Imidacloprid is still highly effective against both laboratory and field strains of Ctenocephalides felis, according to an international study that was initiated in 1999.1
In January 2008, the latest data were compiled by the International Flea Susceptibility Monitoring (IFSM) group.2 The IFSM group assayed C. felis larvae in the United States and in European countries from 2001 to 2007, and each time the results were the same—nearly all
flea isolates assayed for susceptibility to imidacloprid did not survive Step 1 of the bioassay. The few flea eggs that survived
to adulthood didn't survive the second step of the bioassay. A very small number of isolates that survived the first step
of the bioassay never made it through the second step, a complete dose response assay.
The data on file with the IFSM group suggest that variations in the flea population, location, and compliance issues might
contribute to increased client reports of perceived product failure.2 The IFSM group, which was formed in 1999, was charged to scientifically monitor the susceptibility of fleas to imidacloprid
and dispel any misinformation that was being generated. They developed a method for monitoring the susceptibility of C. felis to imidacloprid and assayed its efficacy in a controlled laboratory environment. The results definitively show there was
no reduced susceptibility of the assayed flea isolates to imidacloprid since the inception of the monitoring group.
A study was conducted to monitor the susceptibility of C. felis isolates to imidacloprid.
From 2001 to 2007, 1150 flea egg isolates were collected and assayed in the United States and Europe. Clinic personnel took
flea egg specimens from both dogs and cats in the United States, United Kingdom, and Germany (see Figure 1). Of the isolates collected, 909 contained a sufficient number of eggs to be included in the larval bioassay.
Figure 1. Hosts from which flea eggs were obtained*
Researchers let the eggs hatch, exposing the test group of larvae to flea-rearing media containing a diagnostic dose of imidacloprid.
They monitored these larvae for 28 days, at which time live adult fleas in the treated and control groups were counted.
If on day 28 any isolate had adult fleas numbering 5% or more of the number of larvae assayed, the fleas were propagated on
laboratory cats before being removed and utilized in Step 2 of the bioassay, a complete dose response test. If the number
of surviving fleas in Step 2 was significantly higher than laboratory strains or field isolates, the flea isolate being assayed
would be propagated further for evaluation (Step 3) using an in vivo challenge model on cats.
Of the 909 flea isolates assayed through 2007, only 11 isolates had a survival rate of 5% or more. All 11 of these isolates
progressed to Step 2, the complete dose response test. Adult survival of these isolates in Step 2 was not significantly different
than the laboratory strains or field isolates. Additionally, none of these isolates progressed to Step 3 in the bioassay.
Since the IFSM has never found an isolate showing reduced susceptibility, it has actually never proven or disproven the presence
of resistance of C. felis to imidacloprid. The study suggests that if resistance does actually occur, it appears very difficult to find.
According to the IFSM,2 clients may develop a false impression about the efficacy of a particular flea-control product because they still see fleas
after treatment. But in areas where and years when the flea population is high, these observations will likely occur even
if a product is administered properly. A dog or cat could be treated effectively for a month and then contract fleas from
an external source after treatment (e.g., from a backyard or another animal).
The IFSM group says the veterinary community should be cautious about presuming that perceived flea product failures are caused
by reduced susceptibility. The IFSM group says the capability to identify and document susceptibility to flea-control products
will provide for a better understanding of the molecular and biochemical basis of resistance. It also says similar susceptibility
monitoring programs on other flea control products may be developed in the future.