Spotlight on research: How residual speed of kill affects flea control in dogs and cats (Sponsored by Merck Animal Health)
Does the product prevent fleas from reproducing?
We evaluate a product's ability to prevent reproduction between treatment applications with a reproductive break point study. This study assesses the time following product administration when residual speed of kill slows sufficiently, or ovicidal activity drops below 100%, to allow viable egg production.7-9 In this study design, treated and control animals are reinfested with a known number of fleas at specific intervals after treatment (often weekly, such as on Day 7, 14, 21, 28, 35, or 42). Forty-eight to 72 hours after each reinfestation, researchers carefully collect and count any flea eggs falling from the treated and control animals. These eggs are then incubated to determine their viability, as indicated by the percentage of eggs emerging to adult fleas. Thus we can determine when efficacy of a residual flea adulticide drops below 100% within 24 hours of flea reinfestation and whether the product has additional ovicidal activity. The study identifies the approximate time post-treatment when viable flea eggs are first produced from fleas that reinfest treated animals. This time post-treatment is considered the reproductive break point.
Once the reproductive break point is passed, then the flea treatment will allow fleas to deposit viable eggs into the environment, thereby maintaining the local population. Counting fleas on these treated animals 48 or 72 hours post-infestation may still show excellent efficacy against adult fleas; however, this may be false assurance because the fleas may have laid eggs before dying and reinfested the environment, thus the flea problem will persist.When evaluating the residual performance of a flea product, an effective product's reproductive break point will occur at some time point after the next labeled reapplication interval. If a product is labeled for once-a-month administration and the reproductive break point occurs around Day 21, then the product will probably not be effective in eliminating a flea infestation. Considering the results of a residual speed of kill and a reproductive break point study together gives the best indication of the real clinical efficacy of a flea treatment. A treatment that can control close to 100% of flea adults within 24 hours for the full duration of its treatment interval (as shown in a speed of kill study) and that also does not allow production of viable flea eggs within its treatment interval can be used to drive fleas to extinction in a household. This is because the treatment will not allow adult fleas to survive long enough to reproduce, and no new viable flea eggs are being added to the environment.
A recent reproductive break point study9 demonstrated that the data generated in the laboratory was then directly applicable in a field study in the high-challenge environment of Tampa, Fla.13 The reproductive break point of an indoxacarb-based flea adulticide product in the laboratory trial was well beyond the 30-day retreatment interval.9 When tested in a field trial, regular monthly applications of the product almost completely eliminated flea infestation on pets and in their home environments within 60 days.13
The use of a flea treatment that eliminates fleas within a household is still not quite the end of the story because there is always the possibility that fleas will be reintroduced into your client's household from flea populations on wildlife or from untreated households. As veterinarians, we need to persuade owners to routinely reapply an effective product at the recommended retreatment interval. In most locations, this needs to be continued year round. A treatment gap will allow fleas to reseed the environment with viable eggs.
Michael W. Dryden, DVM, MS, PhD, DAVCM (Parasitology), University Distinguished Professor of Veterinary Parasitology, Department of Diagnostic Medicine and Pathobiology, College of Veterinary Medicine, Kansas State University, Manhattan, Kansas.