Equine protozoal myeloencephalitis is a potentially fatal disease for horses. The parasite that causes the disease attacks the neurological system. In severe cases, the horse loses its ability to swallow or stand and has to be euthanized.
What’s the opossum connection? Cats, skunks, armadillos, raccoons and sea otters become infected with the parasite at one of stage of its life cycle. When an opossum scavenges the carcasses of these animals, the protozoan develops inside its intestines to the stage that is poisonous to horses. Opossum droppings can then contaminate hay, grain, pasture and drinking water for horses. Long a concern for horse owners in the United States, the disease is becoming more common in Canada.
The Virginia opossum is an emblem of the American South, but recently this naked-tailed marsupial has been turning up in garages and garden sheds all the way up to Ontario’s Bruce Peninsula.
Over the past 15 or 20 years, “we had anecdotal evidence that opossums were expanding their range northward and coming up more into Ontario,” says Merilyn Twiss of the Ontario Ministry of Natural Resources. “Somebody in a naturalist club would post a picture of an opossum on the club website or someone would go into a district office and report that there was an opossum found in their garage.”
Normally, opossums are at their northern limit well south of the border. “There are reports from the 1930s and 1940s that report an opossum sighting in the extreme southwest corner of Ontario,” says Twiss, noting the reports treat the sightings as “something very rare and unusual. I think it’s really in the last 30 years or so that we’ve started to see a change in that.”
In January, 2008, Twiss started a research program to have a closer look. Her team dug up fur harvest records and rabies vaccination records to plot where and when opossums had either accidentally been caught by a trapper (opossum fur is sparse and doesn’t have much value, but the creatures occasionally turn up in traps set for another animal) or live-trapped by the province’s vaccinate-and-release anti-rabies program.
Since opossums spread equine protozoal myeloencephalitis, which kills horses, Twiss’s team is also examining whether there is a direct link between outbreaks of the disease and the presence of opossums. (See “Domestic Threat.”) The project is in its infancy, with no findings to report yet.
Phil Myers from the University of Michigan has been looking at the same kinds of things in his state, and what he has found suggests that Twiss’s data will confirm the hearsay that piqued her interest. His team looked at information associated with specimens collected for museum collections and paired it with their own data, including the co-ordinates that Myers collects with his dashboard global positioning system each time he spots a roadkill opossum.
“One hundred years ago, opossums were restricted to the very southernmost part of the state,” Myers explains. “Through the 20th century, they slowly moved northward. But starting in the 1970s and 1980s, they just exploded over the northern part of Michigan.”
So why are opossums coming north? Twiss and Myers both suspect warming temperatures play a role, particularly because opossums are hardly dressed for a typical Ontario or Michigan winter.
For instance, they have relatively little hair on their tails, ears and noses, says Twiss. “They seem to be very susceptible to the cold, so the feeling is that winter weather conditions would limit an expansion of the population. They’re just not capable of overwintering if it gets very cold or if there’s a lot of deep snow. We think there’s a strong link to climate.”
To confirm the connection between opossums moving north and warmer winters, Twiss’s team members are layering their data with climate data to see if there’s a pattern.
Myers thinks human land-use changes have also encouraged opossums’ movement north, particularly since they are adaptable to urban and suburban environments. They can scavenge food from roadkill and garbage dumps and find shelter in barns and sheds.
“We’ve created these microenvironments where possums have an easier time getting food during the winter and can find areas that are warmer during the winter,” says Myers, who points out that the marsupials couldn’t have expanded their range so drastically without a warmer climate.Neither researcher is sure what the northward migration of opossums might mean. Since opossums are active during the winter, they could enter the dens of hibernating species and kill them. On the other hand, they could help to control rodent and insect populations as well as cleaning up roadside carrion. “We don’t know what effect an increase in opossum populations in Ontario will have on ecosystems. There are a lot of interesting questions around these little animals,” says Twiss.