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Mike C. writes: I have seen some odd black spots on some cottontail rabbits. Like the fur is missing and instead is a black spot. If I send pictures, might someone be able to help determine what it could be? I haven’t noticed these black spots in prior years, but this year a lot of rabbits had it.
I did not know the answer to Mike’s question, so I asked Sarah Van de Berg, a wildlife health biologist for the Massachusetts Division of Fisheries and Wildlife. She says that it is difficult to tell from the photos submitted, but she suspects from Mike’s description and the photos that the black spots are warbles, which are caused by a rabbit botfly (Cuterebra). Warbles are not usually fatal, but they do not look pleasant.
The Cuterebra fly is large, up to an inch long, that looks something like a large bee (but does not sting or bite). It lays sticky yellow eggs on vegetation near a rabbit’s den. The rabbit picks up an egg as it grazes on grass or vegetation. The rabbit’s body heat causes the egg to hatch. The larva that emerges from the egg enters the rabbit’s body through its nostrils or mouth, or sometimes through an open wound. The larva travels under the skin for about a week until it finds its preferred location. There are four species of rabbit botflies. Each species migrates to a different area of a rabbit’s body. One type prefers abdominal areas, another the rib cage and shoulders, another the throat and neck areas, another face and ears.
Once the larva finds its preferred location, it grows inside the rabbit, feeding on the plasmalike liquid that fills the spaces between cells. This plasma, called lymph, delivers oxygen, proteins and other nutrients, and it sweeps up debris such as damaged cells.
Parasites such as botflies depend on a living host, so they do not harm their host too much, and botfly larva do not seem to inconvenience rabbits too much. They usually continue eating and acting normally, even as the larva uses pointed mouth hooks to pierce a hole in the rabbit’s skin through which the larva breathes and excretes fluid. As it grows, it creates a lump called a warble. The warble oozes fluid. Often the rabbit scratches off the fur on and around the warble.
After two to four weeks, the larva has grown to its maximum size. It exits the warble through the breathing hole, falls to the ground, and burrows into the soil. It forms a pupa with a hard case around it. The pupa enters diapause (pausing its development) over the late fall, winter and spring. (The pupation process can take up to 10 months.) During warm weather of the following spring and summer (above 68 degrees Fahrenheit), it transforms into a short-lived adult. Adult botflies mate and the female lays eggs. Each egg remains dormant until it sticks to a rabbit. Adult botflies do not have functioning mouths and, therefore, do not eat. They live on energy that they stored as larvae, when they parasitized rabbits. They have only one job as adult flies: to reproduce quickly before they die.
Botfly larvae infest rabbits most often when the weather is warmest in summer or fall (June to October). The females lay eggs near rabbit burrows or vegetation in those areas. Because the chance of a rabbit contacting any one egg is small, the fly lays five to 15 eggs at each site, and may lay as many as 2,000 eggs altogether in her short two-week lifespan. The eggs stick to the rabbit as it passes through egg-infested vegetation. On average, infested rabbits have from one to three warbles.
Rabbits and their botflies have evolved together, so rabbits can endure the flies. Severe effects are limited to only a few – often those with many infections, or very small or young rabbits. The population as a whole does not decline because of botflies.
Rabbit botflies can infest humans, but it is extremely rare. (There is a human botfly, but it is not found in the United States or Canada. It is found in Mexico and Central and South America.) What should you do if you spot a rabbit with warbles?
According to Van de Berg, since the wound usually heals on its own after the larva emerges, you should leave the rabbit alone. Capturing a wild rabbit can injure or even kill it. You do not need to take action unless the rabbit is severely injured or severely ill, unable to perform basic functions. If this happens, contact a licensed wildlife rehabilitator for guidance.
Van de Berg says warbles are not the only reason Mike’s rabbit might have a black spot with missing fur. There could be many other causes, including Shope fibromas (skin tumors caused by the Shope fibroma virus), abscesses, embedded ticks, injuries or even misaligned teeth. It is best to respond to each of these issues in the same manner: Leave the rabbit alone and observe it. If it is in distress, contact a wildlife rehabilitator for advice.
Have you taken photos of our urban wild things? Send your images to Cambridge Day, and we may use them as part of a future feature. Include the photographer’s name and the general location where the photo was taken.
Jeanine Farley is an educational writer who has lived in the Boston area for more than 30 years. She enjoys taking photos of our urban wild things.
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