Causes
Virtually all of the most pervasive threats impacting coral reef ecosystems, including land-based and marine pollution, overfishing, global climate change, and ocean acidification, have been suggested as synergists or facilitators of infectious disease. Infectious disease in corals has increased in frequency and distribution since the early 1970’s when a white band disease outbreak took a heavy toll on Caribbean corals in the Acroporidae family. Since then, there has been an exponential increase in numbers of reported diseases, host species and locations with disease observations.
Dark spot disease on a Siderastrea sideria in Western Sambo, Florida Keys. Photo © Craig Quirolo/Reef Relief
Unlike spatially extensive bleaching events, the causes of coral disease outbreaks are more complex and less understood. There is limited knowledge of the cause of many diseases, the origins of pathogens, life cycles, response to environmental factors such as temperature, or the range of coral species that they can infect. However, research on the causes of coral disease has increased in recent years, especially in terms of identifying the pathogens involved. Most biotic coral diseases are believed to be related to infection by one or a group of pathogens.1 A host of contributing microorganisms2 and macro-parasites such as ciliates have been identified as possible causal agents. To date, the most infectious syndromes of coral for which a causative agent has been isolated involve bacteria.
Significant relationships exist between coral disease prevalence and elevated water temperatures,3,4,5,6 a decline in water quality,7,8,9,10,11 vector and host densities, and intensity of coral bleaching. The effects of environmental factors on coral disease prevalence and modes of transmission are likely to be intricate and synergistic. The following sections discuss environmental factors and modes of transmission in more detail.
Red Band Disease on coral Montastrea cavernosa in Florida Keys. Photo © Craig Quirolo/Reef Relief/Marine Photobank
1 Sokolow, S. 2009
2 Richardson L.L. and Aronson R.B. 2000
3 Jones R.J. et al. 2004
4 Bally M. and Garrabou J. 2007
5 Bruno J.F. et al. 2007
6 Rodriguez S. and Croquer A. 2008
7 Kim K. and Harvell C.D. 2002
8 Kuta K.G. and Richardson L.L. 2002
9 Bruno J.F. et al. 2003
10 Voss J.D. and Richardson L.L. 2006
11 Baker D.M. et al. 2007