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 and the typhus-like rickettsiae.
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 (and even up to a year) 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 8 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.
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. The classic clinical signs include crusted, itchy skin over the hips, base of the tail, and thighs. This often leads to hair loss, scabbing, and secondary infections. As the disease becomes chronic, hair loss, thickened skin, and darkening of skin color are seen. 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. Generally, 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. Extremely sensitive dogs are likely to be virtually free of fleas because of excessive self-grooming. In these cases, it is usually difficult to find evidence of fleas, thus making it harder to convince you that a flea allergy is the problem. 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 premises. 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 goals of flea control are elimination of fleas on pet(s), elimination of existing populations in the environment, and prevention of later infestations of the dog. The first step is to eliminate the existing fleas on the pet. This is necessary to reduce immediate pet discomfort. One common consideration is termed rate or speed of flea kill on a pet. However, it is important to differentiate between speed of elimination of fleas already on the pet and elimination of newly acquired fleas after a product has been applied. When treating a dog with a product applied to the skin externally, it can take up to 36 hours until the medication has spread sufficiently or reached sufficient concentrations to eliminate all existing fleas. If a more rapid rate of kill is needed, a flea spray or one of the new fast-acting oral products may be desirable.
The second step in controlling a flea problem 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. Wash pet blankets, throw rugs, and pet carriers. Vacuum areas where pets sleep, giving special attention to crevices in sofas and chairs and to areas beneath sofas or beds. 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. In severe infestations, a second treatment may be necessary 7–10 days later.
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.
Many pet owners mistakenly think that flea products either kill all newly acquired fleas within seconds to minutes or completely repel them. But many products do no repel fleas, and long-acting products do not kill most fleas within minutes. Often fleas may live for 6 to 24 hours and bite before being killed. Therefore, close scrutiny of treated pets in an infested environment occasionally results in a few flea sightings on dogs for up to 8 weeks and occasionally longer until the infestation is eliminated.
An additional complication is infestation of the yard by wildlife, feral cats and dogs, or other infested pets. Your own dog may have been treated, but new fleas may be constantly added to your dog's environment by wildlife or feral animals (especially cats). Even when pets go outdoors for only brief periods, they are susceptible to becoming infested. Additionally, people can act as carriers, bringing fleas into the household and infesting unprotected pets.
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. Your veterinarian can prescribe medications to control your dog’s skin condition and make your pet more comfortable.
Also see professional content regarding fleas of dogs.