The Population Dynamics of Distemper in Wildlife

Canine distemper is the most common and population-limiting of the viral diseases commonly found in wildlife.

It often sweeps through wild fur bearer populations, such as fox, raccoon, skunk and coyote, leaving behind the stiff carcasses of dead animals who commonly have their lips pulled back in the rictus of a snarl.

Distemper seems to have a particularly devastating effect on raccoon populations, and outbreaks are very common in late Winter and Spring.

While some distemper-related mortality occurs every year, major population crashes are highly correlated to densities, suggesting that in the wild large distemper epidemics may be related to an overshoot of carrying capacity in terms of either food or shelter.

In the Eastern U.S. distemper-related raccoon population crashes seem to occur every four to seven years, depending on food conditions and weather.

Across the U.S., the raccoon population is now somewhere between 15 and about 20 times larger than it was in the 1930s. One reason for this is that the raccoon has dramatically expanded its range northward and westward, across the Plains, across the Rocky Mountains, and up into Canada (where the raccoon is not a native animal). In addition, a deep decline in raccoon trapping, combined with a rise in more fragmented forests bordering houses and cornfields, means that ideal raccoon forage and shelter conditions now exist over much of North America.

Wild North American animals, such as fox, skunk, coyote, wolf, mink, badger and raccoon have very little natural immunity to distemper. The disease itself has varying levels of mortality, ranging from about 50 percent in raccoons to about 90 percent in mink.

Distemper is transmitted from wild animal to wild animal by saliva and urine, and also from occasional contact with the urine or saliva of domestic or feral dogs that have the disease. Along the west coast, sea lions are now coming into contact with distemper -- probably carried to them through raccoon and fox populations patrolling the tidal zone.

Wild animals infected with distemper will present with a wide variety of symptoms, many of which can be very similar to rabies. For example, an animal with distemper will be lethargic, have swollen eyelids, and may have white milky discharge coming from its eyes. The animal will very often have a runny nose and an emaciated look, and will frequently have unkempt fur. At the end stage of distemper, an animal may may also have diarrhea and convulsions, and wander around in daylight with a dazed look. Epileptic-like seizures are not uncommon, and neither are convulsions of the jaw or spasms and rigidity in the back legs.

Death in the wild is not a fun thing, and distemper is a very common and very cruel way for a wild animal to die -- as is rabies and mange, two other population-control diseases common to the Eastern United States.

Though there is no absolute cure for distemper, biologists agree that reducing raccoon population densities through humane hunting and trapping can help reduce the depth of epidemics by relieving the biological stress that occurs when raccoon populations overshoot their carrying capacity in an area (a carrying capacity that may change very rapidly with crop shifts from corn to soy and from soy to oats).

Distemper used to be a major killer of domestics dogs, and it still is among those who do not inoculate litters. That said, beginning in the late 1950s, the prevalence of low-cost distemper vaccines has dramatically improved dog mortality, not only in the U.S., but across the world.

A too-rarely told story is that the creation of this miraculous distemper vaccine came about because of organized fox hunters, and involved not only fox hunters funding the basic research to create the vaccine, but also the use of fox hounds, fox and ferret to create the attenuated virus itself. >> Click here for the rest of that story.
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