Adulticidal and Larvicidal Effects of Imidacloprid

Adulticidal and Larvicidal Effects of Imidacloprid: Two-Stage Control

Dennis E. Jacobs, BVMS, PhD, FRCVS, FRCPath The Royal Veterinary College University of London North Mymms, Hatfield, Herts AL9 7TA, United Kingdom

dult cat fleas (Ctenocephalides ( Ctenocephalides felis) felis ) feed on cats, dogs, and

A

model provided a severe test for the treatment strategy (Table 1). In

other animals, including humans, but other flea life cycle

fact, the control cats needed to be combed frequently to maintain

stages (Figure 1) develop off-host in carpets, upholstery,

their flea burdens within limits acceptable on welfare grounds. Nev-

and other places that provide shade and protection. 1 The domestic

ertheless, apparently complete suppression of the flea population

environment thereby acts as a reservoir of reinfestation for house-

was obtained in the treatment pen. This result exceeded expecta-

hold pets. Consequently, single or occasional on-animal flea treat-

tion because it had been assumed that treated cats would pick up

ments often have no more than a transient beneficial effect because

some fleas in the early stages of the trial while the carpeted pens

treated pets soon become reinfested from their surroundings. Envi-

were still infested with off-host developmental stages. Work pro-

ronmental control is therefore a necessary component of any long-

ceeding in Australia 7 provided a possible explanation. Hopkins and

term flea control strategy, especially if the aim is to prevent recrudes-

colleagues showed that skin debris falling from dogs treated with

cence of skin lesions in susceptible animals. Traditionally, this has

imidacloprid possesses flea larvicidal properties. This suggested that

been achieved by direct application of chemicals throughout the

imidacloprid may have a short-term direct environmental effect as

home. The recent development of new long-acting animal treat-

well as the longer-term impact obtained by stopping flea egg pro-

ments with flea adulticidal or insect growth regulatory activity has

duction. The next study 6 was therefore designed to ascertain

enabled alternative methods to be developed to break the flea life

whether a similar effect could have contributed to the rapid diminu-

cycle. These treatments work on the principle that if all animals in

tion of the overall flea population in the cat model.

2

3

a household are treated so that no viable flea eggs ever drop to the ground, the reservoir of eggs, larvae, and pupae in the domestic

First Larvicidal Study

environment prior to the start of a control program will progres-

To determine whether cats treated with imidacloprid can influ-

sively diminish and ultimately disappear (Figure 2). One such treat-

ence larval flea development in their immediate surroundings, 12

ment, imidacloprid, is a long-acting insecticide that is applied as a spot-on formulation (Advantage ®, Bayer).4 This paper reviews findings from several experimental models

4–6

that have been used to

On the Cat (5% of life cycle)

Adult

investigate various aspects of the efficacy of imidacloprid on cats.

Simulated Home Environment Study The first study in the series 5 showed that the establishment of  adult C. felis on cats reinfested at intervals after a single treatment

In the Environment (95% of life cycle)

Egg

with imidacloprid was reduced by 99% to 100% for at least 3 weeks

Larva

and by more than 96% for at least 4 weeks (Figure 3). The dose used was 10 mg/kg, which is the lowest dose used in practice. Thus, few if any fleas jumping onto a treated cat during this period would survive long enough to produce a significant number of eggs. Theoret-

Pupa

ically, therefore, monthly treatments with imidacloprid should break the flea life cycle and thereby provide environmental control within a household. This hypothesis was tested in a simulated home

Host-seeking Emergent Adult

environment model 4 in which matched groups of cats were kept in identical carpeted pens. The flea life cycle was established in each pen, and one group of cats was treated with imidacloprid at 28-day intervals. Results from the untreated control pen confirmed that the

Figure 1—The flea life cycle.

1 00

BREAKING THE FLEA LIFE CYCLE If fleas on pets in a home are adequately controlled No eggs will drop into environment

   ) 80    %    (   y 60   c   a   c    i    f    f 40    E 20

Reservoir of off-host life-cycle stages will be progressively depleted until

0 0

1

3

4

5

6

Weeks

24 hours

No more reinfestation

2

48 hours

Figure 2 —Principle of environmental flea control by means of animal treatments.

Figure 3 —Protection against reinfestation after a single treatment  with imidacloprid: Percentage of reduction in flea count after  repeated artificial challenge (measured 24 and 48 hours after each infestation).

TABLE 1 SIMULATED HOME ENVIRONMENT STUDY: GROUP MEAN  FLEA COUNTS AND NUMBER OF WELFARE COMBINGSa  

Week 2 4 6 8 10 12 14 16 a

Flea Counts

Welfare Combings

Control

Treated

Control

6 7 70 78 49 47 61 95

0 0 0 0 0 0 0 0

0 0 0 2 4 4 3 5

Treated 0 0 0 0 0 0 0 0

For cats treated monthly with imidacloprid and untreated controls.

domestic shorthair cats were allocated by sex and body weight into

at the end of the incubation period, indicating that the effect was

two separately penned groups: One was an untreated control group,

primarily larvicidal. Adult emergence was reduced by 84% on blan-

the other group was treated with imidacloprid at recommended dos-

kets used by cats during the second week after treatment and by 60%

es (i.e., cats weighing less than 4 kg received 0.4 ml of 10% spot-on

to 74% on those used in the third and fourth weeks. Thus, imida-

formulation while those over this body weight received 0.8 ml ). Each

cloprid was shown to exert significant larvicidal activity in the

cat was allocated an individual cage where it spent 6 hours a day for

immediate environment of treated cats. It is likely, therefore, that

5 days each week from 1 week before to 4 weeks after the treatment

this effect did contribute to the rapid control of the flea population

date. The floor of each cage was covered with a blanket that was

observed in the simulated home environment study quoted above.

changed weekly. After removal from the cage, triplicated samples were cut from each blanket and incubated with flea eggs and flea lar-

4

Second Larvicidal Study

val food following standardized procedures. Larvicidal efficacy was

In real life there are many situations where imidacloprid could

calculated by comparing the percentages of adult fleas developing

collect over periods longer than the 5 days used in the first larvici-

from the eggs placed on the blankets used by each group of cats.

dal study. A further (yet unpublished) experiment was therefore per-

  No fleas were able to develop on blankets used by treated cats

formed to ascertain whether such accumulations could provide

during the first week after treatment (Table 2). No pupae were seen

greater larvicidal efficacy than that shown in the first study. To

TABLE 2

TABLE 3

 FIRST LARVICIDAL STUDY: PERCENTAGE OF REDUCTION OF ADULT FLEA EMERGENCEa

SECOND LAR LARVICIDAL VICIDAL STUDY: NUMBER AND PERCENTAGE OF REDUCTION OF ADULT FLEAS EMERGINGa

Week

Percentage of Reduction

1 2 3 4

1 00 84 60 74

Period of Use Control 10 days 20 days

Number of Fleas

Percentage of  Reduction

41 0 1

— 100 98

a a

Flea eggs were incubated on blankets used by cats during the first, second, third, or fourth weeks after a single treatment with imidacloprid (compared with blankets used by untreated cats). All differences between control and treatment groups were significant (P ( P < .001).

Flea eggs were incubated on blankets used by untreated controls or by imidacloprid-treated cats.

getic activity (such as jumping). It is likely, therefore, that the larachieve this aim, cats from three separately penned groups were individually caged, as before, for 6 hours on 5 days each week, but the same blankets were kept in the cages throughout the investigation period. One group was treated on day 0, another on day 14, while the third was an untreated control. The trial was concluded on day 26. Thus, one set of blankets had been used by treated cats for four 5-day periods during almost 4 weeks, and another set had been used for two 5-day periods during almost 2 weeks. The in vitro tests on the blankets are continuing, but preliminary results (Table 3) indicate a 98% to 100% reduction in the number of fleas able to develop on blankets used by the treated cats.

Conclusion In the home, the greatest concentrations of flea eggs are found where cats spend most time resting or sleeping. 1, 8 These are also the places where the greatest transfer of imidacloprid will occur. The larvicidal activity of imidacloprid is thereby targeted to where it is most needed. If the mechanism of transfer is by adherence to skin debris, as is the case for dogs, 7 then imidacloprid deposits will also accumulate where flea eggs are dropped as a result of habitual ener-

vicidal effect of imidacloprid may be of practical significance in reducing the level of flea infestation in the domestic environment during the early stages of control programs using this compound.

References 1. Rust MK, Dryden MW: MW: The biology, biolog y, ecology, and management of the cat flea. Annual flea. Annual Rev Entomol 42:451– 42:451–473, 1997. 2. Carlotti DN, Jacobs DE: Therapy, Therapy, control and prevention prevention of flea allergy dermatitis in dogs and cats. Vet Dermatol in press, 2000. 3. MacDonald JM: Flea control: An An overview of treatment concepts concepts for  North America. Vet Dermatol 6:121– 6:121–130, 1995. 4. Jacobs DE, Hutchinson Hutchinson MJ, Fox MT, MT, Krieger KJ: Comparison of flea control strategies using imidacloprid or lufenuron on cats in a controlled simulated home environment. Am environment. Am J Vet Res 58:1260– 58:1260–1262, 1997. 5. Jacobs DE, Hutchinson Hutchinson MJ, Krieger KJ: Duration of activity of imidacloimidacloprid, a novel adulticide for flea control, against Ctenocephalides felis on cats. Vet Record 140:259– 140:259–260, 1997. 6. Jacobs DE, Hutchinson MJ, MJ, Ewald-Hamm D: Inhibition Inhibition of immature Ctenocephalides felis Bouche (Siphonaptera: Pulicidae) development in the immediate environment of cats treated with imidacloprid.   J Med Entomol in press, 2000. 7. Hopkins TJ, Woodley Woodley I, Gyr P: Imidacloprid topical topical formulation: LarviciLarvicidal effect against Ctenocephalides felis in the surroundings of treated dogs.  Austral Vet Pract 26:210, 1996. 8. Robinson WH: Distribution Distribution of cat flea larvae in the carpeted carpeted household. Vet Dermatol 6:145– 6:145–150, 1995.