Common Worms in Dogs - Types and Symptoms

The most common internal parasites in dogs are what are commonly known as worms. They can be roundworms (nematodes) or flatworms (cestodes or tapeworms).

These parasites usually settle in a dog's intestine, but can also affect the heart and lungs. Let's look at which internal worms can infect our dog and how to prevent them, to keep them healthy and protected.

What are internal worms, or endoparasites?

A parasite is a living organism that settles in and feeds off another living organism, usually of a different, larger species, called the host. Internal parasites, or endoparasites, are the ones that live inside the animal, and the most common are what we commonly call worms.

The worms that parasitize dogs can be roundworms (nematodes) or flatworms (cestodes or tapeworms), and these parasites usually settle in the intestine, but can affect other organs such as the heart or lungs.

What are the main worms that can parasitize a dog?

There's a significant number of internal parasites that can affect dogs, and some are more important than others due to how common they are, how harmful they are to the host, or their potential to spread to people (zoonotic potential).

We can classify the main worms into two groups based on where they settle in the host:

• Intestinal worms: Toxocara canis, Echinococcus granulosus, Dipylidium caninum, Taenia spp.
• Non-intestinal worms: Dirofilaria immitis, or heartworm.

Most of these parasites are transmitted as eggs or larvae through feces, though Dirofilaria immitis spreads through mosquito bites.

Characteristics of Toxocara canis

Toxocara canis is a roundworm that usually settles in a dog's intestine, and its eggs are shed into the environment through feces.

In adult dogs, this parasite usually causes no symptoms, but in puppies it migrates from the intestine to the lungs and causes respiratory signs (coughing and nasal discharge). If the infection is heavy, worms in the stomach and intestine cause vomiting, diarrhea, and a bloated abdomen.

If a pregnant female is infected with Toxocara canis, her puppies can become infected before birth (in utero) or through her milk while nursing. Because of this, newborns should be dewormed (with specific puppy dewormers such as mebendazole or fenbendazole) at two weeks of age, repeating treatment every 15 days until two weeks after weaning. After that, monthly internal deworming is recommended for puppies until they're six months old.

Characteristics of Dirofilaria immitis

The disease caused by heartworms, or dirofilariasis, is a parasitic infection caused by Dirofilaria immitis. The adult worms live in the infected dog's heart and/or lungs, causing a slow-progressing disease that produces symptoms 5-7 months after infection: weight loss, difficulty breathing, persistent cough, and exercise intolerance.

This parasite is spread by mosquito bites, so coastal and humid areas (rivers, lakes) are at-risk zones, especially during the milder-temperature months (April-October).

Treating dogs that have already developed symptoms can be complicated, which is why preventing this disease is essential. If the dog lives in a risk area, they should be dewormed monthly with products that prevent infection by eliminating this parasite's immature stages before they reach the heart (milbemycin oxime).

If the dog occasionally travels to a risk area, they should be dewormed monthly while there, and the last treatment should be given 30 days after returning from an endemic area.

Characteristics of Echinococcus spp.

The importance of the parasite Echinococcus spp. lies in the fact that it can spread to humans through dog feces, causing a disease involving cyst formation, especially in the liver, which can be fatal (hydatid disease).

Dogs become infected by eating small rodents, offal, or the carcasses of infected animals (sheep, pigs, goats, horses), and this parasite doesn't usually cause symptoms in dogs.

Characteristics of Dipylidium caninum

Dipylidium caninum, or tapeworm, is a common parasite in dogs that lives in the intestine, and each worm can grow up to 60 cm (about 24 inches) long. It's made up of rice-grain-sized segments that break off and are passed out with the feces. These segments can move on their own and cause itching around the anus, leading the dog to drag their bottom along the ground.

This parasite is transmitted through infected fleas, when the dog bites at itself to scratch and swallows these insects.

How can we tell if our dog has worms?

Intestinal worms usually don't cause symptoms in adult dogs, and heartworm also produces no clinical signs until several months after infection, so it's hard to know if our dog is infected without a fecal or blood test.

Infection with any of these worms can be identified through a fecal test, except for Dirofilaria immitis, which requires a blood test for diagnosis.

How can we prevent our dog from getting internal worms?

Preventing internal parasites in our dog is relatively simple if we follow a few general measures and a proper internal deworming schedule.

• We should keep our dog from accessing rodents, slugs, snails, snakes they might hunt, animal carcasses, or raw meat and fish, since eating these can be a source of parasites.
• Dog feces should be picked up when they go to the bathroom outside, since the infective forms of these worms are usually shed into the environment along with feces and can survive there anywhere from a month to several years, infecting other dogs.
• Even if our dogs are properly dewormed and we pick up their waste, the problem is that it's hard to control stray cats' waste, which can be a source of parasites for dogs too. That's why dogs should be dewormed regularly, and, whenever needed, routine fecal tests should be run to confirm the deworming treatment is working.

What's the best internal deworming schedule for dogs?

There are two options for preventing intestinal worms in adult dogs:

• Run periodic fecal tests (monthly or quarterly), and only give deworming treatment if the fecal test shows the presence of a parasite.
• Give deworming treatment on a regular basis — that is, deworm our dog internally every three months. In higher-risk settings (dogs living with young children, hunting dogs, dogs living in areas endemic for Dirofilaria, etc.), monthly internal deworming is recommended.

What internal dewormer should I give my dog?

Ideally, our dog should be dewormed internally with broad-spectrum products capable of eliminating both roundworms and flatworms. A good treatment would be one effective against both intestinal worms and Dirofilaria immitis (milbemycin oxime and praziquantel).

IMPORTANT! Before giving any deworming treatment, check with the vet, since certain products can't be given to puppies, pregnant females, or animals with specific health conditions.

Bibliography ESCCAP Guideline No. 1; Worm Control in Dogs and Cats