publications in the peer-review tube, so we will have opportunities to get back to this topic.

That’s all for now. Live long and prosper – and don’t treat disease simply as a property of the population.

Jonas Waldenström

How to survive the ERC panel

How does a researcher look and behave? Think about it for a while.

If you ask a 10-year-old kid, his perception may already have been tainted by how Nickelodeon and Hollywood portray scientists – a mad lunatic, a vicious bearded guy out to make petty vengeance on the world. Evil geniuses such as Lex Luthor, Dr Evil and others.


If you ask your grandmother, she is more likely to get a soft expression in her eyes. In her rocking chair, with the quilt in her lap, she sees scientists as part of the prodigious progress of the 20th centaury. Dedicated, honest people of societal status, something to proud of, people that changed the world to the better. People that made computers and medicine. Alexander Fleming, Albert Einstein, perhaps even Stephen Hawking – the beautiful mind in the frail body.


If you ask your mate (which you likely shouldn’t do), he’ll laugh at you, punch you in the shoulders, and then going back to discuss what that nice lass was wearing at the party last week, or what the local soccer team should have done, but didn’t do.

Or, is Messi really the best player in the world? Really?

Being a scientist myself, it is quite easy. I know that scientists are just as any other folks. Some ugly, some pretty. Some intolerably smart, some absolutely stupid, but most just like you. Normal folks with an unusual occupation. Sure, there will be an excess of A-personalities with a competitive streak, but not more than you will find among physicians, lawyers or politicians.

Thus, normally I don’t care. I go to the university, fiddle around with manuscripts, teach students about virulence, signing invoices (too many) and make the research lab go a-round.

Today however, wasn’t any normal day in the office. I have spent the day in Brussels to attend an ERC Starting Grant interview. The ERC (European Research Council) are kind enough to distribute money to research, ‘making great ideas work’ is one of their slogans. But to be entitled you need to apply, then the application need to be considered interesting enough, and you as a researcher talented enough, to reach the next level. All very meritocratic, with extensive review processes, ranking of candidates by peers.

Normally it ends there. For instance, at the Swedish Research Council (the major funding agency in Sweden) the 10-15% highest ranked applications get funded and the rest are gently declined. At the ERC Starting Grants, however, that is just the first hurdle. If you pass till the second round you are allowed to present your full research proposal and then go to Brussels for an interview with an expert panel. A make or break interview. Either you get 1.5 million euro, or nothing. Either all fame, or utter disappointment.

And this is where you suddenly start to think about how you look and how you behave.

How should you make a significant impression come across to the panel in a 10-minute presentation and 15 minutes question slot? You train your presentation, you work with the figures, and you start to make answers to putative questions. You start to question yourself – are you really good, are you a fraud? What if they ask you ‘that question’? What if you start stutter and blush? What if you forget to tuck in your shirt, or if the zipper is open in your pants?

I am normally quite cool. But this was, I need to confess, rather nerve-wracking. Fortunately, my university sent me on a course – to Yellow Research in Amsterdam that is doing tailored courses for ERC presentations. I learned to not stand as ‘a construction worker’, stand straight, use my hands, give short answers, appear confident (at least on the outside) – simply prepare like I have not prepared before.

We’ll see whether that helped or not. Today it is done. I had my 25 minutes with the panel. I kept to my allotted time, I answered the questions reasonably well – although in hindsight I could have been better on certain aspects. But what the hell, I survived, and I feel fine. I did what I could, and can only hope it is enough.

And guess what? The panel was all very friendly, no monsters, just normal researchers – in short, people just like you and me. Isn’t that quite lovely.

Now I am waiting for my flight home – at Brussels Airport, probably the only airport in the world where there aren’t any decent bookshops. But what Belgium lacks in books, they have in beers! Lets have a second Leffe and wait for boarding.

Jonas Waldenström