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White plague (WP) is a common tissue loss disease that affects a broad range of scleractinian coral species. White plague is one of the most destructive coral diseases, as it is highly lethal and has a history of widespread outbreak events that have caused extensive losses in coral populations. Due to some variability in lesion characteristics, affected species, and outbreak patterns, White Plague has been further subdivided into types II, II, and III, which are differentiated mainly by their rates of tissue loss. White plague continues to affect reefs throughout the Caribbean, and bears some similarities to the most recently discovered coral disease Stony Coral Tissue Loss Disease (SCTLD).
Like many other white syndromes, white plague lesions are characterized by a distinct band of bare skeleton immediately adjacent to intact, pigmented tissue, with no apparent discoloration or microbial consortium associated with the disease lesion. Exposed corallites where tissue recently receded remain intact and there is no other associated skeletal damage. The rate of tissue loss can range from as little as 1mm/ day to more than 10 cm/ day, and can vary even on a single colony as the lesion becomes more advanced.
The rate of tissue loss is what most clearly differentiates the three different types of white plague, with WP type I being the slowest (<1cm/day), WP type II being between 1 and 2 cm/day, and WP type III being the most virulent (>2cm/ day).
Because white plague affects such a broad range of coral hosts, the lesion shape can be highly variable between circular, irregular, linear, or oblong, depending on the coral species, colony morphology, and stage of disease (newly affected vs. advanced).
Depending on the many factors listed above, particularly on the stage of disease, the distribution of the lesions can also vary. What begins as a focal area of tissue loss can advance to become multifocal to coalescing, and in advanced cases lesions can appear as diffuse over an entire colony.
White plague lesions can form anywhere throughout a colony, beginning at the basal, peripheral, or apical areas and spreading progressively outward.
Transmission and epidemiological studies suggest that white plague is caused by a communicable agent, however, a specific pathogen has not yet been definitively identified for white plague, nor has it been determined whether the agent is bacterial or viral in nature. Only WP type II has had one putative pathogen identified as a novel bacterium Aurantimonas coralicida, though some studies have failed to find this species associated with corals with consistent lesions. In some cases, prevalence of WP on reefs has correlated with environmental stressors, so there is some controversy as to whether the disease is a response to abiotic threats rather than an infectious disease.
White plague is particularly devastating for populations of massive corals and is thought to have caused the more extensive morality among this category of corals than any other known disease. While WP can be chronically present on reefs at a low background level, there are two particularly notable outbreak events have had dramatic impacts in the regions that they occurred, the first being an outbreak of WP type II on Florida reefs in 1995 that killed up to 38% of one coral species, and another outbreak in USVI following a bleaching event in 2005 that caused population losses of up to 80% in some locations.
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