Opossums (Didelphis virginiana) are the definitive host of S. neurona. Opossums shed sporocysts, which are the infective stage for both aberrant hosts, horses, and the natural intermediate hosts, the identity of which are unknown. A flurry of recent research activity has revealed that S. neurona and S. falcatula are distinct species. The geographic distribution of the occurrence of EPM in horses closely matches the geographic range of opossums. This range includes most of South America, all of Central America and most of the US and southern Canada. Regions of the US in which there are no opossums have low rates of both exposure and central nervous system infection with EPM. Those infections that do occur are likely a result of either exposure in horses having left the area, or the rare exposure from hay, grain or other substances originating from an endemic area.
Opossums have roamed the New World for 170 million years, predating dinosaurs. As recently as the turn of the century, opossums in North America were limited in range to the Southeastern United States. However, in the last century, opossums have spread to fill most of the US, and parts of Southern Canada. This increase in population reflects the adaptability of this species. Garbage and other waste is a favorite food source, and declines in fur trapping have resulted in a population explosion of this pest species. The overpopulation of opossums has inevitably resulted in an increase of EPM, and probably has other ecological implications.
Individual opossums have a relatively small range. The range of the males is about 250 acres, whereas the range of the females is smaller, at only about 125 acres. This limited individual range suggests that trapping and relocating opossums may be effective in thinning the population of opossums on horse raising properties
If opossums are to be trapped, live traps are recommended (usually $35-$50 apiece). Dog or cat food, meat, or meat byproducts should not be used, because domestic cats are highly likely to be caught. Rather, apples, oatmeal cookies and peanut butter are more effective bait which minimizes the likelihood of catching domestic animals (personal experience). If the animals become wary of the traps, they can be wired open for several days so that the animals can freely enter and leave. Check with state and local authorities for wildlife laws and regulations governing where you can trap, and where and how you can relocate opossums. Most states at least permit trapping on your own premises.
These mammals have 50 teeth, more than any other animal in the Americas, which permit them to eat just about anything. Opossums are scavengers that eat garbage, grain, and carrion. Part of the reason that so many of them are found dead on the road is that they like to eat other roadkill. Opossums will also eat and mutilate one another.
Additional information may be found at two other excellent sites, where the biology, habits and life styles of opossums are detailed.
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