Fleas in Cats & Dogs

Written by Shula Berg BVSc CertAVP(GSAS) GPAdvCert(SASTS) MRCVS
Clinically reviewed by Elizabeth McLennan-Green BVM&S CertAVP(SAM) MRCVS

Table of Contents

- Overview
- Symptoms
- Causes
- Treatment
- Prevention

What are fleas?

Fleas are tiny insects about 3mm long which feed on your pet's blood. Fleas are mostly found on cats and dogs but will also bite humans. Although fleas can’t fly, they can jump about 20cm into the air, and 30cm in distance! 

What problems do fleas cause?

Flea bites can be itchy and uncomfortable for both people and pets. When a flea bites the skin, it injects a small amount of saliva that some pets are allergic to. This is known as flea allergic dermatitis (FAD) and can cause a severe reaction - even one or two bites can cause intense itching. Pets who experience flea bites can cause trauma to their own skin from chewing and itching, often leading to secondary infections. 

In very small animals, such as young puppies and kittens, severe flea infestations can lead to anaemia (low red blood cell count). This causes weakness, lethargy and even trouble breathing. 

Fleas can also spread other diseases, most commonly tapeworms. The tapeworm egg develops inside the flea which is then swallowed by a cat or dog (usually during grooming). The egg develops inside the dog or cat’s intestines into an adult tapeworm. Less commonly, fleas can spread disease to people such as the bacteria that causes cat scratch fever. 

Where do fleas come from?

Fleas are commonly found on wildlife, such as hedgehogs, so can easily jump on to pets while they are outside. Fleas can also be brought into the house on our clothing or shoes, so don’t only affect pets who go outdoors! 

Female fleas can lay 30-50 eggs per day, which hatch into larvae. The flea larvae hide in dark places, such as carpets and upholstery, and feed on adult flea droppings, crumbs and skin particles. After a week the larvae spin cocoons to become pupae and can remain in this stage for very long periods of time. Eventually, the pupae emerge as adult fleas and find a new host (dog or cat) to feed from. 

Even a few adult fleas can lead to huge numbers of flea eggs. Since the pupae can survive in the environment for days, weeks or even months, treating your pet will not get rid of the infestation. You may kill all the fleas on your dog or cat, but the pupae in the household will hatch and re-infest them. This is why flea infestations are so challenging to get rid of - prevention is much better than cure!  

What do I do if my pet has fleas?

You may see the fleas themselves crawling through your pet’s hair or jumping on or off them. Often, the best way to diagnose a flea infestation is by finding flea dirt (flea poo) on your pet’s skin. This is reddish-brown and looks like small specks of dirt. You can use a special flea comb to rake through the fur before wiping what comes out on some damp paper towel. If flea dirt is present, it will dissolve on the damp paper and become a red-brown smear rather than staying as particles. If you are unsure, your vet or nurse will be able to diagnose a flea infestation for you.


If your cat or dog has picked up fleas, you will need to treat them with a product that will kill and/or repel them. This may be a once-a-month topical treatment, spray, or oral medication – your vet or nurse can advise on the best product to use. 

As explained above, a large part of the flea's life cycle is in the environment rather than on the dog or cat. You will need to treat the house with an appropriate product, otherwise your pet may be reinfected. Treatments used to kill fleas take anywhere from a few hours to a few days to work fully as the fleas must absorb the product to be affected. It is not uncommon to see live fleas a day or two after application; this does not mean the product has been ineffective! It can be very dangerous to dose a cat or dog with multiple products; if you are unsure about the success of your pet's treatment, please call your practice. 

Preventing Fleas

The best way to control flea problems is to prevent them from happening – now that most homes are heated through the winter fleas have become a year-round problem, not just confined to the summer months. 

Several products are available, usually spot-ons or tablets, which can treat the problem and prevent future infestations. They work by killing adult fleas before they can reproduce and preventing eggs from developing into larvae. Most products need to be given/applied every month to provide continuous cover. Speak to your vet or nurse for advice about the most appropriate product for your pet.  

Full flea treatment is included when you are a member of The Healthy Pet Club.


Please note that the content made available on this webpage is for general information purposes only. Whilst we try to ensure that at the time of writing all material is up to date and reflects industry standards, we make no representation, warranties or guarantees that the information made available is up to date, accurate or complete. Any reliance placed by yourselves is done so at your own risk.

Page last reviewed: 7th August 2023
Next review due: 7th August 2025

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