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 0.22µm 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].
