A virus that melts sea stars

Sun flower sea starSea stars are lovely marine invertebrates with a round central body connected to multiple radiating legs (photo credit). In the past year millions of sea stars in the west coast waters of North America have melted into piles of slime and ossicles. Sea star associated densovirus might be the cause of this lethal disease.

Sea star wasting disease (SSWD) is characterized by lesions, limb curling and deflation, and death as the animals rapidly degrade or ‘melt’. The current outbreak began in June 2013 and has killed sea stars from Baja California, Mexico, to Southern Alaska. SSWD might be the biggest marine wildlife epizootic ever observed.

Evidence that SSWD is caused by a virus came from experiments in which extracts of diseased sea stars were passed through a filter with pore sizes small enough to allow passage of viruses but not bacteria or other microbes. When injected into healthy sea stars, these filtrates induced sea star wasting disease. Extracts of diseased sea stars collected in Vancouver, CA contained 25 nanometer virus particles, as determined by electron microscopy.

Nucleic acid sequencing was to identify the viral agent of SSWD. Virus particles were purified from diseased animals, and both DNA and RNA was extracted. Analysis of the nucleotide sequences revealed the presence of giant DNA viruses such as mimiviruses and phycodnaviruses (link to algal virus paper), and among RNA viruses, retroviruses, dicistroviruses, and parvoviruses. With few exceptions, all samples containing parvoviruses were from symptomatic asteroids, and so the authors decided to pursue the study of this virus.

Analysis of the DNA sequence data revealed the presence of a densovirus (a parvovirus) related to viruses found in Hawaiian sea urchins. The authors called this virus sea star-associated densovirus, SSaDV. Like other members of the parvovirus family, these are small (25 nm diameter), naked icosahedral viruses with a ~6 kb single stranded DNA genome. When sea stars were infected in the laboratory with filtrates from diseased animals, virus loads, determined by PCR, increased with time together with disease progression. Field surveys revealed that the virus was more abundant in diseased than in healthy sea stars. The virus was found in marine sediments, plankton, and sea urchins. Viral nucleic acid was also found in sea stars preserved in museums since 1942.

While retrovirus sequences were found in sea star tissues, the authors believe that such ‘Retroviral annotations are likely spurious because they were detected in DNA libraries (and have RNA as nucleic acids).’ The detection of retrovirus sequences in sea stars is not at all spurious! DNA copies of retrovirus genomes are produced during infection and integrated into the host genome, explaining why these sequences were detected in DNA libraries. Their absence in RNA libraries means that virus particles are not produced, as is the case in many other organisms that contain endogenous retroviruses.

The evidence that SSaDV causes sea star wasting disease is strong but not yet complete. The crucial experiment that remains to be done is to isolate infectious virus in cell culture, inoculate it into sea stars, and show that it causes wasting disease.

While the authors’ work reveals that sea star wasting disease virus has been present on the North American Pacific Coast for over 70 years, the disease is not always observed. The current outbreak has been ongoing since just June 2013. Perhaps other environmental conditions have lead to the increased susceptibility of sea stars to disease. It is important to determine if a human activity is involved in precipitating the disease, so that we can prevent future loss of sea stars. The authors suggest one possibility, that increased populations of adult sea stars in small bays and inlets have produced increased virus levels which lead to more infections. It will also be important to determine if the virus has undergone any changes in transmissibility or virulence.

Sea star associated densovirus was also found in non-asteroids, including ophiuroids (brittle stars and basket stars) and echinoids (sea urchins and sand dollars). We need to know the host range of this virus, how it is transmitted, and whether it can cause disease in other species. A troubling scenario is that the recent amplification of the virus in sea stars could lead to infection, and perhaps death of, many other marine species.

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  • Alan Dove

    Someone should look at the pH tolerance of this virus and see whether that has an effect on its transmissibility or pathogenesis. We’re currently engaged in a global experiment in which human activity is gradually acidifying the oceans, with disastrous effects on some marine ecosystems. It’s easy to imagine that decreasing pH could tip the balance of host-pathogen dynamics and produce something like this epizootic.

    Of course, if that does turn out to be the driving factor, it’s unlikely we’ll be willing or able to do anything about it.

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  • Chris

    There was a reason to find out why there was a severe die-back of the sea stars: it affects other things like fisheries, which is a significant part of the economy along the American west coast. First they had to find out what caused the deaths, and it turned out the primary suspect is a virus.

    What other factors would you approve of studying?

    You should probably read the entire study, it is free to read online. The last paragraph says:

    However, it remains to be seen how infection with SSaDV kills asteroids, what the role is for other microbial agents associated with dying asteroids, what triggers outbreaks, and how asteroid mass mortalities will alter near-shore communities throughout the North American Pacific Coast. More generally, viral pathogens are poorly known for all noncommercial invertebrates yet may play an unrecognized, yet important, role in marine ecosystems.

  • Chris


    So who has not emoted a very gloriously happy squee that is the actual name for a star fish? I mean, seriously, first we have to call them “sea stars”, but now “asteroids”? Cool!

    As someone who lives on the Pacific coast this is something that we are all familiar with. I have helped out on more than one school field trip that involved checking the intertidal areas for sea stars and moon snail egg cases. Along with crabs, mussels, clams, limpets, sea urchins, barnacles, sea dollars and skulpin fish.

    They are an important part of an ecology that produces the prawns, crabs, halibut and even the salmon that drive a very robust fisheries industry. This virus is a serious threat to the whole fisheries economy.

  • Chris

    Le sigh. It is about the entire ecosystem. From this news article:

    Sea stars are such an important species to nearshore environments that their absence is already changing many areas. On Whidbey Island, green urchin populations are exploding. In Howe Sound, north of Vancouver, a similar phenomenon already is eliminating sea grasses typically used by young spot prawns.

    Though if you wish, you can contact the authors of the PNAS paper and tell them how to do science better. I am sure they would value your very educated feedback.

  • H. Nasman

    I was just about to e-mail you guys about this research because I was so excited, but then I realized that you were probably already on top of it, and you are! I was SO interested in the die-off while it was happening. How creepy it was, and it feels so satisfying to at least know one piece of the puzzle. I remember reading at the time that sometimes starfish wasting was associated with the overgrowth of vibrio bacteria, and I still wonder whether that could be part of the picture too — it would certainly be affected by temperature and carbonates (I agree with Alan!). Thanks for giving your perspective!

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  • pk_again

    I see, the censor board is back in action here, deleting comments, after I raised some valid objections to why a virus may not be responsible for the SSWD and how shoddy this research was as it missed basic environmental factors. This attitude is very typical of life sciences research institutes in the US who cannot tolerate anyone raising scientific objections to their work. I hope this comment stays a while longer and is read by students planning a career in microbiology to see what they are getting into.

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  • JSPA

    The timing seems right for the virus to have come from (presumably resistant) eastern pacific echinoderms that landed on the west coast with Japanese tsunami debris. (While pH is a huge problem for calcium-depositing organisms, I sort of doubt it’s the deciding factor here (over such a large area and range of temperatures–as temperature also affects the pH of seawater.)

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