going viral

Researchers at the Institute of Marine and Environmental Technology (IMET) in Baltimore, Maryland investigate different marine diseases impacting west and east coast populations in the United States and potential implications of human impact on ocean ecosystems.

By Eden Bartlett


image: IMET in the Inner Harbor, Baltimore, Maryland

Dr. Colleen Burge, Ph.D., has her right arm submerged up to her elbow in a bucket of clear, briny water. In moments, she’s pulled out a silver sieve covered with a collection of what appear to be large grains of light brown sand. 

But these are living, breathing creatures—these are tiny, Pacific oyster spats, the residual population from fieldwork conducted this past summer in Tomales Bay, California, from her research on a form of herpes virus affecting oysters.

“Us, oysters, sea stars—everyone gets sick. Herpes is a virus. It’s not like herpes in humans--it’s just a virus that’s shaped similarly, so it’s morphologically similar,” she says. Burge is an assistant professor at University of Maryland, Baltimore County as well as a scientist at the Institute of Marine & Environmental Technology (IMET) in downtown Baltimore, Maryland.  Currently, her efforts are focused on combating a strain of the Ostreid herpesvirus 1, a form of herpes found in Pacific cupped oysters, Crassostrea gigas, and other bivalves that appears to be impacting populations globally. 

Viruses in oysters work more or less similarly to how they work in humans and other land mammals. Viruses are incredibly small entities that infect the cells of living hosts, injecting genetic information into the cells and recruiting the cell’s enzymes to reproduce copies of the virus.  Some viruses take years to show symptoms, but others, like the Ostreid herpesvirus 1, infect baby oysters or “spats” and usually cause death within only a week.

Efforts to combat diseases in oysters have been ongoing for decades. On the east coast, pollution, overharvesting, and overdevelopment along with viral diseases like Dermo and multi-nucleated unknown (MSX), which are both caused by viruses, have severely impacted oyster populations in portions of the Chesapeake Bay with higher salinity. Fortunately, due to selective breeding efforts, particularly focused on the Virginia oyster, or Crassostrea virginica, restorative efforts are projecting a more positive future for oysters.  

“They’re starting to see some of this oyster restoration working. There’s more folks now engaging in aquaculture too as opposed to just dredging. There’s a concerted effort, and it’s really great to see,” says Burge. Burge, who recently moved to Baltimore from Seattle, is encouraged by the support for the Chesapeake’s oyster populations, and hopes to see the same progress made on the Pacific oyster.

“This oyster virus is one Dr. Burge has been working on for thirteen years now,” says Carolyn Wilkinson, an undergraduate student studying biotechnology at University of Maryland, University College. She just began an internship with Burge at IMET in August, where she helps research marine pathogens by isolating and cloning oyster genes. 

“Why is this virus occurring? And why is it killing off such a large amount of the oyster population, in nature or commercially? It’s fascinating,” says Wilkinson.

As global oyster populations suffer from ocean acidification, temperature changes, and overharvesting, they become more susceptible to disease. Due to the nature of oysters as marine animals, it’s difficult to treat them for viruses like scientists would for any land animal. 

“They don’t have an adaptive immune system so we can’t vaccinate them,” says Burge. Administering a drug or vaccine to prevent them from dying from a virus would be ineffective because they don’t have a sophisticated immune system. “You can actually drill into the side of an oyster and inject something, but they don’t have the T cells for long-term memory.”

 This poses a challenge for the Pacific oyster impacted by the herpesvirus. Selective breeding is difficult and time consuming, so researchers in the United States and France have begun collaborating to isolate specific genes in populations of the Pacific oyster that are more resistant to the virus.

But beyond oysters, Burge notes a bigger problem in the marine ecosystem as a whole—increasingly, as species begin to feel intensified pressures of climate change, pollution, and overharvesting, more diseases are wrecking havoc on dwindling populations.

“Viruses typically move fast,” says Burge as she flips open a copy of Science on the table and turns to a page, pointing out an image of her holding a drooping, bright red sea star. “That’s a sick sea star. It seems neurological, but we don’t know exactly what the virus is doing. One of our studies looked at genes involved and it seemed clear that there were nervous system genes being expressed.”

She points to another image, a map, showing pinpointed areas where sick sea stars have been discovered. This was published in May of 2014—since then, the sea star wasting disease has extended all the way up to Alaska, quickly affecting sea stars all along the pacific coast. In some cases, the disease can cause sea star death in as little as three days.

How does the virus spread? “Probably the water,” posits Burge. Some evidence indicates this disease may be similar to viral diseases that sweep through insect populations. 

Other evidence, like the correlation between rising ocean temperatures and sea star death, implicates rising global temperatures. But Burge says it’s too soon to point to climate change as the definitive cause.

“There’s definitely a temperature link but it’s hard to say. Why did this pop up at a certain point, and how did it pop up at multiple parts of the globe? It just doesn’t quite make sense,” she says. 

The data is not strong enough to indicate a direct connection between global warming and increased emergence of these viral diseases, but it can’t be helping, either. “You can say marine diseases are increasing, and anthropologically there are things that we are doing that don’t help,” she says.

Increasing marine diseases impact the ecosystem as a whole. The removal of a single major species in any ecosystem can result in changes in the densities of other organisms in the ecosystem, upsetting the crucial balance that maintains marine animal interactions.

“It’s called the keystone species concept,” says Burge, pointing to an image of an arch with one block labelled sea star. When one block is removed from the arch, the entire structure crumbles. “The idea that an animal like this has mass mortalities has huge implications for the entire ecosystem. The one I have in there, the big sunflower star, there’s not that many of them left.”

However, Burge notes that the outpouring of public support for the sea star has been enormous on the pacific coast, at least compared with oyster conservation efforts. 

“The interesting thing about this is once people heard about it they wanted to start surveying sea stars. In some places, citizen scientists went out and surveyed before the disease came through,” says Burge. 

These “citizen scientists” collected data from populations before they were affected by the sea star wasting disease, creating a baseline for scientists to compare the stars as the disease progressed. 

“I’ve never seen people so interested as when we were working on sea stars,” says Burge. “There was something just iconic and driving about it.” 

For now, Burge and other marine pathologists hope this enthusiasm continues, as marine ecosystems are under more stress than ever. Next summer, Burge hopes to visit French laboratories on a fellowship from IMET to further study the genes of the oyster herpes virus and find strains of the Pacific oyster that are genetically resistant. 

“What I like most about [this research] is that this field of marine study is one that we don’t know much about,” says Wilkinson. “There is so much more to learn.”