Gross Findings: Affecting approximately 60% of the specimen are multifocal to coalescing areas of tissue loss up to 6 x 10 cm, with a 1 to 2 cm wide border of darkly pigmented, lightly adherent material along the tissue loss margin between living tissue and newly denuded white skeleton.  Older areas of bare skeleton more distant from live tissue are discolored green to brown by algae and sediment. Live tissue is mottled brown to white (bleached).

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Histopathological Findings: There are multifocal to locally extensive areas of necrosis and loss of gastrodermis and surface and basal epithelium with destruction of mesoglea. (Fig. 2A).  Filamentous cyanobacteria with a characteristic segmented appearance are within affected tissues, and endosymbionts are lost, degenerate or necrotic. (Fig. 2B).

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Gross and Morphologic Diagnosis:

1) Acute to subacute tissue loss with a marginal, black microbial mat.

2) Locally extensive epithelial necrosis and mesogleal lysis with intralesional filamentous bacteria. 

Disease: Black band disease

Etiology: This is a polymicrobial disease that involves a complex and variable consortium of microbes, many of which have not been conclusively identified but fall into in four trophic groups: photosynthetic filamentous cyanobacteria (e.g., Phormidium corallyticum), sulfur-oxidizing bacteria (e.g., Beggiatoa sp. or ε-proteobacteria), sulfur-reducing bacteria (e.g., Desulfovibrio sp.), and a more heterogeneous group that includes the Cytophaga-Flexibacter-Bacteriodes (CFB) group and marine fungi (Frias-Lopez and others, 2004; Richardson and others, 2015). 

Distribution: Global

Seasonality: Summer, with greatest incidence during episodes of high water temperature

Host range: Multiple species of scleractinian (stony) corals and some Gorgonians

Transmission: Likely waterborne and possibly via corallivorous invertebrates (e.g. fireworms, snails)

Clinical signs: Black band disease is characterized by a darkly pigmented bacterial mat that migrates across the surface of the coral leaving bare skeleton behind. 

Pathology: Grossly, black band disease presents as progressive tissue loss with a dark-colored band consisting of a membrane-like mat along the tissue loss margin (sometimes absent). Microscopically it is characterized by necrosis of all tissue layers, and invasion by filamentous cyanobacteria along the lesion margin. Initiation and early stage development of black band disease is incompletely understood, but the cyanobacterial component is suspected to be one of the main pathogens carried by water or introduced into a wound when corallivorous invertebrates feed on coral. Other members of the disease consortium can vary between outbreaks and may be carried by the water column, fish, or invertebrates; or are present in the coral microbiota. These microbes are recruited into a small, discolored ‘cyanobacterial patch’, which progresses to tissue necrosis with a trailing black microbial mat as the disease consortium is built. During daylight, the microbial mat is oxygenated, and hydrogen sulfide levels are low. At night, oxygen is depleted, and hydrogen sulfide increases sharply. The combination of low oxygen and high sulfide is lethal to coral tissues and is hypothesized to drive lesion progression (Sato and others, 2016)

Diagnosis: Compatible gross signs seen during warm summer months. Some instances may lack a visible band but have microscopically visible bacterial tissue invasion. Cyanobacteria are easily visible on H&E, and Giemsa staining may reveal other bacteria, though these are present in lower numbers.  Bacterial culture of affected corals is often unrewarding due to both the difficulty of distinguishing pathogenic from commensal or symbiotic microbes and the difficulty of culturing some marine bacteria in laboratory media. Sequencing with 16S rRNA gene primers is used in a research capacity to identify bacteria within the microbial mat.

Public health concerns: None

Wildlife population impacts: The incidence of black band disease in reefs is typically low and the disease regresses during the winter. Warming summers are likely to increase the prevalence and duration of black band disease on coral reefs.

Management: Colonies may be treated by removing the microbial mat followed by treatment of the lesion margin by affixing jute rope saturated with a proprietary antimicrobial ointment or smothering it with clay or epoxy.

References:

• Ainsworth, T.D., Kramasky-Winter, E., Loya, Y., Hoegh-Guldberg, O., and Fine, M., 2007, Coral disease diagnostics: what's between a plague and a band?: Appl Environ Microbiol, v. 73, no. 3, p. 981-992. https://doi.org/10.1128/AEM.02172-06
• Eaton, K.R., Clark, A.S., Curtis, K., Favero, M., Hanna Holloway, N., Ewen, K., and Muller, E.M., 2022, A highly effective therapeutic ointment for treating corals with black band disease: PLOS ONE, v. 17, no. 10, p. e0276902.  https://doi.org/10.1371/journal.pone.0276902
• Frias-Lopez, J., Klaus, J.S., Bonheyo, G.T., and Fouke, B.W., 2004, Bacterial community associated with black band disease in corals: Appl Environ Microbiol, v. 70, no. 10, p. 5955-5962. https://doi.org/10.1128/AEM.70.10.5955-5962.2004
• Raymundo, L.J., and Weil, E., 2015, Indo-Pacific colored-band diseases of corals, in Woodley, C.M., Downs, C.A., Bruckner, A., Porter, J.W., and Galloway, S.B., eds., Diseases of Coral, p. 333-344. https://doi.org/10.1002/9781118828502.ch23
• Richardson, L.L., Miller, A.W., Blackwelder, P.L., and Al-Sayegh, H., 2015, Cyanobacterial-associated colored-band diseases of the Atlantic/Caribbean, Diseases of Coral, p. 345-353. https://doi.org/10.1002/9781118828502.ch24
• Sato, Y., Civiello, M., Bell, S.C., Willis, B.L., and Bourne, D.G., 2016, Integrated approach to understanding the onset and pathogenesis of black band disease in corals: Environmental Microbiology, v. 18, no. 3, p. 752-765. https://doi.org/10.1111/1462-2920.13122
• Zvuloni, A., Artzy-Randrup, Y., Stone, L., Kramarsky-Winter, E., Barkan, R., and Loya, Y., 2009, Spatio-temporal transmission patterns of black-band disease in a coral community: PLOS ONE, v. 4, no. 4, p. e4993. https://doi.org/10.1371/journal.pone.0004993