A dog in Tampa, Fla., had a severe flea infestation.1 An area flea-counting system was used to evaluate pet flea counts.2 The initial area flea count on the dog was 125, which was about 23.5% of the total flea burden; thus, the dog had an estimated
530 fleas (see table). The dog was treated with a topical residual fipronil–(s)-methoprene formulation that initially produced a slight reduction
in flea counts. But on day 21, the pet area flea counts increased to 186 (790 total flea estimate). At first glance, you might
assume that product failure had occurred and that possibly resistance was to blame. In fact, flea counts on the dog were still
high on day 28.
Flea Counts in a Home and on a Dog After Flea Product Administration
In analyzing these data, several important biologic and product-related factors must be considered. In such studies we also
use intermittent-light flea traps3 to evaluate the level of premises infestation since these traps collect fleas emerging from the cocoons. In this case, note
that flea trap counts (counts of emergent fleas) were actually higher on days 7, 14, and 21 than on day 0 and that emergent
flea trap counts were still high on day 28. But keep in mind that the fleas collected in intermittent-light flea traps on
days 7, 14, 21, and 28 were likely produced from flea eggs laid before treatment was initiated. One of the biggest unknowns
in a household is how many flea eggs were laid before treatment was initiated and what percentage of those eggs will develop
into adult fleas—or, stated differently, the level of reinfestation pressure. In such a household, it is possible that a flea
problem may appear to get worse after treatment. Note that in this home, the pet area flea count was highest on day 21 (186)
when the emergent trap counts were also at their highest (678).
Also keep in mind that because of the half-lives of residual insecticides, a product's speed of kill will inevitably slow
toward the end of the month.4 So products may work well for the first two or three weeks after administration but appear to be faltering at the end of
the month since fleas continue to emerge and reinfest the pet at the same time the residual speed of kill is slowing. It is
important to understand that this is not product failure, it is just an indication of a slowing speed of kill.
After the second application of the product to the dog on day 28, there was a dramatic reduction in pet area flea counts of
95.2% on day 42. Again, someone might assume that the second application worked where the first application did not. However,
an evaluation of the emergent trap data combined with knowledge of flea biology indicates that the dramatic drop in flea numbers
is due more to the application on day 0 than to the second application on day 28. The emergent trap flea count on day 42 was
only 34 fleas, dramatically less than the 230 on day 28. But since the fleas emerging on day 42 came from flea eggs laid three
to eight weeks previously, the second product application on day 28 had little if anything to do with this decrease. The precipitous
drop in emergent flea trap counts after day 28 is directly related to the reproductive suppression of the fipronil–(s)-methoprene
formulation during the month immediately after the first application. This example is likely representative of other outlier
cases involving flea control with imidacloprid, selamectin, spinosad, metaflumizone, or other veterinarian-recommended products.
When faced with such a flea infestation, you can readminister the product in a manner consistent with the product label. To
increase the effective dose and speed of kill, administer a short-acting adulticide, such as nitenpyram orally once a day
or every other day, for two or three weeks. Switching products should be a last resort. As evidenced by the data from this
case, if a product switch had occurred, the second product would have received the credit for the dramatic reduction in the
flea infestation, when, in fact, the first product was responsible.
1. Dryden MW, Burkindine S, Lewis T, et al. Efficacy of selamectin in controlling natural flea infestations on pets and in private
residences in comparison with imidacloprid and fipronil, in Proceedings. Am Assoc Vet Parasitol Annu Mtg, 2001; P34.
2. Dryden MW, Boyer JE, Smith V. Techniques for estimating on-animal populations of Ctenocephalides felis (Siphonaptera: Pulicidae). J Med Entomol 1994;31(4):631-634.
3. Dryden MW, Broce AB. Development of a trap for collecting newly emerged Ctenocephalides felis (Siphonaptera: Pulicidae) in homes. J Med Entomol 1993;30(5):901-906.
4. Dryden MW, Smith V, Payne PA, et al. Comparative speed of kill of selamectin, imidacloprid, and fipronil–(S)-methoprene spot-on
formulations against fleas on cats. Vet Ther 2005;6(3):228-236.