Fleas are the most common external parasite of companion animals. Flea allergy dermatitis is the most common skin disease of dogs and cats! Flea control has always been a challenge for veterinarians and pet owners because the adult fleas cause the clinical signs, yet the majority of the flea population (eggs, larvae and pupae) are to be found off the pet in and around the home.  The ideal flea control program utilizes products that target the various stages of the flea life cycle, not only the adult fleas on the pet.

There are over 1900 flea species in the world. Pet owners are concerned with only one: Ctenocephalides felis, the cat flea. This is the flea that we find on our pets (cats, dogs, rabbits, and other species) in 99.9% of cases and in order to understand how to control the damage caused by this tiny little animal, you should learn all you can about it.

What Kind of Damage Can Fleas Cause?

It would be a grave mistake to think of the flea as simply a nuisance. A heavy flea burden can be lethal, especially to small or young animals. This is also a problem for elderly cats who go outside. These animals do not groom well and are often debilitated by other diseases. A heavy flea infestation can be enough to push them over the edge.

The cat flea is not at all selective about its host and has been known to kill dairy calves through heavy infestation. Conditions brought about via flea infestation include:

• Flea Allergic Dermatitis (fleas do not make animals itchy unless there is a flea bite allergy) • Flea Anemia (fleas suck their hosts blood) • Feline Infectious Anemia (a life-threatening blood parasite carried by fleas) • Cat Scratch Fever/Bartonellosis (does not make the cat sick but the infected cat can make a person sick) • Common Tapeworm infection

Despite the TV commercials, the educational pamphlets and the common nature of the parasite there are still some significant awareness problems and a multitude of misconceptions.

Flea Myths Veterinarians Hear Nearly Every Day

Myth: My pet cannot have fleas because he lives entirely indoors.

Reality: Fleas thrive particularly well in the well-regulated temperatures in the home.


Myth:  My pet cannot have fleas because if there were any fleas they would be biting me. Since I’m not being bitten, there must not be any fleas.

Reality: Despite a flea’s ability to feed off a wide variety of hosts, it definitely does not prefer human blood and won’t eat it unless absolutely necessary. A newly emerged flea is hungry and may well take a blood meal from the first warm body it finds. In general adult fleas regard human blood as a last choice and humans tend not to be bitten unless flea population numbers are high.


Myth:  We do not have fleas because we have only hard wood floors.

Reality: Fleas are more difficult to remove from carpets, but the cracks between the boards of hard wood floors can also provide residence for young fleas and larvae. Myth:  My pet cannot have fleas because I would see them.

Reality: You cannot expect to always see fleas, because many animals are adept at licking them away. They may be present in low numbers and may be found with a flea comb.  Sometimes all that is seen is the characteristic skin disease.

Fleas are adaptive and their life cycle is always active: eggs are laid, larvae are developing, pupae are growing, etc. The environmental temperature controls how fast this occurs. If you want to eradicate the flea population in a specific home, it is best to attack when numbers are low in the winter. It is a mistake to stop flea control products in the winter as it will be much harder to gain the upper hand in the spring and summer when the populations are rising.


The Flea Life Cycle

There are four life stages of the flea and it is important to know how to break this life cycle in more than one place. This two-step approach provides the most rapid control and the least resistance to flea control agents in future flea generations.

1.  The Egg About one third of the flea population is in the egg stage. The adult female flea lays up to 40 eggs daily. The eggs are laid on the host and fall off to hatch in the environment. Eggs incubate best in high humidity and temperatures of 65 to 80 degrees Fahrenheit. Larvae are killed at 95 degrees

2.  The Larvae About 57% of the fleas are in the larval stage. Larvae are like little caterpillars crawling around grazing on the flea dirt that falls off the host.  Flea dirt is digested blood that is excreted from the adult flea. Larvae are the stage that picks up tapeworm eggs as they graze.

As larvae grow they molt their outer covering. The first larval stage is called the first instar. After the first molt, the larva becomes the second instar. After the third molt, the larva is called a third instar larva and is capable of spinning a cocoon and pupating.  The time between hatching and pupating depends on environmental conditions. It can be as short as 9 days.

3.  The Pupae  By this life stage most young fleas have been killed off by an assortment of environmental factors. Only 8% make it to the pupal stage but once they have spun cocoons they are nearly invincible. Inside the developing cocoon, the pupae is turning into the flea that we are familiar with. They are especially protected under carpet, which is why carpet has developed such a reputation as a shelter for fleas.  The pupae can remain dormant in its cocoon for up to a year as it waits for the right time to emerge.

4.  The Unfed Adult Flea After the pupa develops it is able to remain in the cocoon until it detects a nearby host. The mature pupa is able to detect the vibrations of an approaching host, carbon dioxide gradients, and sound and light patterns. When these stimuli are present it emerges from the cocoon, hungry and eager to feed on the host.

A common scenario occurs when a dog is boarded during the owner’s vacation. The owner picks up the dog from the boarding kennel and returns home. The mature pupae have been waiting for a host and when the dog enters the home, a large number of adult fleas emerge and jump on the dog, creating a sudden, heavy infestation. Often the boarding kennel is blamed for the dog’s fleas. What really happened was that the mature pupae were waiting for the host to return.

An unfed flea is able to live for months without a blood meal but during that time it is trying to locate a host. Once it finds a host, it will never purposely leave the host.

5.  The Fed Flea  After the adult flea finds a host and takes its first blood meal, metabolic changes occur that alter the flea forever. The flea is now called a fed flea and, if separated from its host, will die in only a few weeks without a blood meal. The female flea begins to produce eggs within 24 to 48 hours of her first blood meal and will lay eggs continually until she dies.  The average life span of an adult flea is 4 to 6 weeks.


Flea control

Years ago, flea control meant foggers, shampoos, powders, collars, and sprays. While these products are still available, they have fallen largely aside in favor of the next generation products.

The next generation started in 1995 with the introduction of Program, an oral product that could be given once a month to a dog or cat and would sterilize – but not kill – any flea that bit the pet. The following year came Advantage and Frontline, topical products that could efficiently kill fleas for a month following application. From there Advantage has been modified to control additional parasites such as ticks and heartworm (Advantix, Advantage Multi), Frontline has been supplemented with a flea sterilizer (Frontline Plus), and new insecticides (Revolution, Comfortis, Vectra, Capstar) have been introduced.

At this point there are so many different products with so many additional effects that it is easiest to review them in table form. (At the end of this article)

Is Resistance Futile?

We learned long ago that insecticide use represents a selection factor in a flea population. The resistant individuals survive and pass their genes on to offspring. Eventually a resistant population is produced. There are two ways of avoiding this:

The First Way to Avoid Resistance: Change Products Periodically This seems simple and even obvious. If you want to make a resistant population, then keep exposing the population to the same insecticide and after enough generations your population will be resistant.  Working against this method is the fact that advertisers encourage people to continue to use a product they like and this is, in fact, what people tend to do. The power of marketing is strong though, technically, it is better in the long run if a household alternates between two flea products each year.

The Second Way to Avoid Resistance: Use a Flea Sterilizer A group of fleas that survives exposure to Frontline or Advantage cannot pass on their resistance genes if they have been sterilized by a second product.  One such sterilizer is methoprene (the “plus” in Frontline Plus). Methoprene was developed as an additive to flea sprays and foggers. It is non-toxic – it doesn’t even kill fleas – and represents a group of insect control agents called insect growth regulators or juvenoids. Methoprene mimics a youth hormone of the flea so that larvae who consume it in flea dirt cannot mature and eggs laid by female fleas who have been topically treated with it cannot develop. Twenty years ago, this compound was a miracle in flea control. For the first time it enabled the life cycle to be broken in two places.  Newer insect growth regulators have been released and are also in use (see flea comparison chart).

Resistance is an important phenomenon and it should not be ignored. You may inadvertently be promoting resistance without realizing it.  Look for this clue: At first the product worked really well but now it must be applied again after 2 to 3 weeks. It doesn’t last the whole month anymore.  In this situation you should add a sterilizer or change to another product.


Additional Questions

Q: On a given dog or cat, sometimes the fleas look smaller than average. Are these baby fleas?

A. No. Remember the flea life cycle. The fleas you see on a dog or cat are adult fleas. They vary in size depending on the nutrients they got in prior development as well as individual genetic make-up. Adult fleas have an exoskeleton made of chitin, as do all insects. They can’t grow bigger than they are.

Q: I put the topical product on the dog 3 weeks ago but now I’m seeing fleas again. Do I need to put more on?

A: That depends on it they are bothering your dog or not.  In the first 20 days or so, the products reliably kill fleas before they have a chance to bite the host. In the last 10 days, they don’t kill as quickly. Seeing fleas does not mean the product isn’t working, it is just working slower.  For most dogs this is not a problem.  However if your dog is flea allergic this can be a problem.  In these cases you may need to use a second product.  Options include using Capstar tablets when you see fleas or applying a long acting flea product every 2 to 3 weeks.  In these cases we recommend using 2 different products and alternating them to reduce the development of resistance.

Q: Is there a difference in buying product from the Internet or from the pet supply store rather than a veterinary clinic?

A: There is not a clear-cut answer to this question.  With the flea product revolution of 1995, the market for flea products changed from being primarily pet supply store / over-the-counter outlets to primarily directly through veterinarians. The pharmaceutical companies liked this arrangement as they felt their products were being marketed by trained personnel who could explain best how the product should be used or not used. The veterinarians also liked this arrangement as they had more input regarding what their patients were using. The pet supply stores did not like this arrangement as they had lost their market-share. In an effort to gain it back, they have resorted to a number of black market techniques (paying large sums of money to veterinarians to order product for them, going to other countries to buy product and smuggling it back to the U.S. for re-sale which increases the risk of counterfeit product).  The products purchased through pet supply stores or from an online source that is not a veterinary hospital are not guaranteed by the manufacturer.  In most cases you are getting what you think you are however there is no guarantee and there have been numerous cases of counterfeit product sold through non-veterinary sources.  Know your source and research that it is reliable!