ROUNDWORMS IN DOGS & PUPPIES

 
(Photocredit: Morguefile.com)

TOXOCARA CANIS AND TOXOCARA LEONINA: ROUNDWORMS OF DOGS AND PUPPIES

There are two species of roundworms affecting dogs and puppies: Toxocara canis and Toxascaris leonina.  While T. canis infects only dogs, T. leonina is also able to infect cats and kittens. Treatment protocols for both worm species is the same but if there are feline household members, it may be useful to specifically identify the roundworm species involved so as to determine which pets are potentially at risk. We will cover each species of roundworm separately for, even though treatment is the same for each, their biology is different.

TOXOCARA CANIS

Toxocara canis is the most common roundworm of the domestic dog and it is not able to infect cats. Its presence can go completely without symptoms though more likely it is going to create some degree of diarrhea and possibly vomiting or general un-thriftiness in its canine host. Its life cycle is somewhat complicated, as we are about to see.

HOW INFECTION OCCURS:

Note: cats cannot be infected with Toxocara canis but humans can. Click here for details.

LIFE AS A ROUNDWORM:

Toxocara canis has one of the most amazing life cycle in the animal kingdom. It is helpful to understand this life cycle if effective treatment and prevention are to be pursued.

(Photocredit for fresh egg: Joel Mills via Wikimedia Commons; Photocredit for developed egg: Flukeman via Wikimedia Commons; all the rest: original graphics by marvistavet.com)

Step One: Eggs Contaminate Environmental Soil: Toxocara eggs are passed in the host’s feces where they can be detected if a fecal sample is tested. Feces, and any worms eggs therein, are deposited on the ground where they are rained on, dried by the sun, stepped on etc. The worms are developing during this time and are not infectious to new hosts until they have developed for about a month. By that time, the original feces has long since melted away into the ground and is no longer evident. It is the dirt that contains infectious eggs. Toxocara eggs are famous for weathering harsh environmental conditions. Eggs can remain infective for months to years.

Note: Fresh feces is not infectious. Soil contaminated with feces is infectious.

Step Two: A Host Eats an Egg and the Larva Encysts: The egg containing what is called a “second stage larva” is picked up from the dirt by a dog or by some other animal. usually in the course of normal self-grooming. The egg hatches in the new host’s intestinal tract and the young worm burrows its way out of the intestinal tract to encyst in the host’s other body tissues. If the new host is a dog, the life cycle proceeds. If the new host is a member of another species, the larvae wait encysted until the new host is eaten by a dog.
	(original graphic by marvistavet.com)	(original graphic by marvistavet.com)

Step Three: The Larva Awakens and Migrates Through the Host: These second stage larvae can remain encysted happily for years. If the host is a puppy under age 6 months of age, the larvae mostly encyst in the host’s liver. In older dogs, the larvae encyst all over the body. When the time comes to move on, the larvae excyst and migrate to the host’s lungs where they develop into “third stage larvae.” They burrow into the small airways and travel upward towards the host’s throat. A heavy infection can produce a serious pneumonia. When they get to the upper airways, their presence generates coughing. The worms are coughed up into the host’s throat where they are swallowed thus entering the intestinal tract for the second time in their development. 

Step Four: Finally Back in the Intestine and Ready to Settle Down: Once back in the intestine, the larvae complete their maturation and begin to mate. The first eggs are laid about one week after the larvae have arrived in the intestine and finished molting into their adult stages (about 4-5 weeks after infection has first occurred). From here the cycle repeats.

WHY IS INFECTION BAD?

Roundworm infection can have numerous negative effects. It is a common cause of diarrhea in young animals and can cause vomiting as well. Sometimes the worms themselves are vomited up which can be alarming as they can be quite large with females reaching lengths of up to seven inches. The worms consume the host’s food and can lead to un-thriftiness and a classical “pot-bellied” appearance. Very heavy infections can lead to pneumonia as the worms migrate and, if there are enough worms, the intestine can actually become obstructed.

It should also be noted that human infection by this parasite is especially serious (see below). It is important to minimize the contamination of environmental soil with the feces of infected animals so as to reduce the exposure hazard to both humans and other animals. In other words, dog feces should be removed and discarded promptly before worm eggs permanently contaminate the local dirt.

HOW DO WE KNOW IF OUR DOG IS INFECTED?

Fecal testing for worm eggs is a must for puppies and a good idea for adult dogs having their annual check up. The Companion Animal Parasite Council recommends fecal testing 2-4 times during a puppy's first year of life and once or twice a year after that. Obviously, if there are worms present, they must be laying eggs in order to be detected but, by and large, fecal testing is a reliable method of detection.

HOW DO WE GET RID OF ROUNDWORMS?

Numerous deworming products are effective. Some are over the counter and some are prescription. Many flea control and/or heartworm prevention products provide a monthly deworming which is especially helpful in minimizing environmental contamination. Common active ingredients include:

• Febantel (active ingredient in Drontal plus)
• Pyrantel pamoate (active ingredient in Drontal, Strongid, Nemex, HeartgardPlus and others)
• Piperazine (active ingredient in many over the counter products)
• Fenbendazole (active ingredient in Panacur)
• Milbemycin Oxime (active ingredient of Sentinel and Trifexis.)
• Moxidectin (active ingredient in AdvantageMulti)

There are two important concepts to keep in mind about deworming. Medications essentially anesthetize the worm so that it lets go of its grip on the host intestine and passes with the stool. Once it has been passed, it cannot survive in the environment and dies.

This means that you will likely see the worms when they pass so be prepared as they can be quite long and may still be alive and moving when you see them.

The other concept stems from the fact that all the larvae in migration cannot be killed by any of these products. After the worms are cleared from the intestine, they will be replaced by new worms completing their migration. This means that a second, and sometimes even a third deworming is needed to keep the intestine clear. The follow-up deworming is generally given several weeks following the first deworming to allow for migrating worms to arrive in the intestine where they are vulnerable.

Do not forget your follow-up deworming.

(Photocredit for fresh egg: Joel Mills via Wikimedia Commons; Photocredit for developed egg: Flukeman via Wikimedia Commons; all the rest: original graphics by marvistavet.com)

Note: Toxascaris leonina can infect both dogs and cats alike.

FOR MORE INFORMATION:

The Companion Animal Parasite Council has put up educational sites for dog owners on roundworms at:

www.petsandparasites.org/dog-owners/roundworms

www.capcvet.org/capc-recommendations/ascarid-roundworm/

 Page last updated: 1/10/2020