Skip to main content


Common Parasites and How They Affect Dogs and Their Owners

There are several parasites that can infect our canine friends. Some infections may cause no symptoms at all while others can be very dangerous. It’s important to know what to look for and to see your vet if you suspect an infection may be present. Below you’ll find information about six of the most common parasites your dog is likely to encounter, and what symptoms you can expect in them and potentially in you.


Hookworms are small intestinal parasites that infect their hosts either orally or through skin contact. Their name comes from the hook-like mouthpiece they use to latch onto the intestinal lining and feed off the small capillaries near the surface.

Hookworms lay their eggs into their host’s digestive system, which hatch into larvae and are transmitted into the environment through fecal excretion. New animals are infected when they sniff or eat infected feces or soil, groom their paws after walking over an infected area, or lay in an infected location, allowing the larvae to burrow in through the skin. They can also be passed in utero from mother to child and via the mother’s milk.

How Hookworms Affect Dogs

Hookworms feed fairly heavily and can cause severe anemia in infected animals. While hookworm infections aren’t generally life-threatening, one of the three common species of the worm can be fatal in puppies. In general, it’s a good idea to seek treatment as soon as a hookworm infection is suspected. There are a number of symptoms that can indicate infection. If any of these are present in your animal, it’s wise to seek veterinary counsel.


Weight loss


Pale gums

Bloody diarrhea

Itchy paws

How Hookworms Affect Owners

Hookworms can infect humans, normally entering the body through the skin of the feet as you walk barefoot around your yard. If you’re infected, you’ll feel a tingling or itching sensation where the larvae burrowed into your skin. In general, they’re not much of a threat to people and treatment is fairly simple.

Treating Hookworm

If hookworms are suspected in your dog your vet will perform a fecal float to look for hookworm eggs in your dog’s feces. If found treatment is fairly simple, involving a course of oral deworming medication. Since these medications don’t kill larvae, a second course is normally prescribed a few weeks later to kill new adult worms that the existing larvae mature into. What’s most important, and can be tricky, is preventing reinfection. Since the worm is spread through infected feces, you’ll need to clean your yard and other places your dog defecates, removing as much of the infected material as possible. Depending on conditions, hookworm larvae can live for months in the environment, so it’s a good idea to add a hookworm preventative medicine to your dog’s treatment, which helps prevent reinfection while you wait for the outdoor larvae to die.


Roundworms are also intestinal parasites, but grow to be significantly larger than hookworms, often reaching several inches in length. Roundworms feed not on blood, but on incompletely digested food as it passes through the small intestines. As a result, roundworms can cause malnourishment problems, particularly in young puppies.

Roundworms are extremely common in dogs. Nearly every dog will have roundworms at least once in its life, most commonly in puppyhood, passed from the mother either in utero or through her milk. A unique feature of roundworm physiology allows larvae to encyst in the infected dog’s tissues, entering a dormant state. During certain illnesses or pregnancy these encysted larvae reactivate and reinfect the mother and then her puppies. Therefore, it’s recommended to treat puppies and their mothers for roundworm as a matter of course.

Dogs can also become infected by eating soil polluted by infected feces or by eating smaller animals, like mice, that can carry worm eggs.

How Roundworms Affect Dogs

Roundworms can grow fairly large and eat quite a bit, depriving the infected dog of much of the food it eats on a daily basis, causing malnourishment. The worms are also known to migrate to the lungs where they can cause irritation, coughing, and in extreme cases, pneumonia. Symptoms to look for include:

Signs of malnutrition: weakness, stunted growth, weight loss

A pot-bellied appearance due to an abundance of worms




Worms in vomit or stool

How Roundworms Affect Owners

There are two different species of roundworm. Only one, Toxocara canis can be transmitted to humans. The worm can’t complete its life cycle in humans, but the larvae can encyst in human tissues, which can cause a host of problems. Roundworm infects humans orally, most often when children play in soil mixed with dog droppings. It’s recommended that children’s hands be washed well and promptly whenever they’ve been playing in outdoor areas frequented by dogs.

Treating Roundworm

Treating roundworm is easy. A simple deworming medication will handle the infection. There are also heartworm control medications that included ingredients to prevent roundworms as well.


Unlike the last two parasites, whipworms opt for the large intestine instead of the small. The worms get their name from their shape, which resembles a whip. In order to leave their mouth free to feed, whipworms embed the thicker end of their body into the mucosal lining of the cecum and colon, causing dogs a good bit of irritation. In large numbers the discomfort the worms cause can be quite severe, leading to anemia, bloody stools, and weight loss.

Whipworm infections are primarily oral, caused by ingestion of the embryonic form of the worm from the environment. Once in the large intestine the worms become prolific egg layers, and these eggs are passed back to the environment to reinfect the host dog and infect new animals. 

How Whipworms Affect Dogs

Many dogs carrying whipworm are asymptomatic. It’s not until the infection grows large enough that visible symptoms begin to occur. That’s why regular parasite checks at your vet’s office are recommended to catch a whipworm infection before it begins to cause problems. But if you see any of the following symptoms you should get your pet checked out.

Bloody stools


Weight loss


How Whipworms Affect Owners

It’s extremely rare for canine whipworms to infect humans, so rare in fact that it’s not something that people generally need to worry about.

Treating Whipworm

Like roundworms and hookworms, whipworms are easily killed with deworming medications. However, whipworm eggs are extremely durable, capable of surviving in the environment for many years. As a result, if preventative measures aren’t taken consistently the chances for reinfection are high. It’s important to treat dogs with a monthly heartworm medication that also prevents whipworms.


Tapeworms are the largest of the canine worm parasites, growing up to 28 inches in length. These segmented worms live in the intestines, eating from the dog’s ingested food as it passes through the digestive system.

The tapeworm life cycle is the most interesting of the canine intestinal parasites. A dog becomes infected when it ingests a flea that carries an immature worm. This generally happens when the dog is grooming itself or another animal, or when it eats a smaller animal like a rabbit or a mouse. Once the flea has been digested the worm takes up residence inside the dog’s intestines and begins to grow. As it does segments break off and are passed out of the body in the feces. These segments look like small grains of rice and are dead giveaway for a tapeworm infection. The segments contain the tapeworm’s eggs, and these eggs will get stuck to the dogs’ fur where they can infect a new flea, restarting the cycle.

How Tapeworms Affect Dogs

Generally, tapeworm infections are asymptomatic and don’t cause any specific illness, but it is common for larger infections to cause weight loss. However, there are certainly ways to tell if your dog has a tapeworm. Look for the rice-like segments in their stool or stuck to the fur around the rectum. The discharged segments can cause irritation, and as a result you may see your dog scooting across the floor or licking and chewing the area.

How Tapeworms Affect Owners

Tapeworms can leap from dogs to humans, but it’s rare and occurs most frequently in children if they accidentally ingest feces infected with tapeworm eggs. Make certain to wash children’s hands frequently if they’ve been rolling around on the ground in locations where tapeworm contamination is possible

Treating Tapeworms

There are several medications that can eliminate tapeworms from your dog’s system. Once treated it’s important to try and prevent reinfection, or prevent infection in the first place. The easiest way is to treat your animal for fleas, and to keep them away from other dogs known to have fleas. A dog without fleas is much less likely to come into contact with infected insects. Also, to avoid the risk of environmental infection try to keep your dogs away from dead and decaying animals.



Giardia is a single-celled parasite that multiplies in your dog’s intestines. As the mature parasites, called trophozoites flourish and spread they begin to form cysts, which are passed from the body in the dog’s feces. These cysts contaminate the environment where they can be consumed by other dogs either when they drink from puddles or other contaminated water sources, eat grass, pick up sticks in their mouths or mouth other objects that have come into contact with infected poop. These cysts travel into the intestines where they start the life cycle over again.

How Giardia Affects Dogs

Giardia often causes no symptoms in dogs, but if they develop giardiasis, the symptoms are unpleasant. Giardiasis causes occasional or continual diarrhea, and a reduced ability to absorb water and nutrients. Both conditions can be dangerous and possibly life threatening in puppies, elderly animals or animals with compromised immune systems. Symptoms to look for include:




Weight Loss

A poor looking coat

How Giardia Affects Owners

Humans can become infected with giardia, but the infection rarely comes from our pets. However, it’s still good practice to wash your hands thoroughly after handling your dog’s droppings.

Treating Giardia

Giardiasis and giardia infections are treatable conditions. It’s important to talk with your vet about your options. In terms of preventing giardia, you want to make sure your dog has access to fresh water all the time, reducing the likelihood that he or she will be motivated to drink from contaminated puddles. Also, you’ll want to clean up any dog waste in areas where your dogs play as quickly as possible to reduce the chances that cysts get spread around the environment. Giardia isn’t something you can prevent with a pill like you can with many of the worm parasites, so maintaining proper hygiene standards is your best bet for controlling giardia infections.


Coccidia is another single-celled organism that invades a dog’s intestines and takes up residence among the cells of the intestinal lining. Like giardia, coccidia is spread in a cyst state through contaminated feces. Coccidia oocysts are hardier than giardia cysts and survive in the environment for quite some time. However, they only become infective when conditions are right, so a local contamination does not automatically mean the organism is in an infective state.

How Coccidia Affects Dogs

Even more so than with giardiasis, coccidiosis, the disease caused by coccidia isn’t common and often isn’t particularly severe, frequently going away without treatment. However, it can be life threatening for young puppies and elderly or infirm dogs if the infection is severe enough. Diarrhea is the most common symptom, but in severe cases dehydration and vomiting can occur.

How Coccidia Affects Owners

Cross species infection with coccidia is impossible for all but one genera of the protozoa, and even then, it’s rare. Like with giardia, cleaning your hands after handling droppings and maintaining proper hygiene should eliminate the possibility of the microbe leaping from dog to person.

Treating Coccidia

Coccidiosis often runs its course and goes away naturally in otherwise healthy animals but in more severe cases it’s generally treated with a course of antibiotics. Because in most cases dogs with coccidia are entirely asymptomatic it’s important to do regular parasite checks with your vet to determine if an infection exists.

Parvovirus (Parvo)

Parvo is a highly contagious virus. It causes an infectious gastrointestinal (GI) illness in puppies and young dogs, and without treatment, it is potentially deadly.

Part of what makes the virus so dangerous is the ease with which it is spread through the canine population. The virus spreads either by direct contact with an infected dog, or through feces, and an infected dog can begin shedding the virus four-to-five days after exposure — often before the dog starts exhibiting any clinical signs of infection. The dog will continue to shed the virus while he is sick and for up to 10 days after he has recovered. This means that accurate diagnosis and quarantine are essential for the health of your dog and of other dogs, as well.

How Parvo Affects Dogs

The virus manifests itself in two different forms. The more common form is the intestinal form, which is characterized by vomiting, diarrhea, weight loss and lack of appetite (anorexia). The less common form is the cardiac form, which attacks the heart muscles of fetuses and very young puppies, often leading to death.

Most cases are seen in puppies that are between six weeks and six months old. The incidence of canine parvovirus infections has been reduced radically by early vaccination in young puppies.

The major symptoms associated with the intestinal form of a canine parvovirus infection include:

Severe, bloody diarrhea





Severe weight loss

How Parvo Affects Owners

In most children, parvovirus infection is mild and requires little treatment. However, in some adults, the infection can be serious. Parvovirus infection in some pregnant women can lead to serious health problems for the fetus. Parvovirus infection is also more serious for people with some kinds of anemia or who have a compromised immune system.

Treating Parvo

Pets with parvo often require intravenous fluids to keep them hydrated; antibiotics to treat infections that can occur as a result of the damage to the intestines; and plasma transfusions to replace protein lost in the vomit or diarrhea and to provide immune support. They may need constant nutritional support as well, to provide them with the nutrients they need to fight off the disease and heal their intestines.

Leave a Reply