Fleas – Where Are They Coming From?

Dr. Colleen Lewis / April 6, 2017

Common scenario: you have treated your dog (or cat) for fleas and they are still scratching. What went wrong? Are you experiencing super fleas that are resistant to the product you are using or are they simply hiding somewhere that is avoiding your treatment? Before you get too excited about product failure, let’s talk about the flea and its crafty ways of survival in spite of what we throw at it.

A quick review of the flea’s lifecycle may explain a lot. The flea that we see on our pet is likely an adult female; she bites your dog and drinks a blood meal to provide nourishment towards reproducing. She is meant to stay on the dog, unless extreme overcrowding is present; here she is warm and her food supply, blood, is always a sip away. She is gravid, with a large egg filling her abdomen, stretching her tiny belly. This egg is laid, but its slippery surface finds it slinking quickly to the ground, into your dog’s bed, in your bed, on the couch; wherever your pet goes, the eggs go too. Each flea can lay hundreds of eggs. Her relatives each lay hundreds of eggs too. The egg will hatch as quickly as 3 days, producing a larval stage. These hairy-looking larval critters live in carpet fibers and will actually feed on flea poop (this is an important concept for a later discussion). They quickly molt into juvenile fleas and begin looking for a warm body to obtain their first blood meal. Commonly the egg to adult life cycle takes 3 to 4 weeks. The flea can actually mature as quickly as 12 days when the temperature and humidity are ideal.

Back to the original question: why is my dog still scratching? Let’s look at some possible answers to that question.

1. The flea’s saliva can be very irritating to the dog and cause anywhere from a minimal irritation to intense itching in the case of a severe flea allergy. The dog’s reaction to a single flea bite can be compared to a mosquito bite on a human, and can be raised and bothersome for a few days. Be patient, if you do not see any fleas; your dog may be simply be having a residual histamine release. Your veterinarian can prescribe a helpful anti-histamine.
2. Your dog still has fleas due to application failure. Getting an accurate body weight for treatment is important to prevent under-dosing. Did you bathe your dog shortly before or after treatment? Access to water can impair the distribution rate or even wash the product off prior to proper absorption. Read the label carefully, and make sure all of the product actually lands on the skin and not on the carpet, washed down the drain or left behind in the tube.
3. The product you used only works for a limited time. Most dips and some topical products only work as a quick kill with no residual efficacy. New fleas are then able to jump right back on your pet and start a new infestation.
4. Fleas are resistant to many of the older products on the market. Pyrethrins and permethrins have been around for many years. There are certainly some fleas that are unresponsive to their presence. But please resist the urge to apply more product when fleas continue to multiply. Unfortunately, they are also one of the most common toxicity issues treated in veterinary emergency rooms due to a narrow margin of safety or use of products labeled for dogs on cats.
5. Your product is killing the adult fleas, yet young adults (smaller and darker) in the environment are still jumping on your dog. The majority of the longer lasting products require that the flea take a blood meal in order to ingest the pesticide. In this instance, your dog may still suffer from the allergic side effects of flea bites. It is tough to find any fleas, since the product is actually working well. If you do find the occasional flea, it will likely be young and dead or dying. This signals a need to treat your environment.
6. You treated the dog with a long lasting product and bombed the living room where your dog spends most of his time. You are seeing fleas again! Make sure you select bombs with a growth regulator (7 month residual) to prevent larval stages from becoming adult fleas. Otherwise, you are just killing the young adult stages in your carpets, but the eggs and larva continue on their way to adulthood. All the carpeted and bedding areas where your dog roams are subject to fleas. Do not forget to treat three-season porches, area rugs, indoor and outdoor fabric furniture and welcome mats when the weather is nice outside. Calling an exterminator may be warranted if large square footage or a lot of wall to wall carpet makes bombing difficult or bombing attempts have been unsuccessful in the past.
7. Your indoor pets only go outside to potty in a fenced yard; suddenly your indoor cat has fleas! Squirrels carry fleas and can drop eggs into your fenced yard; the indoor/outdoor pets can easily bring the fleas inside. Do not forget that your pocket pets and ferrets can have fleas and the eggs can fall off in your home, hatch and jump onto the dogs and cats. Fleas can live 100 days without a blood meal if undisturbed, making a basement or crawl space a potential hideout. A visiting dog can drop a few eggs off, resulting in unsuspected fleas weeks later. Hungry fleas on your front lawn can jump on your socks and ride into the house with you.

Avoiding the pitfalls of flea prevention and treatment can help you to save time, money and misery. Remember, for every flea you see on your dog, there are 100 other fleas in various life stages in your environment. So, make sure your pets are protected. Your veterinarian can help you select from the newest classes of flea medications to get serious about eradicating them from your pets forever.

About the Author
Dr. Colleen Lewis is a 1996 graduate of Kansas State University, College of Veterinary Medicine. Her career has taken her to many places as a practice owner, consultant, embryologist, and mentor. She enjoys mixed animal practice, teaching, traveling, farming and high school sports with her husband, Andrew and their three boys.