Internal Dog Parasites: Heartworm, Hookworm, Ringworm, Roundworm and More

Parasites in dogs take many forms, but they all have one thing in common: sooner or later their presence will almost always have an impact on your pet’s health or comfort. Parasites can cause anything from mild irritation to serious illness. According to the Companion Animal Parasite Council (CAPC), many dogs will be infected with parasites at some point in their lifetime.

In This Article

• • Understanding Heartworm Infection
  • How Hookworms Harm Dogs
  • Keeping Dogs Safe From Roundworms
  • Tapeworm Risks and Prevention
  • What to Know About Whipworms
  • How Liver Flukes Infect Dogs
  • Protecting Dogs From Coccidia
  • Understanding Giardia in Dogs

The first step in protecting your dog from internal parasites is year-long prevention with veterinary-recommended preventatives, such as heartworm pills (which often contain ingredients that kill other internal parasites), and annual exams. “Veterinarians can monitor for parasites such as tapeworms, liver flukes, and emerging pathogens that may not be as common but can still have significant impacts on the health of your pet,” says Dr. Heather Stockton Walden, president of the American Association of Veterinary Parasitologists and Associate Professor of Parasitology at the University of Florida College of Veterinary Medicine.

Another step in prevention is to identify potential parasitic infection risks in areas where you plan to travel with your dog, Dr. Walden advises. “As [the] climate changes, we can see different species occupying new geographical areas,” she says. For example, liver flukes were found for the first time in Colorado last year. “Rat lungworm, Angiostrongylus cantonensis, is another parasite carried in snails, some frogs, crustaceans, and lizards, that is relatively new to the continental U.S., to date mainly found in southeastern states, but has been in Hawaii for decades. Human cases have been reported in Florida, Texas, and Louisiana.”

Here is an overview of the most common internal parasites dogs can get, how they work, and the problems they may cause.

Worms in Dogs

Heartworm

Heartworms enter a dog’s bloodstream from the bite of an infected mosquito. The worms mature in the dog’s heart and can grow up to an amazing one foot in length. Then, they mate and produce offspring. Adult heartworms live in the heart, lungs, and associated blood vessels of an infected dog.

This inflammation in the dog’s arterial wall disrupts blood flow, so the heart must work harder. Once blood flow slows sufficiently, a heartworm-infested dog may develop a mild, persistent cough, become fatigued after only mild exercise, and suffer from a reduced appetite. If left untreated, heartworm disease may cause damage to the heart, lungs, liver, and kidneys, which can result in heart failure and death.

Diagnosis

Though veterinarians can look for some typical symptoms, most dogs harboring this parasite don’t present clinical symptoms prior to the worms being detected via screening tests. The American Heartworm Society recommends that blood tests be conducted annually. While these tests can detect a single worm in a dog’s body, they can only detect the presence of their larvae called microfilaria, which can only be produced by adult female heartworms, so timing is very important. There are also other tests to determine the presence of heartworms, and your vet can walk you through them if they’re an option for your pet.

Transmission and Human Risk

According to the U.S. Food and Drug Administration, heartworm disease cases have been reported in dogs in all 50 states. However, it’s most common along the Atlantic and Gulf Coasts, from the Gulf of Mexico to New Jersey, and along the Mississippi River and its major tributaries. Luckily, heartworm isn’t usually transmitted from a dog to a person.

Treatment and Prevention

Treatment for heartworm can be expensive and hard on the dog and owner and must be administered by a veterinarian. In rare cases, surgery will be required to remove the worms. Luckily, there are many effective options for heartworm prevention. Most veterinarians prescribe monthly chewables, topical treatment, or an annual injection. Collies and certain other herding breeds have a genetically-based sensitivity to specific heartworm preventives, so speak with your vet and ask if they should test for the condition before prescribing a heartworm preventative. Prevention is critical, as Dr. Walden reports the new detection of the French Heartworm, Angiostrongylus vasorum, in the U.S.

Hookworms

Hookworms live inside a dog’s digestive system and are acquired by puppies from their mother (through the placenta or when nursing), or by adult dogs swallowing the parasite’s eggs or having the hookworm burrow into the skin.

Diagnosis

Although difficult to see with the naked eye, these tiny worms can cause major damage. After attaching to the lining of the small intestinal wall, the hookworm feeds on the dog’s blood, which can cause anemia and inflammation of the intestine, making them especially dangerous to very small dogs and puppies. Your veterinarian can detect hookworms by examining a stool sample under a microscope.

Transmission and Human Risk

Hookworm larvae live in soil for weeks and can be ingested when the dog comes in contact via eating infected soil, sniffing dog feces, lying on infected ground, or by licking their feet. Humans can be infected when hookworms penetrate their skin, usually if they walk barefoot or garden in areas where the soil is contaminated with infected dog feces.

Treatment and Prevention

As with a number of intestinal parasites in dogs, diarrhea and weight loss are common symptoms of infection, along with pale gums, fatigue, and dry coat. Keeping your dog’s environment clean can prevent infection.

To prevent hookworms, use an appropriate de-wormer and repeat treatment after 3–4 weeks. Usually oral drugs are administered twice, several weeks apart, to effectively kill the adult worms. Many of the commonly used heartworm preventatives also offer protection against hookworms.

Roundworms

Roundworms are an extremely common parasite in dogs, especially in puppies. Symptoms include coughing, vomiting, diarrhea, malnourishment, dull coat, and sometimes a potbelly and even pneumonia (verminous pneumonia).

Diagnosis

The worms look like white, firm, rounded strips of spaghetti, one-to-three inches long, and can sometimes be seen in the stool.

Transmission and Human Risk

Puppies are most at risk from being infected while in utero or nursing. People are at risk for contacting roundworms. According to the CAPC, people who contact infected soil or dog feces can be infected, and the worms can cause serious symptoms in the eyes, lung, head, neck, and heart.

Treatment and Prevention

The easiest way to prevent roundworms from also infecting humans is to deworm puppies before the worms become adults and produce eggs. Eggs are extremely sticky and can survive for years. Veterinarians can administer effective deworming drugs, which need to be repeated at least once in a period of 3–4 weeks. Annual fecal exams are also essential. Again, many of the more common heartworm preventatives also offer protection against intestinal roundworms.

Children are especially vulnerable if they put a dirty hand in their mouths or play in an infected sandbox. Carefully cleaning up dog feces and covering sandboxes helps prevent the worms from spreading.

Tapeworms

Tapeworms are long, flat worms that are made up of segments, and each segment can reproduce. There are several kinds of tapeworms that affect dogs, some carried by fleas and others by small rodents.

Diagnosis

An infected dog may lose weight, have occasional diarrhea, and scoot their bottom on the ground. You can see flat segments of the worms that look like grains of rice around the dog’s anus or in their stool. Puppies may become anemic or have a GI blockage.

Transmission and Human Risk

The worms are ingested via a host, such as a flea, that is harboring a tapeworm egg and attaches to the dog’s intestines. Some types of tapeworms can cause serious illness in humans, but these are rarely found in the U.S.

Treatment and Prevention

If your dog is infected, your veterinarian will administer medication by injection or orally, which is known to be a highly effective treatment option. But remember, the best protection against tapeworms is to the keep your dog free of fleas and away from dead animals and garbage.

Whipworms

Whipworms are often less than one inch long and look like a fine piece of thread. These parasites live in the cecum and large intestines, causing irritation and inflammation of the intestinal lining. Weight loss and blood in the stool are the primary symptoms of a whipworm infestation, though severe infections can also cause bloody diarrhea.

Diagnosis

Whipworms live in the dog’s large intestine, but unlike other parasitic worms they’re very difficult to spot in a stool sample, so repeated fecal samples may be necessary. A telltale sign, though, is stool covered in mucous, usually at the tip.

Transmission and Human Risk

When dogs lick or sniff contaminated ground or dog feces, they can ingest whipworms. The eggs can live in the ground for several years. People can’t get whipworm.

Treatment and Prevention

Though whipworms are rarely a cause of death, an afflicted dog will need to be treated with a dewormer. Prevention involves promptly removing dog feces from your yard and being diligent about having your veterinarian conduct an annual fecal exam.

Liver Fluke

Liver fluke is a flatworm parasite that inhabits snails and is less commonly seen than the internal parasites listed above. Risk of contamination of dogs is increasing. Originally found in Texas and other Gulf Coast stares, liver fluke was recently found in the Colorado River.

Diagnosis

Testing will first involve looking for eggs in a fecal sample, sometimes followed by bloodwork, X-rays, or ultrasounds.

Transmission and Human Risk

When a dog swims in a body of freshwater containing infected snails, the blood parasite can move to the dog by penetrating the skin. The adult fluke’s eggs then get into the dog’s lungs, spleen, liver, and heart, causing canine schistosomiasis, which affects the liver and intestines. If untreated, it can be fatal.

“The fluke they found in Colorado was Heterobilharzia americana; it’s a schistosome and requires snails for transmission,” Dr. Walden explains. “This is a prime example of parasites occurring beyond ‘historical borders,’ and this is a zoonotic parasite that can cause swimmer’s itch in humans. If dogs become infected with this parasite in endemic regions and travel to other areas of the country, they can spread the parasite if a suitable host is present.”

Treatment and Prevention

The best way to prevent serious illness is to keep your dog from swimming in any location that may put them at risk. Treatment options for affected dogs include antiparasitics, antibiotics, and anti-inflammatory drugs.

Non-Worm Internal Parasites in Dogs

Coccidia

Coccidia, a single-cell, non-worm parasite, is commonly found in the gastrointestinal tract of dogs. The CAPC cites surveys that show coccidia present in up to 21% of dogs in certain areas of the U.S. What makes them particularly dangerous is that they can infect a dog before they appear sick. It may not be clear that the dog is carrying the parasites until they show signs of stress or if another immune-compromising factor arises. Diligent sanitation practices are an important way to stave off these parasites.

Diagnosis

The condition can be difficult to diagnose because adult dogs don’t always display any symptoms. A fecal sample may not detect coccidia early on in an infestation and sometimes must be repeated. Veterinarians sometimes use blood tests for diagnoses.

Signs of an infection in puppies include diarrhea, vomiting, weight loss, lethargy, and dehydration. In rare cases, an infestation can be fatal.

Transmission and Human Risk

Coccidia eggs are present in the stool of an infected dog and can spread when other dogs ingest an egg or an infected animal, such as a mouse. However, the adult parasite isn’t transferred from one dog to another or to humans. It’s found more frequently in puppies because they may acquire it from their littermates or their mother.

Treatment and Prevention

Treatment includes a prescription oral medication, which is often an antibiotic or an antibiotic couple with another deworming medication. Severe cases may require other treatments such as fluids, anti-nausea medication, and a bland diet. Good hygiene is imperative in getting rid of coccidia, so you should also shampoo the dog and be sure to dispose of dog feces right away.

Giardia

Giardia is found throughout the U.S. and is a pervasive protozoon. Symptoms include diarrhea, stomach pain, nausea, and vomiting, but infected dogs might not display any symptoms. Dogs with weak immune systems and young dogs are more likely to experience severe symptoms.

Diagnosis

Giardia is sometimes difficult to diagnose based on stool samples because the cysts are not shed into every stool. However, a veterinarian can also examine a stool sample for proteins produced by the parasite.

Transmission and Human Risk

Transmission occurs when dogs ingest feces-contaminated soil, water, or food. Although rare, giardia can pass from dogs to humans, but normally humans are infected by a different type of giardia.

Treatment and Prevention

Effective treatment includes veterinary-prescribed anti-parasite medication, usually metronidazole, fenbendazole, or a combination of both, for a period of 3-8 days, depending on the medications used. Thorough cleaning indoors and outside the household is also imperative. Cornell University’s Canine Health Center recommends taking the following steps to prevent the infection and spread of giardia:

• Practice good hand hygiene
• Disinfect the environment
• Dispose of feces promptly
• Bathe the dog at the end of treatment to reduce the spread of Giardia cysts
• Prevent dogs from ingesting potentially contaminated water, food or soil