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Field Guide to Western Atlantic Coral Diseases
Coral diseases and Nomenclature
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Coral Disease Nomenclature

The debate surrounding nomenclature of coral diseases

During the first 20 years of coral disease research (1972-1992), relatively few reports of disease appeared in the published literature. These included the widespread conditions referred to as black band disease (BBD), and "white band disease (WBD)" and "plague", which have been used (often interchangeably) to describe conditions resulting in white bands of recently exposed skeleton. The rate of discovery of new coral diseases has increased dramatically since the mid 1990s, with more than 70 different disease names reported in the literature. Although there is good evidence to suggest coral diseases are occurring over a wider range and among an increasing number of host species, we know very little about the ecology or pathology of most of these diseases. In addition, many of the "new diseases" and disease names appearing in the literature do not reflect a correct use of medical terminology, or they incorporate the name of a presumed pathogen which cannot be determined based on visual observations alone.

The proliferation of names for different diseases presents challenges when evaluating geographical distribution of diseases, changes in abundance and distribution over time, and host ranges. This has produced confusion when comparing country or regional reports and when evaluating whether a report of a disease in one location is the same as that reported from another reef or region. It is currently unknown whether diseases observed on different reefs or in different regions have the same causative agent, even if they exhibit similar gross morphological characteristics.

The current state of knowledge and understanding of coral disease

Coral diseases can be biotic or abiotic. Biotic diseases are infectious, and are caused by a number of microorganisms such as bacteria, fungi and cyanobacteria. Abiotic diseases are non-infectious and are caused by environmental stressors including pollutants, sediment and toxicants, climate change, and physical damage. Abiotic diseases also include inherent or congenital defects, and they may occur as a result of a combination of environmental stressors. Diseases can also be caused by microorganisms that are non-infectious under ambient environmental conditions, but they become pathogenic in response to certain triggers (e.g. increase in temperature). In addition, many microorganisms that are the presumed cause of a disease may actually be opportunistic organisms (e.g. organisms that do not ordinarily cause disease but that can, under specific circumstances, become pathogenic) that can become established when the host is stressed or the host's immune system is in some way compromised. Most coral diseases have no known etiology, and it is difficult to explain the source and sudden emergence of these diseases. In addition, few named diseases have been accompanied with systematic morphologic descriptions both at the gross and cellular level.

Diseases of corals include:

  • Bleaching: loss or degradation of symbiotic zooxanthellae in response to physical perturbations such as elevated temperatures and UV radiation, or a bacterial (Vibrio) infection concurrent with elevated temperatures,
  • Non-infectious physiological and morphological changes,
  • Parasitic infections of protozoans and metazoans,
  • Growth anomalies (e.g. tumors), and
  • Infectious agents such as bacteria, fungi and viruses that can be transmitted between hosts.

To date, presumed causative agents have been reported for only five coral diseases using traditional culture techniques. This involves plating of samples of diseased tissue (or mucus from a lesion) on marine agar plates, followed by isolation of abundant bacteria which are grown in pure culture, and subsequently reintroduced to a uninfected host. If these organisms are shown to instigate the disease, and they can be re-isolated, they can be presumed to be the causative agent, based on fulfillment of Koch's postulate. Some of these earlier findings have been questioned based on recent evaluation of the holobiont community using molecular methods. The examination of the microbial community associated with coral mucus and coral disease using molecular methods has helped elucidate the diversity of bacteria and other microorganisms, and shifts in these communities in response to environmental changes. Recent findings also suggest the possibility of multiple causes depending on host species and location.

Because of the lack of consensus regarding the causative agents of several of the better characterized diseases, and the similarity of gross morphological signs defined in the literature as a suite of different diseases, there is a need to:

  1. establish common field names for each disease based on a standardized set of characteristic field signs, that could be applied by scientists conducting routine monitoring;
  2. require a more complete evaluation of the histopathology to develop a morphologic diagnosis for that condition in a specific location or host; and
  3. undertake a more complete evaluation of the characteristics of associated microbial pathogens or other causes using a combination of culture and molecular approaches to develop an etiologic diagnosis.

Examples of nomenclature problems

There are 6 primary features that have been used historically to separate diseases. These include the color of the tissue (yellow band disease) or exposed skeleton (white band disease); presence of a microbial band (black band disease); shape of lesion (yellow blotch disease); pattern and rate of tissue loss (white band type I and II); presence of bleached areas (plague type II and WBD type II), and species affected (white band disease).

Some of the problems with existing descriptions and nomenclature include:

  • Diseases described and named based on a single observation or using limited macroscopic field characters, in absence of standardized diagnostic criteria or recognized medical terminology applied to other wildlife disease investigations;
  • Descriptions based on limited ecological observations, including a lack of information on temporal variability of signs, progression of disease states over time within individual affected hosts, effects of environmental variations (e.g. depth, wave exposure, water clarity) and differences between species;
  • Existing diseases subdivided into multiple "types" in absence of suitable diagnostic criteria;
  • Inference of causality when naming a disease;
  • An absence of a systematic study of morphologic changes associated with the presumed disease when describing the disease;

1) The same name has been used for two diseases that have very different field signs. Yellow band disease is the term given to a condition that affects the genus Montastraea (and other faviids to a lesser degree) in the western Atlantic. It is characterized by pale, lemon yellow blotches or bands that slowly radiate outward as tissue first affected dies. In the Arabian Sea YBD affects four genera of corals. The affected tissue is also yellow, but there is also a yellow-pigmented microbial mat at the interface b/n skeleton and tissue, and skeleton retains yellow pigment after the coral dies.

2) Use of multiple names to describe a single condition. Yellow band disease is also called yellow blotch disease, yellow blotch syndrome, yellow blotch/band and yellow pox syndrome in publications and on web sites. White band disease and black band disease have also been referred to as white line and black line (respectively).

3) One name is used for syndromes with distinct field signs. One of the most confusing syndromes in the western Atlantic is a disease called white pox. Photographs of white pox available on line, on various coral disease websites and in publications illustrate dramatically different diseases signs. These include everything from circular dime-sized lesions, crescent shaped lesions that are the same shape as the jaw of a fish, old lesions that are in various stages of recovery with no evidence that they were caused by a disease, to lesions that show evidence of parrotfish predation. The associated descriptions are also highly variable.

4) Different names have been used interchangeably. Patchy necrosis (PN) is a condition similar to white pox that first reported in 1996. This condition is characterized by small (1-2 cm) circular to irregular patches of denuded skeleton that increase in diameter for several days before stopping. Over time, the lesions then begin to recover, but new lesions may appear in different locations on the colony. There have been suggestions that this is the same syndrome as white pox (pox), except pox has been identified as the major cause of mortality to acroporids in Florida and unlike PN, colonies are not reported to recover once they are infected. Also, one recent study identified an outbreak of patchy necrosis in Puerto Rico that occurred during a period when the sea was very calm (Weil, 2004) yet closer examination of affected colonies revealed that the lesions on the coral were actually due to accumulations of parrotfish feces that smothered the coral tissue.

5) Division of a single disease into multiple types. Perhaps one of the most problematic examples of nomenclature is a tendency to split single syndromes into multiple syndromes based on single observations of an infected coral, without any histological or microbiological evidence that they are in fact different syndromes. Dark spots disease was first reported in the early 1990s among three species of corals in the genus Montastraea, Siderastrea and Stephanocoenia. It was characterized by small round spots that grow in size over time. One researcher recently divided DSD into five different conditions DSD type I, DSD type II, dark band syndrome (DBS), purple band syndrome (PBS) and tissue necrosis (TN). The primary difference between DSD Type I and II was the presence of larger spots and presumably faster rates of tissue loss in Type II. The signs of DBS included the presence of dark spots that turn into bands, while tissue necrosis is similar to DBD, except there were no spots and tissue sloughing was visible on affected corals. Some division has also been based on the species affected. Other researchers who actually followed individual colonies over time have found that dark spots often don’t actually cause mortality at all, and the spots may disappear as water temperatures decline.

6) Misidentification of diseases. There have been at least two different presumed diseases that have been later shown to be caused by fish bites. Ridge mortality disease, first reported from the Flower Gardens National marine Sanctuary is caused by the three spot damselfish (Stegastes planifrons). Rapid Wasting Disease, a condition presumed to be caused by fungi, is the result of predation by the stoplight parrotfish, Sparisoma viride. Even though there have been several detailed studies demonstrating that these are in fact predation, the names still appear in the literature today.

7) Difficulties in giving a name to a disease. Probably the most confusing of the diseases identified in the Caribbean are the various white syndromes that affect most coral species. These include white band disease (type I and II), white plague (type I, II and III), all of which characterized by a distinct white band of recently denuded skeleton separating live tissue from algal colonized skeleton. These have been separated based on the species affected and a variety of gross visible signs that may not be detectable based on a single observation, such as rate of spread and presence of a bleached tissue margin.

Development of standardized nomenclature and diagnostic criteria for diseases

The Coral Disease and Health Consortium (CDHC) have been working to develop standardized approaches for conducting coral disease research, including application of standardized nomenclature and diagnostic criteria. We have developed a three-tiered approach to naming diseases and a decision tree that would help unify disease names. Through application of a series of steps, a researcher can easily determine a common field name for a disease which can be further refined based on a morphologic diagnosis and an etiologic diagnosis. This is based on techniques applied to the study of other diseases, including human and terrestrial animal diseases that include 1) detection of disease, 2) description of the morphologic changes in the host associated with the disease, 3) evaluation of morphologic changes in the host as disease progresses (e.g. pathogenesis), determining the cause (e.g. etiology), and physiologic changes in the host as the disease progresses (e.g. pathophysiology).

Tier I

Level 1 examination involves categorization of obvious gross field signs directly visible to the diver without magnification or sampling. The first step involves the identification of four easily observed descriptive categories - presence of color change, tissue loss, skeletal damage and irregular growth. The observer also records other gross visible signs such as the location and distribution of the lesion, condition of the affected tissue, and the extent of tissue loss. Based on an analysis of these characters, it is possible to identify a common field name to describe the disease in the particular coral and apply the same terminology to other corals (regardless of species) with similar gross visible signs. This approach is used primarily by researchers conducting routine monitoring without the ability to sample corals or conduct detailed evaluations.

Tier II

Level II examination involves a more detailed examination of affected and unaffected tissue taken from the diseased coral (and compared to representative presumed healthy corals) using histology. This approach can help identify the presence of microorganisms and describe morphological changes to the tissue. This could result in a morphologic diagnosis.

Tier III

Level III examination involves a more detailed set of field and laboratory tests to identify and confirm the presence of proposed causative agent(s), toxin, or other factor responsible for the manifestation of the disease. One approach for microbial diseases could be through the demonstration of Koch's postulate - where a presumed pathogen is isolated, grown in pure culture, identified, and used to infect a presumably healthy host. If the disease signs appear, and the presence of the presumed pathogen is confirmed, this can be assumed to be the cause and an etiologic diagnosis can be assigned.

Example of the three tiered approach

A researcher identifies a series of lesions on elkhorn coral (Acropora palmata) colonies. These are characterized by recent tissue loss, but there is no evidence of color change, skeletal damage or abnormal growth. The lesions are 1) irregular in shape, 2) completely surrounded by living tissue, and 3) exhibit a margin of tissue sloughing at the margin between exposed skeleton and normal tissue. The researcher would then assign a field name (e.g. this is currently called white pox) and would use that same name to describe similar signs observed in other corals. The researcher conducts additional studies on the coral, including sampling for histology and microbiology. Using both molecular tools and traditional culture techniques, the researcher identifies the presence of Serratia marcescens, a bacteria reported as the cause of white pox in Florida. By verifying the presence of a known causative agent, the researcher can provide an etiologic diagnosis for the disease, which may be "acroporid serratiosis". If this researcher identifies the same species in a different location with similar gross visible signs, these would also be identified as white pox. However, verification of the presence of Serratia is required in each case to assign an etiologic diagnosis.

This approach is similar to that applied in human medicine. For instance, a person with a sore throat cannot conclude that this is strep throat unless a throat culture is taken and the presence of a Streptococcus bacteria is identified.

Changes to the Global Coral Disease Database to accommodate revised nomenclature

As a first step in standardizing the reporting of coral diseases within the Global Coral Disease Database, all records will be reviewed for consistency with the revised nomenclature. Each will be assigned a common field name whenever possible, using recognized terminology. The original name will be included in the "notes" field. In addition, if additional studies have been completed to characterize a reported disease in a particular location (e.g. the identification of a causative agent) this information, along with an etiologic diagnosis will be included in the notes section.