Guidelines for Veterinarians: Prevention of Zoonotic Transmission of Ascarids and Hookworms of Dogs and Cats

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• Ascarids and Hookworms
• Zoonotic Transmission and Human Disease
• The Public Health Problem
• Veterinarians Can Help Prevent Human Disease
• Preventive Anthelmintic Treatment
• Educating and Counseling Pet Owners
• References

Ascarids and Hookworms

Ascarids (Toxocara canis, T. cati) and hookworms (Ancylostoma spp.) are common intestinal parasites of dogs and cats (referred to here as pets).  Not only can ascarids and hookworms cause disease in their respective hosts, they are also well-known causes of larva migrans syndromes in humans, especially children. While ascarids and hookworms are most commonly diagnosed in puppies and kittens, infections can occur in dogs and cats of all ages.  Dogs can also become infected with Baylisascaris procyonis, the common raccoon ascarid, which can cause serious disease in other animals and humans.¹

Ascarids — Because of the occurrence of both transplacental and transmammary transmission of T. canis, puppies are usually born with or acquire ascarid infections early in life.²   Kittens do not become infected in utero, but like puppies, can acquire ascarids (T. cati) through the queen’s milk.³  The tissue-migrating and early intestinal stages of these worms may cause severe, sometimes life-threatening, disease in the first few weeks of life.  Patent intestinal infections can develop within the first 2½ - 3 weeks of life.  Left untreated, this can lead to widespread contamination of the environment with infective eggs.

Hookworms — Both puppies and kittens acquire hookworm infections (A. caninum, A. braziliense, and A. tubaeforme) through ingestion of or skin penetration by infective larvae, or from infective larvae passed in their dam’s milk (A. caninum).²  Hookworms suck large amounts of blood from their hosts and while infected animals may look healthy in the first week of life, they can develop a rapidly severe, often fatal, anemia.⁴ Patent intestinal infections can occur as early as 2- (dogs) to 3- (cats) weeks of age, leading to environmental contamination with infective larvae.^5,6

The prevalence of these infections varies with climatic conditions; however, they are present in all parts of the United States and must be viewed as a potential public health hazard.^6-9

Zoonotic Transmission and Human Disease

The growing popularity of dogs and cats in the United States, together with high rates of ascarid and hookworm infections, has resulted in widespread contamination of the soil with infective eggs and larvae.  Epidemiologic studies have implicated the presence of dogs, particularly puppies, in a household, and pica (dirt eating) as the principal risk factors for human disease.  Children’s play habits and their attraction to pets put them at higher risk for infection than adults.

Humans become infected with ascarids (Toxocara spp., Baylisascaris spp.) through ingestion of infective eggs in the environment.  When a human ingests infective eggs, the eggs hatch and release larvae that can migrate anywhere in the body, a condition known as visceral larva migrans.  The signs and symptoms seen in humans are determined by the tissues or organs damaged during larval migration. Organs commonly affected are the eye, brain, liver, and lung, where infections can cause permanent visual, neurologic, or other tissue damage.  The common dog ascarid, T. canis, has long been recognized as a cause of larva migrans syndromes in children.  The cat ascarid, T. cati, can also cause disease in humans, although for reasons partly related to the defecation habits of cats, it does so less frequently.  The raccoon ascarid, B. procyonis, is increasingly being recognized as a cause of human disease.¹⁰

Humans can become infected with hookworms through ingestion of infective larvae or through direct penetration of the skin.⁷ When infective larvae penetrate the skin, they undergo a prolonged migration that causes a condition known as cutaneous larva migrans.  These larval migrations are characterized by the appearance of progressive, intensely pruritic, linear eruptive lesions, which are usually more extensive with A. braziliense infections.  A. caninum larvae may also penetrate into deeper tissues and induce symptoms of visceral larva migrans, or migrate to the intestine and induce an eosinophilic enteritis.^11,12

The Public Health Problem

Larva migrans syndromes are not reportable in the United States, so the actual number of human cases is unknown.  However, many human cases continue to be diagnosed and a recent national survey of shelters revealed that almost 36% of dogs nationwide, and 52% of dogs from southeastern states harbored helminths capable of causing human disease.¹³   Every year at least 3,000-4,000 serum specimens from patients with presumptive diagnoses of toxocariasis are sent to the Centers for Disease Control and Prevention (CDC), state public health laboratories, or private laboratories for serodiagnostic confirmation.¹⁴  Zoonotic hookworm infections are more geographically restricted than toxocariasis, with most cutaneous larva migrans and other hookworm-associated syndromes diagnosed in southeastern and Gulf Coast states.  Persons likely to come in contact with larvae-contaminated soil include electricians, plumbers, and other workers who crawl beneath raised buildings, sunbathers who recline on larvae-contaminated sand, — as well as children who play in contaminated areas.  While most hookworm infections are self-limiting, massive infections can lead to infection of deeper tissues.¹⁵

Veterinarians Can Help Prevent Human Disease

Most cases of human ascarid and hookworm infections can be prevented by practicing good personal hygiene, eliminating intestinal parasites from pets through regular deworming, and making potentially contaminated environments, such as unprotected sand boxes, off limits to children.^9,10,14    It is also important to clean up pet feces on a regular basis to remove potentially infective eggs before they become disseminated in the environment via rain, insects, or the active migration of the larvae.¹⁶  Hookworm eggs can develop into infective stage larvae in the soil in as little as 5 days, and ascarid eggs within 2 weeks, depending on temperature and humidity.⁴  To illustrate the extent of environmental contamination that can occur as the result of one infected puppy, a single female ascarid can produce more than 100,000 eggs/day, resulting in millions of potentially infective ascarid eggs per day spread throughout the area the puppy is allowed to roam.¹⁰  Once the eggs become infective, they can remain infective in the environment for years.^4,10

Most pet owners do not know that their pets may carry worms capable of infecting people.  Therefore, practicing veterinarians can provide an important public service by recommending regular fecal examinations, providing well-timed anthelmintic treatments, counseling clients on potential public health hazards, and advising them on any precautionary measures that may be undertaken. Veterinarians are in an ideal position to provide pet owners with this service because of their access to the pet-owning public, their knowledge and training, and their role in the human-animal bond.^17,18

Preventive Anthelmintic Treatment

Because puppies, kittens, and pregnant and nursing animals are at highest risk for these infections, and therefore responsible for most of the environmental contamination and human disease, anthelmintic treatments are most effective when they are initiated early and targeted at these populations.^4,15

While it has long been recognized that transplacental and transmammary infection of ascarids and hookworms could be prevented through prophylactic treatment of pregnant dogs, no drugs are currently approved for this use.  However, the effectiveness of this approach with different drugs approved for parasite control in dogs has been well documented.  Daily treatment of pregnant dogs with fenbendazole from the 40^th day of gestation through the 14^th day of lactation has been shown to inhibit T. canis larvae in tissues, thereby preventing or greatly reducing the incidence of infection in puppies.¹⁹   Alternatively, studies have shown that treatment with ivermectin on day 0, 30, 60 of gestation and 10 days post whelping, reduced the adult T. canis worm burden in pups by 100% and prevented the shedding of eggs.²⁰   In yet another study, treatment with selamectin at 10 and 40 days both before and after parturition was effective in reducing T. canis fecal egg counts in both pups and their dams, and adult worms in the pups.²¹ 

If the mother did not receive prophylactic treatment, puppies and kittens must be treated early and repeatedly in order to prevent patent infections.²² In areas where both ascarids and hookworms are common, begin treating both puppies and their mothers with an age-appropriate anthelmintic at 2, 4, 6, and 8 weeks of age.  Some recommend extending this to 12 weeks and then treating monthly until the pet is 6 months old.  To treat for ascarids alone, begin by 2 ½ - 3 weeks and treat every 2 weeks for at least three additional treatments.  Because prenatal infection does not occur in kittens, preventive treatment should begin at 3 weeks of age, and be repeated at 5, 7, and 9 weeks.  Nursing dogs and queens should be treated concurrently with their offspring because they often develop patent infections along with their young.

Because most puppies and kittens are not routinely brought to a veterinarian before 6-8 weeks of age, they will already have patent infections and be actively contaminating the environment.  For this reason, it is important to reach out to clients who have pregnant or newly born animals at home, and provide these animals with early prophylactic treatment for intestinal parasites.  Early identification of these high-risk animals will provide the veterinarian with the opportunity to educate the owners on the public health risks, provide them with an appropriate anthelmintic, and advise them on how and when to administer it to their pets at home.  This approach to treatment is justified by the frequency with which puppies and kittens acquire intestinal parasites from their mothers, and the difficulties that exist in early diagnosis.  Because young animals may continuously acquire new infections from nursing and from the environment, they may develop a serious illness or even die before a prenatally or lactogenically acquired infection becomes patent and can be diagnosed by fecal examination.

While intestinal parasites are usually less of a problem in young adult and adult animals, they too can develop patent infections and contaminate the environment.  Therefore, they too should be regularly monitored or treated for intestinal parasite infections.  While all adult animals are at risk, those that are allowed to roam or spend most of their time outside run a greater risk of becoming infected.  There are a variety of anthelmintic drugs available that are safe and effective against ascarids, hookworms, and other intestinal helminths of dogs and cats (Table 1).  Mature animals can also be monitored through biannual or yearly diagnostic stool examinations and treated with anthelmintics directed at specific intestinal nematodes.  For animals that live in areas where heartworm (Dirofilaria immitis) infection is enzootic, many of the heartworm preventives are also effective against intestinal parasites (Table 1).²³

Table 1.  Drugs for the treatment of ascarid and hookworm infections in dogs and cats

Educating and Counseling Pet Owners

Pet owner education regarding intestinal parasites and their effects on the health of both their pets and family members should be included in a well-pet exam.  Pet owner education should focus on prevention and include the following:

• Description of ascarids and hookworms that infect dogs and cats, early signs of illness, and when pets are at greatest risk for infection (in utero and when nursing).
• How ascarids and hookworms cause disease in humans, especially in children whose play habits and attraction to pets put them at increased risk.
• How prophylactic treatment of pregnant and nursing pets and their offspring can protect their pets from becoming infected, thus preventing them from shedding eggs into, and contaminating, the environment.
• The need for regular diagnostic fecal examinations of pups or kittens or prophylactic treatment of older pets.
• The need for prompt collection and disposal of pet feces, especially in areas where children play, to remove eggs from the environment before they can become a problem.
• The need to keep children away from areas that may be contaminated with pet feces.

References

1. Samuel WM, Pybus MJ, Kocan AA.  Parasitic diseases of wild mammals.  Second ed.  Ames: Iowa State University Press.  2001;301-41. 
2. Burke TM, Roberson EL.  Prenatal and lactational transmission of Toxocara canis and Ancylostoma caninum: experimental infection of the bitch before pregnancy.  Int J Parasitol 1985;15:71-5.
3. Swerzcek TW, Nielsen SW, Helmbolt CF.   Transmammary passage of Toxocara cati in the cat.  Am J Vet Res 1971;32:89-92.
4. Bowman DD.  Georgis' parasitology for veterinarians.  Seventh ed. Philadelphia:  WB Saunders Company.  1999;178-84.
5. Hendrix CM, Homer SB, Kellman NJ, Harrelson G, Bruhn BF.  Cutaneous larva migrans and enteric hookworm infections.  J Am Vet Med Assoc 1996; 209(10):1763-76.
6. Kalkofen VP.  Hookworms of dogs and cats.  Vet Clin North Am Small Anim Pract 1987;17:1341-54.
7. Glickman LT, Schantz PM.  Epidemiology and pathogenesis of zoonotic toxocariasis.  Epidemiol Rev 1981;3:230-50.
8. Parsons JC.  Ascarid infections in cats and dogs.  Vet Clin North Am Small Anim Pract 1987;17:1307-39.
9. Kazacos KR.  Visceral and ocular larva migrans.  Semin Vet Med Surg (Small Anim) 1991;6:227-35.
10. Kazacos KR.  Protecting children from helminthic zoonosis.  Contemp Pediatr 2000;17(3)(Suppl):1-24.
11. Little MD, Halsey NA, Cline BL, Katz SP.  Ancylostoma larva in muscle fiber of man following cutaneous larva migrans.  Am J Trop Med Hyg 1983;32:1285-8.
12. Prociv P, Croese J.  Human eosinophilic enteritis caused by dog hookworm Ancylostoma caninum.  Lancet 1990;335:1299-1302.
13. Blagburn BL, Lindsay DS, Vaughan JL, et al.  Prevalence of canine parasites based on fecal flotation.  Comp Contin Educ Vet Pract 1996;18:483-509.
14. Schantz PM.  Toxocara larva migrans now.  Am J Trop Med Hyg 1989;41(3)(Suppl):21-34.
15. Schantz PM.  Zoonotic ascarids and hookworms:  The role for veterinarians in preventing human disease.  Compendium on continuing education for the practicing veterinarian 2002;24(1)(Suppl):47-52.
16. Beaver PC.  Biology of soil-transmitted helminths: The massive infection.  H. L. S. 1974;12(2):116-25.
17. Harvey JB, Roberts JM, Schantz PM.  Survey of veterinarians' recommendations for the treatment and control of intestinal parasites in dogs:  Public health implications.  J Am Vet Med Assoc 1991;199:702-7.
18. Barriga OO.  Rational control of canine toxocariasis by the veterinary practitioner.  J Am Vet Med Assoc 1991;198:216-21.
19. Duwel D, Strasser H.  Birth of helminth-free canine pups through maternal fenbendazole therapy.  Dtsch Tierarztl Wochenschr 1978;85(6):239-41.
20. Payne PA, Ridley RK.  Strategic use of ivermectin during pregnancy to control Toxocara canis in greyhound puppies.  Vet Parasitol 1999;85(4):305-12.
21. Payne-Johnson M, Maitland TP, Sherington J, Shanks DJ, Cements PJ, Murphy MG, McLoughlin A, Jernigan AD, Rowan TG.  Efficacy of selamectin administered topically to pregnant and lactating female dogs in the treatment and prevention of adult roundworm (Toxocara canis) infections and flea (Ctenocephalides felis felis) infestations in the dams and their pups.  Vet Parasitol 2000;91(3-4):347-58.
22. Stoye M.  Biology, pathogenicity, diagnosis and control of Ancylostoma caninum.  Dtsch Tierarztl Wochenschr 1992;99(8):315-21.
23. Reinemeyer CR, Faulkner CT, Assadi-Rad AM, Burr JH, Patton S.  Comparison of the efficacies of three heartworm preventatives against experimentally induced infections with Anycylostoma caninum and Toxocara canis in pups.  J Am Vet Med Assoc 1995;206(11):1710-5.

Produced in cooperation with the American Association of Veterinary Parasitologists (AAVP).

Ascaris Infection

Ascaris Infection

Ascaris Infection