Since 1995, the population of California sea otters has plunged to just over 2,000 individuals. Scientists have recently traced the decline in part to an unlikely source: cat feces transported into the ocean by freshwater runoff.
Wildlife veterinarian Melissa Miller at the California Department of Fish and Game in Santa Cruz and her colleagues spent four years testing 223 live and dead sea otters for the parasitic protozoan Toxoplasma gondii. In otters it can cause a fatal brain infection; in humans it forms muscle cysts; but only cats can shed infectious T. gondii eggs in their feces. Miller found that 42 percent of live sea otters and 62 percent of the dead ones carried antibodies of the protozoan. Otters near sites of coastal freshwater runoff—rivers and streams that ferry untreated water from fields and yards into the ocean—were almost three times as likely to be infected as those elsewhere.
Miller suspects the otters are catching T. gondii by swallowing seawater or by eating tainted shellfish. Her findings may raise a red flag for humans. "Otters are a sentinel species," she says. "If the near-shore environment is polluted, they're the first to show problems."