Review of: Chaloupka, M. Balazs, G. H. Work, T. M. (2009) Rise and Fall over 26 years of a Marine Epizootic in Hawaiian Green Sea Turtles. Journal of Wildlife Diseases, 45 (4) 1138-1142.
The long lived and late maturing Green Sea Turtles have been present in our marine environment since the Dinosaurs. Over the years due to exploitation, green turtle numbers have become depleted, leading to their endangered status. Recently the emergence of a pandemic disease, Fibriopapillomatosis, which is associated with the presence of herpesvirus, has been affecting green turtle populations Worldwide. Fibriopapillomatosis (FP) is a disease similar to cancer, leading to tumour growth on the skin of the organism. The prevalence of this disease has increased in green turtle populations in Australia, Indonesia and the US over the past 2-3 decades. This disease emergence is thought to have impaired the already depleted green turtle numbers, especially in Hawaii where FP is a major cause of green turtle stranding.
This study was the first long-term assessment of FP prevalence in any marine turtle population, and has been recording information on FP in green turtles in Palaau, (Molokai, Hawaii) since 1982. The population in Palaau has the highest recorded numbers of FP in the Hawaiian archipelago where the disease is endemic. A programme was set up to annually monitor FP in green turtles by capturing, tagging and recapturing individuals. These turtle were evaluated for FP and given a severity score ranging from 0-3. Data collected over a 26 year sampling period (1982-2007), allowed for estimations of FP prevalence based on the proportion of green turtles with FP at each annual sampling. The data indicated that prevalence of FP increased following the 1980’s outbreak, peaked during the mid 1990’s and has steadily declined since. Observations have also shown that not all diseased turtles die; many have been shown to completely recover from the disease.
In 1978 the green turtle was protected under the US Endangered Species Act, due to its declining numbers; although it has been suggested that the presence of FP will affect recovering stock numbers, green turtle numbers have been shown to be ever increasing. The causes of FP are unknown, along with the role of the alphaherpesvirus in disease causation. However, disturbingly it has been shown that green turtles only contract the disease on entry to coastal developmental habitats, suggesting that the cause for disease is in near shore foraging habitats.
The authors have proposed two plausible explanations for the prevalence of FP. Firstly they suggest that the decrease in turtles infected with FP could be as a result of herd immunity to the infectious tumorigenic agent (if herpesvirus is contributing to disease) and secondly the removal of tumour-inducing agents in the near shore foraging habitats around Molokai. Both explanations seem possible and when compared to FP prevalence in Florida it was mentioned that the population was stable, unlike the decrease in prevalence in Hawaii. This could be an indication that the genetically isolated Hawaiian turtles may have evolved immunity. I believe it is highly possible that the herpesvirus is the tumour causing agent present in the near shore foraging habitats, which the isolated turtle population have managed to develop resistance against.
5 comments:
Hi Arainna, I really enjoyed reading your review. You might be interested in this paper, which talks about the invasive microalgae that the turtles forage on, and how the macroalgae sequesters nitrogen, converting it to arginine which has been linked to the promotion of herpesvirus and tumour formation
http://www.plosone.org/article/info:doi/10.1371/journal.pone.0012900
Sorry, just to clarify that should be macroalgae, not microalgae!
Hi Arainna,
Having just posted my review (on the paper Helen mentioned!)I see you've done a similar one :) Its interesting that your paper highlights the coastal foraging areas as the location for virus contraction as mine supports this idea. It would be interesting to know whether the Hawaiian turtles have developed some immunity as, if so, I think it may be possible that immunity is size (age?) related because it appears that infection rate increases with size until around 75cm then tails off.
Hey Helen, Thank you for recommending that paper, I've just read Natasha's review on it and it is quite interesting.
Natasha: Your paper seems to be the follow up to mine,it does contain some interesting concepts, however I agree with you in that fact that I believe it to be slightly short sighted. where you said that the disease tends to be most prevalent in up to 75cm long turtles, does this mean that turtles measured when they were this size were then measured again later and found that the disease had regressed? The reason I ask is that with marine turtles it is difficult to tell their age, so they tend to relate this to size, if the disease was only present in a group of turtles that were approximately the same size/age it could mean they were exposed to a tumour causing agent or virus when they were young which has affected that generation of turtles. Also the algae concept is interesting but to gain more evidence of it they should see if the same thing is happening in Florida as turtles there are affected more by FP than those in the recovering Hawaiian population.
No the turtles measured had already succumbed I believe, as the study looked at strandings and identified those that had the disease. It also went into a little more detail about the ecology of the species as apparently the juveniles come back to the coastal areas to forage when they are around 30cm in length and stay there, but once they reach around 80cm they become more pelagic and only come back to the coastal areas to feed. So it makes sense that the prevalence of the disease increases from individuals of 30cm up to 75cm, but then tails off from there as the larger/older turtles come into contact with one another less often. This same pattern was repeated for the 28 years that the study looked at so you might be right; there could be a large-ish mortality event in every generation once the disease has built up inside the turtles, which happens to be fatal around the certain size.
Yeah, I guess the algae situation might also be worse in Florida as I expect there will be more coastal runoff there than there is in Hawaii?!
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