Is it Contagious Doc?

The world’s coral reefs are currently undergoing an attack from a variety of different diseases, each with their own symptoms and characteristics. These diseases have received a lot of attention over the last couple of years, however studies have mostly focused on identifying the particular pathogens.

This experiment however examines epidemiology patterns and tries to determine what may have caused the recent increase of coral diseases in the Caribbean - contagion or environmental variation.

Along with laboratory and field work, the use of spatial epidemiology is used when working on coral diseases as it can be used to identify patterns that are key indicators of contagious diseases. Many previous studies of similar nature have been spatially limited resulting in very contrasting results, however this current study is the first to expand the observed area to look over the km2 scale.

Muller et al., (2012) set out to determine whether yellow-band disease, dark-spot syndrome, and white plague disease clustered spatially, a key feature if they followed a contagious-disease model. An area covering 25,300 m2 in the US Virgin Islands and Puerto Rico were studied and 253 still photographs of the benthic community were taken at each 100 m2 site. These images were stitched together allowing them to determine the clustering pattern of diseases over two spatial scales (m and km). Surveys took place in midsummer, when coral disease rate were at their highest, and to avoid seasonal variation.

Coral diseases were identified in situ and if at least one colony of M. annularis spp showed signs of yellow-band disease all colonies of the same species were outlined per site. This outlining procedure was carried out for colonies of S. siderea if they were observed to have dark-spot syndrome or white-plague disease. Locations of each infected colony were then plotted producing a map of every disease coral’s location.

The findings showed that the diseases didn’t cluster, meaning they didn’t follow the spatial pattern that would indicate contagious diseases. What Muller et al. found instead was that the clustering was spread over kilometers, indicating that the disease origin may be due to environmental variation. This change in the environment may lead to stress in the colonies, resulting in the coral becoming more susceptible to infection or providing conditions in which pathogen increase their proliferation and virulence.

The authors acknowledge many limitations throughout the study, especially when trying to identify the diseases. The use of still images and varying progression stages of the disease made them hard to identify. However the biggest short coming is the inability to differentiate between three types of white plague disease leading 3 diseases to be lumped into a single ‘white-plague’ category.

These small but significant problems with the experimental procedure could dramatically affect the observed patterns found in the Caribbean resulting in the survey being inaccurate. Future studies should be carried out hoping to amending these problems the team ran into as it could be very interesting seeing a more complete map with an additional three disease mapped. Further studies would go a long way to help us understand coral disease activity, allowing us to update and maintain disease models, that are essential for the management of coral systems.

References:

Muller, E. and van Woesik, R. (2012). Caribbean coral diseases: primary transmission or secondary infection?. Global Change Biology, 18(12), pp.3529-3535.

Review Article:
Sokolow S (2009) Effects of a changing climate on the dynamics of coral infectious disease: a review of the evidence. Dis Aquat Org 87:5-18