Dogs and worms, unfortunately, often go together. The range of parasitic worms found in dogs includes roundworms, hookworms, tapeworms, whipworms and heartworms. While some worm infestations may be asymptomatic (i.e. not have any symptoms), others can be fatal.

Problems with worms often affect a dog in puppyhood. One of the most common ways puppies can be infected with worms is through their mother’s milk. Worm treatment within the first months of life ensures that a pup’s health is not threatened.


The worms that most often infect dogs are hookworms. Hookworms infect approximately twenty percent of dogs in the U.S. Hookworm larvae are passed through stool and live in the soil.

Transmission of hookworms occurs by contact with larvae-infested soil either through ingestion of the larvae (intestinal) or by the larvae burrowing into the skin (cutaneous). Nursing mothers infected with hookworms can also pass larvae to puppies through their milk. Be aware that hookworm infections aren’t limited to cats and dogs: they can also infect people who walk barefoot on infested soil.

Inside the dog, hookworms seek out the small intestine. Using their hook-like teeth, hookworms attach to the lining of the small intestine where the feed off the blood of the host dog (or person).

Symptoms of hookworms in the dog include:

  • diarrhea
  • loss of energy
  • loss of strength
  • anemia (in severe infestations)
  • sudden death (severe infestations in puppies).

A veterinarian diagnoses hookworm by examining the feces for worm eggs. Treatment includes oral medications and monthly preventive pills.


When many puppy owners think of dogs and worms, they are thinking of roundworms, the intestinal parasites often found in puppies. Roundworms are about four inches long and about the same diameter as a piece of spaghetti.

Dogs contract roundworms by ingesting the worms’ eggs, which are generally found in infested soil, infected feces, infected rodents or maternal milk.

Roundworm eggs hatch into larvae inside the dog. The larvae migrate to the dogs’ small intestine where they survive on some of the food intended for the dog. Although adult dogs can contract roundworm, they rarely fall ill from it. Conversely though, roundworm infections for puppies are considered serious and should be treated immediately.

Symptoms of roundworms in puppies include:

  • bloated “pot bellies”
  • blood or mucus in stool
  • diarrhea
  • lack of energy
  • loss of appetite
  • slow growth
  • vomiting (often containing a worm).

A severe infection of roundworms can cause an intestinal blockage, which can be fatal to puppies if left untreated.

Because roundworms are so common, worm treatment is offered to all puppies. Treatment for roundworms comes in several varieties of oral medication and usually requires multiple doses. Dogs with recurring worms may benefit from monthly preventive medicine.

Roundworms are diagnosed by examining fecal samples for worm eggs. Humans who come into contact with infected feces are also susceptible to contracting roundworms.


Whipworms are transmitted when dogs ingest worm eggs from infected soil. Whipworms get their name from their whip-like tails, which they use to attach themselves to the dog’s large intestine.

Unlike most other intestinal worms in dogs, a hookworm infection does not always produce symptoms. When symptoms are present, they include anemia and dehydration. Diarrhea that contains blood or mucus in the stool can also occur.

A fecal sample can reveal whipworm eggs. Repeated fecal examinations may be required to diagnose whipworms, however, as the worms produce fewer eggs than roundworms or hookworms. Oral medications are available for both worm treatment and preventive care.


Sometimes fleas are responsible for worm transmission to dogs. Some species of tapeworm develop as larvae in fleas. When the dog ingests infected fleas, he may become infected with tapeworms. Other types of tapeworm use different transmission routes, including eggs in soil or in infected rodents.

Tapeworms are long, segmented worms that live in the small intestine. Diagnosis is usually made when segments of worm (often still wiggling) are detected in fecal matter or around the dog’s anus.

Tapeworm infection is not usually serious. Large infestations can cause weight loss, general abdominal discomfort, itching around the anus and vomiting. Tapeworms are treated with a dose of praziquantel, and can be partially controlled with proper flea control and minimal contact with rodents.


One of the most serious worm infections in dogs is heartworms. Heartworms are found in all fifty U.S. states, but are most common in areas around the Atlantic and Gulf coasts and along the Mississippi river and its tributaries. In addition, any area with large mosquito populations is likely to have heartworms.

Mosquitoes are responsible for transmitting heartworms. The mosquito picks up heartworm larvae by biting an infected dog. Once ingested by a mosquito, the larvae change form in readiness for infection. When the mosquito bites another dog, it transmits the larvae.

Once heartworms infect a dog, they settle in the chambers of the heart and in the lung arteries. Heartworms can grow to as much as fourteen inches long and can produce thousands of live young every day. Heartworm larvae can live in a dog’s blood for up to three years.

Heartworms cause a great deal of damage to the heart, lungs and other internal organs. Initial symptoms include a loss of strength and energy, lethargy and coughing with exercise.

Severe infestations of heartworms can cause sudden death. The worms can block arteries and heart chambers, and can cause pulmonary hypertension and heart failure.

Diagnosing and Treating Heartworms

Heartworms are detected through blood tests that reveal either heartworm larvae or antibodies produced in response to the heartworm infection.

Treatment of heartworms is essential to prevent death. However, secondary complications, such as heart, lung or kidney damage, must be stabilized before heartworm treatment begins.

Heartworms are killed with immiticide injections twice a day for two days. After treatment, the dog must be inactive for several weeks as activity may dislodge the dead worms, which can travel to the lungs and result in sudden death due to pulmonary embolism.

Three to four weeks after the initial immiticide injections, dogs are treated daily for a week to kill heartworm larvae. Blood tests are then repeated to ensure all heartworms have been killed, and a follow up exam is scheduled for a year after treatment. Preventive medication for heartworms is available and is the best option for heartworm endemic areas.