Adult Fleas Infesting Dogs & Cats
Biology and Control

In many ways, fleas are incredible creatures.

They jump to record heights. Every day, they eat twice their weight in blood and lay up to 25 eggs.

Of the four life stages of fleas, adult fleas are the only ones you can readily notice, although they represent less than 5% of the total population. They are also the ones that cause nuisance, even health disorders, to your pets and, more rarely, to yourself.

Obvioulsy, adult fleas are the ones you would want to eliminate.

Let's find out who they are and how to control their proliferation.

What do fleas look like?

Adult fleas are small, wingless insects measuring 2 to 4 mm. Their color ranges from orange to dark brown. A darker color indicates a large amount of blood ingested.

The morphology of the flea is remarkable. Its body is laterally compressed, i.e. very thin and relatively high. Its shape allows it to move through the animal's coat. It has three pairs of legs. The last pair is over-developed, hence its ability to jump very high.

Its biting-sucking mouthparts can easily pierce the skin and find the blood it feeds on.

How do I know if my pet has fleas?

Medical history

Only 5% of fleas are adults. The other life forms: eggs, larvae, and pupae are much more numerous but hidden and too small to be noticed.

If one of your pets has had a flea infestation within the last year, it must have infested your home/garden. If left untreated, the infestation will inevitably resume.

You'll need to be particularly vigilant.

Symptoms and behavior of the animal

A flea infestation can cause skin allergy, itching, or redness.

You may notice that your pet has a habit of biting, licking, or scratching its coat. However, some pets may have a flea infestation but do not exhibit this behavior.

You may also see scabs or hair loss.

Pet's feces

Finding Dipylidium egg sacs in the stool means your pet is infested with either fleas or lice.

Either way, you need to treat!

Fleas (and rarely, lice) are the intermediate hosts of the tapeworm Dipylidium caninum. Dipylidium eggs are eaten by flea larvae in which the worm continues its development until it reaches the cysticercoid stage. Once they become adults, the fleas whose tissues still contain the cysticercoid larva may be swallowed and digested by dogs or cats when they lick or bite themselves.

In the animal digestive tract, the cysticercoid resumes its development towards the adult stage: a segmented flatworm that can grow up to 70 cm, with the last segment acting as an egg sac.

Adult Dipylidium caninum flatworm living in dog's intestine

These egg sacs can be easily spotted i n the feces of pets. They look like rice grains (or cucumber grains).

Examination

It is the best way to ascertain the presence of fleas. It involves finding live adult fleas or flea droppings on the animal's coat. But this is not easy. The discovery of a single flea should warrant treatment: experience shows that examination only reveals 5-15% of fleas. If left untreated, the infestation will inevitably explode within a few weeks.

Flea feces are also called flea dirt. They are small dark grains of 1 mm in diameter. They consist of undigested dried blood. If you rehydrate them by placing them on a moistened white paper sheet, you should notice a dark red halo around them.

It is easier to find flea droppings than live adult fleas.

There are two methods:

Bathing your pet (for dogs only)

Once your dog is in the bath, you should look for small floating forms. Adult fleas are around 2 to 4 mm and try to swim in the water. Their body is hard and difficult to crush with fingernails.

You can also examine the towel you wrapped your dog in to dry him. You may notice fleas (they should be moving) or flea feces that leave a bloody red mark.

Combing your pet

Run a fine-teethed flea or lice comb through your pet's coat. In particular, target the neck, the base of the tail, and the belly where the animal cannot lick or chew. You should be able to find and recognize the adults, feces, and eggs.

You may see fleas jumping (worst-case scenario)

You may see fleas hopping around your house, here and there, looking for a host. If they do not find a cat, dog, or another small carnivore, they may turn on to you!

It means fleas heavily infest your home. You should take immediate action.

What is the difference between dog fleas and cat fleas

There is very little difference between the two.

They both infest cats and dogs and cause the same discomfort and health disorders for your pets. The same control strategies apply.

The cat flea (Ctenocephalides felis) is the most common flea in the world. It accounts for 85% of the flea population infecting pets. There are more cat fleas than dog fleas on a dog.

Scientists identified the cat flea for the first time on a cat, hence its name. The Ctenocephalides genome shows the cat flea probably comes from Africa^[1].

Morphologically, cat fleas (Ctenocephalides felis) and dog fleas (Ctenocephalides canis) are very similar. They differ by the size of their head's Ctenidia (also called combs which are raws of chitinous spines), the size of their abdomen, and the presence of a few silks on the body and legs.

Also, their ability to jump differs slightly. Ctenocephalides felis jumps higher, and Ctenocephalides canis jumps farther.

Where do adult fleas live

In short, dog and cat fleas stay on their host until they die.

Flea pupae emerge from their cocoon when they detect signals that a suitable host may be nearby: vibrations, increased CO2 concentration, increased humidity…

When they hatch out, and if they don't find the host they were hoping for, they can survive for about three weeks. This estimate can vary depending on the humidity and temperature conditions.

Once they have located their prey, they jump on it (and they can jump very high for their size) and start feeding almost immediately.

They won't leave the animal unless they are disturbed or estimate there are too many fleas on the same animal. They can live on their host for 100 days and more.

Once they have started to take blood, fleas cannot survive outside a suitable host for more than 2 to 3 days because they can no longer satisfy their increased metabolism.

How do adult fleas feed

Cat and dog fleas (Ctenocephalides felis and Ctenocephalides canis) feed on the blood of different hosts: cats, dogs, wild carnivores, opossums, rodents, rabbits, ruminants… 50 species have been identified.

They can also feed on humans if nothing else is available. They leave as soon as they have found a more suitable host.

Fleas start biting and sucking blood a few minutes after reaching the coat of their host. Their mouthparts are adapted to piercing the skin and sucking blood from peripheral capillaries^[2]. They include:

• 2 labial palps and 2 maxillary palps to locate the best biting place
• 2 stylets (laciniae) to pierce the skin and inject the flea's saliva
• A third stylet, the epipharynx, also pierces the skin, but in addition, gets into capillaries to take blood

Flea saliva contains proteins that soften tissues and prevent clotting. The bite may cause small edema if the animal is hypersensitive. That's what you may see as a papule or a pimple.

Both female and male fleas feed on their host and eat loads of blood. Even more so for females that need the energy to lay up to 50 eggs a day (around 25 on average). Their daily blood consumption can reach up to 15 times their body weight^[3].

How do fleas reproduce

Before mating, both male and female fleas must take a first blood meal. This is what they do immediately, within the first hour of their arrival on the host.

They then mate within 8 to 24 hours, and females begin to lay their first eggs 24-36 hours after their arrival on the host.

Fleas, especially females, are voracious bloodsuckers; they need this resource to produce up to 50 eggs per day. It represents three times their weight!

Throughout her 100-day life, a female flea can lay 1,745 eggs.

So it's no wonder that a flea infestation can develop very quickly.

Flea eggs are 0.5 mm long, oval-shaped, and white. They are moist and sticky right after the female has laid them. Because they dry quickly, they can detach from the coat and fall on the ground. They are potentially present in all the places where the pet has passed by.

More about flea eggs.

The disorders caused by fleas

Anemia

Fleas take a lot of blood from their host. A heavy infestation, if left untreated, can cause long-term anemia and eventually death. Smaller animals, such as kittens or puppies, are especially at risk.

Each flea consumes an average of 13.6 µl a day^[4].

Thus, an infestation of 110 fleas can draw 1.5 ml of blood per day, representing 5% of the total quantity of blood of a young kitten.

Transmission of internal parasites

Internal parasites (worms) transmitted by fleas are not harmful and do not cause any visible health problems for domestic carnivores.

Dipylidium

Dipylidium caninum is a tapeworm that lives, feeds, and reproduces in the intestines of dogs and cats.

Flea larvae ingest Dipylidium eggs as they feed on other organic debris on the ground. Dipylidium larvae begin to develop in the tissues of flea larvae and stop their evolution when they reach a particular larval stage called cysticercoid.

Dogs and cats may swallow adult fleas infested with cysticercoids. The Dipylidium then resumes its development in the animal's intestines. Its segmented body can reach a spectacular length, up to 70 cm!

Regularly, the worm loses its last segment, which contains 20 to 25 eggs. You can readily notice these segments (also called egg sacs) in your pet's feces; they look like rice or cucumber grains.

In most cases, Dipylidium infestation does not cause any symptoms.

Acanthocheilonema reconditum

This parasite is a microfilaria whose biology is similar to that of the heartworm.

It is transmitted by the flea's saliva when it bites.

It is non-pathogenic and causes no symptoms. Therefore, there is no cause for concern.

Transmission of infections to people

Note that it is the rat flea, Xenopsylla cheopis, that transmits the plague; the cat flea and the dog flea are NOT vectors of this disease.

Rickettsial Diseases

The excrement of fleas (=flea dirt) infected with Rickettsia typhi or Rickettsia felis may accidentally come in contact with open wounds, including those caused by flea bites.

Rickettsia bacteria do not cause symptoms in dogs and cats.

However, cats and dogs can introduce cat fleas infected with Rickettsia typhi or Rickettsia felis into the home. Both of these bacteria can infect humans and cause a range of non-specific symptoms: fever, rash, aches, vomiting, etc.

More about murine typhus

Bartonellosis

The bacterium Bartonella henselae causes the “Cat Scratch Disease” (CSD). CSD is transmitted to humans by a bite or a scratch from an infected cat. Symptoms include malaise, decreased appetite, and swollen lymph nodes near the scratch/bite.

Cats are the main reservoir of the disease, transmitted from cat to cat by the cat flea, Ctenocephlides felis.

Infected cats and dogs exhibit no symptoms.

Flea Allergy Dermatitis

Flea Allergy Dermatitis (FAD) is one of the most common skin disorders in cats and dogs.

Pets with FAD are hypersensitive to some proteins contained by the flea saliva injected into the blood capillaries during the bite.

The bite induces local edema followed by erythematous papules. Allergic animals usually have fewer fleas because they groom themselves more actively.

The diagnosis is based on the search for fleas and their excrement on the animal's coat. The veterinarian can then test the animal's hypersensitivity by observing its reaction to an injection of allergens^[5].

A more straightforward method involves applying an effective anti-flea product and seeing if the symptoms disappear.

How to get rid of adult fleas

In the life cycle of fleas, it is the adult stage that you should target in priority, and for several good reasons:

• Adult fleas are more accessible because their presence is limited to the pet's coat
• The effectiveness of a treatment is more easily visible
• The animal is rapidly relieved
• Many very effective products are available on the market

However, this is not a reason not to consider the other life stages of fleas. Indeed, adults represent less than 5% of the flea population. The other life forms: eggs, larvae, and pupae, are necessarily present in your home and continuously feed the re-infestation of your pet.

Treatment goals

Knowing what you expect from the treatment will help define the best control strategy.

Killing the fleas on your pet

The most obvious choice. It will bring relief to your four-legged companion. However, it does not impact the immature forms of fleas that infect your home, which will quickly become adults and infest your pet again. You're right back to where you started!

Clearing your house of immature forms of fleas

It is a long-term choice. It is about drying up the source of new fleas in your home. Some products prevent the eggs laid by fleas from hatching on your pet. It is effective but takes time (a month at least).

However, be aware that your pet can become infested outside your home.

Prevention

You can prevent your home from becoming infected in the first place. It is the best of both worlds and a wise choice. But it comes at a price since it's an ongoing treatment.

Treatment strategies

The sensible objective we recommend here is to eliminate all forms of fleas from your home. Although it is always possible that your pet could become infested on the outside, it will still make the situation much healthier.

The preferred method is to target the adults to prevent them from laying viable eggs.

You must intervene directly on the animal:

• To kill the adult fleas because that's where they all live
• To limit the laying of eggs and their viability, on a limited area of intervention: the coat of your pet

Of course, you must treat all the animals in your home at the same time because untreated animals will inevitably infest your home.

Strategy #1

This strategy is the one favored by most experts at this time. It involves using newer products that kill all fleas within 24 hours so that they don't have time to lay eggs.

Here are the flea medications you should consider:

• Bravecto® (fluralaner) - chewable tablet for dogs and topical solution for cats - 12 weeks effectiveness
• Credelio® (lotilaner) - chewable tablet - 1-month efficacy - for dogs and cats
• Nexgard® (afoxolaner) - chewable tablet - 1-month efficacy - for dogs
• Revolution® Plus (sarolaner + selamectin) - topical solution - 1-month efficacy - cats only
• Simparica® (sarolaner) - chewable tablet - 1-month efficacy - dogs only

Strategy #2

Some older molecules, although still very effective, tend to have a slight decrease in efficacy at the end of the protection period. They are applied topically, and repeated baths can affect their effectiveness.

Insect growth regulators (IGR), that prevent the eggs laid by the few surviving female fleas from hatching, often complement these molecules:

• Frontline® Plus (fipronil + S-methoprene) - spray or spot-on - 8-week efficacy - dogs and cats
• Advantix® II (imidacloprid + permethrin + pyriproxyfen) - spot-on - 4 weeks efficacy - dogs only
• Advantage® II (imidacloprid + pyriproxyfen) - spot-on - 4 weeks efficacy - dogs and cats

Strategy #3

If your pet tolerates a few fleas for a time, there is an economical option that will help get rid of the fleas as long as they do not catch new fleas outside your home.

It's an oral tablet or injectable solution that works only to sterilize female fleas.

• Program® (lufenuron) - flavored tablets - 1-month efficacy - dogs and cats
• Program® 6 Month Injectable for Cats (lufenuron) - injectable - 6-month efficacy - cats only

Note that the 6-month injection for the cats is very convenient thanks to its very long remanence.

FAQ

Conclusion

Adults represent only 5% of the total flea population.

Yet adult fleas are often the only life form that dog or cat owners are aware of. Because they are easier to notice and because they are the ones causing nuisances.

Also, adult fleas should be the target of a modern flea treatment program: they are the most exposed and vulnerable.

Remember that a treatment program's primary goal should be to prevent fleas from contaminating your home either by killing them directly or by preventing them from laying viable eggs.

Gilles Ventejol

Gilles Ventejol is a professional in Animal Health. He holds a master's degree in life sciences. He has worked for more than 20 years in veterinary pharmaceutical companies, where he contributed to the development and promotion of many veterinary drugs. In this context, he has organized numerous conferences for the veterinary profession. His areas of expertise are parasitology, cardiology, nephrology, vaccines, and reproduction.