SCTLD
Stony Coral Tissue Loss Disease (SCTLD)
Stony Coral Tissue Loss Disease (SCTLD), discovered in 2014, is a contagious waterborne disease killing stony corals in the Caribbean off Jamaica, Mexico, Saint Maarten, the US Virgin Islands, Dominican Republic, Turks & Caicos Islands, Saint-Martin, Belize, Sint Eustatius, The Bahamas, Puerto Rico, British Virgin Islands, Cayman Islands, Guadeloupe, St. Lucia, Honduras and Martinique. Sick colonies display multifocal lytic necrosis, appearing as lesions, that start in the gastrodermis and extend out to the surface epithelia. Highly susceptible species are the meandroid corals–i.e., pillar corals (Dendrogyra cylindrus), elliptical star corals (Dichocoenia stokesii), smooth flower corals (Eusmilia fastigiata) and maze corals (Meandrina spp.). Starlet corals that develop numerous "blotchy" lesions, as well as diverse brain and star (boulder) corals, are also dying fairly quickly, followed by star corals (Orbicella spp., Montastraea cavernosa) and other coral species.[1]
One important resource to improve public outreach is the Tracking Map created by the Atlantic and Gulf Rapid Reef Assessment organization.[2] This interactive map displays location based reef assessments of SCTLD by combining numerous coral surveys throughout the world.
There is limited understanding of SCTLD disease, including its cause. Researchers can confirm that it is waterborne, but are unsure whether it is bacterial, viral, or environmental. The lack of research makes treatment rather difficult, but here are a few ways in which scientists are trying to help coral communities.
Genetic Expression to Prevent SCTLD
A 2021 study by the University of Miami (UM) Rosenstiel School of Marine and Atmospheric Science and in collaboration with the Mote Marine Laboratory and the Smithsonian Marine Station has been the first of its kind to document coral gene response against SCTLD. Their work explores the genetic immune response of infected O. faveolata and M. cavernosa corals by exposing these healthy corals to samples of infected corals collected from reefs. The intent of this research is to document the corals' immune responses in order to understand how SCTLD affects the corals' biological mechanisms and to compare species to interpret varied responses and their outcomes.
Experiments between 2019 and 2020 concluded that SCTLD is evoking the feedback of a network of genes important for cell responses including cell death, immunity and tissue rearrangement, indicating that the disease is causing swift cell death and rearrangement of the tissue.[3] Overall, diseased O. faveolata had 2194 differentially expressed genes (DEGs) compared with healthy colonies, whereas diseased M. cavernosa had 582 DEGs compared with healthy colonies.[1]
This research is important to understand the symptoms of the disease throughout the coral body in order to diagnose and treat as soon as possible. The goal is to treat coral disease like human medicine and with further research, scientists can create a toolkit to better diagnose corals and better inform policies to help save them.[4] This analysis has been the first step toward genetically understanding SCTLD and opens doors into further research. Scientists from this study propose that future research examines early time points of infection, before the presence of lesions, to understand the activating mechanisms involved in SCTLD.[1]
Topical amoxicillin treatment for SCTLD
Only a handful of studies have used antibiotics to treat diseased coral. Currently, one of the prime methods of SCTLD treatment is topical amoxicillin, which is commonly used to treat bacterial infections in humans and animals. Before SCTLD interventions included topical antibiotics, it was found that antibiotic-treated water mitigates SCTLD transmission within coral colonies in lab settings.[5][6] Shortly thereafter, methods to apply topical amoxicillin to wild corals infected with SCTLD were developed, and now over 20 coral species in the Caribbean are undergoing amoxicillin treatments.[7][8]
Topical amoxicillin treatments have been found to be highly effective at slowing the spread of SCTLD and in healing SCTLD lesions, but they are not capable of completely stopping its spread. Treated colonies must be re-visited after divers apply the first treatment to see if more treatment is necessary. This varies between species as some are more susceptible to SCTLD. If antibiotics fail, the next step is often amputation of the lesion.[9] Despite its effectiveness in stemming the spread of SCTLD lesions, there are many barriers to effective amoxicillin treatments. Treatments were found to be the least effective in brain corals, possibly because their deep grooves make application more difficult. Also, topical treatment benefits mostly individual corals receiving the treatment and rarely the entire colony, which makes effective applications labor-intensive and expensive.[7] Additionally, offshore sites are more prone to reinfection than inshore sites after being treated with amoxicillin, likely because offshore sites are exposed to more foreign bodies than inshore sites due to contact with more currents, so they require even more monitoring and upkeep.[8]
On top of barriers to effective treatment, there are still many unknowns with using antibiotics for treating coral diseases, such as how they affect the reef microbiome and reef inhabitants. Amoxicillin is a general antibiotic, meaning it kills all bacteria. One study found that the combination of many antibiotics, including amoxicillin, harms cyanobacteria.[10] Cyanobacteria play significant roles in sustaining rock- and coral-dwelling plant communities, make up much of the reef microbiome, contribute to calcification and are the primary nitrogen fixers for reef ecosystems.[11] However, too many cyanobacteria can lead to bacterial mats on coral surfaces, which block photosynthesis, prevent fish from reproducing in affected areas and prevent coral settlement. [12] An overabundance of cyanobacteria has also been linked to black band disease and could also lead to algae overgrowth.[13][14] So, whether amoxicillin harming cyanobacteria populations is a net positive or negative depends on the reef’s health prior to amoxicillin treatment. Healthy reefs tend to have normal levels of cyanobacteria, while unhealthy reefs tend towards an overabundance of cyanobacteria.[15] More research is necessary to gain a broader sense of the effects of antibiotics, specifically amoxicillin, on reef ecosystems as a whole.
Antibiotic resistance is another concern. It is likely only a matter of time before SCTLD develops amoxicillin-resistance and another antibiotic is needed, and as of right now there are no other viable treatments for SCTLD besides amputation. The amoxicillin is also applied by hand by divers, which is regarded as "expensive, time-consuming, and limited in scale to relatively small areas", so while it is an undeniably effective method of treating SCTLD, its benefits on a larger scale may be negligible if interventions lack proper funding and resources.[16]
References
- ↑ 1.0 1.1 1.2 Traylor-Knowles N, Connelly MT, Young BD, Eaton K, Muller EM, Paul VJ, et al. Gene Expression Response to Stony Coral Tissue Loss Disease Transmission in M. cavernosa and O. faveolata From Florida [Internet]. Coral Reef Research. Frontiers in Marine Science; 2021 [cited 2022May25]. Available from: https://www.frontiersin.org/articles/10.3389/fmars.2021.681563/full
- ↑ Stony Coral Tissue Loss Disease (SCTLD) Tracking Map [Internet]. Oref.maps.arcgis.com. Atlantic and Gulf Rapid Reef Assessment; [cited 2022May25]. Available from: https://oref.maps.arcgis.com/apps/View/index.html?appid=6bf1ce3fcd8948598000aac1dda9e84a&extent=-133.8154%2C-0.5271%2C-49.4404%2C38.7200
- ↑ Study finds genes role in immune response of Florida corals to rapidly spreading disease [Internet]. Science News. ScienceDaily; 2021 [cited 2022May26]. Available from: https://www.sciencedaily.com/releases/2021/07/210706115400.htm
- ↑ Staletovich J. Research breakthrough finds hope for corals infected with stony coral disease - they can fight back [Internet]. WLRN. WLRN; 2021 [cited 2022May25]. Available from: https://www.wlrn.org/news/2021-07-12/research-breakthrough-finds-hope-for-corals-infected-with-stony-coral-disease-they-can-fight-back
- ↑ Aeby G, Ushijima B, Campbell J, Jones S, Williams G, Meyer J, et al. Pathogenesis of a Tissue Loss Disease Affecting Multiple Species of Corals Along the Florida Reef Tract. Frontiers in Marine Science [Internet]. 2019 [cited 26 May 2022]; 6, 678. Available from: doi: 10.3389/fmars.2019.00678
- ↑ Miller C, May L, Moffitt Z, Woodley C. Exploratory treatments for stony coral tissue loss disease: pillar coral (Dendrogyra cylindrus). NOAA Technical Memorandum NOS NCCOS 245 and CRCP 37 [Internet]. N.d. [cited 26 May 2022]. Available from: doi:10.7289/V5/TM-NOS-NCCOS-245
- ↑ 7.0 7.1 Neely K, Shea C, Macaulay K, Hower E, Dobler M. Effectiveness of topical antibiotics in treating corals affected by Stony Coral Tissue Loss Disease. PeerJ [Internet]. 2020 [cited 26 May 2022]; pp. 1-12. Available from: https://doi.org/10.7717/peerj.9289
- ↑ 8.0 8.1 Neely K, Shea C, Macaulay K, Hower E, Dobler M. Short- and Long-Term Effectiveness of Coral Disease Treatments. Frontiers in Marine Science [Internet]. 2021 [cited 26 May 2022]; pp. 2-19. Available from: https://core.ac.uk/outputs/481309077
- ↑ Neely, K. Coral Disease Intervention Plan. Florida DEP [Internet]. 2018 [cited 26 May 2022]; Miami, FL. P. 12.
- ↑ González-Pleiter M, Gonzalo S, Rodea-Palomares I, Leganés F, Rosal R, Boltes K et al.Toxicity of five antibiotics and their mixtures towards photosynthetic aquatic organisms: Implications for environmental risk assessment. Water Research [Internet]. 2013 [cited 26 May 2022]; 47(6) pp. 2050-2064. Available from: https://doi.org/10.1016/j.watres.2013.01.020
- ↑ Charpy L, Casareto B, Langlade M, Suzuki Y. Cyanobacteria in Coral Reef Ecosystems: A Review. Journal of Marine Sciences [Internet]. 2012 [cited 26 May 2022]. Available from: https://doi.org/10.1155/2012/259571
- ↑ Kuffner I, Walters L, Becerro M, Paul V, Ritson-Williams R, Beach K. Inhibition of coral recruitment by macroalgae and cyanobacteria. Mar Ecol-Prog Ser [Internet]. 2006 [cited 26 May 2022]; 323:107–17. Available from: https://doi.org/10.1371/journal.pone.0125445
- ↑ Carlton R, Richardson L. Oxygen and sulfide dynamics in a horizontally migrating cyanaobacterial mat—Black band disease of corals. FEMS Microbiol Ecol [Internet]. 1995 [cited 26 May 2022];18(2):155–62. Available from: doi:10.3354/meps323107
- ↑ Larned S. Nitrogen- versus phosphorus-limited growth and sources of nutrients for coral reef macroalgae. Mar Biol [Internet]. 1998 [cited 26 May 2022];132(3):409–21. Available from: https://doi.org/10.1007/s002270050407
- ↑ Brocke H, Polerecky L, de Beer D, Weber M, Claudet J, Nugues M. Organic Matter Degradation Drives Benthic Cyanobacterial Mat Abundance on Caribbean Coral Reefs. PLOS ONE [Internet]. 2015 [cited 26 May 2022]; 10(5). Available from: https://doi.org/10.1371/journal.pone.0125445
- ↑ Forrester G, Arton L, Horton A, Nickles K, Forrester L. Antibiotic Treatment Ameliorates the Impact of Stony Coral Tissue Loss Disease (SCTLD) on Coral Communities. Frontiers of Marine Science [Internet]. 2022 [cited 26 May 2022]; para. 33.