There’s a nice overview article in the May 2013 Scientific American by Christopher Solomon on the multi-directional threat of pathogens associated with humans and human-associated animals on marine life. Called pollutagens, these agents range from Taxoplasma gondii, a protozoan parasite associated with house cats, to Salmonella, to massively drug resistant bacteria. Some mass deaths of coastal sea otters and sea lions, once a mystery, have been traced to various pathogens that originated with pets and livestock, or in the waste streams of human communities.
From my own perspective, I wasn’t surprised 10 years ago or so when the cat-otter death hypothesis began to circulate. I had been working for 10 years along the shores of Monterey and Pacific Grove, CA, towns which seem to harbor a high concentration of “cat ladies”. Not only do the backyards and alleys of Pacific Grove teem with feral cats, but I’ve seen the creatures foraging late at night in the rocky tidepools, just a few yards from drifting sea otters. The now well-established link between cat pathogens and marine mammals has me joining the chorus of bird lovers who implore cat owners to keep them locked up tight inside and get them fixed. Surprisingly, though, even offshore animals such as orcas have also been affected. Moreover, the range of potential pollutagens–including hormone mimicking compounds, which are found in pharmaceuticals and may be too small or hearty to be filtered or killed by waste treatment plants—makes the problem seem especially worrisome.
The article is set up as a detective story, and indeed some great environmental detective work by toxicologists like Melissa A. Miller helped establish the linkage between mass fatalities of marine mammals and these land-based pathogens, but it’s now become an evolution story. These pathogens don’t just sit there in the environment, they adapt. In one alarming finding, a harp seal was found to harbor bacteria that were resistant to 13 of the 16 drugs tested against them. Other studies have shown new viruses evolving from combined human and marine mammal viruses. The ultimate concern for people, even those who don’t care desperately for adorable marine mammals, is that the diseases that we’ve helped to foster in the marine mammals mutate and come back to infect humans—for example, when marine mammals trapped in fishing nets full of fish for human consumption.
In this context, I was a bit surprised to see Michael Moore of Woods Hole take a narrow view of the threat, at least as quoted in the article. Dr. Moore noted, “If there had been any major threat from marine life, I’d have been dead many times over.” The problem, of course, is that when you have systems that can and do evolve as much as pathogens and parasites, something that’s benign today can be deadly tomorrow.