Fleas in Cats and Dogs

Fleas in Cats and Dogs

Animals Affected

Fleas can infest dogs, cats, rodents, and other mammals. People can be bitten by fleas as well.


Fleas are a parasitic pest of dogs, cats, and other animals. They are blood sucking insects. Minor flea infestations can lead to severe skin and health problems in dogs and cats. Major infestations can be life-threatening.

If allowed to progress unchecked, flea infestations will result in a breeding population of fleas in the house. This type of infestation is frustrating to treat, because new fleas will continually hatch from eggs in the house and reinfest the pet.

Allergies to fleas are the leading cause of skin problems in cats and dogs. Most pets suffering from flea-related skin problems will not have large numbers of fleas on their bodies, because these animals groom themselves tirelessly to remove fleas.

Fleas contribute to a number of other health problems in cats and dogs. As well, they carry a wide variety of diseases that can affect pets and people.

Contrary to popular belief, flea infestation is not contagious. Rather, in endemic areas fleas are ubiquitous in the environment. Pets do not need to come into contact with an infested individual to develop a flea infestation.

Preventing flea infestations has been made simple by the development of several products that are extremely safe and effective.


Frequently, animals suffering from fleas will not have a visible flea infestation. Any skin problem in a pet should be taken as a sign that fleas may be present. Specific symptoms of flea infestation or allergies to flea bites include:

  • Itching
  • Flea dirt (actually flea feces, which resembles dirt but consists mostly of the pet’s blood) on the pet or in areas where the pet has been
  • Red skin
  • Hair loss, particularly on the back near the base of the tail
  • Scabs on the skin
  • An unusual skin odor
  • Skin crusts
  • Ear infections
  • Weakness
  • Immune disease
  • Ulcers on the lips (in cats)
  • Fleas seen on the pet or in the house

Risk factors

  • Geography: fleas are more common in warmer areas and low altitude areas.
  • Season: fleas are more common in warmer months.
  • Lack of proper flea control
  • Young, weak or ill animals cannot groom themselves as well and are more prone to infestation.
  • Outdoor pets are at greater risk of fleas. However, in endemic areas, indoor-only pets still can suffer infestation.


Fleas cause skin allergies, hair loss, scabbing, skin infections, and, in the worst cases, anemia that can be fatal.

Fleas spread a number of diseases in pets, including tapeworms (which are contracted when pets eat fleas), feline infectious anemia, and Bartonella, also known as cat scratch disease.

Fleas are associated with or contribute to feline asthma, rodent ulcers (also called eosinophilic granuloma complex), psychogenic alopecia (baldness caused by over-grooming), autoimmune disease, miliary dermatitis (skin rashes), hot spots, ear infections and possibly inflammatory bowel disease and acral lick dermatitis.


Diagnosis of fleas is made by visual inspection of the pet. Even if no fleas are seen, symptoms consistent with fleas may lead to a recommendation of treatment.

Treatment and Prevention

Several safe and effective treatments for fleas are available. Popular over-the-counter options include Advantage® and Frontline®.  The main ingredient in Frontline® (fipronil) has recently become available in other over-the-counter formulations.  The efficacy of these products should be similar to Frontline® and therefore the products should may make good solutions for flea management.

Program®, Capstar®, Revolution®, Comfortis® (for dogs only), and Vectra® are available through veterinarians only. 

Flea collars, herbal remedies, flea bombs, flea combs, and imposter products are not significantly effective and can be very dangerous. Their use is not recommended.

In particular, imposter products containing pyrethrins, pyrethroids, and permethrin have low safety margins.  Cats are especially prone to adverse events after application of these products.  These products should be rigorously avoided.

K9 Advantix® is a product that contains the active ingredient in Advantage® along with permethrin to control tick infestation.  The product is generally well tolerated by dogs, but has significant potential to lead to life-threatening adverse reactions in cats.  K9 Advantix® is not recommended for that reason.


If the flea prevention products listed above are not used consistently, re-infestation is likely.


Fleas spread a large number of human diseases. These include Bartonella, also known (inappropriately) as cat scratch disease, typhus, bubonic plague, and Rickettsia felis. Humans (usually children) who ingest fleas may contract tapeworms. As well, flea feces from infested pets can cause significant contamination of the house.

Due to the ease with which flea infestations can be prevented, there is no justification for the incredible amount of animal suffering that occurs as a result of fleas.

Copyright © Eric Barchas, DVM All rights reserved.
The contents of this page are provided for general informational purposes only. Under no circumstances should this page be substituted for professional consultation with a veterinarian.