Challenges in Reptile Immunology and the Study of GTFP in Green Turtles
Updated: Sep 7, 2019
Ecoimmunology is a rapidly growing study that examines how the immunology of a species is influenced by ecology, biology, and physiology such as sex, age, seasonal variations, and environmental stressors. Most studies in the field focus on endotherms—mammals and birds—however, ectothermic invertebrates are important to consider for a number of reasons. Whereas mammals regulate body temperature from within (endothermically), ectotherms are dependent on environmental factors. The immune systems of turtles function optimally at specific temperatures, and anything above or below this range is problematic. In the last century, an increase in global temperatures has had a significant impact on ectotherms.
Reptilian immunology is still understudied. Reptiles do not have lymph nodes, and instead possess lymphoid tissue (thymus, spleen, gut-associated lymphoid tissue or GALT, and bone marrow). The outer skin of turtles is comprised of keratin, an exceptionally thick structural protein which protects the species against mechanical and microbiological dangers. Abraded skin (e.g., external tumors) becomes a portal of entry for bacteria and parasites.
There are seven species of sea turtles found in tropical and subtropical regions: leatherback, hawskbill, olive ridley, Kemp’s ridley, flatback, loggerhead, and green. Green sea turtles and olive ridleys are particularly vulnerable to herpes-associated fibropapillomatosis, otherwise termed green sea turtle fibropapillomatosis or GTFP; the disease is rare in other turtle species.
Tumor outbreaks, which consist of external and internal hyperplasia, correlate with warmer seasons and warming water.
Green turtles have been the primary focus for fibropapillomatosis research. Virus DNA has been found in leeches that cling to tumor-afflicted turtles (potential vectors); bacteria, fungi, protozoa, and helminths are all associated with GTFP. The virus is thought to be transmitted horizontally (via contact), and vertically (via mom to offspring). Captive tumor-free turtles have developed FP after coming in contact with recycled water from tumor-afflicted turtles. Clinical signs also differ by region. In Florida, liver tumors are most common whereas mouth tumors are prominent in populations in Hawaii.
Green turtles are slow to mature and inhabit both oceanic and coastal environments. Green turtles are the only herbivores species of turtle and feed on sea algae and sea grass as adults. They spend their juvenile life in oceanic habitats and late juvenile life in coastal and estuarine habitats. Hatchlings disappear into pelagic zones and return 4–9 years later as juveniles to near-shore foraging habitats which is when GTFP most commonly presents.
The accumulation of environmental xenobiotics in regions of Brazil (the heavily industrial Espirito Santo Bay)—particularly mercury, aluminum, cadmium, and lead—has lead researchers to study the impact of trace elements in blood panels; correlated oxidative stress parameters have been found to influence tumor growth.
The virus lays dormant in turtle populations that inhabit healthy marine environments and presents in populations near onshore farming. This correlation suggests that pollution and chronic stress may weaken immunity (and elevate cortisol levels) and trigger GTFP. Additional areas of high FP-affliction include Hawaii, Florida (particularly the Indian Lagoon which is prone to toxic algal blooms), the Great Barrier Reef, and coastal habitats in the Gulf of Mexico.