Illustration by Adelaide Tyrol
The strangest animal I had ever seen crossed in front of my car, near the Thetford–Norwich line on the Connecticut River in Vermont last summer. It had a pointed snout, a tubular body and a long skinny tail. Riding high on gangly legs, it looked like a weasel on stilts. The creature lingered beside the road, and I managed a better look. It was a fox, and it was hairless except for a tuft of fur on its head and another on the tip of it tail. The emaciated animal showed no reaction to my passing car; instead it just gazed blankly. Had I swerved, I might have mercifully ended its misery.
What I had seen was the terminal stage of sarcoptic mange. The cause of the disease is a tiny, eyeless mite, Sarcoptes scabei, practically invisible to the human eye. Mites are eight-legged, roundish creatures that are close cousins to ticks. More than 30,000 species exist in the world, and most—such as the ubiquitous house dust mite that eats flakes of dead skin—are benign and aid in the decomposition of plant and animal material. Some are agricultural pests or parasites,
such as the mites that decimate honeybee colonies or
those that can raise havoc with poultry.
So how did Sarcoptes scabei bring such misery to that unfortunate fox? Mites are passed from animal to animal by close contact or in bedding. Male and female mites meet on the animal’s skin and mate. The male mites soon die, while the females burrow into the outermost layer of skin, creating a maze of tunnels and feeding on the body fluids oozing from the minute wounds. As they burrow, they lay eggs. After two or three weeks the female dies at the end of her tunnel. The eggs soon release larvae that work their way to the skin’s surface. There, they migrate to new sites and make other burrows, where they go through a series of molts before adulthood. Males find females, impregnate them, and the cycle starts anew.
During the process, mites deposit excrement that invokes an intense immune response, an inflammation that is terribly itchy. The animal scratches and bites at the irritation, often breaking the skin, allowing in various types of bacterial infections. The scratching also pulls out fur, which worsens the situation because mites prefer skin without hair. The animal is constantly on the move, sleepless and exhausted, and eventually dies from multiple stresses, such as hypothermia, infection and starvation. Shooting a mangy fox is an act of kindness.
Encountering an infected fox does not suggest an epidemic, however. Mange seems largely confined to individuals or certain fox families. The animals pick up mites in a den used by an infested fox – which is possible because an adult mite that drops off its host can survive for up to three weeks without feeding.
The disease is held in check among the greater fox population because usually each fox den is used only about one out of every three years. In areas of high fox populations, however, a single den may get continuous use, and this increases the likelihood that mange will spread. Awful as the disease is, it can help keep a fox population at a healthy level.
In certain urban and suburban areas across the world, where enormous overpopulation can occur, the drop in numbers of red fox due to the disease can be staggering. The city of Bristol, UK, once recorded almost 40 adult foxes per square kilometer, a number that plunged 95 percent once mange arrived.
Anyone in New Hampshire or Vermont who sees a mangy fox, after becoming concerned about the animal’s suffering, is likely to wonder if the disease could travel to pets or livestock or other wild animals. In fact, different animals are cursed with their own variety of Sarcoptes scabei. For instance, there is S. scabiei var. canis for dogs, var. bovis for cattle, var. suisfor pigs, and, yes, var. hominis for us humans. In humans, mites cause the itchy but highly treatable disease known as scabies.
Interestingly, grey fox, the deep forest relative of the red fox, seldom gets mange, possibly because the fox mite does not survive on them very well. There are rare reports of mange transmission from foxes to dogs, but that usually only occurs in places with exceptionally high fox populations. Fortunately, when the pet dog gets mange the condition is easy to treat, either with a chemical dip or with pills.
Li Shen is an adjunct professor at Dartmouth Medical School and a member of the Thetford Conservation Commission.