Inside the Vet's World - Battling the Bugs and Creepy Crawlies: An Explanation of Veterinary Anti-Parasitic

with Dr. Alexandria Horner DVM, MS, DACT

"My dog isn't a risk for parasites; we don't go anywhere but around the neighbourhood." This phrase, in some form or another, is prevalent during many of my consults and, unfortunately, just plain untrue! Each dog is at varying risk for internal and external parasites, and some of these parasites also carry a zoonotic potential, which affects us humans! But they can all acquire an infection in the right situation!

When left unchecked, parasites can wreak havoc on your pet's overall well-being. In severe cases, some parasites can cause further damage than their initial bite by transmitting several severe illnesses such as Rocky Mountain Spotted Fever and Lyme disease. Appropriate treatment and prevention with anti-parasitic medications can help prevent these complications from occurring, improving the overall quality of life of your companion. 

There are several categories of anti-parasitic medication, and some do provide overlap protection. The main categories include:

1.     Flea and Tick Preventatives

2.     Heartworm Preventatives

3.     Dewormers

Each category can then be further broken down into how each medication is administered: oral, topical, collars or injectable. Which one to use depends on the exposure and risk to each dog and, in some instances, breed and previous health history. 

Flea and Tick Preventatives

Flea and tick preventatives have multiple different administration styles and a wide range of different products available.

Oral medication is one of the most popular forms of flea and tick preventatives. This

category is largely made up from medications from the isoxazoline family. They can be seen in products such as Nexgard® (Afoxolaner) and Bravecto®  (Fluralaner). Isoxazoline's work causes paralysis and death of susceptible parasites such as ticks, fleas and mites. However, the medication is only effective once the fleas and ticks have bitten and fed. 

The next group are your topical medications, such as Frontline®  (Fipronil), K9 Advantix® (Imidacloprid, Permethrin, pyriproxyfen) and Revolution® (Selamectin). Each contains a different class of drug compared to our oral external parasite treatments. Frontline is not available in Canada but is available in the United States without a prescription from a veterinarian. It is one of the few flea and tick preventatives that can be safely administered to pregnant and lactating females. These medications work by causing dysregulation of specific neurologic pathways, ultimately resulting in the parasite's death. Most parasites will be repelled, though some still need to bite prior experiencing the medications effects.

Lastly, collars are impregnated with insecticides such as the Seresto®  collar. Seresto®  collars use the medication imidacloprid/flumethrin to repel and kill ticks/fleas through contact. 

Heartworm Preventatives

Mosquitoes that carry heartworm can be found worldwide, with increased incidence in Ontario, Quebec, and interior British Columbia. When an infected mosquito carrying microfilaria (the larval stage of heartworm) feeds, it passes these along to the next dog. The larvae will migrate to the heart, where they will develop to maturity. When mature, they can average 5-12 inches long and can be found within the chambers of the heart and the large blood vessels surrounding it. As one can imagine, having a heavy load of these parasites within the heart and its vasculature can result in chronic coughing, difficulty breathing, edema of the limbs and abdomen, fluid accumulation in the chest, weakness, exercise intolerance and collapse. Treatment is complex and expensive and requires the use of melarsomine, an arsenic derivative, to treat the infection. Therefore, early treatment before larvae reach maturity through preventative medication is the easiest and most cost-effective option. The medication used is under the drug class macrocyclic lactones, which includes products such as ivermectin, milbemycin and moxidectin. The main formulations are available as a monthly oral chewable (Heartgard®, Interceptor®, etc.) or as a long-lasting injection (ProHeart®). Macrocyclic lactones work by disrupting the parasite's nerve and muscle cells, resulting in paralysis and eventual death. 

Intestinal Parasite Prevention

The last group of parasites to discuss are the intestinal parasites, which include roundworms, hookworms and tapeworms. Roundworms are one of the most common intestinal parasites, and it has been estimated that 80% of newborn puppies in the United States will be infected at birth. These worms are spaghetti-like in their appearance, and infection occurs when dogs come in contact with eggs from the parasite. In some cases, roundworms migrate away from the gastrointestinal tract, encyst and then lay dormant in muscle and mammary tissue. This becomes a risk for puppies to contract this parasite shortly after birth as they become active shortly after whelping due to a drop in the dam's immunity, allowing them to be passed on to the puppies when nursing—making deworming protocols essential for young dogs! Medications used to treat these parasites include pyrantel pamoate, fenbendazole and macrocyclic lactones. 

Hookworms are common across North America, especially in regions with warm to moderate climates. Adult hookworms live in the small intestines, where they attach to the wall of the intestine and consume blood. If left untreated, it will eventually cause clinical signs associated with blood loss, anemia, weight loss, dry coat and tarry diarrhea. Similar to the roundworms, these parasites can be transferred to puppies at nursing.

Tapeworms are present across Canada. They are flat, segmented worms that live in the small intestine. There is often an intermediate host (rabbit, domestic herbivore, etc.) that a dog will consume that has infective cysts, resulting in infection. Clinical signs of tapeworm infection may include weight loss and diarrhea. Still, case reports are present with dogs presenting for "tumour-like" masses in the abdomen due to the presence of encysted tapeworms! The primary drug available to treat tapeworm in dogs is praziquantel, which can be found in several oral formulations, including Drontal®  and Interceptor Plus®. In conjunction with preventative medication, owners should also be mindful of other control measures, including avoiding feeding contaminated raw meat, preventing dogs from scavenging on hunted wild animals and controlling fleas. 

Choosing the Right Medication

When choosing which medication to use, it is best to consider the individual dog, their risk level, parasites and diseases prevalence in your area, as well as any current or future travel plans. For example, if your dog frequently travels to regions where heartworm prevalence and tick-borne diseases are common, then treating with good intestinal/heartworm prevention alongside an effective flea and tick product monthly would be recommended. To determine your dog’s risk to certain parasites it’s important to do your research and to use your veterinarian as a resource! I recommend checking the parasite prevalence map (petsandparasites.org), this site demonstrates which parasites are showing up in your area by province or state, providing the most up-to-date information to help you decide what your dog’s risk is and what class of medications you may need to prevent disease in your dog. Your veterinarian can be a valuable resource to understand the diseases present in your area and has the knowledge about potential adverse effects that can occur using your pet’s previous medical history and breed predispositions.

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K9Force (2025, February). CACIT Netherlands https://publuu.com/flip-book/199364/1605518/page/16

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