Pets, worms, and little people

By Patricia Thomblison, DVM, MS

Dogs and cats often come with an unwanted accessory: parasites. Fortunately, transmission of serious infection from animals to humans is rare, and preventive measures minimize even that minor risk.

Pets are becoming an increasingly important part of the family circle. More than half of all households with children in the United States also have a pet.¹ Dogs and cats, among the more popular pets, can be wonderful companions, and can teach children about responsibility (see "Benefits of pet ownership"), but also pose a health risk. The risk is preventable through education and good veterinary care, however.

Most pet owners are familiar with such diseases as rabies, and vaccinate their animals accordingly. But few understand the dangers of other zoonotic diseases, such as intestinal parasites. Some worms found in pets may cause disease in people by accidental ingestion of or contact with eggs or larvae. In a recent survey sponsored by a leading animal health corporation, 73% of responding pediatricians reported seeing cases of zoonotic parasitic disease in any given year.²

"Pets can and do infect children with parasitic diseases," notes Daniel McCrimons, MD, a pediatrician from Sacramento, Calif. "Some children have no reaction to these diseases, while others develop more serious symptoms. Parents should be aware that pets could expose their owners to a variety of zoonotic diseases with varied consequences. When disease is present, the pediatrician should work closely with the veterinarian so that the animal, as well as the child, is treated appropriately."³

The scope of the problem

Almost all puppies and kittens in the US are born with worms or acquire them shortly after birth. Newborn puppies and kittens typically have both hookworms (Ancylostoma caninum, Ancylostoma braziliense, and Ancylostoma tubaeforme) and ascarids (Toxocara canis and Toxocara cati). Puppies routinely become infected with ascarids in utero or during nursing; they acquire hookworms by ingesting larvae during nursing.⁴ As a result, puppies are born with a disproportionately large population of worms in their system. Intestinal infection can occur within the first two or three weeks of life and lead to widespread contamination of the environment with parasite eggs. One study demonstrated that a single puppy shed more than 294,000 Toxocara eggs every time he passed a stool.⁵

Unlike puppies, kittens do not become infected with ascarids until after birth. Infection of kittens with ascarids or hookworms occurs by ingestion of infective larvae during nursing or ingestion of eggs from a contaminated environment.

Adult dogs and cats have fewer worms than puppies or kittens. Whether passed by puppies, kittens, or adult dogs or cats, all parasite eggs passed into the environment are a potential source of infection.

Recently, it has been recognized that dogs also can become infected with the common raccoon ascarid, Baylisascaris procyonis. This parasite can cause serious disease in other animals and humans.⁶

Tapeworms are also common in dogs and cats. Infection has been seen in children as well. The most common tapeworm is Dipylidium caninum, which cats and dogs acquire by ingesting infected fleas.

Tapeworms in the genus Echinococcus also can infect humans. The larval form (hydatid cyst) can cause serious disease. Although human infestation is rare in the US, the potential does exist and appears to be spreading geographically. In North America, the species seen are Echinococcusgranulosus and Echinococcus multilocularis.

Toxocariasis

This zoonotic infection is caused by the worms commonly found in the intestines of dogs (T canis) and cats (T cati). As previously noted, infected dogs and cats shed worm eggs in their feces. After the eggs have been in the environment for about two weeks, they become a potential source of infection for pets, children, and adults alike. Because children play in soil or sand and then put their hands or other objects that may be contaminated with the sticky eggs in their mouth, they are more likely than adults to become infected. Toxocariasis has two major forms: ocular larva migrans and visceral larva migrans.

Ocular larva migrans (OLM). Each year, more than 700 people infected with Toxocara experience permanent partial loss of vision.⁷ After a person ingests infectious eggs, they hatch in the intestine; the ascarid larvae wander aimlessly about the body. When one or more larvae become trapped in the eye, the resulting inflammatory reaction leads to formation of a granuloma. The exact location of the granuloma varies, depending on where the parasite enters the eye; if it enters through the central retinal artery, the granuloma will be peripheral to the retina. If the parasite enters through the short posterior ciliary arteries, the granuloma will likely be at the disk, macula, or elsewhere in the posterior pole.⁸

Visceral larva migrans (VLM). Clinical signs of human infestation with ascarid larvae depend largely on the number of parasites present and the organs involved. Some patients are asymptomatic; others have nonspecific signs, including nausea, fever, and malaise. Physical findings may include hepatomegaly and lymphadenopathy. These patients also may have a high level of circulating eosinophils.⁹ Because visceral larva migrans is not a reportable disease in the US, the true incidence is unknown. Serologic surveys indicate that exposure is quite high, with children having the highest rate of exposure.

Diagnosing and treating toxocariasis

Visceral larva migrans and ocular larva migrans rarely occur together in the same patient. Patients with VLM are usually younger (under 3 years) than patients with OLM (average, 7 or 8 years). Patients with clinical signs of either disease should have further diagnostic testing. Imaging techniques, such as computed tomography and magnetic resonance imaging, can be used to detect the lesions. Serology is also useful to establish a diagnosis in patients with compatible symptoms and signs. An ELISA test with 91% sensitivity and 86% specificity is available that uses T canis larval excretory-secretory antigens. A Western blot test, which has greater sensitivity than the ELISA test, may also be performed.¹⁰

Treatment of toxocariasis includes reducing the inflammation with corticosteroids or antihistamines. Few clinical trials of anthelmintics have been done, and their efficacy is unclear. Antihelmintic agents that have been recommended to treat toxocariasis include diethylcarbamazine (6 mg/kg/day divided into three doses for seven to 10 days) and mebendazole (100 mg to 200 mg twice daily for five days).⁵

Baylisascaris infection

The raccoon ascarid B procyonis is the cause of most cases of neural larva migrans, which can lead to severe or fatal encephalitis in a variety of birds and mammals, including humans. The larvae are larger and more aggressive than Toxocara larvae. Although the raccoon is the natural host, dogs also may acquire these parasites and shed eggs into the environment.

After the eggs are ingested, the larvae migrate to the viscera, eyes, and central nervous system. The severity of neurologic disease they cause depends on the number of larvae that enter the CNS. Larvae in the CNS cause an inflammatory reaction and tissue damage. They can become encapsulated within granulomas in the CNS. A diagnosis of Baylisascaris encephalitis should be considered in people with a sudden onset of eosinophilic encephalitis and a history of potential exposure.¹¹

Unfortunately, by the time encephalitis is diagnosed, treatment with an antihelmintic may not be effective and might even cause further complications by exacerbating inflammation. If disease is diagnosed early or CNS involvement is mild, albendazole or diethylcarbamazine may be helpful.⁵

Hookworm infection

Pets infected with hookworms (A braziliense and A caninum) shed eggs into the environment that hatch into larvae within two or three days. When larvae penetrate the skin of people, they cause cutaneous larva migrans (CLM).

The larvae of A braziliense form a pruritic papule at the entry site and then migrate within the skin, leaving a characteristic serpiginous track that advances each day, giving CLM its common name, "creeping eruption." The lesion caused by the larvae of A caninum is characterized by papules and pustules.

CLM is more common in people who have walked barefoot on the beaches along the southern Atlantic and Gulf coasts and at Caribbean resorts. It can occur anywhere larvae thrive, however, such as the damp soil in crawl spaces under homes or porches, sandboxes, and shady areas under bushes. Prevention of CLM by avoiding exposure to contaminated areas is the best management strategy. Topical thiabendazole cream can be used to treat the lesions.⁸

In rare cases, the larvae of A caninum reach the small intestine and cause eosinophilic enteritis. Patients with this condition have recurrent abdominal pain, eosinophilia, small bowel thickening, and focal areas of inflammation and ulceration in the terminal ileum and colon. Diagnosis is based on clinical signs, colonoscopic findings, and serologic results.⁵ Fecal examinations are negative because human infections have not so far resulted in egg production by the parasites.

Treatment with oral mebendazole, 300 mg once a day or 100 mg twice a day for three days, usually eliminates the clinical signs. Patients may experience a relapse up to several months after treatment caused by larvae that were initially dormant in skeletal muscles.⁵

Tapeworm infection

Dipylidiasis, infection with the tapeworm D caninum, is usually asymptomatic. The normal life cycle of the parasite involves the flea most commonly found on cats and dogs (Ctenocephalides felis). Larval fleas ingest tapeworm eggs. The eggs hatch and a cysticercoid (larval tapeworm) develops in the flea as it matures. Dogs and cats swallow the adult fleas during grooming. The cysticercoids develop into adult tapeworms and lay eggs, completing the life cycle.

People (usually toddlers) become infected when they accidentally swallow an infected flea. Some children may have mild gastrointestinal upset and lack of appetite. They also may be irritable and develop anal pruritus caused by excretion of the proglottids (tapeworm segments) through the anus. Parents become quite concerned when they see the segments in diapers, the perianal area, or feces of young children. Tapeworm infection can be treated with a single 5 to 10 mg/kg dose of praziquantal.⁵

It is important to differentiate tapeworm infection from infection with the human pinworm, Enterobius vermicularis. There is no evidence that human pinworm infection is a zoonotic disease.

Echinococcosis: Serious tapeworm infection

Echinococcus granulosus has been found in the contiguous 48 states. In areas of the Southwest where dogs are used to herd sheep, the dog is the definitive host for the domestic strain of E granulosus, and sheep are the intermediate hosts. A sylvatic strain of E granulosus, which is found in the northern states, uses wolves and coyotes as definitive hosts and large wild ungulates such as moose as intermediate hosts.

The adult worms in dogs and other carnivores are very small (about 2 mm to 7 mm long) with only three to four segments. When gravid proglottids are passed in the feces, they disintegrate, releasing eggs that are ingested by an intermediate host. Although sheep are the most common intermediate hosts, other species, including humans, can also serve as hosts. The eggs hatch in the intermediate host, and the hexacanth embryo penetrates the intestinal wall and travels through the body via the lymphatics or blood system.

The majority of the embryos lodge in the liver and produce slowly developing hydatid cysts. The hydatid cyst—which contains numerous protoscolices (the larval form of tapeworm), brood capsules, and daughter cysts—is a unilocular structure with a thick, laminated outer layer that is surrounded by a fibrous tissue capsule made by the host rather than the parasite. The definitive hosts, canids, are infected when they consume the protoscolices by predation or scavenging of intermediate hosts. When ingested by the canine host, each of the protoscolices has the ability to grow into an adult worm.

In humans, hydatid cysts can grow quite large, reaching several centimeters in diameter. If ruptured—as a result of routine surgery or a sharp blow to the abdomen, for example—the protoscolices within the cyst are released into the body cavity, where each can grow into a new hydatid cyst. The expression of disease depends on the size of the cysts and their location.

Another Echinococcus species, E multilocularis, is enzootic throughout much of the north central US and south central Canada. There is evidence that its range is expanding southward. The life cycle of E multilocularis is similar to that of E granulosus. The primary definitive hosts are foxes, but other canids and cats may also serve as definitive hosts. They become infected when they consume infested rodents.

Adult tapeworms are quite small (1.2 mm to 3.7 mm) and live in the small intestine of the definitive host. They produce eggs that are shed in the feces and contaminate the environment where feces are dropped. Rodents and other intermediate hosts, including humans, become infected by ingesting eggs, which contain thousands of protoscolices.

The cyst of E multilocularis is multilocular with multiple thin-walled vesicles. In contrast to the hydatid cyst of E granulosus, a thick fibrous tissue capsule does not surround it. Therefore, the cysts progressively expand into surrounding host tissue, much like a highly invasive, proliferative malignant neoplasm. E multilocularis is often considered the most lethal of all zoonotic helminths. Treatment of disease caused by both E granulosus and E multiocularis consists of surgical removal of the cyst and long-term use of an anthelmintic (mebendazole or albendazole).⁵

Pets: Reducing risk

The special relationship between children and pets, coupled with the fact that children are often less careful about hygiene than adults, puts them at a greater risk of parasitic infection transmitted from their pets. Others at increased risk include immunosuppressed people and the elderly.

Transmission of serious parasitic disease from pets to humans is rare, and regular veterinary care and good hygiene can reduce the risk further. "Zoonotic parasitic disease is a preventable condition," emphasizes Peter M. Schantz, VMD, PhD, a veterinarian specializing in epidemiology and zoonotic disease for the CDC.¹² Broad-spectrum deworming agents that specifically treat and control the worms that cause zoonotic parasitic disease are available. The parent guide lists some easy steps to help prevent the spread of infection—and thereby protect two-legged and four-legged family members.

REFERENCES

1. Kyone M, Staffort K: The benefits of pet ownership. NZVA Companion Animal Society Newsletter (official publication of the New Zealand Veterinary Association) 2001;12(3):15

2. Bayer Corporation: Unpublished data from survey of pediatricians concerning zoonotic parasitic disease, in Kazacos KR: Protecting children from helminthic zoonoses. Contemporary Pediatrics March 2002 (supplement)

3. McCrimons D: Personal communication, 2003

4. Bowman D: Georgis' Parasitology for Veterinarians, ed 7. Philadelphia, WB Saunders Company, 1999

5. Kazacos K: Protecting children from helminthic zoonoses. Contemporary Pediatrics (supplement), March 2000

6. Kazacos K, Boyce W: Balyisascaris larva migrans. Zoonosis Updates from the Journal of the American Veterinary Medical Association, ed 2. Schaumburg, Ill., American Veterinary Medical Association, 1995, pp 20–30

7. http://www.cdc.gov/ncidod/dpd/parasites/toxocara/factsht_toxocara.htm

8. Juckett G: Pets and parasites. Am Fam Physician 1997;56(7):1763, 1777

9. Prociv P: Zoonotic hookworm infections (ancylostomiasis), in Palmer SR et al (eds): Zoonoses. New York, Oxford University Press, 1988, pp 803—822

10. Glickman LT, Magnaval JF: Zoonotic roundworm infections. Infect Dis Clin North Am 1993;7:717

11. Centers for Disease Control and Prevention: Raccoon roundworm encephalitis—Chicago, Illinois, and Los Angeles, California, 2000. MMWR Morb Mortal Wkly Rep January 4, 2002;50(51):1153

12. Schantz PM: Personal communication, 2003

DR. THOMBLISON is a consultant for Hill's Pet Nutrition, Inc., Veterinary Consultation Service and medical consulting editor for NAVC Clinician's Brief (official publication of the North American Veterinary Conference), Topeka, Kan. At the time this article was written, she was employed by Stephens & Associates agency, which represents Bayer Animal Health.

Benefits of pet ownership

People can benefit from pet ownership in many ways. Here are a few:

• Jennings and colleagues at the Baker Medical Research Institute in Prahan, Victoria, Australia, report that people who own a pet have, on average, lower blood fat levels and lower blood pressure.¹

• Children turn to companion animals for reassurance when they are feeling stressed.

• Contact with pets can alleviate symptoms of attention deficit hyperactivity disorder, according to a study by the Philadelphia Devereux Foundation, a nonprofit organization that provides services to people with special needs.²

• By taking care of a pet, children learn about responsibility, which encourages emotional development. Children can learn about life and death and how to grieve from owning a pet.

REFERENCES

1. Wilson CC, Turner DC (eds): Companion Animals in Human Health. Thousand Oaks, Calif., Sage Publications, 1997

2. Fine AH (ed): Handbook on Animal Assisted Therapy: Theoretical Foundations and Guidelines for Practice. San Diego, Academic Press, 1999

GUIDE FOR PARENTS

Tips on preventing parasitic infections from pets

Pets, especially dogs and cats, can be wonderful companions, but they may harbor parasites (worms) that can cause disease in people. Parasites spread from animals to people when people come into accidental contact with the parasite's eggs or larvae. The following measures can help you protect your child, and yourself, from infection.

• Practice good hygiene. Wash your hands, and have your children wash their hands, regularly, especially after handling pets or cleaning up pet waste.

• Remove pet droppings from your yard at least two or three times a week. Daily removal is best.

• Wash all fruits and vegetables thoroughly before you or your children eat them.

• Do not allow children to go barefoot, sit, or lie in areas where they may be exposed to animal stools.

• Remove fecal waste from cat litter boxes daily and wash your hands afterward.

• Do not drink water from streams or other sources that may be contaminated with animal feces.

• Deworm your pets on a regular schedule, as recommended by the veterinarian.

• Keep your pets free of fleas.

• Keep your pets clean. Bathe them after they have been dewormed.

This guide may be photocopied and distributed without permission to give to your patients and their parents. Reproduction for any other purpose requires express permission of the publisher.