In many ways, possums are cursed animals. Not only do they have small brains, they are not very fast, and are easily killed by coyotes, farm dogs, fox and owls.
The possum's competitive edge is in the reproduction arena, and here it is a true champion. A typical female possum will have two litters a year, each with as many as 18 young. The gestation period for a possum is just 13 days -- the shortest gestation period of any mammal in North America.
Possums are the primary transmitter of equine protozoal myeloencephalitis (EPM), the most commonly diagnosed neurological disease in horses. Common horse symptoms include incoordination, with one side usually more pronounced than the other; muscle loss; and weakness. Some horses may die, but even those that aren't severely affected may not be able to run or ride as well as they once did.
About 50 percent of all horses, regardless of breed, show exposure to S. neurona, the parasite which causes EPM, and which has been traced only to the opossum. Only a small number of these infected animals, perhaps one percent, actually get sick, but about 30-40 percent of those that do get sick don't respond well to existing treatments.
The bottom line: any farm that has horses should be glad to see you and your terriers, as you can rid the pasture of groundhogs and their holes which can break a horse's legs, and you can also get rid of possums in the barn and surrounds that might transmit a serious equine disease.