Not fleas again?!

Sarah J. Wooten, DVM

If you're tired of fleas, just imagine how your poor creeped-out veterinary clients feel facing the devil's jumping beans. One more time, doctors, with gusto: Let's go get those blood suckers!

A flea infestation can get hairy in a hurry. (Getty Images)

Even with the advent of modern flea products with excellent residual activity and speed of kill, flea control has always been a thorn in veterinarians' sides and fleas continue to be a leading cause of pruritus in dogs and cats. Fleas cause everything from alopecia to fulminant dermatitis to behavioral changes, and they can stump the general practitioner with their persistence even in the presence of potent adulticides and insect growth regulators.

The worst part for clients, of course, is when they show up as a surprise-or won't go away. Why does an indoor-only cat get fleas? Why is a dog that gets a treatment every month still crawling with the bloodsuckers? What do you tell the irritated client who, three weeks ago, started the flea control you recommended and now sees even more fleas? In this article, Michael Dryden, DVM, MS, PhD, shares new strategies, tips and tricks on how to manage challenging flea cases.

Birthing a menace

Ctenocephalides felis. (Getty Images)The target:Ctenocephalides felis felis, the cat flea

Common hosts: Cats, dogs, opossums, raccoons, domestic rabbits, ferrets and hedgehogs. FUN FACT: Squirrels, birds and wild rabbits-you're safe! BUM FACT: Wildlife and feral cats can serve as outdoor sources of flea infestations and reinfestations.

How it works: Once a flea lands on a host, it starts eating in seconds to minutes, breeding soon after and laying eggs in 24 to 48 hours. BUM FACT: A female flea can lay 40 to 50 eggs per day. These eggs rolling off the host will hatch into larvae in three to five days, depending on temperature and moisture. (Visit to download a client handout on the whole flea cycle.)

That's hot: Larvae are the most sensitive life stage of fleas, requiring flea feces for nutrition, protection from direct sunlight, temperatures in the range of 45 to 95 F and a relative humidity of 50% to 85%. Most don't make it to adulthood because of desiccation or starvation. The higher the humidity, the more larvae survive to adults and the worse the flea problem becomes. The cooler the temperatures, the slower the fleas develop and the longer the problem exists.

Armed with this knowledge, we veterinarians try to control fleas in pets by breaking the pest's life cycle, rather than focusing on environmental control. But what do you do when your therapy doesn't seem to be working?

The horror show that is biomass

By the time a pet owner notices fleas, immature flea stages have been developing in the home for up to two months, and the environment is full of fleas hatching and developing. Even when the owner repeatedly applies insecticide for two to three months, developing fleas can continue to show up on the pet and cause problems. This creepy concept is called the biomass and represents the entire load of immature flea stages in the home environment.

Up to 95% of a biomass in a home will develop and complete its emergence in one to two months, depending on temperature and relative humidity. Biomass hatches more quickly in warmer and more humid environments. Unless you address this biomass, the flea problem persists.

In one week ---> no more flea eggs ---> they have all become larvae

2 weeks later ----> no more larvae ----> they have all died or become pupae

2-4 weeks after that ----> no more pupae ----> they have all become adults, ready to feed and breed!

Time to get down to some sleuthing

In the aggravating event of a flea infestation that won't quit-or a frustrating reinfestation-it'll be up to you to dust off the Sherlock Holmes hat and help the pet owner determine where the fleas are coming from. Here's a start:

> Hitchhikers. Fleas can get into the house on visiting animals (cats, dogs, ferrets, domestic rabbits, hedgehogs). Ask, “Have any animals visited your home in the past six months?” and “Have you brought in any used animal bedding?” and “Have any family or friends who own pets visited?”

> Wild animals. Raccoons, opossums and feral cats get around and serve as a source of recurrent flea infestations. Ask, “Where does your pet like to spend its time outside?” Fleas can drop out of trees where wild animals hide and hitchhike in on clothes. Flea larvae love any shady, cool area-piles of leaves in shady spots, under the porch and shady mulched areas. You can see that fleas don't need to directly interact with pets to give the gift of fleas. If wildlife can't be kept out of the yard, pets will need to be on lifelong flea control.

> Typhoid Mary. Client needs to know that all pets in a household must be on strict flea control, including indoor-only pets. Ask, “Are there animals in the household that aren't being treated?”


Home is where the hassle is

Pets' favorite spots are where the most fleas in various life stages will be-and where you and the client should focus your environmental treatment. Why? Because this is where most eggs are deposited and larvae require flea feces for nutrition.

Indoor areas such as pet and human beds, the underside of couch cushions, carpets and area throw rugs harbor the lurking biomass, ready to hatch and terrorize pets and people alike. Outdoor areas where your pet likes to rest such as under porches, leaf piles and mulched areas, are suspect areas for fleas and need to be treated.

Seek and destroy

When choosing a product to eradicate fleas, the most important factors are safety and residual speed of kill (how fast the insecticide kills fleas over time). The longer the product is on, the higher likelihood that the residual speed of kill will decrease, eventually slowing down enough to allow fleas to lay viable eggs before they die. (Remember, they only need 24 to 48 hours.)

To prevent reinfestation, you need an insecticide that kills the biomass within 12 to 24 hours as larvae emerge over the next one to two months. This is considered the reproductive breaking point. Here's your arsenal:


> Excellent for speed of kill, but has no residual activity.

> Must be followed by a product with residual speed of kill high enough to kill fleas before they feed.

Products with fipronil and s-methoprene

> Labeled for one month, but may not keep residual speed of kill high enough over time to take out fleas before they feed.1

> The insect growth regulator in these products is an effective way to disrupt reproduction, but you may need reapplication every four weeks for four months until the biomass is eradicated, especially in pets with flea allergy dermatitis.

> The study says … A one-month study of residual speed of kill following a single treatment of selamectin, imidacloprid and fipronil-(S)-methoprene against C. felis infestations on cats seems to suggest multiple applications are needed in challenging flea infestations. Here's the lowdown:

  • Eighty cats were randomly allocated, with 20 cats in four different treatment groups.

  • On days -2, 7, 14, 21 and 28, each cat was infested with 100 adult C. felis from the Kansas 1 flea strain. After initial application, only imidacloprid had caused a significant reduction in adult fleas on treated cats within six hours, but by 24 hours all three formulations had killed 96.7% of the fleas.

  • On day 7, all three formulations reduced flea populations within six and 24 hours by 68.4% and 99.4%, respectively.

  • On days 21 and 28, none of the formulations killed significant numbers of fleas as compared to controls within six hours of infestation.

  • On day 28, selamectin, fipronil-(S)-methoprene and imidacloprid had killed 99%, 86.4% and 72.6% of the fleas within 48 hours of infestation, respectively.

> The residual speed of kill of flea products on cats decreases throughout the month after application. Products should be reapplied every three weeks in challenging control situations.2

Oral products with spinosad

> Start killing fleas within 30 minutes, with 100% flea kill within four hours

> Labeled to last one month, but in laboratory settings residual speed of kill wasn't found effective 23 days after administration in dogs

> The study says … One study found selamectin took up to 48 hours to control (> 90%) existing infestation in dogs. For subsequent weekly flea infestations, selamectin had similar or better efficacy than spinosad or spinosad/milbemycin oxime at 24 and 48 hours after infestation. Spinosad/milbemycin oxime and spinosad were > 90% effective against the KS1 strain from day 1 to day 23. In contrast, selamectin was > 90% effective against the KS1 strain of C. felis from day 2 to day 30.3

Oral isoxazolines

> Newer class of drugs with excellent initial and residual speed of kill

> Start killing fleas within two hours; 100% of fleas dead within 12 hours4

> Shown to last up to 12 weeks4

> Like spinosad, cannot be washed off

> Biggest advantage? Residual speed of kill long enough to eradicate biomass



Redline homes: When things go from bad to worse

Sometimes the number of fleas in a home increases substantially in the month after treatment of all pets with flea control product. Pet owners will (incorrectly) conclude that the flea control product isn't working.

These are called redline homes, where fleas emerge from eggs laid weeks before treatment. Until the source of these new fleas, the biomass, is exhausted, pet owners will continue to see new fleas on pets and in their homes. Depending on the size of the biomass and temperature, this can take anywhere from three weeks to several months for the biomass infestation to run its course.

Don't let this be a surprise or a frustrating new development to clients. Communicate. Communicate. Communicate.

Getting real with clients

One of the toughest challenges about managing fleas is managing client expectations. Most clients with flea-ridden pets are in denial about the presence of the flea biomass that exists in the carpet, furniture and bedding and how long it will take to get rid of it. Here are three important things to let clients know about taking care of their flea infestations:

1. It'll take a while. To be on the safe side, advise clients it can take anywhere from three to four months to eradicate the biomass from their home, and they should expect to see new fleas hatching up until then.

2. We need to be diligent. During that time, all pets in the household are treated with a product with excellent residual speed of kill. Depending on the level of infestation, treating additionally with a quick-kill product such as nitenpyram might be necessary.

3. Some products need repeats. Let clients using monthly products know they may need to be administered every three weeks for the next four months, especially in patients with flea allergy dermatitis.

> Show, not tell. Start with an initial physical examination and thorough history. Check for tapeworm segments, and do an initial flea count on patients in the exam room. Comb and count fleas in one of five areas: the dorsal midline, the base of the tail, lateral thorax on left and right, and the inguinal region. The number of fleas seen in these areas represents 23% of the total adult flea burden on that animal. Then schedule biweekly rechecks on all animals in the household to count fleas or teach the owner how to check for fleas at home to make sure the treatment plan is working.

> Wash up. Depending on the size of the biomass, explain to your client the flea problem may seem to get worse before it gets better, and explain the concept of a redline home (see the sidebar “Redline homes: When things go from bad to worse”). Recommend that clients wash all bedding at least once and vacuum under couch cushions and all carpets weekly for three to four months.

What happens next …

> Good job! If the client follows your instructions completely, the problem will be gone in two to four months. If the problem persists beyond four months-or goes away and then comes back-you'll need to look for an undetected source of reinfestation (indoor, outdoor, or both).

> Here we go again. If the client is in denial about the flea infestation in the home, you or a veterinary technician can offer to do a home visit (charge appropriately for a home visit) and quantify the number of fleas every two weeks (or, again, teach the client how to count fleas). Traps are another way to capture and count fleas. Clients can buy flea traps online and plant them in or near pets' favorite spots. If you do a home visit, flip the couch cushions or bedding onto a white sheet and look for fleas, flea eggs or larvae. Count what you find. (Warning: The owner will likely be shocked and repulsed by what you find. Good … ?)

> Patience is overrated. In all likelihood, your client wanted the problem solved yesterday. If slow-and-steady methods aren't cutting it, let your client know that it's biologically impossible to eliminate the biomass overnight. If your client simply can't wait for the biomass to die off with treatment and time, here are some strategies to speed up the process:

  • Flea treatment on all pets as prescribed by a veterinarian

  • Insecticides for outdoor areas that harbor fleas (Bayer Advanced Complete Insect Killer is an excellent product for fleas and ticks)

  • Daily: Carpet and undersides of cushions vacuumed and pet carriers cleaned

  • 3x/week: Pet bedding and area throw rugs washed

  • 1x/week: New flea traps placed


Embrace your inner super sleuth

The first time a client returns unsatisfied because the pets are becoming reinfested with fleas is your clue to become a flea super sleuth and expert client expectation manager. OVERCOMMUNICATE. Advise your client from the very beginning on the timeline of solving the problem: If you tell them three to four months and it only takes three months then BAM! You are a hero. Put EVERYTHING in writing. Give a four-month estimate for treatment costs so the client knows what to expect and give costs for year-round flea control. Set up all the rechecks visits at that appointment to ensure that you are controlling the follow-up. Determine whether the client will need to bring the pets in or you will need to do home visits. Make it clear to clients that it is up to them to protect their investment in the treatment costs by following instructions EXACTLY and doing all the follow-up that is schedule. I find when you put it in those terms, clients tend to view it as much their responsibility as it is yours.

Successfully solve a difficult flea case and you will become something of a local legend to pet parents. You may be surprised how much your expertise in this one area will grow your practice, so go forth and conquer Ctenocephalides felis.


1. McCoy C, Bruce AB, Dryden MW. Flea blood feeding patterns in cats treated with nitenpyram and the topical insecticides imidacloprid, fipronil and selamectin. Vet Parasitol 2008;156:293-301.

2. 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(3):228-236.

3. Dryden MW, Payne PA, Smith V, et al. Efficacy of selamectin, spinosad, and spinosad/milbemycin oxime against the KS1 Ctenocephalides felis flea strain infesting dogs. Parasites Vectors 2013;6:80.

4. Beugnet F, Liebenberg J, Halos L. Comparative speed of efficacy against Ctenocephalides felis of two oral treatments for dogs containing either afoxolaner or fluralaner. Vet Parasitol 2015;207:297-301.