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Fleas, flea allergy, and flea control: a review

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Fleas, flea allergy, and flea control, a review
Candace A. Sousa, DVM.
Dermatology Online Journal 3(2): 7

Figure 1
Ctenocephalides felis felis
Flea infestations of pets and the home environment are a common occurrence. Fleas are responsible for the production and transmission of several diseases of humans and their pets. The flea that causes most of the problems is Ctenocephalides felis felis, the common cat flea. In one study it accounted for 92 to 990f the fleas found on dogs and cats respectively.(1)
The flea passes through 4 stages in its life cycle. It undergoes a complete metamorphosis at each stage.(2) At any time, about 570f the fleas are eggs, 34% are larva, 8% are pupa, and only about 1% are present as adults. The life cycle can be completed in as little as 12 days or take as long as 174 days and is dependent upon the ambient temperature and humidity.(3)

Flea eggs are oval, pearly white, non-sticky and about 0.5 mm in length.(4) The egg hatches between 1 and 10 days of being deposited on the host and falling off into the environment, depending on the ambient temperature and humidity (ideal conditions are 70% relative humidity and 35° C (95°F).(2-4)

Flea larvae emerge from the eggs after hatching. The larvae of C. felis felis has three stages or larval "instars." The larvae are about 2 mm long, slender, white, and covered with short hairs. They feed on organic debris and blood-containing feces from adult fleas. The larvae are negatively phototactic (move away from light) and positively geotactic (move toward the ground).(2,4) Therefore, indoors the larvae avoid direct sunlight and move under furniture, appliances, and into carpet fibers. Outdoors, they move into shaded areas under bushes, trees, and leaves. Five to 11 days are required for the larvae to molt twice during which they grow to about 5 mm in length. The larvae are extremely susceptible to heat and desiccation. They can survive only if the relative humidity is greater than 500r when soil moisture is between 1% to 20%.(3) Temperatures greater than 35°C (95°F) and cold less than 3°C (35°F) for more than 40 hours per month is also deleterious to survival. The mature larvae produce a sticky cocoon in which to pupate. Environmental debris may adhere to the cocoon which helps it to go undetected and provides excellent protection against insecticides. Pupation lasts from 5 to 9 days. Environmental locations suitable for a high rate of larval survival are termed "hot spots" or "source points."

The pre-emerged adult flea is the stage that can extend the longevity of the flea. They can survive for up to 140 days in the cocoon if protected from desiccation. In the cocoon they are also protected from most insecticides. Physical pressure and changes in light, temperature, and carbon dioxide are thought to be stimuli for emergence of the adult flea.

Newly emerged fleas can survive in the environment from 10 to 62 days, again depending on the temperature and humidity. (8) Once on the host, the flea begins feeding within seconds and becomes an obligate parasite. The animals grooming activity is the primary cause of mortality of ectoparasites. The flea feeds by piercing the skin of the host and inserting the tip of the labrum epipharynx to extract capillary blood. (5) Saliva is introduced by way of the salivary pump and used as an anticoagulant. (4) The female flea consumes an average of 14 microliters of blood per day (equivalent to 150f her body weight.)(6) About 72 female fleas will remove 1 milliliter of blood daily. Males consume less blood than females but feed more frequently. (2) Once fleas feed and initiate reproduction they become dependent on a constant source of blood or they will die within a couple of days. During feeding, female fleas excrete large quantities of incompletely digested blood ("flea dirt") in long tubular coils or fine pellets. (2)

The first of multiple matings occurs on the host within 8 to 24 hours. Egg production begins within 36 to 48 hours of the first blood meal, reaches maximum production between 4 and 9 days, and may continue for more than 100 days. Egg production peaks at 40 to 50 per day and averages 27 eggs per day for the first 50 days. A single female flea may deposit over 2000 eggs during her lifetime .(4,7)

Flea allergy dermatitis (FAD) is the most common veterinary dermatologic condition in the world. It begins with the bite of a flea. The saliva of the flea contains amino acids, aromatic compounds, fluorescent materials, polypeptides, and phosphorus. (9) In the dog the antigenic substances carry a molecular weight of 18,000 to 45,000 daltons with the major allergen of MW 30,000 to 32,000 daltons. (10) The main thing that dog owners notice is scratching, chewing, licking, biting, and other signs of pruritus. Sixty one percent of flea allergic dogs develop clinical signs between 1 and 3 years of age. (11) As animals age, with continued exposure to fleas, the degree of hypersensitivity may wane. FAD is uncommon in dogs less than 6 months of age. Patients usually have papules, crusts, salivary stains, excoriations, and erythema in a wedge-shaped pattern over the lumbosacral region, caudal thighs, proximal tail, ventral abdomen, and around the umbilicus. With chronic itching the areas become alopecic, lichenified, and hyperpigmented and the dog will develop an odor related to secondary infections with Staphylococcus intermedius and Malassezia pachydermatis.

Figure 2
A diagnosis of FAD is based on the age of onset of the pruritus, the distribution of the pruritus and clinical signs, and the observation of fleas and/or flea feces. Many dogs who are allergic to the bite of a flea have very few fleas on them at any time because their excessive grooming activity removes the fleas. Some of those patients will have recurrent tapeworm (Diplydium caninum) infestations from ingestion of the fleas. The diagnosis of FAD can be confirmed with an intradermal skin test with flea antigen.

Figure 2: Flea allergy resulting in scratching and severe alopecia.

The goals of flea control are to eliminate the adult fleas on all the animals in the house as well as immature fleas in the environment. The best approach incorporates mechanical, physical, and chemical measures. Source points should be identified and treated aggressively. Carpets, pet bedding, and resting areas in the home should be well-vacuumed using a vacuum with a power head. Pet bedding should be washed. Dead vegetation should be cleaned away from animal resting areas outside. There are a wide variety of chemicals that can be used in the environment and on pets and each has a place where it is indicated. There is no "one" best or miracle flea product.

Botanical products are the closest things we have to "natural" insecticides. Pyrethrins are derived from a certain species of chrysanthemum. There are six naturally occurring pyrethrins. They are minimally toxic to mammals (oral LD50 in rats = 1,500 mg/kg body weight).(12) Since pyrethrins are unstable in the presence of ultraviolet light, moisture, and air, most are found combined with synergists (e.g. piperonyl butoxide) that inhibit oxidative and hydrolytic degradation of the compounds. Pyrethroids (e.g. permethrin, sumethrin, resmethrin, d-trans-allethrin, tetramethrin, fenvalerate) are synthetic insecticides derived from the molecular structure or sharing the same mechanism of action of pyrethrins. They are more stable than pyrethrins but are slightly more toxic. The mechanism of action of pyrethroids is based on changes in ion conductance, mainly through delayed closing of the sodium activation gate of the nerve cells. (13) Rotenone is an extract of the root of the derris plant and contains two active ingredients. It is slightly more toxic than pyrethrins and is very toxic to fish. (12) It is safe for use in small animals but not used in many products. D-limonene and linalool are derived from citrus pulp. They have a solvent action on the cuticular lipids of the exoskeleton of the flea that results in desiccation and death of the flea. (14) They are relatively effective but very short-acting and there have been reports of severe toxic reactions in cats.

Carbamates (e.g. carbaryl, propoxur, bendiocarb) and organophosphates (e.g. malathion, ronnel, chlorpyrifos, fenthion, dichlorvos, cythioate, diazanon, propetamphos, phosmet) are cholinesterase inhibitors. They act mainly as adulticides but are potentially very toxic to pets, particularly to cats and young animals. These ingredients are found in a variety of formulations for use on the animal and in the environment. Cythioate and fenthion are also formulated as systemic insecticides.

Imidacloprid (Advantage TM, Bayer )is a topical adult flea killer that acts by binding to nicotinergic receptors in the nervous system of the insect. Claims are that most fleas are killed within 24 hours, before they have a chance to lay eggs. It is used as an on-animal spot treatment once a month and is very safe for use on mammals but can be washed off if the pet is bathed which will decrease it's efficacy.

Fipronil (Frontline®, Rhone Merieux) is another topical adult flea killer that is found as both a spot treatment and spray. It is a phenylpyrazole and acts by blocking the passage of chloride ions through GABA regulated chloride channels. It is highly specific to invertebrates. One hundred percent of adult fleas are killed within 24 hours. Once used, claims are made that fleas will be killed for up to 3 months and that since the active ingredient is incorporated within the sebaceous secretions, after 48 hours the animal can be bathed as often as needed without affecting the efficacy of the product.

Sodium polyborate (SPB) is a powder that is used in the indoor environment to interrupt the flea life cycle. It is believed that the flea larvae ingest the powder and are killed before they can pupate. Elimination of fleas in the indoor environment may require 3 to 6 weeks. SPB has a high margin of safety around mammals (oral LD50 in rats = 3,479 mg/kg.)

Insect growth regulators (IGR's) (e.g. methoprene, fenoxycarb, pyriproxyfen) mimic the juvenile hormone that the flea larvae produce during pupation. When the egg and larvae are exposed to this hormone, the egg will not hatch and the larvae will not pupate. These products have no effect on pets and people since these hormones are specific for insects. (Oral LD50 in dogs for methoprene = 5,000-10,000 mg/kg; oral LD50 for fenoxycarb in rats = 16,000 mg/kg) IGR's are commonly found in both on- animal and premise products.

Insect development inhibitors (IDI's) (i.e. lufenuron, Program®, Novartis Animal Health) are chitin inhibitors that prevent flea larvae from hatching from the egg. The product is fed to pets once monthly and deposited in the fat stores of treated animals. It is slowly released from tissues allowing maintenance of effective blood levels of drug for weeks after administration. The female flea ingests the product when feeding where it becomes incorporated into the flea egg. These products will not kill the adult flea. There are no known contraindications or side effects in mammals and lufenuron is very safe (oral LD50 in rats > 2,000 mg/kg.)

There are many formulations for the deliverance of the insecticides to the pet. Shampoos act to mechanically remove the fleas but because they are rinsed off they have minimal residual action. This problem with residual flea control can be overcome by using a final rinse (dip) that contains an insecticidal product. Many flea sprays are alcohol- based and quickly kill adult fleas. Unfortunately, most contain pyrethrins and unless the chemical is microencapsulated, their duration of action is less than 1 day. Powders, foams, concentrated solutions (spot treatments), and collars are all available formulated with a variety of chemicals, and several of the chemicals are sold in oral formulations.

In the indoor environment, hand spraying either by a professional or the owner is the preferred method of delivering the chemicals. This allows the product to be applied directly on the areas most frequented by the pets ("source points".) Large pieces of furniture must be moved to insure that the spray reaches the areas of larval migration.

Outdoors, sprays are very useful and their application should be concentrated in the areas frequented by the animals, especially those that are shaded, have a mild temperature, and contain organic matter.


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