Fleas Likely Not Resistant to New Flea Control Products

Product failures with new topical and oral flea treatments are probably caused by failure to use the products correctly rather than product resistance, according to a review recently published in Insects.

Ctenocephalides felis (the most common flea species) has been shown to have resistance to carbamates, pyrethroids, pyrethrins, organochlorine, and organophosphates, writes Dr. Michael K. Rust, of the University of California Riverside. However, resistance to newer oral and topical flea control products has not been demonstrated.

The number of flea products on the market has increased in the past several years. “Unlike many situations in agricultural pest management, [the increase in new products] is not in response to a failure of new products to control fleas or the development of insecticide resistance,” writes Dr. Rust. “Rather, it is due to the lucrative nature of the flea control market and to the consumer’s desire for extra convenience.”

Lufenuron, fipronil, and imidacloprid became available in the 1990s, and C. felis has not developed significant resistance to these compounds over the years. These compounds, as well as fluralaner, pyriproxyfen, and selamectin, disrupt the flea life cycle by killing fleas before they feed and lay eggs. In this way they may avoid the problem of resistance.

Although insecticide resistance is sometimes the blame for product failures, operational factors are often the true culprits and wild animals also harbor fleas that can re-infest pets. Reported causes of product failure in treated pets include the following:

• Failure to properly apply the product
• Failure to follow label instructions
• Failure to treat all household pets
• Failure to apply the product all year round
• Excessive bathing, which may alter the efficacy of some topical products

Investigating insecticide resistance in fleas is complicated by the lack of a standard susceptible flea strain to use in laboratory studies. Many laboratory strains of fleas have genetic mutations linked to pyrethroid resistance. Field populations of fleas should also be monitored to assess their response to new insecticides. “Vigilance is necessary if we are to protect and preserve our active chemistries for flea control,” says Dr. Rust.

Dr. Laurie Anne Walden received her doctorate in veterinary medicine from North Carolina State University in 1994. After an internship at Auburn University College of Veterinary Medicine, she returned to North Carolina, where she has been in companion animal general practice for over 20 years. Dr. Walden is also a board-certified Editor in the Life Sciences and owner of Walden Medical Writing.