Any seasoned pet owner has almost certainly come across this crisis situation at least once and it always starts the same way. “I think my dog has fleas!”. To be fair maybe this has happened with your cat instead but the point remains that fleas are stressful. Fleas can turn an entire household on its head with worries about tiny creepy crawlies walking all over you. Not to mention the itching for both you and your pet.

Finding Fleas

Usually the first thing a pet owner will notice is their pet is scratching all of a sudden. Other times you may notice actual fleas on your pet or in your apartment. If you’re not sure exactly what a flea looks like, here’s a picture below to familiarize yourself with. On occasion people may even notice flea bites on themselves. If you notice any of these signs, you and your pet may be a victim of a flea infestation.


Picture of a flea on a human nail

Size of a flea compared to a human nail.


When veterinarians search for fleas they will often use a flea comb and thoroughly comb the pets coat. We are looking for either fleas or secondary signs that signal the presence of fleas. This means flea dirt. Flea dirt is actually not dirt at all but flea feces containing partially digested blood. This flea dirt will have a reddish tinge especially if made wet. It is also a key part to the fleas life cycle but we will talk about this more later. Another way of looking for fleas is to thoroughly rub or shake out the pet and then collect the debri. Flea dirt is often found in this debri when pets that have a flea problem.

Flea Season

In the Toronto and Southern Ontario region, fleas are most common during the later summer months with prime flea season usually starting in late August and extending until the end of October. This time frame is dependent of the weather patterns of that particular year. The ideal environment for a flea is temperatures between 21°C and 30°C and moderate to high humid weather. The reason fleas are so abundant at this time of year is due to the climate and because they have been breeding aggressively for the previous 4-5 months.
Fleas first start to pop up around April or May and tend to reach a critical mass around late August. This is the time when we start to see many pets carrying fleas. In Toronto and Southern Ontario, it is recommended to start preventative flea treatments around April or May and continue until November. When the weather rises above 10°C for a prolonged period of time, this signals the beginning of the flea season and it is recommended to start your pet on flea preventatives at this time.

Flea Life Cycle

The most frustrating part about treating fleas is that the gross little bugs you’re finding on your pet are only the tip of the iceberg.  The bigger infestation is all the other life stages of a flea that are residing somewhere in your house or backyard and can easily lead to your pet being repeatedly reinfected.  Properly treating and cleaning the environment will significantly decrease the risk of a lingering flea issue.  Newer veterinary flea medications are also quite effective at eliminating the adult stage (the fleas you see) and preventing progression of other stages (the fleas you can’t see).

metamorphosis of a flea

The different life stages of a flea. Only adults live on our pets.

The Adult

We’ll start with the most well known villain, the adult.  Adults emerge from the pupa or cocoon when stimulated by vibrations, heat, or carbon dioxide.  This means when your dog is sleeping quietly in his bed, or even worse paddling in his dreams, your dog is unintentionally doing everything he can to make the flea cocoons around him hatch.  His motion, body heat, and breath (CO2) are the perfect environment for hatching fleas.

Once the fleas hatch they jump when they see shadows.  This is their tactic for latching on to your pet.  The majority of fleas start to feed on their new host within the first hour and often within the first 5 minutes.  The natural host for a flea is either a dog or a cat (and chickens for a particular type of flea).  Fleas rarely use humans as their hosts unless there are a large amount of fleas in the environment.

Adult fleas can survive on their host for more than 100 days.  If taken off their host they can only survive for 2-4 days.  When on your pet, fleas will continue to feed and with a large enough population can cause an anemia or low red blood cell levels.  Fleas will also produce “flea dirt” which is flea feces containing a partially digested blood meal.  This is one of the key signs of a flea infestation.  This flea dirt is an important part of the flea circle of life because it becomes food for the flea larvae.  Very disgusting but from a fleas point of view, delicious and efficient!

Most flea preventatives are active against this life stage.  Some preventatives can eliminate all adults within 24 hours.


Once adult fleas start feeding on their new walking hosts the female fleas start making eggs within 2 days.  Adults lay their 1-2 mm, white, ovoid eggs in their hosts hair coat.  Adults can lay 40-50 eggs per day.  These eggs then fall to the ground and hatch within 2-12 days.  Some flea preventative treatments sterilize the female fleas and help to break the flea life cycle.


Eggs hatch to reveal equally unappealing larvae.  These larvae live in the environment.  They tend to prefer low light areas with a limited amount of foot traffic.  Carpets and beds make excellent homes for these creatures.  Over 1-2 weeks they will spin cocoons or pupae, often out of carpet fibres.  This life stage is susceptible to multiple different flea medications and is an integral part of the flea life cycle to eliminate when combating a flea infestation.

Pupae (Cocoon)

The final stage is the most difficult stage to eliminate.  The flea pupae are incredibly resistant to freezing, drying out, and flea medications.  This stage can survive and hibernate for up to 9 months in some cases.  In another 1-2 weeks the pupae will hatch when stimulated by a pets body heat, motion, and breath (CO2).  The hatched fleas will often find a host within minutes.

flea life cycle proportions

The majority of fleas in the environment are not adults.

The entire flea life cycle can be completed in as few as 16 days.  Only 3-5% of all fleas are the adults that we find jumping on our pets.  This is why treating the environment and interrupting the different stages are keys to successful flea decontamination.  I hope you enjoyed learning about the circle of life, when it comes to fleas.  Stay tuned for more information on how to fix your flea problem and get your home back to normal.

Find those fleas!
Dr. Tim Julian