Sea star wasting disease (SSWD) has affected over 20 species of asteroids on the North American Pacific Coast. The current geographic extent of this disease is from the Kenai Peninsula to Baja California, making this the most geographically extensive marine disease ever observed. The cause of the disease is not yet fully resolved, however previous work by the Team Aquatic Virus and colleagues established that SSWD signs can be elicited by taking virus-sized particles and injecting them into asymptomatic animals, and through comparisons of the microbiota inhabiting asymptomatic and symptomatic (i.e. healthy and diseased) sea stars we found a strong association between the disease and the Sea Star Associated Densovirus (SSaDV) (Hewson et al., 2014, PNAS). Furthermore, by comparing genes expressed by healthy and diseased animals, we found that SSWD signs are associated with apoptotic and degradative processes, as well as changes in the way sea star tissues are bound (Gudenkauf and Hewson, 2015, PLoS One).
There remain enormous questions about SSWD that are the focus of this 3-year study.
First, how might SSaDV actually cause disease? Densoviruses belong to a unique group of parvoviruses that in some cases cause disease, while in others do not. From what we know about parvoviruses generally, they do not cause disease in isolation, but rather are associated with other pathogens that elicit symptoms. The project will examine progression of the disease, from entirely healthy and uninfected, to SSWD and eventually animal death through monitoring temporal changes in the microbiome and the host gene expression response.
Second, we seek to understand how SSaDV moves around. Our previous work highlighted that SSaDV can be found in plankton and in sediments, but as-yet unpublished decay studies suggest that as a free particle it decays rapidly (2 – 12% per hour). One avenue we haven’t yet looked at is their transport in larval sea stars. Many sea stars are broadcast spawners, which means they release eggs and sperm into plankton, where they fertilize and become larvae which spend days to weeks in surface waters. We previously observed very high SSaDV abundances in juvenile sea stars. Hence, we seek to understand whether larvae may also bear SSaDV, whether settled larvae have the possibility to transmit SSaDV to adults, and finally what impact SSaDV has upon juvenile sea stars as they develop.
Third, our previous work highlighted great heterogeneity in the association between SSaDV and SSWD signs, with many individuals bearing SSaDV infection, but lacking SSWD signs. While we attribute this to the ‘incubation period’ (i.e. preclinical infection) at the time those sea stars were sampled, we will examine whether there is a relationship between SSWD signs and genetic variation of sea stars at the population level.
Finally, we discovered that SSaDV, based on detection of two genes on its genome, has been present on the west coast for at least 72 years. A key question is why the disease has happened now? One clue comes from parvoviruses generally, which experience random mutation in genes responsible for viral entry that can strongly influence their ability to infect (i.e. their virulence). We will go back in time through museum specimen studies to understand what changes, on the nucleic acid level, have occurred in SSaDV, to try and understand whether such mutations correspond with the current event.
This project is a collaboration between the Team Aquatic Virus at Cornell University, Seattle Aquarium, Vancouver Aquarium, and the Natural History Museum of Los Angeles County.
Stay tuned for the adventure ahead!