Hawai'i turtles recovering from severe tumor disease

The debilitating tumor disease that racked Hawaiian green sea turtle populations starting in the 1950s appears to have declined since its peak, and some individual turtles seem to be able to overcome it.

It's a hopeful sign in the upbeat story of turtle recovery in the Islands.

(Image: A healthy green sea turtle. Credit: NOAA.)

Hawaiian green sea turtle were so depleted in numbers in the late 1960s that hunting for them was banned four decades ago, and they were placed on the endangered species list. They have since recovered significantly in population.

A new report by Hawai'i researchers an Australian ecologist, in the Journal of Wildlife Diseases, documents their strong recovery, against both their depletion and also against the disease known as fibropapillomatosis or FP.

The paper is “Rise and Fall over 26 Years of a Marine Epizootic in Hawaiian Green Sea Turtles,” by Milani Chaloupka of Ecological Modelling Services at the University of Queensland, George Balazs of NOAA Fisheries' Pacific Islands Fisheries Science Center and Thierry Work of the U.S. Geological Survey-National Wildlife Health Center's Honolulu Field Station.

The tumor disease has been found worldwide, but nowhere was its severity like Hawai'i's outbreak. The writers call it an epidemic, and cite Hawai'i as the “world's main endemic hot spot.” It is the single most significant cause of turtle stranding and death in Hawai'i.

The disease can result in horrific globular white tumors, some on the outside of the turtle's body, some on the inside. They can grow around the eyes, blinding the animal, and around the mouth, starving the animal, and in the throat, starving the animal, and in the windpipe, choking the animal.

In the late 1980s and early 90s, the tumor-ridden turtles were common sights among those who watched turtles or came across them during marine activities.

During a sampling study over a 26-year period at Pala'au, Molokai, researchers inspected hundreds of green sea turtles. While the disease has been found throughout the Hawaiian chain, it was particularly prevalent off south Molokai.

They found that the tumor disease appeared to spread rapidly through the late 1980s and early 1990s, and then peaked in the late 1990s. Since then, its prevalence has dropped off steadily. At one time, nearly half of turtles showed symptoms, and now that number is around 10 percent.

On top of that, in a remarkable finding, researchers were able to show that some tumor-ridden turtles showed up later tumor-free.

“Not all diseased green turtles die, and our observations suggest that many green turtles with FP in Hawaiian waters can recover,” they wrote.

There are still plenty of mysteries in this story. Researchers don't know just how the fibropapilloma virus, a herpes-type virus, works. They just know that a turtle with tumors also has the virus. And they suspect, since the disease seems to be worse in some areas than others, that there might be environmental factors as well as disease factors at play, causing turtles to be more or less susceptible.

“Because we do not know the cause of FP, the reasons why this disease was absent before the 1950s, peaked in the late 1990s, and has declined since are purely speculative. Two plausible explanations would include the development of herd immunity... to an infectious tumorigenic agent (if herpesvirus is contributing to disease) and/or removal of a tumor-inducing environmental insult in the nearshore foraging habitats around the island of Molokai,” they write.

The Hawaiian green sea turtle population is genetically isolated from those in other areas. And the FP virus may be slightly different in other areas. In Florida, for instance, where the disease has been noticed much longer than in Hawaii, its effects appear to be stable, and not declining like Hawai'i.

But the Hawaiian research suggests that the turtles can thrive, in spite of this debilitating disease.

“The FP epidemic decline at Palaau is encouraging news for other marine turtle populations afflicted more recently with this chronic and often fatal disease,” they write.

© Jan TenBruggencate 2009