Since June 2013, Sea-star wasting
disease (SSWD) has caused mass mortality in Sea-star populations on the west
coast of North America. This out-break is on a scale never before seen, with
twenty species being affected over a wide geographical area. Sea-stars
initially become lethargic, before developing lesions and deflating as death of
epidermal tissue occurs. Infected individuals rarely recover. However, until
now we have had little idea what was causing the disease. Hewson et al. (2014)
provide evidence which strongly suggests SSWD is caused by a virus.
Firstly it was established that
virus sized particles induce SSWD in healthy Pycnopodia helianthoides.
This was done by injecting healthy animals with homogenate from afflicted
individuals. Homogenate had previously been filtered through 1 and 0.22ยตm polyethersulfone filters to
exclude particles larger than viruses. Signs of disease appeared after 10-17
days in previously healthy individuals. This pattern was repeated when homogenate
from the previously healthy group was used to infect a third group. The results
confirmed suspicions that bacterial or eukaryote pathogens were not the culprits.
The next step was then to identify a potential virus. This was done by
comparing the viral metagenomes of both healthy and afflicted Sea-stars. This revealed
a parvovirus-like sequence, named Sea-star associated densovirus (SSaDV), which
was most abundant in diseased tissue and most closely related to viruses from Hawaiian
Sea Urchins. SSaDV abundance was also shown to increase with the appearance of symptoms
using qPCR. Interestingly SSaDV was also found to be associated with suspended
particles in the water column, which may explain why the disease is able to jump
between spatially isolated populations.
One of strengths of the study was
that the authors went beyond the confines of the laboratory and investigated
SSaDV in animals from the field over fourteen different species. This clearly
showed that SSaDV was much more abundant in diseased organisms than healthy and
a later experiment into the presence of viral RNA confirmed its activity. Three
species were then examined closely to determine the factors which most likely predicted a
diseased state. This was unsurprisingly viral load, although for Pisaster ochraceus, animals in southern areas were more likely to be diseased and
for P. helianthoides smaller
individuals had a greater frequency of disease. The historical presence of SSaDV
was assessed in Sea-stars again using qPCR. Amazingly it was revealed that the
virus was present in preserved Sea-stars from as far back as 1942. So why has
it struck now? Possibly due to overpopulation in years leading up to the
outbreak or a mutation which has increased its virulence, but we honestly don’t
know.
In review, I feel this study was
highly comprehensive and provided convincing evidence that SSaDV is the most
likely agent of SSWD. Future work will investigate the mechanisms by which
SSaDV causes death, what factors triggered the outbreak and the ecological
impact which the disease has caused on near-shore communities. Depressingly it
would seem impossible for us to do anything about it and we can only watch as
it runs its course. But I am sure we will see many more publications on this crisis
in the future.
Reference: Hewson, I., Button,
J.B., Gudenkauf, B.M., Miner, B., Newton, A. L., Gaydos, J.K., Wynne, J.,
Groves, C.L., Hendler, G., Murray, M., Fradkin, S., Breitbart, M.,
Fahsbender, E., Lafferty, K.D.,
Kilpatrick, A.M., Miner, C.M., Raimondi, P., Lahner, L.,
Friedman, C.S., Daniels, S.,
Haulena, M., Marliave, J., Burge, C.A., Eisenlord M.E. & Harvell C.D.
(2014). Densovirus associated with sea-star wasting disease and mass mortality.
PNAS (published ahead of print), doi:10.1073/pnas.1416625111.
Video Reference: New Scientist,
2014. Mystery sea star disease causes arms to twist off. Available at:
<https://www.youtube.com/watch?v=BVd1JyknIXY> [Accessed 29 November 2014].
Hi Tom, Thanks for the unique post, very interesting. Do they know if this virus only infects one genus then? What if it can infect others? It may even be linked to climate change that it has suddenly boomed- if they could culture the virus they could confirm if this was the case by exposing it to different environmental conditions. But you are right- in the long run we have no control :( Poor sea stars!!
ReplyDeleteHi,
ReplyDeleteThe virus infects 20 species in multiple different genera, such as Pisaster (it was first observed in Pisaster ochraceus), Pycnopodia, Evasterias and Patiria to name a few. And the virus is able to infect between Asteroid family's. It maybe that viruses are infecting via a common cell-surface feature. SSaDV was also found in Sea Urchins, which although the impact it has on them is unquantified, could plausibly act as a reservoir for the disease.
I think if you were to chase up the climate change aspect, it would be most fruitful to look at how several environmental factors effect susceptibility or increase transmission with a focus on the animals I think. Climate change is an obvious one but it could easily be other factors such as nutrient pollution etc stressing the animals. But a mutation leading to a change in the virulence of the virus is a very plausible candidate (if you think about human disease this is the key driver of new outbreaks). Also mentioned in the blog, it could be due to overpopulation of starfish in previous years, high density populations are more susceptible to diseases etc. But then what causes this? It could be due to a number of factors.
For more general information see http://www.eeb.ucsc.edu/pacificrockyintertidal/data-products/sea-star-wasting/index.html
Hi Tom, Yes you are rights about it may increase transmission... What do you think lead to this overpopulation? Thanks for the comment.s
ReplyDeleteHonestly don't know! The authors said the reports of overpopulation of P. helianthoides were anecdotal, so it was not based on a specific study etc. Perhaps due to an overabundance of prey species or the local loss of predators (such as king crabs or larger fish) due to overfishing.
ReplyDeleteHi Tom,
ReplyDeleteHave you considered the idea that the virus could be lysogenic. Some change in environmental conditions may have now set the lytic cycle in motion. Might explain why they can see the presence of this virus as far back as the 80's with no disease outbreak.
I find it interesting that the paper suggests that one virus is infecting so many different genera of star fish - my impression has always been that viruses are very host specific and that seems like a large range. Very interesting. Thanks
Hi Kat,
ReplyDeleteThey did not mention lysogeny at all in the research paper, so I would say no? I should mention that out-breaks have occurred in the past and the term SSWD has been used from 1979. So based on this I would say that its more likely to be changes in the viruses virulence and immunity of sea-stars that drive events rather than the virus coming out of lysogeny. But saying that the Lysogeny article on wikipedia, does say that lysogeny can occur in eukaryotes, so its plausible. I will email the author and get a better answer.
With regards to specificity it is amazing! The authors said that Parvoviruses have been know to infect between Familys in other cases. As they gain access to host cells via transferrin receptors (which are highly expressed proteins in coelomic fluid), so it may be the case that similar receptors are shared between species. But this is definitely an area for further research.