When going back over the notes of the marine mammal disease lectures I found a slide on turtle diseases which I don’t think we covered; so I thought it might be interesting to delve a little deeper into the area...
Turtles are susceptible to a tumour-forming disease caused by a fibropapilloma-associated herpesvirus. This results in tumours growing on the turtle’s body and on its internal organs which can interfere with feeding, reproduction, etc. in many species. This paper looked at Green turtles (Chelonia mydas) in particular, which are widely affected by fibropapillomatosis. The disease is thought to be transmitted by contact as it has low genetic variability and mutability, suggesting that the pathogen is quite specific to its host and so presumably could not survive as a free living virus. This has led to the suggestion that other factors could be important in disease development such as environment and diet.
The authors suggest that environment could be particularly influential, as in the Hawaiian study population, the disease was only contracted after the turtles spent time in shallow coastal waters once they reached a certain size. Furthermore, coastal runoff enhanced non-native macroalgae growth, suggesting a link between land use and disease. Therefore the paper aimed to investigate the interplay between land use, environment (i.e. macroalgae), size and disease; as well as trying to identify the correct spatial scale for investigating the disease.
Van Houtan and colleagues focused on the Hawaiian population of C. mydas over 28 years by looking at stranding records and indentifying disease cases. They standardised disease rates by size (as it is a known factor in disease susceptibility) in order to gain a detailed picture of the disease through space and time in each size class. They also characterised land use at each stranding site and calculated the nitrogen footprint of each area.
The study found strong links between disease and all factors. Size was shown to be a consistent risk factor as turtles of around 75cm were most affected; a correlation was apparent between land use and disease according to nitrogen levels; and it was also linked to macroalgae. Furthermore, local spatial scales were found to be the most suitable for investigation of fibropapillomatosis as it varied locally, suggesting local causations ought to be looking in to. According to the paper, it generally seems as though human impacts may increase disease rates, as agriculturally dominant areas of land had high nitrogen footprints and also high disease rates. These high nitrogen levels also corresponded to increased disease as they encouraged the growth of non-native algal species which uptake a lot of the nitrogen. As this is passed on to the turtles during feeding and digestion, it may become available to the herpesvirus and promote infection.
Although the paper is quite frankly a little confusing, I think it is important that it has attempted to link several factors together in order to understand Green turtle fibropapillomatosis as ecological processes are complex and diverse and do not just occur between the host and its pathogen. Moreover, it is also interesting that human ecological impacts are considered instead of focusing purely on the marine aspect, especially as in this case human activities seem to be the main promoter of infection. Having said this however, I think the authors have been a little short sighted as many other factors could influence disease rates, especially in terms of nitrogen as oceanic cycling may unfold more of the story.
A review of: Van Houtan KS, Hargrove SK, Balazs GH (2010) Land Use, Macroalgae, and a Tumor-Forming Disease in Marine Turtles. PLoS ONE 5(9): e12900. doi:10.1371/journal.pone.0012900
2 comments:
Author of the paper Kyle van Houtan somehow got a link to this post and made this very helpful comment
"Dear Natasha, I'm sorry that you found parts of the paper confusing. Let me sum up the study, briefly. We found that turtle with tumors were spatially clustered in watersheds were both N enrichment and nuisance algae blooms were issues. The tumors are caused by a herpes virus, and herpes requires a particular amino acid -- arginine -- to do its dirty work. The relation between arginine and the nitrogen and algae is simple. Kind of like how animals store carbohydrates as fat, plants and algae store nitrogen as arginine. So when there is a lot of nitrogen, there will be a lot of algae. But that algae will be rich in arginine. So when the turtles have the herpes, and they reside a nitrogen rich coastal area, they'll be literally eating themselves sick."
The fact that the author of the paper saw your blog post is quite cool! His summary definitely makes the links in the paper more understandable!
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