Flea Control

Contrary to popular belief, flea problems don't have to be persistent and recurring as a rule. It can sometimes seem that way, though, especially for PCOs who don't take the time to study fleas and comprehend all the factors that cause infestations.

The recurrence of fleas days, weeks, or months after treatment with insecticides and insect growth regulators is almost routine, with many pet owners combatting flea infestations repeatedly. There are several explanations for these recurrences.

Following application of insecticides and insect growth regulators (IGRs), pet owners will often observe fleas again within a few days.^1,2 This apparent continued presence of fleas after application of insecticides and IGRs has been called the "pupal window."³ Flea pupae and pre-emerged adults present a major problem in control programs. Most flea control programs fail because insecticides applied to the carpet kill few flea pupae and pre-emerged adults.⁴

When a flea product containing an insecticide and an IGR is applied, most if not all of the resident adult fleas are killed and further larval development is stopped. However, pupae will continue to develop and adult fleas will continue to emerge from cocoons for the next two to four weeks (occasionally longer).^2,5

While most residual insecticides exhibit activity for several weeks, this often causes a false sense of security. These residual compounds generally do not have a quick "knockdown" rate.

Therefore, when a flea emerges from the cocoon it may survive several hours before the residual insecticide causes its death.⁶ This often allows sufficient time for the fleas to find a dog or cat, or to bite the pet owner.

HITTING THE HOT SPOTS. The pest control operator can successfully treat the home environment by using directed aerosols or pump sprays containing an insecticide and insect growth regulator. In conjunction with whole-house treatment, spot treatment of "environmental hot spots" should also be conducted.

Spot treatments should be conducted in areas where pets spend most of their time, since that is where flea eggs and larvae accumulate. These areas should be treated with a hand sprayer or directed aerosol containing both an insecticide and an insect growth regulator. Spot treatment with this type of product will ensure that adult fleas receive a lethal dose of insecticide, and that further development of flea larvae will be inhibited.

The fully formed pre-emerged adult cat flea residing in the cocoon is the stage that can extend the longevity of the flea. Once pupae have fully developed, the adult flea residing inside the cocoon (pre-emerged adult) is stimulated to emerge from the cocoon upon proper stimulation physical pressure, carbon dioxide, substrate movement and heat. If the pre-emerged adult does not receive the proper stimulus to emerge, it can remain quiescent in the cocoon for up to 140 days until a suitable host arrives.⁷

Prolonged survival of pre-emerged adults may explain reports of fleas biting people or their pets after extended absences of the hosts from the home (such as vacations); it may also explain many of the reinfestations that occur within a few weeks or months after treatment.

Under conditions where there is almost constant pet and human activity in the home, cat fleas will usually emerge from the pupal cocoon during the third or fourth week of development. However, emergence may be delayed for several weeks if pre-emerged adults are not stimulated.⁸

When humans or their pets re-enter the home following an absence, these fully developed but previously unstimulated fleas residing in cocoons are stimulated to emerge and are immediately ready to respond to host cues.

This phenomenon explains those situations that occur when the pet is returned to the home after spending several days or a few weeks in the veterinary clinic or boarding kennel. Upon returning the pet to the house, the owners often notice fleas on their pet within a few minutes to a few hours. The owners mistakenly believe the fleas must have originated where the pet was boarded.

ANNUAL RECURRENCES. While delayed emergence may explain flea recurrences after a few weeks or a few months, alternative explanations may be needed for yearly recurrences in northern temperate climates. While certain flea species survive freezing temperatures as pupae or adults off the host,^9,10 no life stage of Ctenocephalides felis can survive for ten days at 3ºC (37ºF), or for five days at -1ºC (30ºF).¹¹

The explanation for yearly recurrences may be found in the wide host range of the cat flea. In North America, a wide variety of nondomesticated hosts have been reported harboring cat fleas. Among them: coyotes, red and grey foxes, bobcats, skunks, several rodent species, raccoons, opossums, and ferrets.^8,12,13 With such a large number of alternative hosts, several of which often live in close proximity to humans and their pets, it is likely that flea-infested wild animals or feral dogs and cats are serving as continual sources of reinfestation.

A study was conducted in northeastern Kansas to determine whether small and medium-sized wild mammals living in close association with humans and their pets are serving as reservoir and overwintering hosts of C. felis. Small and medium-sized mammals were live-trapped within the city limits of Manhattan, Kan., and in rural areas surrounding this city.

Trapping was initiated June 20, 1994 and continued through December 27, 1995. (The data presented in this article represent this six-month trapping period. However, trapping continued through the winter and spring of 1995.) Animals were sedated (Ketamine & Acepromazine). They were treated with a pyrethrum flea spray and fleas were removed and identified by species.

THE RESULTS. Cat fleas were recovered from 70% (41 of 59) and 30% (15 of 50) of urban trapped opossums (Didelphis virginiana) and raccoons (Procyon lotor), respectively. On the other hand, cat fleas were recovered from only 10% (2 of 20) and 8% (2 of 24) of rural trapped opossums and raccoons, respectively.

The percentages of urban trapped opossums and raccoons infested with any species of flea were 90% and 80%, respectively. Flea numbers and the percentages of urban trapped opossums and raccoons infested with fleas increased during the fall and early winter. Predominant flea species were Pulex species and C. felis.

The percentage of rural trapped opossums infested with fleas (65%) also increased during the fall and early winter (42% summer; 100% fall and early winter). However, those increases were not due to cat fleas or Pulex species. Rural trapped raccoons had very low rates of flea infestation (13% 3 of 24). Overall, ten different flea species were recovered from opossums, and seven different flea species were recovered from raccoons.

Cat fleas were also recovered from one Eastern cottontail, Sylvilagus floridanus (1 of 11), and two red foxes, Vulpes (2 of 3). Other mammal species trapped included Sciurus niger, 16; Neotoma floridana, 53; Sigmodon hispidus, 7; and Rattus norvegicus (commonly known as the Norway rat), 5.

THE LIKELY CULPRITS. It appears that in northeastern Kansas, urbanized opossums and raccoons are serving as reservoir hosts of cat fleas. It is also apparent that cat fleas are not indigenous to these mammals. This is based upon the scarcity of these fleas infesting rural opossums and raccoons in our study. As these animals have adapted to man and the urban environment, they have come into close and more frequent contact with fleas from our pets.

These mammals are apparently serving as overwintering hosts for cat fleas in the urban environment, and may be responsible for continued reinfestation of pets. Opossums and raccoons often are found feeding out of pet food bowls, and residing temporarily under porches, in attics, in woodpiles, in brush or weeds, and in sewers. As these animals pass through yards in the spring or set up nesting sites, the flea eggs and feces drop off and a new generation of adult fleas develop. This results in the periodic or continuous infestation of pets.

Although these wild animals may be serving as a source of flea reinfestation for dogs and cats, they potentially are serving as a reservoir of susceptible genes (fleas). This untreated "refugia" of fleas would have the value of diluting the insecticide resistance developing in the urban cat flea populations.

Michael W. Dryden, DVM, Ph.D, is with the Department of Diagnostic Medicine/Pathobiology, College of Veterinary Medicine, Kansas State University, Manhattan, Kan. Alberto B. Broce, Ph.D is with the Department of Entomology at KSU.

REFERENCES

1) Osbrink W.L.A., Rust M.K., Reierson D.A.: Distribution and control of cat fleas in homes in southern California (Siphonaptera: Pulicidae). J Econ Entomol 79:135-140, 1986.

2) Dryden M.W., Prestwood A.K.: Successful flea control. Comp Cont Ed Pract Vet 15:821-831, 1993.

3) Dryden M.W.: Concepts of flea control. In Knapp FW (ed): Proceedings of 1st Symposium On Ectoparasites Of Pets. 1991, pp 56-60.

4) Rust M.K., Reierson D.A.: Activity of insecticides against the pre-emerged adult cat flea in the cocoon (Siphonaptera: Pulicidae). J Med Entomol 26:301-305, 1989.

5) Dryden M.W., Breault B., Galland J.: Evaluation of an adulticide and insect growth regulator to control flea populations in naturally infested homes. Entomological Society of America Annual Meeting. Dec 1993. Indianapolis IN.

6) Dryden M.W., Reid B.: Rate of kill of residual insecticides directed against adult cat fleas on carpets. American Association of Veterinary Parasitologists Annual Conference. July 1994. San Francisco CA.

7) Silverman J., Rust M.K.: Extended longevity of the pre-emerged adult cat flea (Siphonaptera: Pulicidae) and factors stimulating emergence from the pupal cocoon. Ann Entomol Soc Am 78:763-768, 1985.

8) Dryden M., Rust M.: The Cat Flea Biology, Ecology and Control. Vet. Parasitol. 52:1-19, 1994.

9) Humphries D.A.: The host-finding behavior of the hen flea, Ceratophyllus gallinae (Schronk) (Siphonaptera). Parasitol 58:403-414, 1968.

10) Schelhass D.P., Larson O.R.: Cold hardiness and winter survival in the bird flea, Ceratophyllus idius. J Insec Physiol 35:149-153, 1989.

11) Silverman J., Rust M.K.: Some abiotic factors affecting the survival of the cat flea, Ctenocephalides felis (Siphonaptera: Pulicidae). Environ Entomol 12:490-495, 1983.

12) Morlan H.B.: Host relationships and seasonal abundance of some southwest Georgia ectoparasites. Am Mid Nat 48:74-93, 1952.

13) Rust M.K.: The biology and ecology of fleas of domestic animals. In: F.W. Knapp (Editor). Proceedings of 1st Symposium On Ectoparasites Of Pets, Lexington, KY. 1991, pp. 2-12.