What Parasites Can Dogs Get?

Dog owners who fail to protect their animals against heartworm, fleas, and intestinal worms increase the risk that their dog will suffer the life-threatening complications of heartworm disease, not to mention the discomfort of flea bites and painful symptoms of worm infestation.

What are common parasites in dogs?

The five main canine parasites are heartworm, fleas, roundworm, hookworm, and whipworm. These common parasites can all be picked up by a dog during its lifetime, especially if the dog owner does not diligently practice preventative measures.


Every unprotected dog is at risk of contracting heartworm – a parasite spread by mosquitoes that can cause canine heart disease and organ failure, often leading to death.

The deadly heartworm cycle begins when a mosquito bites an infected dog and becomes a carrier of heartworm larvae. These infective larvae are passed on to the next canine victim when the mosquito feeds.

The larvae escape into the dog’s body through the mosquito’s tiny puncture wound and migrate to the heart, where they grow and breed. Heartworm grows to about 30cm long, making it very difficult for the heart to function properly.

The first noticeable sign a dog has contracted heartworm is a cough. The dog will then show a lack of energy, shortness of breath, weight loss, inability to exercise, and loss of coat condition. Heartworm-infected dogs left untreated are likely to die of congested heart failure.

Untreated dogs serve as dangerous reservoirs of infection for other dogs. The adult worms breed, producing baby worms, or microfilariae, that circulate in the blood to start the cycle again with the next mosquito and the next dog.

Highly effective once-monthly heartworm prevention is now available, however, dogs must be tested for heartworm before commencing prevention therapy.


The flea most commonly found on both dogs and cats is the cat flea. Fleas affect every dog at some stage in its life. The only time a flea will bite a human is if there is no food source available and it is very hungry.

Flea bites and scurrying over the surface of the skin causes intense itching and the dog can severely damage its skin in an attempt to relieve the irritation.

Many dogs become hypersensitive to the saliva of the flea so that even the bite of one flea can induce a severe generalized skin reaction.

Fleas can stay in their cocoon for up to 8 months in a dormant state, emerging only in response to stimuli indicating a potential host is present or passing by.

Fleas can jump 10,000 times in succession without stopping when looking for a host animal.

From egg to death a flea’s natural life cycle can range from 12 to 240 days.

After all the effort of finding a host, fleas mate and lay eggs continually. Female fleas lay 50 eggs a day and consume 15 times their body weight in blood.

To keep up with its mating performance, the penis of the male flea is one-third its body length.

Fleas found on a dog are the adults, the last stage of the life cycle. These adults account for only five percent of the flea population; the rest of the flea population of fleas is found in the dog’s environment.

Once laid, flea eggs fall from the dog’s coat to the ground everywhere a dog goes, developing into larvae within 1-10 days. Eggs account for half the flea population.

The larvae live in the base of carpet in homes feeding off organic debris and spinning a cocoon before they develop into pupae.

The pupae are fully developed fleas that can remain encased in a cocoon for months on end while waiting for the right conditions before emerging. However, in some conditions, a flea can go through all four stages of its life cycle (egg, larva, pupa, adult) in just 12 days.

The only way to successfully treat fleas is to break the breeding cycle. New medications available from vets provide a unique birth control mechanism, which prevents flea eggs from hatching into larvae and eliminates the problem at its source.

Unfortunately, flea sprays and powders only kill adult fleas, without targeting eggs, larvae, or pupae in the dog’s bedding or in the carpet. So after spraying a pet, re-infestation is not far away.

Insecticides are often unsuccessful against fleas because flea larvae move using bristles, meaning they hardly ever touch the ground. Flea pupae are just as difficult to kill because a thick cocoon protects them from insecticides.

There are three main types of intestinal worms that can cause serious debilitating illnesses in dogs.


Hookworms have a voracious appetite for blood. They cause bloody diarrhea and anemia, which can kill puppies. The worms attach to a dog’s intestinal lining with hook-like teeth, leaving bleeding internal wounds that are particularly dangerous to puppies.

Hookworm eggs pass through the feces of infected dogs and hatch into larvae, which are then swallowed by healthy dogs or penetrate the dog’s skin. Puppies can contract hookworm while suckling from their mother.

Humans can also pick up hookworm, which can be dangerous for children, from contaminated soil in backyards and parks.


Roundworms virtually all puppies are born with roundworms or may acquire them through their mother’s milk.

Roundworm eggs are passed in the feces of infected dogs and are picked up by healthy dogs from contaminated soil or by eating infected rodents. Infected dogs often have a pot-belly and suffer from diarrhea and vomiting.

These worms can also be picked by humans around the garden, or by playing with an infected dog and can be extremely dangerous for children.


Whipworms are transmitted in the same way as hookworm and roundworm and are prolific everywhere in the United States.

Whipworm infestations cause bloody diarrhea, weight loss, anemia, and dehydration. Once a dog is infected with whipworm it is very difficult to control the infection as the eggs are very persistent in the environment.

What to do if my dog has parasites

Your vet can provide advice on how to ensure your dog is protected against the five most common parasites – heartworm, fleas, hookworm, roundworm, and whipworm – now and for the rest of its life.

  • Worm your dogs (and cats) every 3 months, at the change of the season.
  • Always wash your hands after handling or playing with dogs (and cats).
  • Teach your children to wash their hands after playing with pets.
  • Control fleas on your pet and their environment.
  • Clean your pet’s sleeping areas regularly.
  • Remove droppings daily from the garden and litter trays and always wear gloves or wash your hands afterward.
  • Base your pet’s diet on good quality prepared pet foods.
  • Don’t let your pets eat birds, rodents, or rabbits.
  • Don’t feed raw offal or raw meat.


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Hannah Elizabeth is an English animal behavior author, having written for several online publications. With a degree in Animal Behaviour and over a decade of practical animal husbandry experience, Hannah's articles cover everything from pet care to wildlife conservation. When she isn't creating content for blog posts, Hannah enjoys long walks with her Rottweiler cross Senna, reading fantasy novels and breeding aquarium shrimp.

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