Intestinal Parasites And Your Dog

A “positive” fecal test?!

It is not uncommon for a dog on an excellent parasite control product to still have a positive fecal test. Why? Because parasite eggs and immature stages are everywhere: in the soil, on grass, in dog-walking areas, by the lake or in other natural spaces where other dogs or wildlife are found. Common ways for eggs to be ingested, and your dog to become infected, are when your dog:

• licks his paws
• comes in contact with these microscopic eggs by picking up a contaminated ball, stick, or other toy
• licks or eats grass

Some parasites, like the hookworm (Ancylostoma caninum), have an immature life cycle stage that can penetrate through your dog’s skin (typically the paws). Due to differences in geographic region and lifestyles, some dogs have lower exposures to parasites, and others live in areas that are heavily contaminated with these microscopic eggs and immature stages. Many of the eggs can last for years in the environment.

Since maintaining a good quality of life for most dogs depends on exercise, fresh air and being outdoors, your dog’s exposure to parasite eggs cannot be controlled. What can be controlled (not “prevented,” which is a common misnomer) are the parasites in your dog’s digestive system. It is important to understand that immature stages of the parasite may migrate to “protected” areas of the body (like internal organs or muscles) where they can remain for years! Development of the parasite continues inside the dog’s body ending in migration to the digestive tract where they mature into adult worms and begin laying eggs. These eggs pass in the dog’s feces to the environment, which completes the life cycle, and explains a positive fecal test.

How the medication works: It is important to know that none of the monthly oral control products can kill all immature stages of the parasite, nor can they kill the ingested eggs. Monthly oral intestinal parasite control products work by killing the adult worms present in the digestive tract at the time of dosing. The medication peaks rapidly, kills the adult intestinal parasites it is labeled for, and effectively controls clinical disease under most exposure conditions. The medications have a short half-life, meaning they will be out of the dog’s system within days.

Post-dosing, it is possible for a dog to still harbor immature stages of the parasite or become re-infected from a contaminated environment. In these situations, it is possible for the parasite to complete maturation and begin egg shedding between monthly doses of medication. For that reason, it may be recommended to have a fecal test performed 7-10 days after the last dose of medication and that test should result in a negative (no parasites seen) outcome. If not, you and your veterinarian may begin investigating the possible reasons, for example: compliance, dosing, exposure, testing procedures, speciation of eggs.

Understanding parasite life cycles: Parasites (eggs or immature worms) can enter your dog’s body, mature to an adult, and begin laying eggs in less than a month! This time period (infection to egg production) is called the prepatent period. This short part of the life cycle may explain the majority of positive fecal results. Depending on when your dog became infected, parasites could mature into an adult worm just after deworming, so they may not be susceptible to elimination until the next month, which can also explain a positive fecal result.

Prepatent periods:
Hookworm (Ancylostoma caninum): 2-3 weeks¹
Roundworm (Toxocara canis): 2-4 weeks²
Roundworm (Toxascaris leonina): 8-10 weeks²
Whipworm (Trichuris vulpis): 74-90 days³

The good news is that year-round parasite treatment and control can minimize the impact of these parasites, and that we can all take an active role in minimizing the parasite burden in our neighborhoods by picking up dog waste which helps break the life cycle of many of these worms.

To learn more about intestinal parasites, please visit the following websites:
https://www.youtube.com/watch?v=7HNf4Sjswww
http://www.capcvet.org

References:
1. Companion Animal Parasite Council Recommendations: Hookworms (2015)
http://www.capcvet.org/capc-recommendations/hookworms
2. Companion Animal Parasite Council Recommendations: Ascarid (Also Roundworm, Also Toxocara) (2015)
http://www.capcvet.org/capc-recommendations/ascarid-roundworm
3. Companion Animal Parasite Council Recommendations: Whipworms (2015)
http://www.capcvet.org/capc-recommendations/whipworms