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Compendium May 2011 (Vol 33, No 5)

Flea Control: Real Homes, Real Problems, Real Answers, Real Lessons: Where Are All These Fleas Coming From?

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


    This is the second 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 why pet owners may see many fleas on their pet even when the flea control product they applied is working as it should. You will also see where these fleas can come from and that there can be flea development “hot spots” in the home and yard.


    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: Where Are All These Fleas Coming From?

    Signalment: A 1-year-old, spayed cocker spaniel and a 5-year-old, spayed domestic shorthair cat.  

    History: At the initial examination, there were no fleas on the cat, and the dog had 40 fleas. Twenty fleas were collected in the flea traps. The cat was not included in the study, but it was treated with Frontline Plus at the same time the dog was treated. In this home, the flea trap counts initially increased (redline home) but then decreased to zero as the indoor flea infestation ran its course (FIGURE 1A and FIGURE 1B). The flea counts on the dog, after initially declining, steadily increased, even as the indoor emerging flea counts (trap flea counts) declined. This increase prompted a thorough on-site 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 dog is restricted to the backyard when outdoors.

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

    The owner’s brother had his dog over for a few days last month, but that dog is being treated with Frontline Plus.

    Where does your pet sleep or rest?

    The dog sleeps in the utility room but rests outside under the pool deck.

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

    There is an elevated pool deck.

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

    Yes, a raccoon lives in a tree in the yard. 

    Examination of the pets: The site investigation was performed on day 45 of the study. The dog was examined at 8:00 a.m. by the Flea Team. (All examinations at this home were performed at this time because of the homeowner’s schedule.) Many fleas were found crawling on the dog’s abdomen. Area thumb counts revealed 33 fleas.  

    Backyard with shed and tree

    Figure 2. The backyard, with the shed and the tree in which a raccoon (probably more than one raccoon) lived.

    Examination of the premises: The homeowner showed the Flea Team a large tree hanging over a shed in the backyard (FIGURE 2). This tree, he explained, was home to a raccoon. The owner had pruned branches from the tree in an attempt to prevent the raccoon from getting into the shed, where he kept an African macaw. There was also a deck surrounding an elevated pool. Wooden lattice covered the sides of most of the deck, but one section of lattice had been removed to allow the dog to go under the deck to rest in the shade (FIGURE 3). The soil under the deck was sandy, which is typical for Tampa. Dr. Dryden explored under the deck and behind the shed, where the owner had discarded cut limbs from the tree. A few minutes later, he found several fleas on his socks!  

    Diagnosis: The source of the dog’s fleas was the backyard. Fleas were developing under the deck, where the shaded, sandy soil provided ideal conditions for flea development. The source of flea eggs was suspected to be the raccoon.  

    Area under pool deck

    Figure 3. Area under the pool deck. Removal of the wooden lattice gave the dog (and the raccoons) access to the shaded, sandy area under the deck.

    Follow-up examination: On day 60 of the study, the Flea Team conducted its final examination. There were no fleas in the two traps placed in the home overnight, but the dog had 62 fleas! These fleas were combed off the dog and, later, examined under a microscope. Most were female, all were relatively small, and only a few contained any visible blood (FIGURE 4).  

    Conclusions and lessons learned: The dog picked up a large number of fleas every time it went out to the backyard. The Flea Team concluded that the raccoon (likely raccoons) used the area under the deck as shelter, so this became a flea infestation hot spot. Treatment with Frontline Plus stopped flea reproduction on the pets, and the indoor flea infestation ran its course. Frontline Plus continued to kill new fleas that the dog picked up when it went out to the yard. Because the Flea Team always conducted its examinations at 8:00 am, they always examined the dog within an hour of it having used the yard. This explains why the dog always had so many fleas and why those fleas were typically small, with no visible blood. This was not a failure of the flea product, because no monthly flea product kills all fleas within 1 hour. In the weeks following Frontline Plus application (FIGURE 1A and FIGURE 1B), fewer fleas were found on the dog. This reduction was due to the immediate effect on the adult flea population on the pet and in the household. The dog was still picking up a large number of fleas when it went outdoors, but they were being killed most quickly in the week after treatment compared with later in the month. This is true for any monthly flea control product, whether topically or systemically active.13,14  The key lesson is that fleas can quickly acquire a host, so whenever a dog encounters a flea infestation hot spot, many fleas may be seen on the dog, often for a number of hours. If the dog is appropriately treated with a monthly flea product, these fleas ultimately die.


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

    In next month’s case, “Hitchhiker Fleas and the Indoor-Only Cats,” you will learn how even animals that do not go outside can get fleas.

    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 MW. Flea and tick control in the 21st century, challenges and opportunities. Vet Dermatol 2009;20:435-440. 

    14. 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;6:228-236.

    References »

    NEXT: Focus on Nutrition — Optimal Nutrition for Older Cats


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