Parasites, Your Pet and Your Family

Spring is here!  As the weather warms away winter’s chill, it also brings with it a higher threat of parasites in southern Ontario.    Examples of some parasitic infections that are in our area include roundworms, hookworms, Giardiaˆ, and heartworm.

Parasites can affect your pet in a variety of different ways, ranging from simple digestive upset to potentially life threatening conditions if left untreated.  As well, some parasites are zoonotic, which means they can be passed from animals to people.  It is important to screen your pet regularly for parasites in order to treat the parasite appropriately, prevent transmission of some parasites to the human members of your family, and to create prevention strategies.

Intestinal Parasites

Common intestinal parasites in Ontario include roundworms, coccidia, hookworms and Giardia.    Most intestinal parasites are spread through fecal-oral transmission.  Roundworms are the most common intestinal infection in dogs and cats, with some studies showing up to 25-30% of all dogs and cats actively infected.[1]  Roundworms can also pass to puppies and kittens across the placenta in pregnancy, and in small numbers in the milk when nursing.  Therefore, at birth many puppies and kittens are infected.  Untreated, intestinal parasites can cause ill thrift, poor hair coat, a failure to gain weight, diarrhea and/or vomiting.  Furthermore, it is also important to note that roundworms can be passed to people by accidentally ingesting small amounts of fecal material.  Cleaning up fecal matter in the yard, hand-washing, testing and deworming your pet as appropriate are the best modes of prevention!


Heartworm is a parasite in which the adult stage of the worm lives mainly in the pulmonary arteries (large blood vessels in the lung) and the heart.[2]  Larval stages can be found in the bloodstream.  Heartworms can cause heart failure, which is why this is such a serious, potentially fatal disease.  Transmission occurs through the mosquito:  a mosquito that bites an infected dog can pick up larvae in the bloodmeal. The larva then undergoes a developmental stage in the mosquito, and when the mosquito bites another dog it can transmit the larvae while feeding.


Prevalence of heartworm disease varies over North America, and is directly related to the mosquito population.  As such, warmer climates such as the southern United States have a very high prevalence of heartworm disease as the transmission period is year round.  In Ontario, we have a limited transmission period from June through November. (Winter is good for something!)  While preventative medication has been extremely effective in limiting heartworm disease in our pets, we will likely never be free of this parasite in our area because of a wild dog population that will serve as a reservoir for our pets.  In 2010, there were 431 dogs reported with heartworm, up from 268 in 2002.[3]  The focus of infection in our province continues to be in southwestern Ontario.

 Diagnosis, Treatment and Prevention

Luckily, effective treatment and prevention of parasites is available through your veterinarian.  Intestinal parasites are detected through examination of feces.  This should be performed yearly as part of your pet’s wellness examination.  Many parasite infections in adult animals can be completely asymptomatic, and detection is important to prevent transmission  and illness.  An effective medication can then be selected for the particular parasite.

Heartworm is a disease in which we are fortunate to be able to prevent.  Prevention is advantageous to treatment, as the treatment of heartworm disease requires use of an expensive drug with many serious side effects. Alternatively, preventative medications are very safe, very effective and can often be used to manage other parasitic infections as well, such as fleas, ticks, and some of the aforementioned intestinal parasites.  Once a blood sample has been obtained to determine that your pet does not carry the heartworm parasite, options for prevention are numerous, and can be tailored to your pet’s specific lifestyle and health concerns.  These can include oral and topical formulations.  Your veterinarian can guide you on which medication is best for your particular pet!



[3] Slocombe, Heartworm disease in Canada (2010)
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