Parasite control

parasite-controlHow Do we Know if our Pet has Worms?

Q. How do we know if our pet has intestinal worms?

A. Many times an animal may have worms even though you see no evidence of it.

Roundworms (ascarids) are several inches long, look like spaghetti, and may

occasionally be seen in the stool or vomit of an infected pet; usually, though, you

will not see them.

Hookworms and whipworms are very small and virtually impossible to see in the

stool or vomit.

Segments of Tapeworms can be seen; they may appear as rectangular segments

moving around the anal area of the animal, or as white rice-like or cucumber

seed-like segments around the anus.

So basically, except for tapeworms, the best way to diagnose worms in a pet is to

have a fecal exam performed by your Veterinarian. In a fecal exam, we look for the

microscopic eggs of the worms. Eggs may not always be present in the stool, even

though a pet has worms. This is why regular deworming is performed even though

evidence of worms may not be present. Fecal exams should still be performed

regularly to detect the presence of species of parasitic worms, which may not be

killed by our usual wormers.

Roundworms, often called ‘ascarids,’ are the most common parasite of the digestive tract in dogs and cats. Most puppies are infested with roundworms and when we look at the life cycle, we will understand why. All of these roundworms are widely distributed in North America. They are of considerable importance in young animals and in kennels. Because they can cause disease in humans, they are also very important to our health as well.

Even birds and reptiles can have roundworms, although they are a different genus and species than those found in dogs and cats.

The adult roundworms all live in the small intestine of the host and their eggs look very similar. All the roundworms are prolific and an infested animal can pass millions of eggs in the feces each day. The roundworms differ, however, in their life cycles. These differences are very important when we look at how we can eliminate these parasites from our pets.

What are the life cycles of the roundworms in dogs and cats?

There are 3 types of roundworms that affect dogs and cats and each has transport hosts.


Primary Host

Transport Host

Toxascaris leonina Dog, cat, fox, and other wild carnivores Small rodents

Toxocara canis Dog, fox Small rodents

Toxocara cati Cat Small rodents, beetles, earthworms

T. leonina: Of the roundworms, T. leonina has the simplest life cycle. After an animal ingests infective eggs, the eggs hatch and the larvae mature within the small intestine. The adult female worm lays eggs which are passed in the feces. The eggs become infective after remaining in the environment for at least 3-6 days. Animals become infected if they eat something contaminated with infected feces.

Mice can act as intermediate or transport hosts of T. leonina. The rodent ingests the eggs, the eggs hatch, and the larvae migrate through the tissues of the rodent. If a carnivore eats the mouse, the larvae are released in the digestive system of the carnivore and develop into adults in the intestine.

T. canis: Roundworms of the species T. canis have a more complicated life cycle and a very effective way of making sure its species will be passed from generation to generation. Let us take a look.

Most puppies are born infected with T. canis.

An animal can acquire a T. canis infection several ways: ingestion of eggs, ingestion of a transport host, or by larvae entering the animal while in the uterus or through the milk. First let us follow the ingestion of infective eggs.

Ingestion of eggs: After a dog eats the eggs, they hatch, and the larvae enter the wall of the small intestine. The larvae migrate through the circulatory system and either go to the respiratory system or other organs or tissues in the body. If they enter body tissues, they can encyst (become walled off and inactive). They can remain encysted in tissues for months or years. This is the migration most commonly seen in older dogs. In very young puppies, larvae move from the circulation to the respiratory system, are coughed up and swallowed. The larvae mature into adults. The adult worms lay eggs which pass out of the animal in the feces. The eggs need to remain in the environment 10-14 days before they become infective.

Ingestion of transport host: If an animal ingests a transport host having encysted larvae, the larvae are released when the transport host is digested and mature in the intestine.

Larvae through the uterus: A pregnant dog that has T. canis encysted larvae in her tissues can pass them to her puppies in two ways. The larvae that were dormant in her tissues can migrate through the uterus and placenta and infect the fetal pup. This is called in utero transmission. The larvae enter the lungs of the fetal pup. When the pup is born, the pup will cough up the larvae and they will mature in the pup’s intestine. This is why so many puppies have roundworms – they are infected before they are born.

Larvae through the milk: Larvae can also enter the female’s mammary tissues. The puppies can become infected through the milk while nursing. The swallowed larvae mature in the pup’s intestine. If the larvae are passed out in the pup’s feces before they can mature, they can infect the mother when she licks her pup.

About 4 weeks after a dog eats an infective egg or a puppy with a prenatal infection is born, the adult worm has matured in the animal’s intestine and the next generation of eggs is passed.

T. cati: In some ways, the life cycle of T. cati is similar to that of T. canis. The infective eggs are swallowed. The larvae hatch and penetrate the stomach wall. From there the larvae migrate through the liver, other tissues, and lungs. Some larvae may encyst in the tissues. Larvae that enter the lungs are coughed up and swallowed. The larvae mature in the stomach and small intestine, and the adult female worms start laying eggs.

Dog nursing her puppiesUnlike T. canis, in utero infection does not occur with T. cati. However, during the perinatal period dormant larvae in the queen can start to migrate and can be passed from the mammary tissues to the young kittens through the milk.

Mice can serve as intermediate hosts for T. cati in a manner similar to the other roundworms. Cats can also become infected by eating other transport hosts such as earthworms and beetles.


Remember, for all roundworms, the eggs need to remain in the environment for days to weeks before they become infective. Larvae encysted in the host’s tissues can remain dormant there for the host’s lifetime.

How do roundworms may cause disease in pets?

In the intestine, roundworms absorb nutrients from what the animal eats, interfere with digestion, and can damage the lining of the intestine. Animals with mild infestations of roundworms may not show any signs of disease. Animals with more severe infestations may be thin, have dull hair coats, and develop a pot-bellied appearance. Some may become anemic and have vomiting, diarrhea, or constipation. Rarely, in severe infestations, the roundworms can cause obstruction of the intestines. A cough may be observed in some animals due to the migration of the larvae through the respiratory system. In young puppies the migration of the T. canis larvae in the lungs can cause pneumonia.

How are roundworm infestations in pets diagnosed?

Adult worms are usually 3-4 inches long, although some T. canis roundworms can be up to 7 inches. Adults may be seen in the feces or vomit. The worms are round on cross-section (hence the catchy name) and look a bit like spaghetti.

An individual T. canis female worm can produce 200,000 eggs per day.

The eggs are identified in the feces. A flotation solution is used to separate the eggs from the rest of the stool, and the resulting sample is examined microscopically. Very slight differences in appearance of the eggs of the three roundworms can allow experienced persons to distinguish between them.

Surprise! Occasionally we will see eggs of T. cati in dog stool. How could that happen? The dog has made a raid on the cat’s litter box and has eaten cat feces. The eggs pass through the digestive system of the dog and are found in its stool.

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