The lesions are unusual, however, prompting outside experts to question the diagnosis of cancer
A British-Australian research team has just found coral trout living on the south side of the Great Barrier Reef sporting dark skin lesions. Fifteen percent of the affected populations at two sites bore brown-black growths, some of them raised and almost scablike. The authors conclude that they’ve stumbled onto an epidemic of melanoma — a type of skin cancer — in iconic and commercially important fish.
Marine epidemiologist Michael Sweet of the Newcastle Institute for Research on Sustainability in England and his coworkers describe their findings August 1 in PLOS ONE.
If confirmed, the findings would be quite troubling, other scientists agree. However, after perusing the paper, three outside experts each concluded independently that the new lesions don’t appear to be cancers. But even if they were, these experts say, skin cancers wouldn’t constitute the novelty being claimed.
Although skin cancer is on the rise in humans, Sweet's group contends that it’s occurrence in fish has not been reported outside of the laboratory. When asked to clarify this point, Sweet said: “We have not found any reports of cancers — including melanomas — in wild fish.”
Go back to the library, then, and look a bit more, advises Vicki Blazer, a pathologist at the U.S. Geological Survey’s National Fish Health Research Laboratory in Kearneysville, W. Va. Skin cancers affecting fish in the Great Lakes and Chesapeake Bay have been reported “for decades,” she says. It’s something with which she has more than a little familiarity: Blazer served as lead author for a 38-page Pennsylvania Sea Grant report documenting liver and skin lesions — including melanomas — in brown bullheads. While acknowledging that melanomas are a rare cancer, she says at least 10 or 12 papers over the years have confirmed the malignancy in wild fish, including marine species.
Moreover, she notes, in Europe, piscine cancers have been considered a recognized symptom of unhealthy seas for at least 20 years. It’s been “a standard bio-indicator there,” she says, with research papers documenting a reduction in tumor prevalence as water quality improves.
What the new study found
Whatever afflicts the Australian fish, the problem didn’t emerge until 2010. That’s when a reef biologist studying sharks happened to notice dark patches on neighboring coral trout. The observation prompted the researchers to deliberately hook 136 of these fish between August 2010 and February 2012.
Twenty displayed dark, almost-scarlike blotches covering anywhere from a few percent to virtually the entire body of a fish. Young and old animals were equally likely to be affected and no part of the skin was immune. The lesions showed up on three types of coral trouts: the so-called common, bar cheeked and blue-spotted species.
A microbiologist by training, Sweet initially assumed the animals were afflicted with some kind of fungal infection. So he and his colleagues probed extensively for fungi and other types of microbial pathogens. But fish with the dark blotches harbored the same types and quantities as normal fish. Sweet’s team then probed the lesions from five fish on a cellular level. And they turned up no evidence here of microbial DNA.
So the scientists turned to the remaining 15 affected fish and started examining their lesions microscopically. To their surprise, these tests showed especially dense tissue and a deeper distribution of melanin — the dark pigment for which melanoma is named. Ordinarily, fish cells containing melanin are “well organized” and clustered in tight groups throughout the lower skin, or dermis. But within the coral trout’s lesions, cells looked abnormal with nuclei that varied dramatically in size and shape.
These deeply pigmented cells appeared to be migrating from the deep skin up to the outer layers, or epidermis. In no instances did the altered tissues with excessive pigmentation invade into the muscle or internal organs, Sweet and his colleagues report. They concluded the discolored skin tissue represents early-stage melanoma.
“As pathologists when we say melanoma, there are certain things we need to see — like local invasiveness,” explains Wolfgang Vogelbein of the Virginia Institute of Marine Science in Gloucester Point. He's referring to where cells begin migrating from the initial tumor toward blood vessels, from which they can serve as seeds for new cancers throughout the body. The cells comprising melanomas also look strange — even bizarrely shaped. “And these characteristic cellular features of malignancy — well, they’re just not there” in the images accompanying the new PLOS ONE paper, Vogelbein says.
That no “hyperpigmented” region appeared invasive within these fish is a telling feature, agrees Michael Stoskopf, a toxicologist and aquatic clinician at North Carolina State University’s College of Veterinary Medicine in Raleigh. Micrographs of the lesions depict cells with membranes “that are all smooth. They don’t look ‘angry,’” he says, as he’d expect with a melanoma.
Acknowledging that the discolorations do look odd, “what impact they will have on the health of the fish remains highly questionable,” he says. To him, the blotches appear more analogous “to a mole or liver spot that comes with aging — not a malignant cancer that’s going to spread into other organs.”
“I kind of agree,” says Blazer. “They might be similar to moles, which are really benign.” And she maintains that their lack of invasiveness suggests it’s too early to indict the admittedly strange blotches as being even pre-cancerous.
But they’re still weird
Still, Blazer says, “there’s definitely something wrong here.” And this disease, for want of a better term, may be emerging in places far from the Great Barrier Reef. In fact, she says, at the behest of the State of Pennsylvania, “We are actually pursuing a very similar thing.”
Stories carried by local newspapers this spring noted that anglers had been hauling in smallmouth bass from the Susquehanna River sporting strange dark blotches. The Chesapeake Bay Foundation’s online publication in May quoted the executive director of the Pennsylvania Fish and Boat Commission as saying that in some patches of the Susquehanna, last fall, up to 40 percent of adult smallmouth bass hosted blotches and sometimes open sores. A related story by an NPR affiliate cited a retired family physician as worrying that the spots are melanomas triggered by hormone-mimicking water pollutants.
These media reports triggered a lot of concern among local anglers. But as with the coral trout, lesions in the blotchy bass don’t resemble melanomas, Blazer reports. The hyperpigmented cells again appear to be migrating up toward the skin surface, not down toward muscle and internal organs, she has found.
These lesions could reflect some hormonal abnormality or pre-cancerous transformation triggered by water contaminants, she says. Those blotches might even be the equivalent of sun burn from ultraviolet radiation penetrating unusually deeply into the water. “We just don’t know,” she says. “It’s something we need to investigate more.”
Followup on the reef epidemic
Arguing against the coral trout lesions being a pollution effect, Sweet observes, is that the reef regions where his team sampled fish remains relatively pristine. A more likely trigger: ultraviolet light.
Portions of the Great Barrier Reef spend part of each year under the edges of a stratospheric ozone hole that forms above Antarctica. So these waters could experience prolonged, seasonal exposures to elevated UV light, he explains. And in at least a few lab-reared fish, UV can initiate the development of melanoma (especially among hybrids which inherit a tumor-promoting gene from one parent but no accompanying tumor-suppressor gene from the other). Sweet would like to irradiate the coral trout to see if they're similarly susceptible to UV-fostered cancer.
In the mean time, “We’re currently radiotracking fish [with implanted sensors] to see if the ones that have melanoma behave differently,” Sweet says. And his group has applied for funding to run lab studies to see if the fish hosting blotchy lesions develop more subtle — and disturbing — symptoms with time, such as a slower swimming speed or changes in blood-cell counts.
M.J. Sweet, et al. Evidence of melanoma in wild marine fish populations. PLOS ONE. August 1, 2012. doi:10.1371/journal.pone.0041989 [Go to]
Susan Phillips. Retired doctor and fisherman says black spots on fish could be cancer. State Impact: Pennsylvania. April 16, 2012. [Go to]
V.S. Blazer, et al. Manual for the microscopic diagnosis of proliferative liver and skin lesions in the brown bullhead (Ameiurus nebulosus). A Pennsylvania Sea Grant report, February 2007, 38 pp.
Tom Pelton. Smallmouth bass epidemic highlights need for pollution limits in Susquehanna River. Bay Daily (a publication of the Chesapeake Bay Foundation). May 2, 2012. [Go to]
J. Hayes. Outdoors Notebook: State says 'blotchy bass' not harmful to fish or humans. Pittsburgh Post-Gazette. April 8, 2012. [Go to]