Fleas are small wingless insects that feed on animal blood. In addition to being a nuisance, they can also transmit diseases and cause allergies or anemia. There are more than 2,200 species of fleas recognized worldwide. In North America, only a few species commonly infest house pets. Two common species of flea are the cat flea (Ctenocephalides felis) and the dog flea (Ctenocephalides canis). However, most of the fleas found on both dogs and cats are cat fleas. Fleas cause severe irritation in other animals and humans. They also transmit a wide variety of diseases, including tapeworm infections (see Digestive Disorders of Dogs: Tapeworms) and the typhus-like rickettsiae (see Brain, Spinal Cord, and Nerve Disorders of Dogs: Rickettsial Diseases).
Transmission and Life Cycle
Cat fleas begin reproduction about 1 or 2 days after a blood meal from a host. Female fleas lay eggs as they feed and move about on the surface of the skin. A single female flea can produce up to 50 eggs per day and about 2,000 in her lifetime. The eggs are pearly white, oval, and tiny. They readily fall from the fur and drop onto bedding, carpet, or soil, where they hatch in 1 to 6 days. Newly hatched flea larvae are mobile and free-living, feeding on organic debris found in their environment and on adult flea droppings. Flea larvae avoid direct light and actively move deep into carpet fibers or under organic debris (grass, branches, leaves, or soil).
Larvae can easily dry out, and exposure to relative humidity under 50% will kill them. However, they are capable of moving as far as 3 feet (1 meter) to find locations suitable for their survival. Indoors, flea larvae best survive in the protected environment deep within carpet fibers, in cracks between hardwood floor boards, and on unfinished concrete floors in damp basements. Flea development occurs outdoors only where the ground is shaded and moist. The larval stage usually lasts 5 to 11 days but may be prolonged for 2 to 3 weeks, depending on the availability of food and the environmental conditions.
After completing its development, the mature larva produces a silk-like cocoon in which it pupates. The pupa is fully developed in 1 to 2 weeks, but the adult flea may remain in the cocoon for several weeks until a suitable host arrives. When it emerges from the cocoon, it can survive 1 to 2 weeks before finding a host on which to feed. It is the newly emerged, unfed fleas that infest pets and bite people. Fleas generally do not leave their host unless forced off by grooming or insecticides. Cat fleas in any stage of the life cycle cannot survive cold temperatures. They will die if the environmental temperature falls below 37°F (3°C) for several days.
Depending on temperature and humidity, the entire life cycle of the flea can be completed in as little as 12 to 14 days or last up to 350 days. However, under most conditions, fleas complete their life cycle in 3 to 6 weeks. Fleas mate after feeding, and females lay eggs within 1 to 2 days of their first blood meal.
A flea-infested dog or cat can easily introduce fleas into a home where they deposit eggs that then develop into newly emerging fleas. These then infest other pets and bite people.
Flea Allergy Dermatitis
When feeding, fleas inject saliva into the host on which they are living. Many dogs and cats are allergic to flea saliva. Even nonallergic animals will scratch due to the annoyance of flea bites. Allergic dogs itch intensely in some or all areas of the body. They are likely to be restless and uncomfortable, spending much time scratching, licking, rubbing, chewing, and even nibbling at their skin. This often leads to hair loss, scabbing, and secondary infections. In heavy infestations (or in young puppies), anemia may develop due to the loss of blood.
Most cases of flea allergy dermatitis occur in the late summer, corresponding to the peak of flea populations. Animals younger than 1 year old do not usually have flea allergy dermatitis. Usually, diagnosis is made by visual observation. Slowly parting the hair often reveals flea excrement or rapidly moving fleas. Flea excrement is reddish black, cylindrical, and pellet or comma-shaped. Placed in water or on a damp paper towel, the excrement dissolves, producing a reddish brown color. Examination of the pet's bedding for eggs, larvae, and excrement is also useful. The presence of fleas does not exclude another disease being at least partially responsible for the dog's itching and skin condition. Your veterinarian may do skin testing to eliminate other causes for the itching and confirm a diagnosis of flea allergy dermatitis. Other conditions that can look like flea allergy dermatitis include respiratory allergies, food allergies, mange and other skin parasite infestations, and hair follicle infections.
Flea control measures have changed dramatically in recent years. Flea control previously required repeated application of insecticides on the animal and the prem-ises. Recently, new insecticides and insect growth regulators have been developed that provide residual control and require fewer applications. The most effective of these products are sold by veterinarians. Insect growth regulators prevent fleas from reproducing. Flea treatments include topically applied liquids, oral and injected medications, and “fogger” sprays. You should discuss flea control products with your veterinarian and select one that works well for your individual pet and the environment in which it lives.
The overall goal of treatment is to kill all fleas in all life stages on the affected animal and everywhere in the pet's environment. The first step is eliminating fleas currently on your dog. Topical spot-on treatments can take 12 to 36 hours until the medication has spread sufficiently to eliminate all existing fleas. The second step is eliminating fleas in the pet's environment. In-home studies have shown that in many cases the newer topical and oral flea control products can effectively control flea populations without the need to treat the environment itself. By using these products, it is possible to eliminate a flea infestation in a household; however, the amount of time necessary to achieve flea control will vary because of the flea's life cycle and conditions in the environment. Typically, control of an infestation can take 6 weeks to 3 months. In cases of massive flea infestation or severe pet or human flea allergy, treatment of the household environment may still be necessary. Control may be achieved using insecticides with residual activity or by repeated application of short-acting products. Areas where flea eggs and larvae gather, such as bedding, furniture, carpets, the tiny spaces in hardwood flooring, behind baseboards, and within closets, should be treated.
If your dog spends time outside regularly, also treat the outside areas it frequents. Spraying flea control products over the whole yard is not worthwhile. Instead, concentrate outdoor treatments on shaded areas, including dog houses, garages, under porches, and in animal lounging areas beneath shrubs or other shaded areas. Other outdoor spaces where fleas can be found include cracks in shaded or moist brick walks and patios and areas under decks and steps.
Despite your best efforts, it may not be possible to totally eliminate fleas rapidly enough to prevent signs of flea allergy dermatitis in your dog. Treatment may also be required to control itching and secondary skin disease in hypersensitive animals (see Skin Disorders of Dogs: Itching (Pruritus) in Dogs). Your veterinarian can prescribe medications to control your dog's skin condition and make your pet more comfortable.
Last full review/revision July 2011 by Karen A. Moriello, DVM, DACVD; Patricia D. White, DVM, MS, DACVD; Michael W. Dryden, DVM, PhD; Carol S. Foil, DVM, MS, DACVD; William W. Hawkins, BS, DVM; Thomas R. Klei, PhD; John E. Lloyd, BS, PhD; Bernard Mignon, DVM, PhD, DEVPC; Wayne Rosenkrantz, DVM, DACVD; David Stiller, MS, PhD; Patricia A. Talcott, MS, DVM, PhD, DABVT; Alice E. Villalobos, DVM, DPNAP; Stephen D. White, DVM, DACVD