Fibropapillomatosis is the most common disease affecting turtles. This condition is characterized by benign epithelial tumors that eventually weaken biological tissue surfaces. The herpes virus is believed to be the agent causing the disease, while turtle leeches are suspected mechanical vectors, transmitting the disease to other turtles. This disease is suspected to be caused by various factors, including the tumor growth phase, which may be triggered by biotoxins or other contaminants. Fibropapillomatosis is found worldwide but is most prominent in warmer climates, affecting up to 50%-70% of some populations.
Fibropapillomatosis is a benign tumor disease primarily affecting green turtles (Chelonia mydas). However, this disease has also been reported in loggerhead turtles (Caretta caretta), olive ridley turtles (Lepidochelys olivacea), Kemp’s ridley turtles (Lepidochelys kempii), and leatherback turtles (Dermochelys coriacea). The tumors are most commonly found externally around the armpits, genitals, neck, eyes, and tail of the turtle, but they can also occur inside and around the mouth, and rarely in internal organs or the carapace. This, in turn, interferes with vision, feeding, and movement. Around 25%-30% of turtles with external tumors also have internal tumors, primarily in the heart, lungs, and kidneys.
This disease predominantly affects young and juvenile green turtles, and is rarely seen in adults. It is known that these tumors can regress and heal, which has been documented in some individuals, even when the tumors are severe. However, the factors causing tumor regression remain unknown. The second possibility is that juvenile individuals with this disease may die before reaching adulthood.
The first documented case of the disease was in 1938 in Key West, Florida. Long-term studies did not find signs of this disease in the Florida Atlantic coast during the 1970s, but by the 1980s, Fibropapillomatosis was recorded in 28%-67% of the population. Today, an incidence of up to 92% has been reported in Kaneohe Bay, Oahu, Hawaii. In general, Fibropapillomatosis is most prominent in warmer climates. Recent research has found that FP is caused by stress, and tumors have been observed in turtles involved in tourism. It is believed that the presence of tourists causes stress to the turtles.
Fibropapillomatosis is a contagious disease with horizontal transmission. A herpesvirus called fibropapilloma-related turtle herpesvirus (FPTHV) is believed to be the agent causing the disease, although there is no definitive evidence of its causality. This belief is supported by the fact that almost all tissue samples tested from affected turtles contain genetic material from this herpesvirus, varying between 95% and 100% depending on the study and location. The amount of herpesvirus DNA in tumor tissue is 2.5-4.5 log higher than in uninfected tissue. The herpesvirus FPTHV has been found in turtles that are free from Fibropapillomatosis, which suggests that the development of FP is multifactorial and may even involve a tumor-promoting phase. Research indicates that this disease has multifactorial causes, not a single factor or agent. Possible factors include various parasites, bacteria, environmental pollutants, UV radiation, water temperature changes, and biotoxins. Additionally, physiological factors such as stress and immunological status appear to be related to the disease.
The Ozobranchus leech species is considered a mechanical vector for the herpesvirus, transmitting the virus from one turtle to another. These leeches are common ectoparasites of turtles, exclusively feeding on turtle blood, and some leeches have been found carrying more than 10 million copies of the herpesvirus DNA. Green turtles are herbivores, feeding on seagrass and macroalgae. While causality has not been concluded, there appears to be a relationship between the distribution of dinoflagellates and the occurrence of Fibropapillomatosis. Turtles with Fibropapillomatosis are found to have a weakened immune system. They have higher numbers of phagocytic leukocytes (especially heterophils) compared to healthy turtles, which seems to be a result of Fibropapillomatosis, as most cases are observed in individuals with severe tumors. This further supports the herpesvirus hypothesis as the causative agent.
Fibropapillomatosis is closely related to nitrogen footprints and invasive macroalgae. The strongest correlation with FP is with habitat type, especially increased anthropogenic activity (human activities) causing high nitrogen footprints in the environment where green turtles are found. Observations support the hypothesis that coastal habitats are strongly correlated with the disease, as individuals that have recently exited the pelagic life phase (open ocean) have never been found with tumors. When migrating to shallower marine zones, such as neritic zones, individuals remain free of Fibropapillomatosis, but upon entering lagoon systems, turtles may become infected. Fibropapillomatosis is also associated with poor habitat quality, while it is absent in some habitats with good quality.
