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Zoonosis – Pets and Kids

What completes the image of contemporary family life better than a family pet? Many families with young children also have dogs or cats living in their homes. While pets and kids have long since made a natural pairing, it is important for parents to be aware of the possibility that children may catch certain bugs from their furry friends. It needs to be underscored that although the risk for disease is low, and often the consequences of infection are mild, the potential for cross-species illness (also known as “zoonosis”) does indeed exist. Awareness, coupled with responsible pet ownership, can make a significant impact on disease risk.

Everybody who has brought a new puppy or kitten into the vet’s office for their first visit is well aware that these young animals are highly susceptible to intestinal parasites, or “worms.”

In fact, ‘deworming’ programs make up a large component of the routine health care protocols for animals of this age. Which worms are your pets most likely to encounter, and of those, which are potentially transmissible to kids? More importantly, what can you do about them? Read on to find out.


Also known as ascarids, roundworms are among the most common intestinal parasites of dogs and cats. They are acquired by a number of different routes, most notably through the ingestion of eggs in the feces of other infected animals and cause diarrhea and poor growth, especially in young animals. The risk of transmission increases wherever pets are exposed to high animal traffic areas, such as dog parks, where the hearty roundworm eggs accumulate in, and therefore heavily contaminate, the environment.

So why should parents be concerned? Although they are not immediately infective in fresh feces, ingestion of so-called larvated eggs (that is, eggs that have become infective after about 2 weeks in the open) can cause infection in children, even though humans are not roundworms’ natural hosts. It is important to state that most cases of roundworm in kids have no serious symptoms at all. However, two very notable, yet rare, exceptions exist, referred to as Visceral Larval Migrans (VLM) and Ocular Larval Migrans (OLM).

VLM describes the syndrome associated with the aberrant migration of the roundworm larval stages as they wander from the gut, where they developed after being ingested as eggs, through major organs of the body such as the lungs or brain. The nature of and degree of illness that manifests depends entirely on the location and extent of the organ damage caused by the larval stages of the worm. It appears that children under the age of five are most often affected, especially if they have close contact with puppies and are known to be in the habit of eating dirt or soil. OLM refers to the clinical manifestation of aberrant larval migration through the eye and optic nerve. OLM is a very serious illness that can sometimes lead to blindness. In contrast to VLM, children 7 to 8 years old make up the bulk of OLM cases.

What can parents do to prevent this potential zoonosis? Firstly, routine monitoring of the family pet for all intestinal parasites, not just roundworms, is a key step in preventing problems for everyone concerned. Fecal exams should be performed by your vet 2 to 4 times yearly during puppyhood and 1-2 times a year for adult pets. Equally, if not more important, is the owner’s responsibility to deworm all pets in the house with antiparasitic medication. Talk to your vet about which medications and treatment or prevention regimens best fit the needs of you and your pet. Since eggs need time before they become infective in the environment, it is imperative that all pet owners pick up after their animals defecate – immediately. Lastly, avoid having kids play in sandboxes (or “litter boxes” from the point of view of outdoor cats) or in areas likely to be highly contaminated with pet feces. And by all means, tell your children that soil or gravel is never to go in their mouths! What may be common sense to you, may not be to a toddler.


Hookworms are another intestinal parasite, with different species of worms affecting dogs and cats. Infection in puppies and kittens occurs primarily by egg ingestion, but other routes are possible. Hookworms can cause life-threatening anemias in young animals due to their blood-sucking behaviour in the gut. Humans can ingest their eggs or the adult worms themselves can penetrate skin, causing mainly a syndrome known as Cutaneous Larval Migrans (CLM). As was the case with roundworms, hookworm larvae that develop can travel aberrantly around the body, causing tissue damage. With hookworms, the migration is through skin, creating linear lesions just below the skin surface. CLM is a phenomenon more closely associated with tropical climates. Therefore, it is more common in the southern U.S. and the Carribean. Usually, these infections go away on their own with time.

Prevention is addressed with an approach similar to roundworms.


Giardia is another relatively common cause of diarrhea and weight loss in young animals, especially puppies. Infection occurs when pets drink water or eat food contaminated with parasitic cysts from fresh feces of infected animals. Drinking from dirty puddles is commonly linked to Giardia cases. Giardia is the most common intestinal parasite of people in North America, causing a watery diarrhea associated with having been drinking water from a contaminated source. What is interesting about this parasite, however, is that humans are the major source of their own species of Giardia infection, but can also get sick from animal-derived species of the bug.

As with the other intestinal parasites, frequent, routine monitoring of poop samples from puppies under 6 months of age and yearly checks in adults are a good idea in homes with children. Hand washing is also recommended in light of the oral route of transmission. Talk to your vet about specific medications used to treat overt cases of Giardia diarrhea.


Toxoplasma gondii is a protozoan parasite of cats. Cats can eat egg-like forms (or “oocysts”) from other cats’ feces or can ingest cyst-like forms from the muscle tissue of other animals. Most infections in cats show no associated clinical signs! The life cycle of the organism is complicated, but the bottom line is that newborns, pregnant women, and immunocompromised people can become seriously ill if they ingest life stages of the parasite. Pregnancies can be lost entirely or birth defects may occur.

Prevention of toxoplasmosis focuses on very specific guidelines. At-risk individuals should not come into contact with their cats’ litterboxes, since it is the feces that can contain oocysts. Frequent litter changes (by non-susceptible family members) are of the utmost importance, because oocysts take time to become infective. Fresh feces are not yet infective, so should be removed from the house immediately, before they are. Gardening should be avoided, since many outdoor cats bury their feces in soil.


This coccidian parasite is uncommon. Although most dogs shedding the organism show no signs of illness, it can cause bloody diarrhea in young puppies following the ingestion of infective egg-like forms in feces. In human, cryptosporidia can also cause profuse watery diarrhea that usually goes away on its own. Due to their still-developing immune system, children are more likely than healthy adults to become infected.

Prevention of cryptosporidian infection is centred on minimizing contact with pets’ feces. Hand washing is very important. Talk to your vet about specific strategies for treatment.

The Bottom Line

Pet owners and parents have a lot in common: both groups want to keep their loved ones happy and healthy. With a little background knowledge and a healthy dose of preventative vigilance, pets and children can really be one big happy family.

By Rebecca Greenstein – writer

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