Rapid Tissue Necrosis- A New Understanding.

Rapid Tissue Necrosis- A New Understanding.

by Jonathan Lowrie

Rapid Tissue Necrosis is a term coined by the aquarium populace to describe a common malady of corals kept in captivity.  It is present predominantly in members of the coral families Pocilloporidae (Pocillopora, Seriatopora, Stylophora, Madracis) and Acroporidae (Acropora, Anacropora, Montipora, etc.).  The condition is characterized by the rapid sloughing of tissue from the base of the skeleton outward which, if left unchecked, can result in coral death in a period of hours to days.  There has been much talk and writing on the subject over the past several years in magazines, Internet groups, and at conferences, regarding this affliction.  However, despite numerous theories and remedies put forth, the lack of cohesive methodology, including observable and inexplicable pathophysiology, prevented our willingness to accept much of the available information.  As coral pathology is an evolving field of research, there will be continual advances in the understanding of the etiology of these conditions and how they afflict corals.

After observing the deaths of many specimens to rapid tissue sloughing, we were a bit surprised and hesitant to accept the current model of Rapid Tissue Necrosis (henceforth "RTN") as being pathogenically causative.  Based on many years of observations, we had both seen numerous cases of RTN which were not explainable by these models, nor were they likely to be so in the future.  This series of observations has been corroborated by many others keeping these families of corals, through personal observation and interview.  Furthermore, we had observed individual cases where the pattern of tissue necrosis did not fit the common base-up loss.  We had experienced corals which did not respond to the now accepted treatment method involving the use of the antibiotic, chloramphenicol.  We had data to suggest that other inducements of a non-pathogenic nature existed.  We had observations and data of unusual morphologic and physiologic changes immediately prior to and concurrent with extant "disease."  We had surveyed the literature in depth and found that there were many seemingly similar or possibly same- symptom diseases that had been reported for over twenty years.  We also had a working hypothesis.  Through subsequent conversations and sharing of experiences and research, we have delved into a dimly lit area of marine science; Cnidarian immunology. 

Coral pathology is a relatively seldom studied aspect of the Cnidarian group.  Some of the early works on this topic, are strictly descriptions of conditions noticed in wild corals on reefs.  In the late 1970’s Arnfried Antonius first described a natural coral disease called White Band Disease (WBD).  Coral propagation and collection for home aquaria was not commonplace at this time.  As the work continues, a multitude of natural diseases have been found, many with as yet to be determined etiologies.  This broadly classifies a multitude of maladies in which various rates of tissue sloughing may be present.   Years later, other researches like Esther Peters determined that many of these disease seem to have a component in which environmental stress has a role.  She has further classified the diseases into appropriate categories.  In some cases, a specific pathogenic causative agent has been discovered.  In the case of RTN, the results of such a search have yielded a potential pathogen for some tested corals.

Dr. Craig Bingman, a biochemist at Columbia University, found evidence of large numbers of Vibrio vulnificus colonies present in tissue from RTN afflicted corals.  He also showed that the gram-negative antibiotic, chloramphenicol has been successful in arresting the progression of RTN in these and other samples.

Some have said that a marine Vibrio- vibrio vulvinificus is the caustitive agent of RTN. Based on studies that show Vibrio in large numbers in affected tanks, this conclusion may seem valid.  When one understands more about Vibrio- it becomes less clear  Vibrio are gram negatice, comma shaped bacteria.  They are found worldwide.  Most are marine spoecies.  These bacteria are extremely hard to differentiate from one another.  Many Vibrio are pathogentic to certain species, but usually only one species per bacteria.  Meaning Vibrio chlolera is pathogenic to humans, while Vibrio vulvinificus is pathogenic to Atlantic Oysters (crassostrea virginicata).  It is unlikely that one coudl accurately tell if a Vibrio is the causitive agent to RTN in corals.

Unfortunately, other gram-negative antibiotics have little effect over the Vibrio within the tissue.  Because other antibiotics fail to achieve a similar level of success, one must investigate the nature of chlormaphenicol as a pharmacologic agent.  This compound is sometimes attributed with a form of bone marrow anemia, and may cause other immunity suppression.  We feel that this may be why the drug has a more pronounced affect on RTN affected corals than equally competent gram-negative antibiotics.  A  downside of this medication is that it is a prescriptive, requiring a medical doctor or veterinarian to dispense.  It also has been implicated as a potential hazard to humans.  Furthermore, as of 1998, chloramphenicol is NOT approved for aquaculture use in the United States.

Another form of treatment for RTN involves dipping the afflicted coral into a solution of iodine. Lugol’s solution (.5% iodine, and 1% potassium iodide) is commonly used as a bacteriostatic or sterilizing agent in many aquaculture applications. While some assert it is effective through the theoretical explanation of reducing surface bacterial populations, other aquarists have noted that certain sources of wild corals are prone to more frequent RTN outbreaks following a preventive Lugol’s dip.  A possible explanation lies with the oxidative nature of the iodine solution.  During the dipping, a large portion of the mucosal layer is stripped off, potentially lessening the effectiveness of the protective mucus. 

There has also been a perceived increase in the incidence of RTN within the hobby.  Imports to the US have steadily grown over the past 5 years, and captive propagation has grown considerably with advances in husbandry and technique. Some of the perceived increase may be attributable to the increased demand of RTN-susceptible corals by aquarists as methodologies of maintaining these animals successfully has improved.  Of course, with increased demand comes lowered prices, and the concurrent lowering of care for collected specimens to preserve profit margins.  It is not a new revelation that most of those who have theorized about the causes of RTN have come to the almost incontrovertible conclusion that this syndrome is related to stress. Captive raised RTN-susceptible corals have a decreased report of showing signs of the affliction.   Their tissue has already had time to acclimate to the environment of a home system, and they are normally subjected to a much less traumatic shipping procedure.  It is notable that most of the cases of RTN are arising from specimens collected from the wild which are often stressed to an extreme in their shipments and holdings.  

Through our investigations, we proposed that the corals afflicted with RTN were responding in a way that did not correlate well with models of any pathogen.  The rate of infection (pathogenecity) was too quick, and the spread of disease occurred faster than what seemed likely with any viral or bacterial agent.  But, we had also observed incidences of RTN that occurred after specific instances of sedimentation, lowered oxygen levels, temperature changes, etc.  All  of these causes are environmental stressors. Whether stressed by environmental conditions, or infection, death of these corals was consistent with RTN.  This was not all together acceptable.  Certainly it was possible that increased stress would allow for the introduction of an infectious agent.  But why?  Stress in humans decreases our immunity...and decreased immunity had been purported as part of the reason these corals became "sick."  We began to look at Cnidarian immunity to ascertain what was occurring. We then developed a hypothesis.  RTN is not necessarily exclusively bacterial in nature, but can be likened to an allergic reaction.  Allergies are a heightened immune response that results in the appearance of disease, though no pathogen is necessarily present.  In fact, only an antigen must be present, and antigens can be as insignificant as a particle of pollen.  Founded in no small part by his earlier graduate work in assessing natural damage to reef populations, Jonathan contributed the data to this paper.  We proposed that corals were possibly reacting to any number of stressors in an auto-immune fashion, effectively causing their own deaths.  We are tentatively calling this reaction, the immune-response hypothesis.  As we have conducted a variety of related and ancillary studies to draw these conclusions, it is beyond the scope of this publication to detail the data and experimental method. A modicum of understanding by the reader will allow for the presentation of our data in a readable form.

Phylum Cnidaria, of which corals are a member, possess a relatively unstudied immune system. Cnidarians are primitive organisms that have one of the most ancient immune systems in nature.  It is an effective one, owing to their long term success over millions of years.  However, it is also a primitive one that is adept at coping with the relatively stable conditions of the ocean over time.  It was never evolved to deal with the stresses inherent to collection for aquariums, and it was not evolved to deal with the stressors being placed on wild communities today by human action.  Hence, there is a measurable increase in the incidence of diseases by both field observations and aquarists.  One of the features of the Cnidarian immune response is that they are inherently capable of releasing all the known cell types and enzymes that are capable of eliciting self-digestion.  Furthermore, the time it takes for non-RTN affected corals to "contract" RTN fits in well with the amount of time required for immune responses.  And it has already been found that Cnidarians, and even corals, participate in autoimmune behavior under other circumstances. Recently, Jonathan injected a non-pathogenic Vibrio species into a cold water anemone.  He found a dramatically increased number of immunoresponsive cells at the injections siste, which was later followed by the host-mediated local digestion of its own tissues.  In other words, an autoimmune response was found that caused tissue necrosis.  As this is a onetime observation, it requires further study.  We plan to extend this experimental protocol to tropical species and corals. 

Coral mucus is important in the immune response of corals to potential invaders.  It is not merely a protective coating, but also hosts immunodefensive cell types from the interstitial layers that are responsible for action against potentially dangerous environmental stresses and antigens such as sediments, bacteria, and chemicals.  Many other animals of the ocean realm have developed the use of a skin coating or mucus as a mechanism to prevent disease or infection.  Fish have a highly specialized ‘slime’ coat that allows for rough scrapes and prevents many parasites from getting an internal start. Under normal circumstances of active water flow, oxygen levels, and pH ranges, the corals' immune system can effectively deal with such antigens.  They produce immune responses that are normal to these levels.  When  exposed  to non-natural levels, their immune response is not coordinated.  They may over or underreact, and related and/or nearby corals may even counterreact to the immunoreactive corals because of  substances the "stressed" corals release.  Cnidarians  release chemicals known as histocompatibility factors into the water through their mucus. These factors are involved both in recognition of self and in the immune response, and they confer a natural immunity to antigens normally encountered in a population.  

So why Acropora?  Or at least, why is Acropora so often affected?  This coral is one of the fastest growing corals and is running at a brisk metabolic rate.  Therefore, it must be able to elicit quick action against invaders.  It is also a relatively recent coral from an evolutionary standpoint, and has unique or specialized physiologic attributes.  Immunity may to be one of them.  They are voracious consumers of plankton, and consume a large portion of bacterioplankton and bacteria they culture in their mucus (many of which are potentially pathogenic species). In a marine system, the majority of microfauna have pathogenic tendencies. Acroporids, et al. are also prolific mucus shedders.  This is not only as a physical and immune related protection against the normally high stress environments in which they are found, but also because they depend on mucus capture of nutrients to a larger degree that many corals.  Such mucosal secretions would mean that the factors which would stimulate an autoimmune response (especially in closed systems) would also more easily trigger an RTN outbreak.  Not coincidentally, the other frequently RTN affected coral family, Pocilloporidae, shares many characteristics with Acroporidae. 

Fortunately, many observations, experiments, and available information already support our hypothesis. Further research will hopefully close any gaps in our research and methodology. Previously collected field observations and data in 1992 showed that stressed corals had an increased number of phagocytic and granular type cells in their mucus.  It was later found that RTN could be induced in healthy corals by environmental stressors alone. By using methods that would affect the incidence of RTN if caused by either chemical (immune) mediators or pathogenic organisms, the results showed that a causative organisms was either not present, or not necessarily involved.  The implications of this work is truly fascinating and detailed, as they represent many years of study.  After we had combined our research, we did some preliminary exposure of RTN-affected corals to various steroidal and non-steroidal antihistamines and anti-inflammatory agents. Several of them halted the spread of RTN.  Unfortunately, the agents, or their dosages proved to be cytotoxic and resulted in the death of the previously affected specimens.  Nonetheless, this was hopefully a harbinger of possible treatment protocol in the future.

Although our work thus far has been exciting and novel, it is not without implications for the future and for the hobby.  We plan on continuing our research into immune responses in Cnidarians.  As we begin to understand more and more of the immune response, perhaps we will better understand the levels of stress required to bring about an autoimmune response and the role of pathogens in this and other diseases.  While this hypothesis is promising, we hold no allusions about the immensity of the challenge ahead; not only in providing sufficient evidence that such behavior is responsible for at least some forms of RTN, but also in beginning to assess possible ways to prevent the damage it causes to both captive and wild corals.  Hopefully, the results will see the decreased mortality of corals and other Cnidaria, and the increased success and longevity of specimens in the care of aquarists; in whom the future survival of many corals may rest.

This entire article was written without reference to any source save our own understanding, experiences, and work.  Any readers who wish to see the specific exerimental and literature references for the information contained herein may feel free to write to either of us for said information, or may consult with the in-depth paper we have authored.