Progress in Understanding Coral Diseases in the Caribbean

Coral disease research in the Caribbean initiated in 1972 with the discovery of black band disease (BDD) by Dr. Arnfred Antonius. Since this time, there has been an expansion in the number of researchers working in the Caribbean, including studies to document the prevalence and incidence across large spatial scales and at increased temporal frequencies, evaluation of the linkages between coral disease and environmental drivers, identification of disease vectors, and laboratory studies to characterize causes, physiological responses, histological changes, defense mechanisms and mechanisms of resistance and susceptibility. Since 1998, the Caribbean has emerged as a “hot spot” for coral disease due to the large number of named diseases, their wide distribution, expanding host ranges, and increasing abundance and severity, with over 30 diseases now reported from this region. Localized epizootics of three diseases (BBD, WBD, WP-I) were first documented in the 1970s; one of these (WBD) expanded throughout the Caribbean to become the most significant factor in the region-wide decline of acroporids. Five diseases (WP-II, YBD, WPX, DSD and ASP) emerged in the mid 1990s, and have expanded their geographic distribution and host ranges over the last ten years with several of the diseases causing substantial coral mortality since the late 1990s. By 2005, at least 41 scleractinian corals, 8 gorgonians and two hydrozoans were observed with one or more diseases. The most abundant and important group of corals found on Caribbean reefs today (M. annularis complex) is susceptible to at least 8 different diseases, and individual colonies may show signs of 2-3 diseases at the same time. The average prevalence of coral disease at the community level is generally low, although it is highly variable between and within sites, during different years, and seasonally. Coral disease outbreaks have affected up to 91% of certain susceptible populations in localized areas, and often (but not always) exhibit a clumped distribution. Disease prevalence and severity is generally greater during warm water periods, and recent disease outbreaks have been associated with mass bleaching events. Over the last five years, there has been an increase in the numbers of studies that have reported a correlation between coral disease and environmental factors, including higher prevalence rates and greater rates of spread in areas affected by nutrients, sediments and other pollutants. Causative agents have been identified for relatively few diseases, three of which (WP-II, ASP and WPX) have been verified through application of Koch’s postulate. In these and other diseases, complex microbial communities have been identified using new molecular techniques, including biota on diseased tissue that is absent from control samples and suites of microorganisms that differ from those identified using traditional (microscopy and culture) techniques.  Furthermore, pathogens identified using traditional microbiological approaches are no longer infective, including the proposed causative agent for white plague type II, suggesting 1) the pathogens may have lost their virulence and/or corals have gained immunity, or 2) causation was determined based on a relatively small number of corals from a single location/event and other microbial agents are capable of causing similar signs. In addition to key advances in understanding the coral holobiont, and how microbial communities associated with coral tissue and coral mucus change during periods of stress, some advances have been made in identifying possible vectors of coral disease, including linkages between a coral eating snail (Coralliophila abbreviata) and a white syndrome that affects acroporids as well as three spot damselfish and BBD. Efforts have been made to mitigate disease, through removal of the microbial community, antibiotic treatments, use of putty and/or clay to cover the affected area, and addition of urchins to reduce algal abundance, however these exhibited only limited success and they do not appear to be feasible treatments on a larger scale. One of the major limitations in advancing our understanding of coral disease has been the lack of standardized nomenclature and diagnostic criteria for diseases, which has resulted in a proliferation of names and the identification of new presumed diseases that later have been shown to be caused by other factors. Some of the key needs for the Caribbean include: 1) greater geographic coverage and more frequent surveys to characterize prevalence and incidence; 2) more emphasis on population dynamics and impacts, including size structure of diseased and healthy corals, extent of partial and whole colony mortality and impact to individual corals and coral populations; 3) concurrent monitoring and assessment of environmental factors; 4) revision of existing disease nomenclature and adoption of standardized terminology and diagnostics; 5) application of traditional culture and histopathology techniques in combination with new molecular tools to characterize causative agents and sources of pathogens and development of molecular probes to facilitate screening of corals; and 6) more emphasis on cellular diagnostics, including biomarker characterization, to assess stress levels in corals and underlying causes; and 7) a coordinated rapid response program to address coral disease outbreaks and unusual mortality events.