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Compendium July 2011 (Vol 33, No 7)

Flea Control: Real Homes, Real Problems, Real Answers, Real Lessons: The "Deep Dive"

by Michael W. Dryden, DVM, MS, PhD, Doug Carithers, DVM, EVP, Michael J. Murray, DVM, MS, DACVIM


    This is the fourth of a five-part series of cases that formed part of a field-study research project conducted by the Kansas State University Flea Team in 2009 in Tampa, Florida. BOX 1 describes the criteria for inclusion and the methods used in this study.

    In this case, you will learn how complicated and tangled flea control cases can be. Flea cases are diagnostic challenges just like other medical problems, requiring a thorough understanding of the science of flea biology and the mindset of pet owners.


    Flea control has always been difficult. The advent of modern flea control products with excellent month-long activity against fleas and/or their eggs has had a tremendous impact on flea control; however, for some pet owners, flea control still seems problematic. Consequently, veterinary practices continue to get cases in which it appears that the flea control product they sold failed to work. 

    It is often not possible for the veterinary clinic to get to the root cause of all of these cases. Flea biology and its interaction with the environment and flea hosts are complex, and it is not feasible for veterinary staff to conduct in-home investigations. This series of cases illustrates real examples of challenging flea control situations in which owners continued to see fleas on treated pets. By understanding the results of these investigations, you and your staff can see how complicated flea infestations can become and why simply blaming product performance is not a solution. 

    Essentials for Understanding Flea Biology and Flea Control

    Before examining this case, it is essential to understand the following facts about flea biology and flea control:

    • As an adult, Ctenocephalides felis felis, the cat flea, is an obligate parasite that is metabolically and reproductively bound to its host.1–3 This revelation dramatically changed approaches to managing fleas: by applying products to and on pets, owners and veterinarians attempt to control reproduction and “break the life cycle” rather than focusing on the environment.4–6
    • Once on a preferred host, cat fleas begin feeding within minutes and start breeding soon after. A female flea can begin laying eggs within 24 to 48 hours of jumping onto a host. In a few days, a female flea can lay 40 to 50 eggs/day.2
    • Flea eggs roll off the host, and larvae typically hatch in 3 to 5 days. The larval stage is the most sensitive. Flea larvae require flea feces for nutrition, protection from direct sunlight, temperatures in the range of 45°F to 90°F, and relative humidity in the range of 50% to 85%.7,8
    • Most larvae do not survive to become adult fleas. 
    • The rate of flea development depends on the temperature. Development from eggs to fleas can take less than 3 weeks at 85°F and can take 7 to 12 weeks at 65°F.7,8
    • New fleas develop and emerge where pets or other flea hosts spend most of their time because this is where the most eggs and feces are deposited, and larvae require flea feces for nutrition.  
    • Common hosts for C. felis include cats, dogs, opossums, raccoons, domestic rabbits, and hedgehogs.7,8 Squirrels and birds are not hosts for cat fleas.  
    • Feral cats, opossums, and raccoons move throughout neighborhoods. These “urban wildlife” hosts for the cat flea often seek shelter in covered, protected areas, and wherever they rest, they leave behind flea eggs and flea feces.  

    With some indoor flea infestations, the number of fleas emerging into the home increases substantially in the month after all pets are treated with a monthly flea control product.5,6 These are called redline homes, and in these cases, pet owners will see more fleas on their pets—and possibly themselves—than before treatment. When confronted with increasing numbers of fleas, pet owners inevitably conclude that the flea control product is not working at all. In fact, the fleas emerging into the home after the pets were treated came from eggs laid weeks before treatment. Those eggs will continue to develop into larvae, pupae, and adult fleas, and the pet owners will continue to see new fleas on their pets until all the adult fleas have emerged. Depending on the temperature, it can take 3 weeks to several months for the infestation to run its course.9–12

    Case Presentation: The “Deep Dive”

    Signalment: A 2-year-old, spayed Parson Russell terrier in an upstairs apartment and a small, 5-year-old, neutered terrier mix in a downstairs apartment. The apartments were in a detached dwelling, and the dogs shared the same yard and had access to both apartments.

    History: At the initial examination, there were 41 fleas on the upstairs dog and just six fleas in the upstairs flea traps. Conversely, the downstairs dog had nine fleas on it, and there were 22 fleas in the downstairs traps. During the month after the first treatment with Frontline Plus, the flea counts in the traps in the upstairs apartment remained relatively low, but the upstairs dog still had several fleas (21 to 51 at different counts). In the downstairs apartment, the flea counts in the traps increased after treatment (redline home), as did the flea counts on the dog. As is typical for redline homes, the flea counts in the traps and on the dogs declined by 28 days after the first treatment, indicating that the flea infestation in the apartment was running its course (FIGURE 1 and FIGURE 2).

    However, flea trap counts on study day 45 increased considerably in both apartments (FIGURE 1 and FIGURE 2). This increase was extremely odd, and the Flea Team conducted a thorough investigation (TABLE 1).

    Table 1. Questions Asked of the Owner



    How many hours a day does the pet spend outdoors? Where does it go: the backyard, shared courtyard, sidewalk, or dog parks?

    The dogs have free access to the yard.

    Do other pets visit your household, or does your pet visit another home?


    Where does your pet sleep or rest?

    The upstairs dog sleeps on a couch in the den; the downstairs dog sleeps in the daughter’s room.

    Do you have an elevated porch, a crawl space, or another structure under which your pet, stray animals, or wildlife might go?


    Do you see feral cats, dogs, or wildlife in your yard or neighborhood?


    Examination of the pets: Although the owner of the downstairs apartment indicated that no other pets visited the building, the Flea Team students reported finding two pit bulls at the home when they placed flea traps on study day 45. The owner said that the dogs had just come over for the day. The students noted fleas on the visiting dogs and treated both dogs with Frontline Plus.

    Examination of the premises: Because the surge in flea trap counts, both upstairs and downstairs, was so unusual at this point of the investigation (day 45), the fleas from the traps were examined under a dissecting microscope to test the hypothesis that a flea infestation running its course should consist of more male fleas than female fleas.13 Dr. Dryden examined and determined the sex of each flea collected in the traps from study day 45 (TABLE 2).

    Table 2. Flea Counts on Day 45


    Number of fleas

    Upstairs traps

    Number of fleas

    Male:Female ratio

    % groomed fleasb




    Downstairs traps

    Number of fleas

    Male:Female ratio

    % groomed fleasb




    aIndicative of late-stage flea infestation.

    bBased on finding fleas with ingested blood in the traps.

    cIndicative of early or replenished flea infestation.

    dOne gravid female flea was found in the trap from the parents’ room.

    The first fleas to emerge from a cohort of C. felis eggs are female, followed by both male and female fleas, and finally almost exclusively males. Therefore, when flea reproduction in the home has been stopped by killing fleas on the pets before they can reproduce (adulticidal effect) or preventing flea eggs from developing (insect growth regulator effect), over time, the ratio of male to female fleas should increase to a mostly male-dominated population. If this shift is not seen in trap collections, then new, viable flea eggs are likely being introduced into the environment by untreated animals.

    Diagnosis: The male:female ratios in the flea traps from the upstairs apartment reflected an aged population. It was anticipated that the flea trap counts would subsequently decline as the flea population ran its course, and this was verified by flea trap counts performed on study days 60 and 74 (TABLE 3). The male:female ratios in the flea traps from downstairs reflected an active infestation with more female than male fleas. Also, most fleas in the traps downstairs had blood in them, including one gravid flea, indicating that they had been groomed off an animal on which they had fed and had subsequently jumped into the flea traps (FIGURE 3).

    Table 3. Flea Counts on Days 60 and 74


    Number of Fleas on Day 60

    Number of Fleas on Day 74

    Upstairs traps

    Number of fleas

    Male:Female ratio

    % groomed fleasb





    Not done

    Not done

    Downstairs traps

    Number of fleas

    Male:Female ratio

    % groomed fleasb





    Not done

    Not done

    aIndicative of late-stage flea infestation.

    bBased on finding fleas with ingested blood in the traps.

    cIndicative of early or replenished flea infestation.

    Follow-up examination (“deep dive”): Based on microscopic examination of the fleas, it was clear that there were different phenomena occurring upstairs and downstairs. Because the two pit bulls had been observed by the students, and with the knowledge that fed and gravid fleas were being groomed off animals that were probably untreated, the Flea Team again asked the family in the downstairs apartment about visitor pets. Other than the one-time visit by the pit bulls, the family said, there were none. However, when the daughter was asked separately whether she had any concerns about the visiting pit bulls harming her little dog if left unsupervised, she replied, “Oh, no, they know each other well and they get along great together.” As it turned out, these dogs belonged to the owner’s adult son, and in fact, they were over at the apartment frequently.

    On the same visit, the Flea Team learned from the resident of the upstairs apartment that he had gone out of town for 6 days before the flea trap was placed in his home on study day 45. With no pets (or people) in the upstairs apartment, emerging fleas jumped at the traps. The high number of fleas caught in the trap on day 45 demonstrates just how many fleas were infesting the dog every day. Yet few fleas were found on the dog because Frontline Plus was killing them quickly.

    Follow-up flea trap counts confirmed that the upstairs infestation had run its course. The flea trap counts in the downstairs infestation were declining, possibly in response to treatment of the two visitor dogs on study day 45.

    Conclusions and lessons learned: Treatment with Frontline Plus stopped flea reproduction on the treated pets, upstairs and downstairs, and the original indoor flea infestations ran their course. However, a secondary infestation from flea eggs coming from the visiting, untreated dogs resulted in a second surge of emerging fleas downstairs and increased fleas on dogs in both apartments.

    Examining the fleas in the traps provided critical insights into why there was a secondary surge of fleas in the downstairs apartment and led to the conclusion that there must be an external source of flea eggs. It also allowed the Flea Team to predict that the upstairs infestation was running its course. Thus, despite the huge surge in fleas in the traps placed upstairs on study day 45, finding more male fleas than female fleas indicated that this was an aging population that was not being replenished by a new source of flea eggs. Regardless of the cause for the surge of fleas in the traps, this population was headed for extinction.


    Click here to take a quiz and see what you have learned!

    In next month’s case, “The What?”, you will learn how client perspective affects flea control.

    This information has been peer reviewed. It does not necessarily reflect the opinions of, nor constitute or imply endorsement or recommendation by, the Publisher or Editorial Board. The Publisher is not responsible for any data, opinions, or statements provided herein.

    © 2011 Merial Limited, Duluth, GA. All rights reserved. Sponsored by Merial.

    The study from which these cases were selected was funded by Merial Limited, Duluth, GA. Dr. Dryden is a professor at Kansas State University College of Veterinary Medicine, and he performs studies and consulting work for Merial Limited and other animal health companies. Dr. Carithers and Dr. Murray are employees of Merial Limited.


    1.Dryden MW. Evaluation of Certain Parameters in the Bionomics of Ctenocephalides felis felis (Bouché 1835) [master’s thesis]. West Lafayette, IN: Purdue University; 1988:115. 

    2. Dryden MW. Host association, on-host longevity and egg production of Ctenocephalides felis felis. Vet Parasitol 1989;34:117-122. 

    3. Dryden M, Gaafar S. Blood consumption by the cat flea, Ctenocephalides felis felis (Siphonaptera: Pulicidae). J Med Entomol 1991;28(3):394-400. 

    4. Dryden MW, Broce AB. Integrated flea control for the 21st century. Compend Contin Educ Pract Vet 2002;24(1 suppl):36-39. 

    5. Chin A, Lunn P, Dryden M. Persistent flea infestations in dogs and cats controlled with monthly topical applications of fipronil and methoprene. Aust Vet Pract 2005;35(3):89-96. 

    6. Dryden MW. How you and your clients can win the flea control battle. Vet Med 2009;Mar (suppl):17-26. 

    7. Dryden M, Rust M. The cat flea: biology, ecology and control. Vet Parasitol 1994;52:1-19. 

    8. Rust M, Dryden M. The biology, ecology and management of the cat flea. Ann Rev Entomol 1997;42:451-473. 

    9. Dryden MW, Perez HR, Ulitchny DM. Control of fleas on pets and in homes by use of imidacloprid or lufenuron and a pyrethrin spray. JAVMA 1999;215(1):36-39.  

    10. Dryden MW, Denenberg TM, Bunch S. Control of fleas on naturally infested dogs and cats and in private residences with topical spot applications of fipronil or imidacloprid. Vet Parasitol 2000;93(1): 69-75. 

    11. Dryden M, Denenberg TM, Bunch S, et al. Control of fleas on dogs and cats and in private residences with the combination of oral lufenuron and nitenpyram. Vet Ther 2001;2:208-214. 

    12. Dryden MW, Burkindine T, Lewis L, et al. Efficacy of selamectin in controlling natural flea infestations on pets and in private residences in comparison with imidacloprid and fipronil. Proc Am Assoc Vet Parasitol Annu Meet 2001:34. 

    13. Dryden M, Smith V. Cat flea (Siphonaptera: Pulicidae) cocoon formation and development of naked flea pupae. J Med Entomol 1994;31(2):272-277.

    References »

    NEXT: Artificial Insemination and Embryo Transfer in Mares


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