Understanding flea biology key to busting resistance speculation

During the past decade, there has been enormous improvements made in flea protection products for use on pet animals and in the environment.

The emergence of the topical adulticides, such as imidocloprid, fipronil and selemectin, has revolutionized flea control in the United States. In addition, the pre-adulticides that include lufenuron, methoprene and pyriproxyfen also have proven effective in flea control. Unfortunately, there has been speculation recently that the common cat flea (Ctenocephalides felis) has demonstrated resistance to some of these chemicals. This is considered to be a misconception among most flea biologists and veterinary dermatologists, and the true "resistance" stems from the lack of knowledge of flea biology, existing resistance to older adulticides and poor compliance (See suggested reading).

Biology revisited

Ctenocephalides felis

(

C. felis

) is the common flea found on most domesticated animals. This cat flea is found on cats, dogs, raccoons, opossums, domesticated rabbits, ferrets, cattle, foxes, coyotes, bobcats, koalas and some avian and rodent species. It is also found to infest the mongoose population in Hawaii. They are rarely found on squirrels or wild rabbits.

C. felis

stays permanently on these hosts, where they feed, breed and eventually die.

They do not survive for extended periods of time off the host, however, survival rates of 12 days or more were reported under moist conditions in homes. In addition, adult fleas may, on occasion, leap onto clothing of humans and be carried away to a new location.

Fleas are obligate parasites and are a permanent parasite on the host until it dies, usually in about 100 days. The average pet has twice as many female fleas as male fleas. The female flea begins feeding on the pet and starts to lay eggs within 24 hours of hatching and can lay up to 40-50 eggs per day.

The eggs are not sticky and fall off the pet into the environment. It has been clearly shown that many eggs do not survive and undergo desiccation in dry environments. However, due to the massive reproductive capabilities of the adult flea, a substantial infestation can still occur in the home as well as the outdoors.

The ova of C. felis begin hatching within a few days after they enter the environment (home and outdoors). The proper conditions for survival include adequate temperature (probably the most important factor) and humidity. The ideal temperature is 40-85 degrees F and humidity above 50 percent. Dry conditions with humidity below 50 percent can be lethal to ova. Ova and larvae simply desiccate when exposed to hot and dry conditions with inadequate moisture.

The surviving eggs hatch into larvae within a few days or weeks. The larvae also undergo several moltings. Larvae survival is also dependent upon similar temperature and humidity values as the ova but also are reliant on a food source: adult flea feces (dried host blood). The larvae enter the pupal stage by spinning a loosely packed silk cocoon. This sticky cocoon often is coated with environmental particles and is much more resistance to extreme environmental conditions and insecticides. Pupae will hatch in 13 days or less under ideal conditions, but may survive for five months or more without an adequate host. The time for pupal development and hatching is termed the pupal window.

The time from deposition of eggs to adult-flea emergence is called the developmental window and can be up to three months in duration. These pre-adults are found in the carpeting fibers, pet bedding, under furniture, cracks in hardwood floors, under sofa cushions, soil, grass, sand and in animal burrows.

When encountering a flea infestation on a pet, especially a pet with symptoms of flea allergy dermatitis, a three-step program is recommended to eliminate or reduce flea bites on the pet.

Initially, the proper application of adulticides, such as fipronil, imidocloprid or selemectin, on all pets is advised. Some of these products demonstrate larvicidal properties.

Cats tend to lick the non-dried product after application, so these products are applied to the base of the head in an area that cannot be groomed. Most animals that have adult active fleas on the coat are most likely exposed to an infested environment thus overwhelming the adulticide product.

Frequent shampoo therapy (especially in dogs) with strong soaps or stripping ingredients may remove some of the product as well. It is recommended to apply adulticides on a dry coat and delaying shampoo several days after application.

Treating the home

The second objective is to apply treatment to the home environment. There are at least two insect growth regulators (methoprene and pyriproxyfen) available on the market in the Unites States, and they are designed to interfere with egg hatchability and larval development. These two stages can comprise more than 50 percent of the pre-adult population.

Most house sprays are water-based and contain an accompanying adulticide, such as permethrin or tetramethrin. The spray should be applied to pet bedding, all carpets, hardwood floors, under furniture, under sofa cushions, closets, carpeted-based cat condominiums, door-mats and carpeted areas in the automobile. These insecticides also demonstrated some ovicidal and larvicidal properties but may not persist in the environment for long periods of time.

It is therefore recommended to treat the home with a second application two weeks later (pupal-window) thus achieving a quick knockdown of newly emerging adult fleas. A third application is also advised three to six months later.

Finally, sodium polyborate powder has been shown to be effective in controlled indoor pre-adult stages. The powder acts as a desiccant and, when ingested by larvae, acts as a toxin.

The third and final treatment focuses on the outdoor environment. This area may not be as important as the home due to the extreme variations in temperature and humidity or to the pet's environment.

If dogs and cats frequent the yard of a home, it is advisable to treat these areas monthly with either malathion or diazinon. Newer and safer products are also available and include biologics, such as a nematode spray. The nematode, Steinernema carpocapsa, preferentially parasitizes flea larvae and other destructive insects, such as cut worms and army worms.

The nematodes are reported to be safe for outdoor use but are effective only in moist-shade and part-shade areas and should be applied every one to three months.

With the knowledge of flea reproduction and the proper use of adulticides and environmental treatments, one can deal swiftly and adequately to control flea infestations and the associated flea allergy in pets.

Consistency with treatment is also critical and will enhance treatment success. Finally, client education is very important, and an adequate flea-control program is not complete without the education of the owner and the monitoring of compliance.

Dr. Vitale received his veterinary degree from Mississippi State University, College of Veterinary Medicine. He completed a residency in veterinary dermatology at the University of California, Davis and is a diplomate of the American College of Veterinary Dermatology. He is a clinical instructor/lecturer at UC-Davis and a staff dermatologist at East Bay Veterinary Specialists (formerly Encina Veterinary Hospital), Bay Area Veterinary Specialists and San Francisco Veterinary Specialists.