KAREN Neill doesn’t need an alarm clock to wake her up. This morning, like every morning, her six-year-old tabby cat Lucas heralds the dawn by kneading his paws on Karen’s stomach and purring loudly in her ear. Karen rises, pads down the hall, opens the front door and watches as her pet slinks into the bushes that line the driveway. As Lucas pauses to sniff the scat left behind by a local stray, Karen turns and heads back down the hall toward the bathroom. She notices that Lucas used his cat-litter box in the night and with one quick and easy action she scoops, tips and flushes the toilet.
Miles away on the central coast of California, Andy Johnson’s morning has been anything but routine. Awakened by the beep of his pager, Johnson hurries to the veterinary lab at the Monterey Bay Aquarium and arrives in time to witness an adult female southern sea otter in the throes of a grand mal seizure. Treating stranded and injured sea otters is an important part of Johnson’s job as manager of the aquarium’s Sea Otter Research and Conservation programme, but the severity of this seizure takes him aback. He fears it is probably another case of a debilitating brain infection that first showed up in the local otters a few years ago and has killed dozens of them since.
Until recently, nobody imagined that there might be a connection between the ordinary events in a cat-lover’s house and a mysterious brain disease in sea otters. But in July, Melissa Miller’s team at the University of California, Davis, identified the deadly link. Their analysis of the brains of sick and dead otters reveals a telltale parasite called Toxoplasma gondii (International Journal for Parasitology, vol 32, p 997). As pregnant women and people with AIDS are well aware, it’s cats that usually carry T. gondii. Yet Miller and her team found signs of the parasite in 60 per cent of the dead otters and 40 per cent of living ones. “To me that’s astonishing,” she says. If the parasite does originate in our pets, what’s it doing infecting all these marine mammals?
Advertisement
Southern sea otters (Enhydra lutris nereis) are classified as a threatened species under the US Endangered Species Act. The total population numbers little more than 2000 individuals. And recent surveys confirm what many have feared: the population is declining, with a drop of 10 per cent since 1995. With boat strikes, shark attacks and disease all taking their toll, the additional impact of toxoplasmosis could be devastating.
More ominous still, T. gondii doesn’t appear to be sticking to the normal rules. In the case of cats and humans, a healthy immune system is well able to handle the parasite – only the very young and the infirm are at risk. Yet, with sea otters, adult animals in their prime seem to be succumbing to the disease. No one is sure why this is. The researchers are currently trying to discover whether the otters are particularly susceptible because they are a new host, or because their immune system has been weakened by a genetic bottleneck early last century, when population levels fell dramatically. Another possibility is that the high mortality rate is due to massive exposure. This might explain why twice as many adult males are infected with T. gondii as females. They are more likely to come into contact with infection “hot spots” as they range over a large area, setting up and defending their territories.
But how could a parasite that is spread through cat faeces end up infecting sea otters living off the coast of California? Think back to Lucas. It’s less than half a century since the late Ed Lowe invented cat litter when he handed his neighbour a bag of industrial absorbent. Today, the US’s 72 million pet cats get through $700 million worth of the stuff each year. The latest innovation is “flushable” kitty litter: it’s convenient, it’s hygienic, but it may also be bad news for southern sea otters. Guides at the Monterey Bay Aquarium are so concerned about the possibility that T. gondii is getting from our homes into coastal waters in flushable kitty litter that they are urging cat lovers to put pet waste in plastic bags and dispose of it with their garbage. Patricia Conrad, a pathologist at the University of California, Davis, is even more forthright. “We’re depositing all our faecal waste and the faecal waste of our animals into that marine environment,” she says. “And with that faecal waste come pathogens – organisms that can be infectious for other humans and for wildlife.”
But if domestic cats are a threat to otters, strays may be an even greater problem. Conservative estimates put the number of feral cats in the US at 60 million. When a cat defecates outdoors, the scat eventually ends up in storm drains, in streams and in rivers that flow into the ocean.
In a search for clues about how T. gondii got into sea otters, Miller launched an epidemiological study along the California coast to identify “hot spots” for infection. She found that sea otters living in coastal waters where there is high water flow from streams or rivers or storm drains are three times as likely to have come into contact with T. gondii.
Conrad, Miller and colleagues, including Kristen Arkush at the Bodega Bay Marine Labs, are now testing the idea that sea otters may be acquiring T. gondii from one of their main sources of food – mussels, clams, oysters or other benthic bivalves. Recent studies at East Coast sites including Chesapeake Bay have revealed high levels of protozoal parasites such as Cryptosporidium, Giardia and Cyclospora in oysters, mussels and clams. Bivalves are filter feeders. As they filter seawater, they concentrate tiny reproductive products of these parasites. Conrad’s group has developed a new technique to search for these “oocysts” from T. gondii in Pacific coast bivalves.
The thing about these oocysts that makes them a “little bit scary”, according to Conrad, is that they’re designed to be environmentally resistant. “They’re as hardy as any parasite product you’re going to find.” Conrad says that when his group toured a state-of-the-art water treatment facility, it found no process that would reliably destroy Toxoplasma oocysts. And it’s not just otters at risk either. Once these parasites get into the water system, humans can get infected too.
Already the evidence is there. In 1995, the largest ever reported outbreak of toxoplasmosis in humans was traced to the municipal water supply of the Greater Victoria area of British Columbia. Perhaps we should take this as a warning shot. Last year, the Intergovernmental Panel on Climate Change published a report pointing out that the number of feral cats in Canada is growing and climate change is expanding the area in which the hardy T. gondii oocysts can survive.
“We may be exposing ourselves to the same kinds of parasites that are devastating southern sea otters,” says Conrad. “In these days filled with talk about toxic pollution and about contaminants going into the marine ecosystem, it’s amazing that no one has been looking at specific pathogens that cause diseases in wildlife and people.” Miller and Conrad are changing that. The new hot topic is pathogen pollution.