Fleas and Flea Allergies

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Fleas and flea allergies are the most common type of skin disease. Dogs can develop crusting, itching, redness, and hair loss on their rump. Cats will overgroom their bellies and back legs, or develop “eosinophilic granulomas.” Check out the images of pets with classic skin disease related to fleas and flea allergies and listen to the episode for more information.

Transcript and Images:

[00:01:08] There are some diseases in veterinary medicine where I can walk into the exam room, take a look at a patient and know almost immediately what’s going on with the pet and what to do. It’s a special treat for me when I have those patients and with the disease we’ll be talking about today. It’s even more of a treat because I know that animal’s going to start to feel better very quickly. And in return, I feel like I’m wearing my superhero Cape. 

[00:01:31] So let’s talk about today’s disease that’s easily recognizable and that’s fleas and flea allergies. If I walk into an exam room and a dog is missing a bunch of hair on his rump at the base of his tail and it’s red and there’s crusting, I can almost guarantee you that dog is either allergic to fleas or it’s infested with fleas. Anytime a dog gets a hotspot, I can almost guarantee you that dog is not on flea prevention because fleas are the most common cause of hotspots, also known as pyotraumatic dermatitis. 

dog with hair loss, crusting, and redness on tail base and thighs due to biting and scratching from flea allergies

Classic appearance of tail base and rump of a dog with severe flea allergies. Notice the hair loss, redness, and crusting of the skin.

cat with hair loss on rump and thighs due to overgrooming from flea allergies

Overgrooming resulting in broken and lost hairs on the rump of a cat with flea allergies.

[00:02:14] Cats are a little bit different. Fleas can go anywhere in a cat, but most often I see cats with hair loss on their thighs and big bald bellies. Cats can also have an allergic phenomenon called eosinophilic granulomas, and these can occur on different parts of the body. usually the belly or the limbs. Eosinophilic granulomas are ulcerated red plaques that commonly have secondary bacterial infection. These plaques can also occur on the upper lip, and you may have heard of this referred to as a rodent ulcer. There’s pictures of eosinophilic granulomas and rodent ulcers in the show notes as well for you to see. These eosinophilic granulomas on cats can occur due to several different types of allergies. However, the most common cause is a hypersensitivity or allergy to flea, saliva or insects.

Cat with ulceration, redness, and secondary infection on upper lips from flea allergy, known as a rodent ulcer
same cat as previous image with complete resolution of lips good before and after pictures of effective flea control treatment

Cat with eosinophilic granuloma, also known as “rodent ulcer,” secondary to flea allergies. Before, with severe swelling, ulceration, and granulation tissue on the lips. After, following treatment and effective flea prevention.

Cat with hair loss on rump and thighs from licking and overgrooming due to flea allergy

Severe overgrooming from flea allergies. Photos courtesy of Dr. Austin Richman.

Cat with overgrowing on legs and belly from flea allergies

[00:03:11] Now dogs and cats can either have a flea infestation or a flea allergy dogs and cats that have an infestation can be uncomfortable because of their flea bites. It’s pretty easy to diagnose this using a very simple tool called a flea comb. The flea comb gets combed through the hair and as it does, it picks off fleas and flea dirt from the pets fur. Flea dirt is a very diplomatic way of saying poop. It’s flea poop. It looks brown when it’s dry and when you put a little bit of water on it and smash it, it turns red from the blood that the flea has eaten. I know it’s gross. Small animals, such as puppies and especially kittens, when they’re infested with fleas, they can actually become anemic because of how much blood the fleas are drinking. 

[00:03:59] So what’s the difference between an animal with a flea infestation and an animal with a flea allergy? Dogs and cats can also be allergic to the protein that’s in flea saliva. And with these animals, you won’t see fleas on them. The reason for that is because that flea saliva creates an allergic reaction where the flea bites, the pet and the pet becomes so overwhelmingly itchy that the dog and cat, they can’t help themselves, but whip their heads around and start chewing and licking the area that’s having the allergic reaction to the saliva. And you know what happens when they do that? They eat the flea that bit them. Boom, evidence gone. So it’s a lot harder to actually find the fleas on these particular animals than it is on normal dogs and cats that are infested with fleas. Flea allergic animals are comparable to a human that has an allergy to a bee sting. It’s this exaggerated immune response to something that should only be mildly obnoxious. However, instead of having an allergic reaction that involves their respiratory tract like humans do with bee allergies, dogs and cats have an allergic reaction to the flea saliva that involves their skin. 

[00:05:06] So why are fleas important enough to dedicate the first episode of Your Vet Wants You to Know? Well, fleas are one of the most common reasons for dogs and cats to go to the veterinarian. And the incidence of this has been steadily rising. Banfield Pet Hospital’s 2018 state of pet health report, which focused on skin allergies and pets highlighted a 67% increase in flea allergy dermatitis cases diagnosed in cats at Banfield clinics since 2008. And a 12.5% rise in the disease in dogs. The great news is that fleas and flea allergies are usually very easy for most veterinarians to recognize and even better, they’re so easy to safely prevent nowadays. 

[00:05:49] I’d like to talk a little bit about fleas themselves for a moment, because they are pretty impressive little creatures, even if they are incredibly pesky, the most common species of flea is Ctenocephalides felis, also known as the cat flea. And they don’t just like cats. They like dogs, possums, raccoons, skunks, rabbits, foxes. And if any of those animals aren’t around, the cat flea has no problem enjoying a meal from a nearby human. 

[00:06:17] A lot of people think fleas are seasonal and that is mostly true, but they are present year round, depending on the region. I always like to tell my clients that here in Southern California, even though it’s January, it’s 65 degrees outside. The fleas have no idea it’s winter and neither do I, if I were just to base it on the weather. They don’t have a calendar and they don’t have frost to kill them off. In North central Florida from September to November, survival of fleas was as high as 84.6%. In June and July, when temperatures are optimum for fleas, the eggs developed into adults within 20 to 24 days, whereas in the “winter” in Florida, it took 36 to 50 days. So they’re still developing. They’re always around. Immature stages of fleas can actually survive frost when they are in protected microhabitats. And if there were fleas or flea eggs inside your home or on your pet before that frost set in, well your home, especially your rugs and your pet’s bedding, is a protected microhabitat. 

[00:07:22] Fleas are incredibly athletic, little creatures. The adult flea is about an eighth of an inch or 3.2 millimeters in length. And the mean height of jumps for the cat flea is 5.2 inches. So an eighth of an inch body length can jump 5.2 inches. The highest jump that they measured was 6.7 inches. And the mean length of jump was 7.8 inches. Now to put that in perspective for you, that’s like the average person jumping 230 feet into the air. That is pretty impressive. When I put together this episode, I found a great GoPro video on YouTube of someone at the top of an electrical tower that was 230 feet tall. And I’ve posted the link in the show notes so that you can get an idea of the comparison between how high these little creatures jump for their size. So for that athleticism, some people would say those fleas earned a refreshing drink once they get onto their host. 

Video of the view from a 230 foot tall tower, a comparable height to how high a flea can jump for its size!

[00:08:24] Not only are they very athletic adult female fleas are prolific breeders. And as soon as the environmental conditions favor larval development, populations of fleas can explode in the home. One female flea can lay up to 50 eggs per day. 

[00:08:41] Unfortunately, a lot of people are in “flea-nial” when it comes to their pet having fleas. The most common thing I will hear a pet owner say is, “I haven’t seen any fleas.” And that’s great because by the time you wind up seeing a flea, game’s over, they’ve already won and they have infested your home. One of the reasons that it’s uncommon for you to see a flea before the animal develops problems is because cats with fleas will groom more than twice as much as normal cats. So they’re getting rid of those fleas before you can find them. 

[00:09:12] I’ll also hear people say my cat can’t have fleas. It’s indoor only.  Well, fleas love being inside our homes just as much as we do. And they’ll try and catch a ride on your clothing, your bags, your dog, to get into your house. Once a flea’s inside, if your animal isn’t on flea prevention, well, then you’ve just picked up some new roommates. I’ll never forget that one appointment I had where I was flea combing a cat because I was suspicious of fleas. As I was combing, I asked the owner about what flea preventative the cat was on and the last time that it was given. Just as she was responding, “Oh, well, we don’t need that. She’s an indoor only cat,” a flea jumped off of my flea comb and landed on the owner’s shirt! That cat is now on year-round flea prevention with no further skin issues.

[00:09:57] In addition to the skin problems that fleas cause, there’s a lot of other ways that they can harm our pets and the humans in the household too. Fleas can transmit a number of zoonotic diseases. Zoonotic diseases are diseases that can be passed from animals to people such as cat flea typhus, and murine typhus, two rickettsial diseases can be transmitted from cat fleas to humans. The bacteria that causes cat scratch fever, called Bartonella hensellae, gets inoculated into the host, so into the person, when they scratch an area that has contaminated flea poop. Unfortunately cat scratch fever is commonly diagnosed in children. So it’s really important to protect ourselves and protect our children in our homes by having pets on flea prevention. 

[00:10:48] In addition to those zoonotic diseases, fleas are also the intermediate hosts for tapeworm. The larval cat flea becomes infected with the taperworm by consuming  the egg, and then the infective form of the tapeworm develops inside of the adult flea. Dogs or cats become infected with the tapeworm when infected adult fleas are consumed. So all those dogs and cats that are whipping their heads around to chew and eat at the fleas that are biting them, well, they’ve potentially eaten tapeworms too. I also mentioned earlier that if animals are infested with fleas, it can lead to a pretty severe anemia from all the blood that they’re drinking. 

[00:11:27] So what can we do about these pesky little bugs? Well, it’s a big problem with a very simple solution. Many of the flea preventatives available nowadays target neurologic channels in insects that are not present in mammals, or if the channel is present in both insects and mammals, the medication is much more targeted towards the insect form of the channel.

[00:11:47] You’ll hear me talk about risk versus benefit a lot on this podcast. I always weigh the risks of a medication against the benefit to try and determine if I should use that medication. Flea medications are largely safe when they’re used appropriately. And of the 750 cases over two years of permethrin spot on intoxication reported in Australia, all but one of those cases where the result of using a product that was labeled for dogs only on a cat. Many of the other commonly reported side effects are very mild.

[00:12:22] I mentioned earlier, the zoonotic diseases that fleas can transmit to humans. So there is a significant risk to people in the household by not using these preventative medications. For pets, especially those with flea allergies, repeated courses of steroids to decrease inflammation can cause significant side effects and multiple rounds of antibiotics to treat the secondary infection can lead to multi-drug resistant infections.

[00:12:46] So for me, the benefit of using a flea prevention far outweighs the risk of side effects and the risk of not using a flea preventative is too great. It is recommended that dogs and cats be on flea preventatives year round, but unfortunately in a US survey of 24 veterinary hospitals, it was estimated that dogs were given preventative treatments about 6.1 months each year, based on their medication purchase history. Even though the staff hospitals recommended protection for 12 months, only 62% of dog owners remembered that recommendation. 

[00:13:20] The good news is there are a lot of options out there for flea prevention. And as new research comes out and new products become available, those recommendations will definitely change. So I highly recommend that you talk to your family veterinarian about what options are available for you now, as the information may change if you’re listening to this episode in the future. Many of the products that are available nowadays are highly effective and one of the most common reasons for the preventative to not work is because it was never given to the pet. So if your vet sends you home with a flea preventative, please use it. 

[00:13:51] As far as environmental control, all stages of cat fleas, including the adults on the larva are killed by vacuuming. So get at that vacuum and start cleaning. Because of the life cycle of the flea, it’s important for you to understand that it may take several months to get a flea infestation in the household under control. If you have an infestation, all animals in the household should be on a preventative. Doing as much vacuuming and washing of pet bedding as you possibly can will help to speed up getting rid of the fleas in your home as well. 

[00:14:23] Now that you’ve learned a little bit more about fleas, I hope you realize they are not something that you want in your house or on your pets. Please talk to your family veterinarian about what flea prevention is right for your pet specifically. Visit the Facebook group to let me know what type of flea prevention you’re using, and if you’ve ever had an issue with fleas and how you got through it.  You can sign up for the email list to receive a fun infographic with different types of allergies, including flea allergies. If you’re concerned about fleas or allergies in your pet, please talk to your family veterinarian about options or visit our website for a link to where you can find a veterinary dermatologist near you. 

Scratching the Itch

[00:15:08] I’d like to close each episode with a segment called “Scratching the itch.” The segment is designed to highlight something, whether it’s a human interest story, a product or a website that provides relief or just makes you feel good. Hence, scratching the itch. You can let me know through the Facebook group or contact me through the website to let me know what you think about the “Scratching the itch” segment and if you have a suggestion for something we should feature in the future.

[00:15:36] Some of us have been isolating recently due to the COVID-19 global pandemic. If you like to travel as much as I do, I’m sure it’s been hard having the same view from your window over and over again. That’s why this episode’s “scratching the itch” is window swap. This website, window-swap.com, offers you a much needed change of scenery. It features video from different windows all over the world, transporting you, if only briefly, from that same space you’ve been isolating in since March of 2020 to the other side of the county, the other side of the state, your country, and all over the world. It costs nothing. And there’s no risk of being infected or spreading infection when traveling. I highly suggest you check out window-swap.com, which will be in the show notes. If you’re lucky, you may even see a cat enjoying the view there too.


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  4. Nelson,C.A.;Saha,S.;Mead,P.S.Cat-scratch disease in the United States,2005–2013. Emerg.Infect.Dis.2016, 22, 1741–1746. 
  5. Rust MK. The Biology and Ecology of Cat Fleas and Advancements in Their Pest Management: A Review. INSECTS. 8(4).

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