Parasites in dogs

Written by Dr Andrew Miller MRCVSDr Andrew Miller MRCVS is an expert veterinary working in the field for over 10 years after graduating from Bristol University. Andy fact checks and writes for Pure Pet Food while also working as a full time veterinarian. Pure Pet FoodPure Pet Food are the experts in healthy dog food and healthy dogs featured in media outlets such as BBC, Good Housekeeping and The Telegraph. Working with high profile veterinary professionals and nutritionists, Pure Pet Food are changing dog food for the better. - Our editorial process

Parasites are some of the peskiest problems you’ll encounter when you become a dog owner. They take several different forms, and the impact they have on your dog can range from a minor itch to a severe illness.

Ticks and fleas are amongst some of the most recognised parasites, possibly because they’re extremely common and they’re quite easy for pet parents to spot. Despite this, our pooches can contract various other parasites. Many of these are internal parasites, which are some of the most dangerous as they can be living inside your dog without you even having the foggiest idea that they’re there.

We’re going to give an overview of all the different types of parasite, whether they’re internal or external, what symptoms they each create and the best method to treat them.

What is a parasite?

The word ‘parasite’ originates from Latin and Greek, meaning ‘eating at another’s table’, which is a great way to summarise what parasites do. They’re greedy little creatures that turn up where they aren’t wanted!

Parasites are organisms that can live either internally or externally on what is called a host, which is essentially just a word for another living being. They gain their food from their host. To further differentiate the different types of parasite, they can be split into three categories: protozoa, helminths and ectoparasites.

Protozoa are tiny, single-celled organisms that typically aren’t visible to the human eye, for example, coccidia and giardia. These ones are pretty sneaky and can go undetected by owners for quite a while.

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Helminths are multi-celled and a lot larger than protozoans, we can usually spot them when they’re in their adult stage. Examples of helminths are roundworms and tapeworms, which usually appear as visible worms in your pooch’s stool.

Finally, ectoparasites are probably the most well-known type, as it groups together all the blood-sucking parasites such as fleas, ticks, mites and lice.

Life cycle of parasites

Every parasite has its own life cycle, each requiring different factors to mature fully and start causing disturbances to your dog. Parasites tend to either have a direct life cycle, where they only need one host to survive, or an indirect cycle, in which they need a transitional, intermediate host (such as a mosquito) to become infectious, before moving onto their main host.

Most parasites will lay eggs at one point in their life cycle, and these eggs can also be referred to as larvae. In most cases, the larvae are infective and can dwell in the environment for several months waiting for the right host to pick them up so they can continue their full life cycle.

The idea of a strange creature residing in or on your dog and feeding from them is definitely a disturbing, creepy thought. However, every dog will most likely have to confront one of these creepy crawlies at some point in their life so it’s important not to worry too much if you spot one.

How do dogs catch parasites?

There is no hard and fast rule about the way in which our pooches can catch a parasite.

However, most internal parasites, such as hookworm and roundworm, are usually transmitted to a dog when they somehow ingest the parasite’s eggs, whether this be inadvertently or not. The eggs can live in an infected animal’s faeces, soil, water and food. Parasites can even be passed on from puppy parent to their pups through nursing.

External parasites work slightly differently, however. Fleas are sneaky little critters, jumping from one infected animal to start infecting another. Ticks on the other hand usually lurk in the long grass that your pup probably likes to bound through, latching on to your dog as they pass.

All in all, these little insects can be creeping around in many different places, and even more so in places that have a high pooch population such as dog shelters and kennels. These environments facilitate the transmission of parasitic infections, especially those that are caught through the skin or ingestion of faeces.

Every single parasite works in a different way, causing different symptoms, living in different places and latching onto your dog through various different methods.


ParasiteWhat does it do?Main symptomsCan I catch it?
HeartwormClogs heart and blood vessels, causing heartworm diseaseMild cough, fatigue, reduced appetiteNo
HookwormFeeds on dog’s bloodBlood in stools, diarrhoea, pale gums, weaknessYes, but only indirectly through touching infected soil
RoundwormFeeds on food your dog eatsVisible worms in stool, malnourishment, vomiting, diarrhoeaYes, but unlikely
TapewormFeeds on food your dog eatsVisible worms in stools and around anus, scooting rear end across the floorYes, but extremely unlikely
WhipwormFeeds on dog’s blood and the food they eatWeight loss, bloody and watery diarrhoeaYes, but unlikely
Coccidia and giardiaFeeds on skin debrisDiarrhoea, blood in stools, weight loss, vomitingYes, but unlikely
FleasFeeds on dog’s bloodVisible fleas and ‘flea dirt’ on their skin and coat, itching and patchy furYes
TicksFeeds on dog’s bloodVisible ticks attached to dog’s skinYes, but it’s rare you’ll catch it directly from your dog
LiceFeeds on dog’s skin and bloodItchiness, dry skin, patchy furNo, lice that live on dogs are different to the ones that humans can get
MitesFeeds on dog’s skin and bloodItchiness, dry skin, patchy furNo

What are internal parasites?

Internal parasites reside inside your dog’s body and can do for quite some time without getting noticed.

Many dogs can be completely asymptomatic to these parasites, or their symptoms could be dismissed as they are not specific and could be indicative of various other concerns. This is why you must go for annual parasite check-ups at the vet.

If you think your puppy, senior dog or your already unwell dog has contracted an internal parasite, you must act quickly as these dogs have a weakened or undeveloped immune system that can’t fight off the parasite as well.

Types of internal parasites:

  • Heartworm

  • Hookworm

  • Roundworms

  • Tapeworms

  • Whipworms

  • Coccidia and giardia


Heartworm, or Dirofilaria immitis, is a large parasite that flows into the dog’s bloodstream and primarily lives in the lungs, blood vessels and the heart, eventually causing heartworm disease.

It’s transmitted through a mosquito bite and can be incredibly dangerous to your dog, as they mature into adults and produce offspring inside your dog’s body. A dog is the main host, however, the mosquito acts as the transitional host, living for a little while in the mosquito until it reaches the point of infectiousness. Once the parasite is infectious enough to cause heartworm disease, it’ll be passed onto a dog and finish its life cycle there.

It takes a number of months from the point of the mosquito bite for the larvae to develop into adult heartworms, but in the end these adult heartworms can reach a shocking 30cm in length!

It can take many years for your dog to become symptomatic, sadly meaning that by the time the signs start to show, the disease will have developed significantly. The disease is triggered when the worms start to clog up the heart and vital blood vessels, predominantly the pulmonary artery.

Due to this obstruction, the heart must work much harder and blood flow is diminished, causing the organs to crash.


Heartworm disease will typically start to show itself through a continual, mild cough and your dog struggling to catch their breath, most noticeably after exercise. Paired with fatigue, lack of stamina and a reduced appetite, heartworm disease could eventually result in heart failure.


To treat this disease, your dog will be injected with a medication that will kill the worms. When your dog is being treated for heartworm disease, it’s absolutely fundamental that your pooch does nothing but rest.

As the worms are dying, they decompose into several fragments and they can cause obstructions in the vessels, which can have fatal consequences if your pooch doesn’t rest properly.


Hookworms are tiny parasites that are barely visible to the human eye. They attach themselves to the intestinal wall and suck extensive amounts of blood from your pooch, aided by their hook-shaped mouth.

As a result, they cause inflamed intestines and a condition called anemia, which is the dangerous reduction of red blood cells.

Female hookworms lay eggs in the dog’s body which will pass through the faeces of the infected dog and subsequently infect the surrounding area. Several dogs display the unpleasant habit of eating poo and this is a common way for hookworms to infect your hound.

Hookworm can even be ingested unintentionally. Our curious canines love a good sniff around, but this means they could accidentally pick up the parasite by simply sniffing at any contaminated soil or faeces.

Once ingested, hookworm larvae will eventually travel to the intestinal tract to finish off their life cycle, or they can even burrow into the skin if your dog walks over any infected soil.

Unfortunately, puppies can also acquire hookworms from their mother through nursing. If a pregnant pooch has already suffered with hookworms previously, the pregnancy may reactivate larvae that were dormant. Hookworms can be severely detrimental to a puppy, due to their underdeveloped immune system.


Being aware of the symptoms can save your pup a lot of problems in the long run, with the main ones being bloody diarrhoea, weakness, pale gums, weight loss and anemia. Many of these symptoms could be signs of other medical concerns, so seek guidance from your vet immediately to work out what the issue is.


Hookworm will typically be diagnosed through a stool sample, and dogs suffering with the parasite are usually prescribed with oral de-wormers, called anthelmintics. This medication will effectively eradicate all the adult hookworms.

However, your pup will need to be treated again in a few weeks to kill any newly developed adult worms that have formed from any larvae that were still residing in your dog’s body during the initial treatment.


Roundworm is a greedy little parasite that probably every dog will encounter at some point. Living in their intestinal tract, roundworms feed from your pup’s partially digested food.

Eventually, roundworms will cause malnourishment, as they essentially steal the nutrients your dog should be receiving from the food they eat themselves. This can be incredibly dangerous for puppies who have a weakened immune system as they need all the nutrients they can get to grow into healthy adult dogs.

Dogs can become infected with the parasite by ingesting larvae that are living in their environment. Your dog can consume these infectious eggs through eating faeces, soil or even by treating themselves to a strange snack such as an infected bird, rodent or insect.

Puppies are susceptible to roundworm in the same way they are to hookworm. This is when any dormant larvae residing in the mother become active during pregnancy and the pups subsequently become infected by nursing.


Roundworms are small, white and stringy little creatures, having a similar appearance to spaghetti. Even though this is very unpleasant and possibly quite creepy to think about, you should notice the worms in your dog’s stool. Other symptoms include, malnourishment, coughing, a pot-bellied look, diarrhoea and vomiting.


The treatment for roundworm is relatively easy and effective, your vet will simply administer your dog with a de-worming drug to eliminate the worms.


The thought of tapeworm will definitely give you the creeps. Your pooch will develop a tapeworm by somehow eating a host that is holding tapeworm eggs, this host is commonly a flea.

The eggs will travel to the intestine, mature into an adult and then break up into lots of small segments called proglottids. These tiny segments of the parasite will pass through and end up in your dog’s faeces, in which the cycle of infection can begin again.

Tapeworms are usually not severe, but sometimes they can cause anemia, a slowed growth rate and blockages in the intestines, especially in young puppies.


Generally, the tapeworms can be visible near your dog’s anus or in their stools; they look like a grain of rice.

One of the most tell-tale signs of tapeworms is your dog dragging their rear end against the floor, known as scooting, as this may momentarily soothe the itch.


Your vet will provide your pup with either an oral or injected medication that will cause the tapeworms to dissolve into the intestine. The treatment is usually very quick, simple and successful.


Another incredibly common parasite, whipworms burrow and embed themselves into the walls of the large intestine, taking their name from their whip-like appearance.

Their life cycle begins by tiny eggs, that are similar in appearance to a small piece of thread, appearing in faeces. These eggs can go completely unnoticed by the naked eye, but this is the point where they start to become infective as they mature within the environment.

Just like many of the other parasites in our list, your pooch will somehow ingest these eggs and they’ll travel to the intestinal tract so they can mature into adults.

Puppies, older dogs and dogs with a weaker immune system are the ones that’ll suffer the most from the effects of whipworm.


Our dogs may live with whipworms for quite some time without owners noticing, as many dogs don’t show symptoms for a while.

However, the main symptoms are weight loss, anemia and bloody, watery diarrhoea.


To treat whipworm, your vet will provide a de-worming medication to kill the parasite.

However, whipworms are very resilient little creatures, and their eggs can last several years in the environment. This means the rate of reinfection is quite high.

Your vet is likely to advise that if your pooch has been infested with whipworm before, that they come back for treatment every few months to completely eradicate the parasite.

Coccidia and giardia

Coccidia and giardia, are tiny, non-worm parasites that can live in the intestinal tract.

Both coccidia and giardia have a continuous life cycle, which begins when they’re attached to the intestinal wall. Here, they will multiply, and some will develop into cysts which are passed through the dog’s faeces.

A dog will catch this parasite by somehow ingesting these infected cysts. Sometimes, coccidia will cause the disease coccidiosis and giardia will cause giardiasis.

Until an external factor arises, such as stress or a different medical concern, these parasites can live for a long time in your dog completely undetected. If a veterinary expert finds coccidia or giardia cysts within your dog’s stool, they won’t be too worried about it unless it was causing your dog any issues.

Similar to many other parasites on this list, it’s the most dangerous to puppies, senior dogs or dogs with weakened immune systems.


As stated, you may not even know your pooch is living with these pesky parasites until they begin showing symptoms of the subsequent illnesses, coccidiosis and giardiasis.

The symptoms of these are diarrhoea, vomiting, weight loss and blood in your dog’s stools.


Both coccidiosis and giardiasis require a course of antibiotics to treat the disease, and many will need a repeated course to completely eradicate all signs of the illness.

Treatment is usually very effective. However, diarrhoea is one the main symptoms of both of these parasites, which can cause severe dehydration. In the event that your poor pup is positively parched, a drip may be required to replace any missing fluids.

Typical symptoms of most internal parasites:

What are external parasites?

Parasites that inhabit your dog’s body and skin rather than inside their body are called external parasites.

Even though these irritating little insects are enough to make your skin crawl if you spot one on your dog, it means that they are usually detected much quicker than internal parasites. Consequently, external parasites generally get treated quicker, limiting the negative impact they can cause.

All parasites will usually cause some level of discomfort, even if it’s only a minor itch. However, sometimes they can transmit serious diseases to your dog, so it’s important to get it treated as soon as you spot one of these little critters.

Types of external parasites:

  • Fleas

  • Ticks

  • Mites

  • Lice


You may start to notice your dog is scratting and scratching at themselves more than usual, in which many dog owners will instantly jump to the conclusion of a flea infestation, which is a perfectly reasonable conclusion.

Fleas are really common pests for several species of animals, thriving in hot, humid weather. Although they can’t fly, they can leap from host to host or jump from the environment onto your pup.

No matter how well-groomed and clean your dog is, they can still contract fleas. Your dog may have caught fleas from another dog, your home, the garden, basically anywhere in the environment. Fleas have the capacity to lay hundreds of eggs in just a few days, which can fall onto anything.

Once hatched, the fleas will need a host, so they’ll jump to your dog, feed from their blood and lay more eggs. Even though fleas can only survive for around 2 weeks, the constant reproductive cycle means they will keep hatching, breeding and feeding from your poor pooch until they receive treatment.


It really isn’t too tricky to spot the signs and symptoms of a flea infestation. The main sign is the constant itching and scratting, which will often be accompanied by bites, rashes and patchy fur if your dog is persistent with their scratching.

Some dogs don’t have such an extreme reaction, but you’ll still be able to notice an increased level of itching.

Sometimes, you might be able to see the deep reddish-brown coloured fleas, about the size of a sesame seed, scampering on the skin. They most frequently reside near your dog’s rear end, groin or at the base of the ears.

However, you’re more likely to notice little black specks, which is actually flea faeces, or ‘flea dirt’ as it’s often referred to.


To relieve your pooch of these irritating insects, you’ll need to pick up the suitable flea treatment from your vet.

Flea treatment is relatively accessible from several shops, however, it’s best to ask the vet to get the best one that’ll be long term and effective for your pooch’s specific case.

It’s also extremely important to give your house a thorough clean to make sure you’ve properly booted out those bothersome bloodsuckers. Vacuuming, cleaning floors and skirting boards and washing any other soft furnishings should do the trick to eliminate the fleas at whatever stage of their lifecycle.


Ticks are some of the most common parasites and your dog will no doubt encounter one of these irksome little insects at some point.

They have a spider-like form and have a knack for climbing and dropping onto your dog’s coat when your pooch is just minding their own business. They’re commonly found in long grass and woodlands, so if your pet often prances around the grass, it’s a good idea to frequently check for this little parasite.

Initially, ticks are tiny, oval-shaped insects and they’re pretty hard to notice, but after feeding on your dog’s blood they’ll enlarge and darken in colour. You’re most likely to spot a tick when you’re giving your pooch a well-deserved stroke, and you’ll randomly feel a strange bump.

There are hundreds and hundreds of tick species worldwide, and they can sometimes transmit various illnesses to your pest-infested pup, with the predominant one being Lyme disease.


Of course, the main symptom of a tick infestation is spotting the actual presence of the tick itself.

Unlike flea bites, ticks don’t really cause that much itchiness unless your pooch has a reaction to the bite. This means you probably won’t notice any strange behaviour from your dog, it’ll be you that notices the little creature burrowing onto your dog’s skin. This is why it’s important to frequently check your dog for any ticks.

However, keep an eye out for lethargy, fever, lack of appetite and a red rash surrounding the bite as this could be indicative of Lyme disease.


As stated, even though those ticks are tiny, they can cause a lot of unnecessary trouble. Therefore, you must remove them straight away to prevent any illnesses.

Tick removal tools are readily available at pet stores and it’s usually pretty easy for you to remove the tick yourself.

You just need to be sure not to squeeze the tick’s body or leave their head burrowed into your pooch’s skin. This could cause blood to inject back into your dog’s body, increasing the infection risk.

Mites & lice

Although mites and lice are two different types of pesky parasite, they’re both tiny, annoying, itchy and they operate almost identically.

Mites burrow down into your dog’s fur or around their ears, causing many skin problems, with the most common condition being mange, which causes significant hair loss. Microscopic mites are usually passed between dogs, but they can be found anywhere in the environment.

On the other hand, lice are a lot less common, only really affecting dogs with less efficient immune systems or that have been in an unhygienic environment. Luckily, the lice that impact our dogs are different to those that impact humans, dog lice need dog’s blood whereas human lice need our blood.


The symptoms of both of these parasites are very similar, mainly causing itchiness, dandruff, dry skin and patchy fur. You may even spot visible mites or lice and their eggs lurking in your pup’s fur.


To free your pup of these irksome little insects, you’ll need a product to kill the lice or mites, such as a ‘spot on’ treatment or a medicated shampoo. Your vet will be able to provide guidance on exactly which.

Unfortunately, lice and mites can leave their eggs behind, ready to try and cause another infestation. Make sure to do a deep clean of your house, using a parasite-killing product, washing soft furnishings, flooring, brushes, anything you can think of really to eradicate any possible pests.

Typical symptoms of most external parasites:
  • Scratching

  • Chewing

  • Red and inflamed skin

  • Hair loss

  • Crusting

  • Discoloured skin

  • Dry coat

  • Scaly appearance

  • Black debris

  • Fleas or flea dirt visible

  • Restlessness

How can I prevent my dog catching a parasite?

We all know the saying that ‘prevention is better than cure’, and the same goes to stop any parasites disturbing your poor dog.

Treatment is definitely very effective, but for internal parasites it could sometimes be too little too late since symptoms don’t often arise until the disease has started advancing.

Preventing pesky parasites is mostly about maintenance, keeping up with your pooch’s preventative medication will stop parasites from pestering your dog at all. Constantly keeping a keen eye out for any parasites lurking in your dog’s skin and fur, alongside attending regular screening tests at the vet to look for those sneaky internal ones will work wonders for catching the little critters early on.

If your pooch lives in clean surroundings with a nice clean bed to sleep in, they’re less likely to acquire an infestation.

Try and stop your dog from sniffing at faeces, rubbish, dead animals and other dogs that you think are possibly infected. Canines are curious by nature, so it’s hard to keep them away from disgusting, possibly parasite ridden things. However, these are breeding grounds for parasites so do your best to keep them well away.

Even picking up your pooch’s poo immediately can make a world of difference, both for reinfecting your dog and other dogs.

Parasites, pooches and people - can they be transmitted to me?

Humans can catch some parasites from their pets, and as a result catch the diseases they carry. A disease that can be transmitted from pet to person, or vice versa is actually called a zoonotic disease.

Don’t be too worried though, it’s quite rare for you to catch anything from your dog. Intestinal parasites pose the most risk, roundworms, hookworms and whipworms are ones that could possibly be passed onto you. Although it’s incredibly unusual, you can become infected if you become exposed to contaminated soil.

Some forms of external parasites can hop from your dog to you, but again its very rare. In all honesty, you could catch a parasite accidentally that has nothing to do with your dog whatsoever!

Again, by practicing all the hygiene tips detailed above, you should be able to avoid the chances of catching anything. Getting rid of any eggs is the most important thing! If you know your pooch has an infestation, be extra careful.


All in all, parasites are 100% one of the most infuriating things you’ll come across in your time as a pooch parent. It makes it even harder that there are that many different types of parasite, all with various symptoms and treatment methods.

Even though some parasites can cause your dog some serious bother and you a lot of stress, be reassured that all parasites are incredibly common and most of the time it’s really easy to solve the issue. As long as you’re clued up on the signs of parasites you’ll be able to keep your dog happy and healthy with ease!

  1. Zoonoses World Health Organisation, July 2020
  2. Parasites Centers for Disease Control and Infection
  3. Pet roundworms and hookworms: A continuing need for global worming Parasites & vectors, 5 (91), May 2012, doi:10.1186/1756-3305-5-91
  4. Gastrointestinal parasites in shelter dogs: Occurrence, pathology, treatment and risk to shelter workers Animals : an open access journal from MDPI, 8, (7), July 2018,