Coral 'White Plague' Epidemic Could Be Caused by Virus

Coral reef with white plague — A coral with the "white plague" disease that has spread through reefs in the Caribbean Sea.

(Image credit: Marilyn Brandt)

The Caribbean Sea is battling an epidemic — a nasty plague that spreads and kills quickly. Unlike the historical Black Plague, which killed millions of people in the Middle Ages, this so-called white plague is devastating populations of marine corals.

Scientists long believed the scourge, which first popped up in the 1970s, had strictly bacterial origins, but research now suggests viruses may play a prominent role in causing white plague.

During a 2010 white plague outbreak in the Virgin Islands, researchers analyzed the viruses present in diseased and healthy corals. While all of the sampled corals carried numerous viruses, the tissue taken from corals that had white plague predominately contained one, specific group of viruses. Known as small, circular, single-strand DNA viruses (SCSDV), these could potentially have caused the disease.

White plague is characterized by rapid tissue loss, which exposes the white skeleton of the coral. "White plague is not that difficult to tell apart from other coral diseases," said Nitzan Soffer, a microbiologist at Oregon State University and lead author of the new study. "You have living, healthy tissue, and then immediately below that you have a straight band of white on the bottom of the coral." This white band quickly expands upward to the rest of the colony.

Researchers first identified white plague in 1977 in the Florida Keys, but the disease wasn't much of an issue at the time. "It was just this new thing that people were noticing," Soffer told LiveScience. The disease reemerged in the same reefs in 1995 and quickly spread — scientists had reported white plague outbreaks throughout the Caribbean by 2001, according to the World Conservation Monitoring Centre. Recently, outbreaks of white plague have wiped out 70 to 80 percent of some Caribbean coral reefs, Soffer said.

Scientists currently recognize three types of white plague that differ in how quickly the disease progresses. Type I, the variety identified in 1977, causes coral tissue mortality at a rate of about a tenth of an inch (3 millimeters) per day. Type II, identified in 1995, progresses at up to 0.8 inches (2 centimeters) per day and can kill a small colony in one or two days. First identified in 2000, type III causes tissue loss at more than 0.8 inches a day and mainly affects the largest reef-building corals, including mountainous star coral (Montastraeafaveolata) and giant brain corals (Colpophyllia natans).

Regardless of type, the disease tends to develop in corals that have recently bleached, a process in which some stressor, typically heat, causes coral to expel the symbiotic algae that provides the coral with food. Relatedly, white plague seems to be correlated with high temperatures. "My collaborators have been seeing that this disease often comes out at the end of summer, when corals are the most heat-stressed," Soffer said. (Bleaching is an increasing concern as the world's oceans heat up due to climate change.)

Adding another layer to the mystery, other coral diseases resemble white plague. For example, white band disease produces symptoms similar to white plague, but it only affects Elkhorn coral (Acropora palmata) and staghorn coral (Acropora cervicornis). White plague is not known to infect either of these types of coral. Soffer suggests there may be more than just three types of white plague out there, particularly since very similar diseases have been found in other waters.

Viruses are another potential cause of white plague and other coral diseases, though few studies have tested this idea. "Viruses are challenging to work with and thus are often overlooked," explained Collin Closek, a University of Pennsylvania biologist who studies yellow blotch disease among western Caribbean corals and who wasn't involved in the study. But a viral cause of white plague makes sense because the disease spreads and kills quickly, as viruses are known to do, Soffer said.

To see if certain viruses are associated with white plague, Soffer and her colleagues investigated a September 2010 white plague outbreak — likely of type I — among colonies of boulder star corals (Montastraea annularis) in the Virgin Islands. "Originally, we were just going to sample diseased corals and healthy corals," Soffer said. "But when we got there, we realized that corals were bleaching on the top portion of the colony, while the bottom had white plague."

Up to 90 percent of the colonies the team examined were bleached, and 7 percent showed signs of both bleaching and white plague (no colonies had only white plague). The researchers took tissue samples from both the diseased and bleached portions of seven coral colonies, as well as samples from five colonies that were bleached but not diseased. Scientists also took samples from the only two coral colonies in the entire reef that appeared to be completely healthy.

Herpes-like viruses dominated the tissues of the healthy corals — the team believes the viruses establish long-term, nonlethal infections in corals, just as they do in humans and other animals. For bleached-only colonies, nucleocytoplasmic large DNA viruses — a group that includes the poxvirus — were in the slight majority, but the tissue samples also contained herpes-like viruses and SCSDVs. Diseased tissues from the colony overwhelmingly contained SCSDVs. [Tiny & Nasty: Images of Things That Make Us Sick]

"[The research] is a great addition to the study of coral diseases, as it highlights the potential role of viruses in coral disease," said Christian Voolstra, a marine scientist at the King Abdullah University of Science and Technology in Saudi Arabia, who wasn't involved in the study. However, he notes that scientists have yet to pinpoint exactly what causes coral diseases (white plague included), and that a more comprehensive look at the issue is needed.

Joseph Bennington-Castro is a Hawaii-based contributing writer for Live Science and Space.com. He holds a master's degree in science journalism from New York University, and a bachelor's degree in physics from the University of Hawaii. His work covers all areas of science, from the quirky mating behaviors of different animals, to the drug and alcohol habits of ancient cultures, to new advances in solar cell technology. On a more personal note, Joseph has had a near-obsession with video games for as long as he can remember, and is probably playing a game at this very moment.