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Intestinal and External Parasites and Zoonotic Impact


  • Head lice infect only humans; dogs might carry, transmit or become infected with human head lice. False – dogs do not carry, transmit or spread human head lice.
  • Pinworms infect only humans but dogs can carry and transmit pinworms. False-it is a human only disease.
  • Companion animal intestinal parasites only cause problems for companion animals. False – see below.
  • The cat, the definitive host for Toxoplasma gondi, is the only potential source of human cases of infection with Toxplasma. False – meats and undercooked meat are also a source of infection.
  • Other tapeworms or parasites of food producing animals don’t cause problems for people.
  • Pets are not the source of most human cases of Giardia infection and Cryptosporidia infection. Most canine and feline strains of these parasites are not readily transmitted to people.
  • Mange is a problem seen in animals only. False – it is a contagious and zoonotic disease.
  • Ticks drop down from trees to attach to small animals. False, they tend to live where other host animals reside.
  • Cats are not as affected or do not suffer tick attachment. Also false.

General information:

A recent study showed 24 percent of pets nationwide are infected with parasites; in the Southeastern US, the incidence can be as high as 54 percent. Children are primarily at risk. The Centers for Disease Control and Prevention has estimated that between 1 and 3 million people are infected with larval migrans from roundworm infection each year. Larval migrans is a condition resulting from infection with the larval stages of roundworm larvae; the larvae can migrate through the tissues of the abdominal organs OR eye tissues. This eye condition in children was formerly misdiagnosed as a retinal tumor. Zoonotic parasitic diseases have no reporting requirements to public health organizations by health professionals; an American Journal of Tropical Medicine and Hygiene article also suspects that millions of humans are infected annually. Most people’s knowledge of zoonotic diseases is limited to high profile diseases such as Lyme disease and rabies virus exposure. Human health counterparts have little training in parasitic diseases. Pet owners tend to be largely unaware of parasitic diseases that can be spread from pet to human or from food sources to humans.

Micro-issues center around controlling parasites in juvenile pets to minimize family-pet contact problems. Larger issues relate to environmental contamination (efforts towards decontamination and sanitation) from infected animals including wildlife.

CAPC, an organization formed and constituted with the input from MDs, veterinarians, parasitologists and epidemiologists with the goal of providing information and establishing recommendations for parasite surveillance and control measures on the veterinary side as well as  awareness and risk factor avoidance on the human medicine side (pediatrics). To help protect the unique relationship of dogs and cats and their human family, CAPCs  guidelines lead with the recommendation that all family pets be treated year-round with broad-spectrum heartworm anthelmintics that also protect against intestinal nematodes. It is, after all, our children who live and play in closest proximity to pets and their environments and are, consequently, most vulnerable to accidental infections. CDC is committed to helping reduce and eliminate these infections that are entirely preventable. For more information, see CDC’s web site: http://www.cdc.gov/healthypets/index.htm

Who is at risk? Plumbers, gardeners and sunbathers are most likely to suffer cutaneous larva migrans caused by hookworms (usually Ancylostoma braziliense) or Strongyloides species. Pica and geophagia, most often seen in children and people who are mentally disabled, are predisposing factors for visceral larva migrans (Toxocara canis, T. cati and Baylisascaris procyonis), toxoplasmosis and hydatid disease (Echinococcus granulosus and E. multilocularis).

Flea bites can occur in anyone in contact with an infested dog, cat or home. Some pet infesting mites (Cheyletiella and Sarcoptes spp.) are transmitted between animal and human companions through direct contact. Ticks such as Ixodes, Dermacentor and/or Amblyomma spp. will feed on humans, dogs and cats, although these infestations are almost always picked up from the environment. Contrary to popular belief, ticks rarely move from pets to humans or vice versa to feed.

Who is most at risk? Immunocompromised individuals — including fetuses, infants, the elderly (in the nursing care setting), and those with immunosuppressive diseases (such as HIV-AIDS related syndromes) or persons taking immunosuppressive medications (in the treatment of cancers and auto-immune diseases) can suffer severely from opportunistic infections.

Toxoplasmosis is more likely to be transmitted to people from contact with contaminated meats during food preparation or from eating undercooked meats. The garden is another source of infection with Toxoplasma infection. While most infections in adults occur without any clinical signs, the hazard in pregnant women, more specifically the developing human fetus, is very significant. The infection can lead to damage to developing brain, eye and other tissues. Additionally there has been an association (without causal evidence yet) to a notable percentage of human schizophrenic patients with evidence of immune system response to Toxoplasma.

Other immunocompromised individuals include patients with AIDS; certain types of hematopoietic system tumors; and those receiving radiation, corticosteroids or immunosuppressive agents for organ transplantation or various autoimmune, inflammatory or neoplastic conditions. These individuals can suffer severe, potentially fatal toxoplasmosis, strongyloidiasis and flea-borne infections such as bartonellosis. Patients without a spleen are likely to become severely, often fatally, ill with babesiosis, a tick-borne infection.

Flea bites are generally uncomfortable but rarely cause problems except in people with a general skin hypersentsitivity. (However recall that in medieval times, Bubonic Plague transmission was vectored by fleas from infected rodents, rodent bites as well as food borne contamination with the causative bacteria.  For more information, see http://www.history.com/topics/black-death . There are small pockets of Bubonic Plague affecting feral cats today in the southwestern part of the United States, complete with the appearance of swollen lymph nodes called buboes.)

Risk factor avoidance:

  • regular deworming programs (CAPC and CDC recommend lifetime, year-round prevention with broad spectrum deworming medications that treat both heartworm and intestinal parasites.
  • annual pet stool exams to survey for the presence of intestinal worms as well as the use of deworming medications where appropriate (and heartworm prevention).
  • treatment of pet and their premises (if appropriate) against fleas and ticks.
  • daily disposal of canine and feline feces and compliance with animal waste municipal laws and ordinances.
  • covering children’s sand boxes when not in use.
  • preventing predatory behavior (which leads to increased parasite exposure) by keeping cats indoors.
  • feeding cooked, canned or dry pet food diets.
  • washing or cooking vegetables for human consumption.
  • adequate hand washing after contact with anything contaminated with dog or cat feces. (**especially important for children of all ages as hand to mouth activities are more likely.)
  • wearing gloves when working in the garden as well as thorough washing of your hands wiwth soap and water when gardening work is complete.
  • preventing your pet from toilet bowl water drinking (important for prevention of the spread of human strains of Giardia, and for Cryptosporidia.

Environmental controls:

                The importance of proper disposal of animal waste cannot be overemphasized. While it is less labor intensive to leave pet waste disposal for someone else (ie. the property owner), all pet owners have an ethical responsibility to provide proper care for their pets and to not endanger the community at large. Proper leash control and allowing pet eliminations to take place at non-child play common community areas is and should be a common courtesy, particularly as the risk of infection to children is high. Eliminating free roaming dog and cat activity in high density population areas is also a customary community courtesy. Intestinal parasites prefer moist soils and the larval stages need to molt before they become infectious to people or pets. Similarly, Toxoplasma egges need air exposure before they will transform into a stage that is infectious for people and pets. Some parasite stages are immediately infectious to people including the tapeworm Ecchinococcus species, Giardia eggs and Cryptosporidium eggs. For more information you can go to http://www.capcvet.org.


Authored by Robert Z Berry, DVM


Source material: Companion animal Parasite Council and CAPC articles by Eugena Marcus, MD, Leonard C.  Marcus, VMD, MD, Kevin R Kazacos, DVM, PhD, and CDC source material.