Why some green turtles develop tumors

A sea turtle found off the coast of eastern Brazil with tumors from Fibropapillomatosis disease. (Credit: Ralph Vanstreels/UC Davis)

Water temperature, salinity, and proximity to environmental stressors may trigger the development of a tumor disease associated with a herpesvirus in endangered green turtles in Brazil.

The new study, published in PLOS ONE, also finds that the distribution of the tumors from fibropapillomatosis, or FP, can provide clues to better understanding the disease.

“FP is one of those things that has troubled scientists for a long time,” says senior author Marcela Uhart, a wildlife veterinarian with the One Health Institute at the University of California, Davis School of Veterinary Medicine. “It could be that where the tumors show up on the turtle’s body is not as random as we used to think.”

Researchers first documented the disease in 1938, and cases began exploding in the 1980s. All sea turtle species are susceptible, but juvenile green sea turtles, which are endangered, represent the most cases and the highest disease severity.

The FP tumors grow on the neck, eyes, flippers, and limbs, and although they are not invasive, they may affect feeding, swimming, and other activities.

Green turtles in Brazil

Although the disease is caused by a virus, the Chelonid herpesvirus 5, not all turtles infected with the virus will develop FP. The tumors tend to be more frequent in juvenile turtles that live near urban areas, which suggests that habitat quality may play a role in the progression of the disease.

“The presence of the herpesvirus is not sufficient for the disease to show up, so we need to explore what other factors may play a role in triggering tumor development,” Uhart says.

Seeking to better understand the disease, the researchers and experts from several Brazilian institutions studied the ecology of FP in green turtles on the coast of Espírito Santo, in eastern Brazil. This area is considered one of the global hotspots for FP, with more than 40% of the turtles presenting tumors.

One factor that may have contributed to the high prevalence of FP in Espírito Santo is the presence of metallurgical plants, which discharge large quantities of warm water into the sea. The warm waters near those plants are known to attract juvenile green turtles. Underwater surveys show that the average FP prevalence can reach 76% in those gathering areas.

The researchers looked at two datasets: daily beach surveys documenting stranded or dead turtles along 250 miles of coastline over four years, and a database about the size and location of tumors on 271 turtles from the same region.

The analysis found that turtles with marine leeches, which were previously suggested to transmit the herpesvirus, were four times more likely to have tumors. The findings also suggest that a combination of individual factors (age and body condition) and environmental factors (salinity, temperature, and proximity to metallurgical plants) play a role in the likelihood of FP tumors.

Complex virus

The researchers also found there are at least three different patterns of FP, with tumors concentrating either in the front flippers, back flippers, or in other parts of the body. These patterns were also found to be influenced by the environment.

“The fact that there are these distinct anatomical patterns, which are linked to a suite of environmental factors, suggests that FP might behave in more complex ways than we previously thought,” says lead author Ralph Vanstreels, an associate researcher with the School of Veterinary Medicine.

The study suggests that looking at how the tumors are distributed on the turtle’s body can provide hints about what individual and environmental factors triggered the growths.

Researchers have long sought for the environmental factor that could best explain why FP is more common in some regions than others, but the results from different studies have often disagreed.

“Our findings suggest that we should look at this disease as a mosaic of anatomical presentations, and that perhaps these different presentations are associated with different environmental factors,” Vanstreels says.

This information will be useful when trying to understand why the disease emerges and how to control it.

“If the trends continue, our expectation for sea turtle health is it will get worse rather than better,” Uhart says. “These factors are linked to global change.”

Sea turtles have existed for over 150 million years and play a key role in the ecosystem as they consume and transport nutrients and act as both predator and prey.

“Protecting these species is essential to ensure the health of our oceans, and understanding the recent increase in FP is an important part of this puzzle,” Uhart says.

Additional researchers are from UC Davis, Instituto de Pesquisa e Reabilitação de Animais Marinhos, Universidade Federal de Alagoas, Universidade de São Paulo, Universidade Federal de Alagoas, Instituto Argonauta para a Conservação Costeira e Marinha, Universidade do Estado do Rio Grande do Norte, Universidade São Judas, Universidade Paulista, and Instituto Federal de São Paulo in Brazil.

Source: UC Davis