Ok, so, unless you've been living under a rock in the marine world for the past 24 h, you'll no doubt have been informed that the mystery of sea star wasting disease (SSWD) has apparently been solved (1). Srsly, it's already in the Wikipedia entry for Sea Star Wasting Disease, get with the program. For those of you who would like a reminder of what SSWD is and why we care: as you wish(2).
|This is why we care, because I almost stepped in a gross pile of sea star. |
December 3, 2013, Shilshole marina. Evasterias troschelii(?)
Are we all caught up now?
Good because it turns out that it might not be as simple as we would want. This weekend, I spent a few days at a conference, and while in the past I have complained of SCAD, this year, I've really been enjoying the conferences I've been to (more to follow?). I saw a talk this weekend presenting additional data that will soon be published complicating the story.
But first, let's see where we are:
The Hewson et al. paper that is receiving so much attention this week appears in an early edition of PNAS, showing a bunch of evidence that this densovirus(3) is associated with the collection of symptoms known as SSWD, e.g., melting, wayward errant arms, complete disintegration of the body within 24 hours of first showing symptoms(4).
Here is some of the evidence the authors are presenting:
1. Researchers took infected individuals, mashed them up, then filtered the diseased mash through a filter with extremely small holes, designed to only allow particles as small as viruses to go through. Then they took the virus slurry and heated some of it to very high temperatures, which should cause the DNA in the viruses to break apart. Then they injected some asymptomatic sea stars with the heat-shocked virus slurry (which was subsequently cooled), and some with virus slurry that hadn't been heated. Stars that got the unheated slurry got sick, stars that got the heat-shocked slurry did not.
- What this means: This shows pretty clearly that a virus, one that is susceptible to heat damage, causes SSWD.
2. Researchers took tissue samples from infected and uninfected stars (total of 28 stars) and looked for whatever virus DNA they could find. Then, after characterizing as many of the viruses as they could, the looked at whether any of the individual viruses was more common in infected stars than uninfected ones. The densovirus they currently attribute to SSWD appeared in 7 of 15 symptomatic stars, but only 2 of 13 asymptomatic ones.
- What this means: A single densovirus was more associated with the symptoms of SSWD in this sample of seastars.
3. Having a guess about the thing doing the stuff allowed researchers to follow up with a bunch of experiments looking at the amount of the virus (viral load) present in symptomatic versus asymptomatic individuals. They measured viral load in a bunch of stars collected from the field, and found that, generally, the stars that already look messed up have more of the densovirus in them, BUT asymptomatic stars also contain virus.
- What this means: Having more virus means a star is more likely to be showing symptoms of SSWD. However, the fact that stars that look just fine also have a non-trivial viral load is a bit tricky to interpret. It could be that they are incubating the virus and haven't started to show symptoms, it could be that they are resistant and the virus doesn't cause disease progression in that individual. It could also be that something else causes SSWD that makes stars more vulnerable to this densovirus (5).
Ok, there's a bunch of other fun data in the paper as well, which makes excellent leisure reading. I suspect no disease of non-commercial marine invertebrates has ever received this much attention.
Now to the caveat that started this post:
It's a lesson in ecology that as soon as you think you have a story, the next thing you do almost always blows your theory to bits.
Ben Miner gave a talk this weekend at the Western Society of Naturalists Annual Meeting in Tacoma (one of about 40 on SSWD), and showed new data that will soon be published that thicken the plot. Similar to the data shown in the PNAS paper (which he also co-authored), Ben took samples from stars and had them measured for viral load. However, rather than calling them symptomatic/asymptomatic at the time of sampling, he watched each star for several weeks after the sampling. This way, any infections that were incubating would not be erroneously called "asymptomatic". It turns out when you look at the sea stars this way, the densovirus load is quite a bit LOWER in individuals that ultimately end up with SSWD.
Well, shoot. Was the previous association of viral load with symptoms just a fluke? Is there another possible explanation? Too soon to tell!
Science isn't a romance novel that ends all tidily with the one person cozily ensconced (in the PG version) in the other person's arms (trying to avoid being heteronormative here). Science is like one of those radio serial cliffhangers, where you have to "TUNE IN NEXT WEEK to find out how our heroes get themselves out of this jam!" The joy of science is that it is iterative, and this paper represents a substantial advancement of the knowledge on this disease which has been stymying scientists for nearly two years now. We can only wait with baited breath until the next installment!
References and Miscellany:
(1) Actually if you've been living under a rock in the marine world, you might have SSWD, check with your Dr. (*Editorial Note: J/K, people don't get SSWD, our arms don't walk away like that)
(3) This virus is in the Parvoviridae family - which also includes canine parvovirus, to bring it back to something some of us might be able to use as a reference point)
(4) You know, things that generally suck when they happen to you.
(5) But, for the love of God, not radiation from Fukushima-Daichi. If anyone puts this forward as a serious suggestion, I have nothing left to say to you.