To understand why you may still see
fleas on your cat despite being diligent with applying a flea
product regularly, we need explain what the products are up against,
that is, explain how well fleas multiply.
Each flea can lay up to 2000 eggs and
the life cycle is only 3 weeks. Let’s assume a cat starts with only
1 flea and is somewhere that has never been exposed to fleas and
that each flea lays only 100 eggs. After 3 weeks, there will be 100
fleas ready to jump on the cat. If they all hatch and infest that
cat and lay 100 eggs and this cycle continues, then after 6 weeks,
the cat will have 10, 000 fleas and after 9 weeks, the cat will be
burdened by 1 million fleas!!!!
Back in the real World, not every flea
egg laid hatches but they can survive in the environment for years.
There is usually exposure to multiple cats and dogs over years so a
typical backyard in a neighbourhood of an average 5-10 cats over the
last 20 years will literally have BILLIONS of fleas waiting to jump
up. Of course, any fleas killed by a product in a 24-hour period
will be replaced in each subsequent 24-hour period.
The most commonly used products to
kill fleas are the ‘spot-ons’ that are usually applied to back of
the cat’s head. The newer versions of these are all effective at
killing fleas and are safe for cats (some older versions are
available at supermarkets and are not as safe for the
The main products like this all have
their strengths and weaknesses. Some of them are effective for
worming and other parasites as well as fleas. [Their spectrum
against parasites can be seen here.] These products also vary
in how quickly they kill fleas as well as the percentage of fleas
still being killed after 4 weeks1 (when the next
application is due).
These differences are summarised in
the following table:
Time to kill
at 4 weeks
(NB Advocate® has the same drug to kill fleas as
Advantage® but has another drug added for the activity against other
To summarise this table, Advantage®
kills fleas quickest but has the least effect in the last week while
with Revolution®, you may still see fleas since they are taking
longer to die (even though they mostly all dying). Some
dermatologists will recommend Advantage® because of this faster
action but recommend for it to be given every 2 weeks. We prefer to
recommend Revolution® because of the greater number of fleas killed
by the fourth week as well as its ability to make flea eggs sterile.
If you are using Revolution® but your cat is still bothered by
fleas, Capstar® tablets can be given as required (these kill fleas
within 24 hrs but only last a day or 2).
1. Dryden MW, Smith V, Payne PA, McTier TL (2005)
Comparative speed of kill of selamectin, imidacloprid, and
fipronil-(S)-methoprene spot-on formulations against fleas on cats.
Veterinary Therapeutics;6:228-36. Link to PubMed