Swenson’s Wild Midwest Exotic Petting Zoo is apparently a popular attraction in Clermont, Iowa, and in August 1999 they featured a bear cub.
It was really cute, and it let people touch it and wrestle with it. Occasionally it would nip at people, but it never really hurt anyone. The cub was also brought to a celebration at a barn warming party in Holy Cross, Iowa.
By August 27, when the cub died after several hours of acute nervous system symptoms, about 400 people had had contact with the animal. This was too bad, because the bear cub died of rabies, a transmissible disease that, if left untreated, is always fatal.
It is impossible to know if anyone contracted the rabies virus from the cub, so the Iowa Department of Public Health began a campaign to try to trace everyone who had had contact with the bear in order to treat them all prophylactically, the only sure way to protect them. Rabies became more common in animals in the United States and Europe during the 1980s and 1990s, and it continues to spread in the wildlife population. While rabies is quite rare in humans in the United States, no one who touched this bear would want to skip the prophylactic treatment.
Although wild animals can be and often are rabid, dogs are the most common transmitter of rabies to humans, and there are about 30,000 cases worldwide every year, very few of them in the United States, where routine vaccination of pets has practically eliminated them as source of the virus. In fact, in the United States cats are more likely to be rabid than dogs. Cats, dogs, and ferrets should all be vaccinated against rabies by a veterinarian.
But dogs in this country carry other diseases that, if not as serious as rabies, may be of more immediate interest. Toxocariasis is one of these, and Toxocaris canis, a parasitic nematode, is the culprit. This is usually called dog roundworm (cats can have it, too, in the form of Toxocaris cati, but it is much less common in cats).
T. canis usually lives its entire life cycle in dogs, but humans can also be an accidental host of the parasite. Puppies (and sometimes kittens) are infected by their mothers, either in the uterus or after birth through suckling. The nematode, which has been living in the mother’s tissues in an undeveloped state, then grows in the puppy’s or kitten’s intestines and is shed in feces. You get it by oral contact with the toxocara eggs, expelled in dog feces into soil or onto other surfaces.
Kids, as you might expect, are more likely to get it than adults, but adults, especially dog owners, are also at risk. There are two ways in which the nematode infects people. The first is called visceral larva migrans. The nematodes don’t usually reproduce in humans; instead, the larvae migrate through organ tissues, causing inflammation as they go. While most infections are asymptomatic, preschool children often get sick with fever, malaise, loss of appetite, and weight loss. Death from this parasite is rare, but when it occurs it is usually because the heart, the lungs, or the brain have become infected.
When toxocara invades the eye, the illness is called ocular larva migrans. A mass develops around the entrapped larva that looks like a retinoblastoma, a malignant tumor. Careful diagnosis is important because a retinoblastoma often requires removal of the eye, something you don’t want to do if the problem is a temporary infection caused by a nematode parasite. The infection causes eye pain and inflammation in addition to visual disturbances. Unlike visceral larva migrans, which almost always affects young children, it is usually older children and adults who get the ocular form. Usually toxocara infections of both kinds resolve by themselves without treatment. There are some drugs that can be used, but their effectiveness is questionable. Treating the ocular form usually doesn’t work at all.
Dogs also carry Ancylostoma braziliense, another nematode parasite, known commonly as hookworm. This one causes an illness whose symptoms are even more revolting than those of toxocara. The scientific name of the disease is cutaneous larva migrans, but its common name is much more vividly descriptive: creeping eruption. The larvae hatch from eggs in dog or sometimes in cat feces and grow in the soil.
Skin contact with the parasite is enough to cause infection. Children and people who live in warm, humid climates, such as the American Southeast, are more susceptible.
This worm’s habits sound like something lifted out of a grade B horror movie: it drills into the skin and then migrates between skin layers, advancing an inch or two a day and leaving red marks and puss-filled blisters as it travels. If the person has been lying on the ground these lesions can be all over the body, and wherever they appear they itch intensely. Fortunately, the larvae do not grow in humans, and after several weeks of torturing their victim, they die and the patient gets better. There are some medicines that help relieve the symptoms (thiabendazole administered orally or topically, ivermectin, or albendazole). In short, this organism is one more reason, if another is needed, not to touch dog feces or let it touch you.
These diseases are not, however, a reason to get rid of the dog. Dogs and cats can and should be treated for both hookworm and roundworm. Deworming of puppies and kittens and their mothers is the best procedure, because they’re most likely to harbor the parasites. You should have your puppy treated immediately after birth and then again at two, four, six, and eight weeks of age. Kittens are less often infected, and when they are they are usually infected at an older age. Treatment of kittens can start at about six weeks, then be repeated at eight and ten weeks.
There are many safe and effective drugs a veterinarian can use, and you can also administer them yourself at home. You don’t need to wait for a diagnosis, the worms are so common in puppies and kittens that all of them should be treated prophylactically.
