How To Prevent Worms Transmission

How To Prevent Transmission of Intestinal Roundworms from Pets to People

(Excerpts taken from CDC report and recommendations to veterinarians found on the web at

These guidelines address transmission of intestinal ascarids and hookworms from dogs and cats to people and recommend counseling of dog and cat owners and well-timed preventive treatments (strategic de-worming) for pets.

Ascarids (Roundworms) and Hookworms

Ascarids (Toxocara spp.) and hookworms (Ancylostoma spp. and Uncinaria stenocephala), the common intestinal roundworms of dogs and cats, can cause larva migrans syndromes in persons who accidentally ingest eggs or larvae or have direct skin contact with hookworm larvae in soil contaminated with the feces of infected animals.
Pups and kittens are often infected by transfer of larvae from their dams in utero (T. canis) or via milk (A. caninum, T. cati, and to a lesser extent, T. canis), and the tissue-migrating and early intestinal stages of these worms may cause severe, sometimes life-threatening, disease in the first few weeks of the animal’s life. Furthermore, pups and kittens may have patent intestinal infections as early as the first 2 (hookworms) to 3 (ascarids) weeks of life, and may contaminate their environment with huge numbers of infective eggs and larvae.
The prevalence of these infections varies with climatic conditions; however, they are present in all parts of the contiguous United States and must be viewed as a potential public health hazard.

Zoonotic Transmission and Human Disease

When zoonotic ascarids and hookworms infect humans, the parasites rarely mature in the intestine; rather, the larval worms migrate in the host’s tissues (larva migrans). The characteristics of the particular tissues and organs in which the larvae migrate determine the signs and symptoms humans have.
The common ascarid of dogs, T. canis, has long been recognized as a cause of larva migrans syndromes in children. The ascarid of cats, T. cati, can also cause disease in humans, although for reasons partly related to the “toilet behavior” of cats, it does so less frequently than T. canis. When the eggs are accidentally ingested, they hatch, and infective-stage larvae migrate through human liver, lungs, and other organs and tissues where they produce damage and induce allergic responses. Infection may leave children with permanent visual or neurologic damage.

In the United States, the popularity of pets together with high ascarid and hookworm infection rates in dogs and cats, especially pups and kittens, result in widespread contamination of soil with infective-stage larvae. Epidemiologic studies have implicated the presence of dogs, particularly pups, in the household and pica (dirt eating) as the principal risk factors for human toxocaral disease. Children’s play habits and attraction to pets put them at high risk for ascarid and hookworm infection.

Hookworms of dogs and cats, A. caninum, A. braziliense, A. tubaeforme, and U. stenocephala, can also infect people when larvae in soil are ingested or directly penetrate the skin on contact. Cutaneous larva migrans syndromes, characterized by progressive, intensely pruritic, linear eruptive lesions caused by prolonged migration of the larvae in the skin, are the most common manifestation of zoonotic hookworm infection. A. caninum larvae may penetrate into deeper tissues, however, and induce symptoms of visceral larva migrans or migrate to and partially mature in the intestine, inducing eosinophilic enteritis.

The Public Health Problem

Larva migrans syndromes are not reportable in the United States, so the number of clinical cases of zoonotic ascarid and hookworm infection in humans is unknown; however, many human cases continue to be diagnosed. 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. Zoonotic hookworm infection is more geographically restricted than zoonotic toxocariasis; however, cutaneous larva migrans and other syndromes caused by hookworms are diagnosed commonly in southeastern and Gulf states. Electricians, plumbers, and other workers who crawl beneath raised buildings, sunbathers who recline on wet sand contaminated with hookworm larvae, and children typically become infected.

Veterinarians Can Help Prevent Human Disease

Most cases of human toxocariasis and zoonotic ancylostomiasis can be prevented by simple measures, such as practicing good personal hygiene, eliminating intestinal parasites from pets, and making potentially contaminated environments off limits to children.
Most pet owners do not know that intestinal worms of pets may infect people; therefore, they may have neither the knowledge nor the incentive to take precautionary measures. By recommending well-timed anthelmintic treatments and by counseling clients on the potential public health hazards, the practicing veterinarian can provide an important public service. Veterinarians are in an optimal position to provide pet owners with this service because of their unique training, frequent contact with the high proportion of pet owners who use veterinary services, and their rapport with clients.
Prevent the transmission of intestinal ascarids and hookworms from pets to people. Avoid environmental contamination by emphasizing good hygiene and sanitation and providing well-timed preventive treatments, especially for pups and kittens.

Strategic De-Worming

Deworming is most effective in preventing environmental contamination and human illness when it is aimed at pups and kittens and their dams because they harbor the most worms and thus produce the most infective-stage larvae.
For optimal prevention of environmental contamination and illness in pups and kittens, initiate anthelmintic treatment of pups and kittens soon after birth. Where both ascarids and hookworms are commonly transmitted, anthelmintic drugs should be given to pups at ages 2, 4, 6, and 8 weeks of age. If only ascarids are present, preventive anthelmintic treatments may begin at 3 weeks of age. In kittens, because prenatal infection does not occur, egg excretion begins later than in pups, and in most areas, rates of acquisition of ascarids and hookworms by cats are comparatively lower; therefore, preventive treatment for kittens can be started effectively at 6 weeks of age and be repeated at 8 and 10 weeks. Treat nursing dams concurrently because they often develop patent infections about the same time as their offspring. For the earliest treatments provide clients with medication to administer to their pets at home. Thereafter, in areas where heartworm (Dirofilaria immitis) infection is enzootic, ascarid and hookworm prevention can be maintained in dogs by using one of the heartworm prevention medications that are also effective against intestinal nematodes. Control in older dogs and cats can be -achieved by periodic treatments with drugs whose efficacy is limited to intestinal nematodes, or by treatments based on the results of periodic diagnostic stool examinations.
Severe illness and even death may occur before prenatal or lactogenically acquired ascarids and hookworms become gravid and can be diagnosed by stool examinations.
The prophylactic approach to treatment is justified by the frequency with which pups and kittens acquire ascarids and hookworms from their dams and the difficulties in diagnosing these infections in their early stages. Severe illness and even death may occur before prenatal or lactogenically acquired ascarids and hookworms become gravid and can be diagnosed by stool examinations. Because many pups and kittens are not brought to a veterinarian before 6-8 weeks of age, delaying treatment until that time allows infections to become patent and contaminate the environment with eggs or larvae. Because young animals acquire new infections continuously from dam’s milk and from the environment and many worms are not yet fully mature, fecal examinations are often falsely negative in pups and kittens.

Educating and Counseling Pet Owners

Pet owners should be informed about intestinal parasites and their effects on the health of pets and people. Pet owner education should provide the following information:

  • Types of intestinal helminths that infect dogs and cats, and the illnesses they cause in these pets.
  • How intestinal helminths are transmitted to dogs and cats, with special emphasis on helminths acquired through prenatal (T. canis) and transmammary (A. caninum, T. cati and, to a limited degree, T. canis) routes.
  • How ascarids and hookworms can cause problems in humans, especially children, whose play habits and attraction to pets put them at increased risk.
  • How infection in both pets and people can be prevented by well-timed prophylactic anthelmintic treatment of pups and kittens and routine diagnostic examinations or periodic preventive treatment of older pets.
  • How to collect and dispose of pets’ feces, especially in areas where children play.
  • How to keep children away from areas that may be contaminated by dogs or cats infected with helminths.
Prepared by the Division of Parasitic Diseases, MS F22
National Center for Infectious Diseases
Centers for Disease Control and Prevention
Atlanta, Georgia 30341-3724
the American Association of Veterinary Parasitologists
Endorsed by the National Association
of State Public Health Veterinarians
and the Conference of Public Health Veterinarians
March 1995
reprinted January 1999