Most pet owners are familiar with the adult flea, a tiny dark brown or black insect that consumes the blood of host animals. However, the adult flea is only part of the flea population. After consuming blood, eggs are laid by female fleas. Eggs are often found in areas where the host sleeps. After incubation, the eggs hatch into larvae, the worm-like second stage of the fleas’ life cycle. Larvae eat bits of organic material, including “flea dirt,” the feces of adult fleas. After molting several times, larvae cocoon themselves, forming pupae. Within the pupa, the larva transforms into the adult flea, beginning the flea life cycle again. Proportionately, 50% of the flea population are eggs, 35% are larvae, 10% are pupae, and 5% are adults. This means that for each adult flea you see, there are many more immature fleas hidden away.

Why do I see fleas when my pet is on a flea medication?

Flea treatments are often a case of the flea or the egg. Some flea medications are adulticides, meaning they kill the adult flea, leaving the immature fleas alive to grow into adults but preventing new eggs from being laid. Other flea medications kill the eggs and larvae (think “flea birth control”), meaning that adult fleas live out their normal lifespan, but are unable to reproduce. In the case of adulticidal flea medications, fleas that jump onto your pet will be killed when they bite and will not be brought into the home. However, if flea eggs have been laid in your house, these fleas will continue to hatch, prolonging the infestation period. Medications that kill the flea eggs and larvae will not kill any adult fleas that your pet may pick up when they go outside, meaning you may still see fleas on your pet when they come in. However, flea infestations in the house will be killed and no new infestations will begin. So which is better? If often comes down to a matter of personal preference. Another factor to consider when choosing a flea medication is what other parasites it treats. While some flea medications treat only fleas, others will treat a variety of external and internal parasites, such as ticks, ear mites, heartworm, roundworm, hookworm, and more. If your pet will be roaming outdoors or meeting other animals, treating for a variety of parasites will help to keep your pet happy and healthy.

Copyright 2012 Lynne Semple, Rowan, and Acres Animal Hospital