Solving the Riddle of Skin Tumors in Brown Bullhead Catfish
Mouth lesions on a brown bullhead from the South River, later diagnosed as squamous cell carcinomas. Photo by Fred Pinkney USFWS
Scientists and environmental managers monitor bottom-dwelling fish, like brown bullhead catfish, for tumors as an indicator of habitat quality. Bullheads feed on worms, insect larvae, and small crustaceans living in the mud. Because of their feeding habit, these fish often develop liver and skin tumors due to cancer-causing chemicals that accumulate in the sediment.
There is a strong link between carcinogens in sediments, especially polynuclear aromatic hydrocarbons (PAHs), and liver tumors. PAHs can are found in gasoline, oil, coal, car exhaust, and road particles.
In 2005, a Fish and Wildlife study revealed an unexpectedly high rate of skin tumors (53%) in bullheads from the South River near Annapolis. Twenty percent of these fish also had liver tumors. The river has no point sources for cancer causing chemicals and is largely bordered by homes and buffered in many areas by woods.
With support from the South River Federation, the Fish and Wildlife Service sampled the South River four more times, as well as the Severn, Rhode, and Choptank Rivers. A team from George Washington University, U.S. Geological Survey, Stockholm University and Penn State Erie provided expert analysis.
We found consistently high skin tumors in the South River but few or no liver tumors in these collections. One collection of the Severn River had 52% skin tumors but two others were lower (10% and 2%). The Choptank was low in both types of tumors in two collections. The Rhode had 6% of each type but the sample size was small.
In addition to examining the fish for skin and liver tumors, we looked at the DNA in the liver and skin tissues for PAHs and another class of chemicals, alkylating agents. Attachment of chemicals to DNA is an early step in the cancer process.
Neither the DNA nor the sediment data suggest an association between fish with tumors and exposure to PAHs or alkylating agents. Thus, although we were able to rule out some causes, questions remain. Follow-up studies will examine the immune system of fish from some of the rivers to see why they frequently develop tumors.
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