Fleas are well-adapted parasites, obtaining their nutrition from the blood of their host.  The two species most
often encountered are Ctenocephalides canis and C. felis, and either may inhabit the dog.  The flea is probably the
most common cause of dermatitis in the dog, particularly in the southeast.  Resistance to insecticides has increased the problem. 

Flea Life Cycle
The adult flea lays 20-28 eggs per day, and eggs hatch in the environment approximately 4-7 days later.  The newly hatched fleas are first stage larvae which feed on organic material and molt to form second stage larvae within a week.  Within another week the second stage larvae molt to form the final third stage larvae.  This larva spins a cocoon and goes into a pupal stage.  The adult flea emerges from its cocoon in several days to 2 weeks.  The entire life cycle normally takes 18-21 days, but high temperature and humidity accelerate the process.  Fleas do not live in cold climates, nor do they thrive in hot and dry climates, or at high altitudes.  It is the adult flea that spends its time on the host, feeding on its blood, leaving to lay eggs in the environment.  Eggs and pupae are fairly resistant to insecticides, making control of these stages difficult.

Adult fleas prefer to feed on dogs and cats, and will usually only attack humans if they are very hungry.  The life span of the average adult flea is usually 3 months, although it may live longer if environmental conditions are very favorable. 

Fleas and Tapeworms
With an infestation of fleas comes the occurrence of tapeworms, Dipylidium caninum.  Through grooming or chewing, the dog ingests an adult flea containing tapeworm eggs.  Once the eggs are released, they grow to maturity in the small intestine.  Segments of the tapeworm (proglottids) are passed in the dog's feces, and the segments rupture releasing eggs.  Flea larva feed on the ruptured proglottids, eating eggs.  The flea larvae finish their own life cycle producing an adult flea containing tapeworm eggs.  This entire cycle takes 2 to 4 weeks.

Tapeworms damage the small intestine, causing loss of nutrient absorption, and altered motility.  Drugs such as Droncit and Yomesan are very effective in removing tapeworms from the dog, but to prevent immediate reinfestation with tapeworms, the fleas must be removed from the environment.

Flea Allergy Dermatitis
During feeding, the flea injects saliva into the skin (intradermal) of the dog.  This saliva contains a number of irritating substances causing the dog to itch or bite at the site.  There is also the potential to develop hypersensitivity to the injected saliva.  It is not known why some dogs develop an allergy, and others do not, although genetics is likely to be involved.  Also, the age at which exposure first occurs may play a role in the development of allergy.

The clinical signs depend on the degree of severity of the allergy.  In the allergic dog, a small red circle of inflammation (wheal) develops at the flea bite site.  This may develop into a pimple, and will crust over.  The allergic dog will show intense itching, even to only a few flea bites.  Self-trauma can be extensive, with widespread redness, crusting, hair loss, and thickening of the skin.  The lower part of the back, the rump and the backs of the thighs are most commonly involved.

Most diagnoses of flea allergy dermatitis are made on the basis of appearance of signs.  To be absolutely certain the dog is allergic to fleas and not some other allergen, an intradermal skin test using isolated flea allergens is performed.  Treatment consists primarily of getting rid of the fleas.  Methods of flea control depend on personal preference and geographic location.  What works in the north may be totally ineffective in the south!  Hyposensitization (desensitization) by injection has been tried, but has not been shown to be very effective.  Short- acting corticosteroids such as prednisolone may also be used in conjunction with flea control for short periods of time to control the intense itching of flea allergy dermatitis.

AN UPDATED LOOK AT INSECTICIDES 
or What's in All Those Flea Products

Fleas continue to be a problem, but with new types of products available such as fipronil, imidacloprid, and lufenuron, flea control can be achieved a little easier.  There are a vast number of products available, and most of us use a combination of products to keep fleas under control.  Combining products can be a problem, and can result in toxicity if the wrong products are combined.  The best way to prevent toxicity is to know what you are using.  Read every label of every product, and be aware of which chemicals can be used in combination with other chemicals, and which can't.  Hopefully the following will help you understand how the different chemicals work. 

Botanicals
Pyrethrin is an extract of the chrysanthemum and has a very low toxicity.  It is relatively safe for cats and young animals.  Pyrethrin has a very fast knockdown effect, killing adult fleas almost immediately, acting on the flea's central nervous system.  Pyrethrins are never used without synergists, because synergists are needed for maximum kill.  Pyrethrins are very rapidly degraded after application especially by ultraviolet light (4 hours or less).  Microencapsulation of the pyrethrin may prolong the anti-flea activity.

Pyrethrin is the most common ingredient in pet sprays, shampoos and powders.  It is also found in foggers and house sprays for an immediate kill of adult fleas.  Pyrethrin works more rapidly in an organic solvent base such as alcohol, rather than in water, where it is fairly insoluble.  There has been no evidence of resistance of fleas to the action of pyrethrin.  Pyrethrins have very little residual activity, so daily application is required.

Rotenone is extracted from the root of the derris plant.  It is more toxic than pyrethrins, and is limited to use in pour-on insecticides.  It is very toxic to fish.  The use of rotenone is uncommon, but is sometimes used in combination with pyrethrin, lindane, or carbaryl.  Rotenone paralyzes the flea's cardiovascular and respiratory systems, and does not require synergizing for activity.

D-Limonene, an extract of citrus peel, has been used for a long time as an insecticide.  Its effectiveness is questionable when compared to the other botanicals, and must be used at a relatively high concentration to have any effect.  Limonene may be toxic to cats, and some dogs (and humans) are sensitive to it as well. 

Synthetic Pyrethroids
These compounds are synthesized chemicals that resemble the chrysanthemum extract.  They produce a quick kill of adult fleas, are more stable than pyrethrins, and have higher potency.  Permethrin is the most commonly used pyrethroid, and is a component of flea sprays and pour-on treatments.  Permethrin binds to the hair, but is removed if the pet is bathed or goes swimming.  Sprays containing permethrin are typically applied weekly, and spot-on formulations (which use higher concentration of permethrin) are effective for approximately one month. 

The use of pyrethroids on dogs is safe, but cats are very sensitive, and toxicity can result. Pyrethroid- containing products for use in cats contain very low concentrations of pyrethroid. Signs of toxicity in the cat include salivation, depression, muscle tremors, ear flicking, paw shaking, loss of appetite, seizures and death. 

Synergists
Synergists that are added to pyrethrin compounds work by preventing enzyme breakdown of the pyrethrin by the flea.  The two major synergists are piperonyl butoxide (PBO) and N-octyl bicycloheptene dicarboximide (MGK).  These compounds are extremely safe on dogs, but side effects such as tremors and incoordination may appear in cats with the use of PBO.

Petroleum Distillates
Petroleum distillates or kerosene is often listed on product labels as an active ingredient.  It is usually the solvent for the pyrethrin or synthetic pyrethroids.  It has some insecticidal properties, and may also be listed as an inert ingredient. 

Carbamates and Organophosphates
This group of insecticides has been used extensively for years.  Both types of compounds act by interfering with the acetylcholinesterase enzyme.  Acetylcholinesterase normally breaks down the acetylcholine that is involved in nerve transmission.  When acetylcholinesterase is inhibited, acetylcholine builds up, resulting in continuous nerve stimulation, leading to death of the flea. Organophosphates change the structure of the enzyme, thus reversal of the process is very slow.  Carbamates block the enzyme rather than change it, so its action is more rapidly reversible.  This is an important concept when considering toxicity.  Carbamate toxicity of the dog is more easily reversible than is toxicity of organophosphates.  Intoxication results in neurologic signs such as vomiting, diarrhea, muscle twitching, weakness, or seizures.  If a dog develops signs of toxicity after being dipped or sprayed, it should be washed immediately and taken to your veterinarian for treatment.  Death can result if the animal is not treated. Carbamates and organophosphates should not be used on cats, nor on pregnant, lactating, or young animals.

This group of insecticides must be used with caution, and should not be used with other drugs that also act by inhibiting acetylcholinesterase.  Directions for use should be followed explicitly, and two organophosphates or carbamates should not be used simultaneously in the flea control program.  One should also be aware that some wormers and anesthetics work via their acetylcholinesterase inhibiting action, thereby potentiating toxicity. 

Chlorpyrifos (Dursban) is a popular organophosphate premise spray, and may be present in some dog sprays and dips.  Fenthion is an organophosphate that was used as a spot-on treatment for dogs. Some humans are extremely sensitive to fenthion, and may show signs of toxicity after use on their pets.  Carbaryl (Sevin) is the most commonly used carbamate, and is used as a premise treatment.  Propoxur (Baygon, Sendran) is a carbamate used in shampoos, sprays, and flea collars.  Chlorfeniphos is an organophosphate present in some dips, and has also been used as a premise spray.  Dichlorvos (Vapona) is not very popular anymore, but is sometimes used in flea collars. Malathion is an organophosphate that is widely used as a premise spray, and in some dips.  Phosmet is an organophosphate present in some dips.  Ronnel is an organophosphate known as Ectoral.  It can be found in some dips and premise sprays.  Fleas have become increasingly more resistant to these compounds.

Insect Growth Regulators (IGR)
Insect growth regulators mimic the natural growth hormone of the flea.  This growth hormone is necessary for normal development of the larva, but high concentrations of it inhibit the maturation of the larvae.  IGR in the environment prevents maturation and causes death of the larvae.  IGR is present in many sprays and foggers to prevent emergence of new fleas.  One must take into account the life cycle of the flea.  Since IGR acts late in the life cycle, it will take 1 to 4 weeks to actually see a beneficial effect from application.  This time period depends on the temperature of the environment, and rate of larval development.  They are safe to use on both dogs and cats. 

Methoprene
Methoprene (Precor) is a common IGR used in premise sprays and topical products.  Methoprene is light sensitive and will not last outdoors.  Pyriproxyfen (Nylar) is used in premise sprays, shampoos, sprays, spot-ons, and collars. Pyriproxyfen is not sensitive to light.  Flea control can last up to 60 days with spot-on application. 

Fipronil
Fipronil (Frontline) is effective against both fleas and ticks.  It acts by binding to GABA receptors, resulting in excitation of the nervous system and death in insects. 

Fipronil is available as a 10% spot-on formulation (Frontline Top Spot), or as a 0.25% alcohol-based spray (Frontline Spray).  Both the spray and spot-on formulation show a greater than 95% efficacy after 36 days.  Fipronil is not easily removed by shampoos or water.  It accumulates in the sebaceous glands of the skin, and is slowly released via the hair shaft ducts.  Fipronil is very safe, since the GABA receptors of mammals are different from those of insects.  It is safe for very young puppies and kittens, and pregnant/lactating dogs and cats.  Incidence of adverse reaction to the application of Frontline is less than 0.0005%.

Imidacloprid 
Imidacloprid (Advantage) was originally used for crop pest control, and is approved for use in dogs and cats.  It works on contact by preventing transmission of neuro-impulses, resulting in death of the adult flea.  Imidacloprid is not effective against ticks. 

Imidacloprid is applied monthly as a pour-on.  Within 24 hours after application, it is evenly distributed over the body, with no systemic absorption.  It has no repellent activity, and is not inactivated by sunlight.  It is partially removed by swimming and bathing.  Imidacloprid has low toxicity in mammals, and is safe in the environment.  It is safe to use in pregnant or lactating dogs and cats, and puppies and kittens. 

Lufenuron
Lufenuron (Program) targets the development of immature stages of fleas by disrupting the synthesis of chitin, which is essential for survival for flea eggs and larvae.  Lufenuron is given orally once monthly with food, and concentrates in fat, from where it is slowly released into the blood of the dog or cat.  When fleas bite the dog or cat, they ingest lufenuron, and subsequently lay eggs that cannot hatch.  This effectively breaks the life cycle of the flea.  Lufenuron is not ideal in the flea-allergic animal, since the flea must bite the dog or cat to ingest it.  Also, in heavily infested areas, it takes time to break the life cycle and eliminate fleas. Also any untreated animals entering the area can reinfest the environment.  Best results are achieved with indoor pets, or those in a confined environment.  Lufenuron is very safe in mammals, since they do not have chitin.  It is safe for pregnant/lactating dogs and cats, and puppies and kittens. 

In conclusion, choosing a flea control program can be a difficult decision.  Your own veterinarian should be consulted, and you may wish to talk to other dog people in your area to see what works for them.  The new flea control products such as fipronil, imidacloprid, and lufenuron are excellent choices, but are quite expensive when managing a kennel situation with multiple dogs.  However many kennels do use these products effectively, since their use does decrease the expense and time involved with purchasing and applying topical sprays and premise control.  When using other topicals and premise treatments, usually an insecticide from one class can be safely used with an insecticide from another class.  For example, one could safely use a pyrethrin spray combined with an indoor insect growth regulator spray and an organophosphate outdoor spray.  Extreme caution must be taken when combining two different chemicals from the same group, especially the organophosphates/carbamates because toxicity may result.  For example, one should not use a flea collar containing propoxur, a dip containing phosmet, and a chlorpyrifos premise spray.  However, pyrethrins may be used in combination with each other due to their low toxicity.

Patricia Schenck, DVM, PhD
Veterinary Nutritionist