Normally, doctors worry about how human illness will spread between people. But wildlife, too, can sometimes become infected with germs shed by people or their pets. And sometimes those germs may hit wild animals as hard — or harder — than they do people.
This is something that Melissa Miller has been studying for the California Department of Fish and Wildlife in Santa Cruz. As a veterinary pathologist, Miller studies animals to determine their cause of illness or death. She and her coworkers think of microbes as biological pollutants, which can harm animals in the wild. Human or animal feces — poop — are usually the source of these household germs that can wash into rivers and the ocean.
Natural wetlands can help slow the flow of polluted water. This gives beneficial bacteria in stream water the time they need to break down pollutants. But many communities have been converting wetlands to farms and residential areas. Now pipes and culverts move water quickly through what used to be slowly draining wetlands. The result, says Miller: Germs that typically live in land animals and people are sickening ocean mammals. These include sea otters, sea lions, dolphins and whales.
In the past few years, Miller and other researchers have discovered dead sea otters infected with a microbe called Toxoplasma gondii . This germ can cause  nerve damage (including blindness) and retardation in people. It can cause deadly brain damage in otters. Miller and her coworkers now regularly examine the brains of dead California sea otters for signs of this infection, such as swelling and scarring.
Feces of infected land animals — typically wild and domestic cats — can spread the microbe. Feces and the germs they harbor get washed downstream and into the ocean. Clams, mussels, crabs and other filter-feeding animals can then ingest the germs. Animals that eat filter feeders can pick up the germ. “We think that is a big method by which sea otters are getting exposed,” explains Miller.
Sea otters can tell us a lot about the health of the environment, she says. One reason: These animals eat a quarter of their weight in food every day. Miller says that’s like a 160 pound person eating 40 pounds of hamburgers every day. Their large appetites and their living close to shorelines make sea otters particularly vulnerable to pollutants washed off of land. “What the otters are trying to teach us is that as much as we think [pollution] is going to go away, it actually just gets sent downstream and comes back to haunt us,” says Miller.
To help reduce sickness in wildlife — such as sea otters — keep pet cats indoors. In addition, seal pet wastes from litter boxes into bags before putting them in the trash. Towns near coastlines should also focus on preserving some natural areas as habitat for wildlife — and as a way to boost the breakdown of pollutants.