What is flea dirt?

what is flea dirt

Dr Zak

Img 1 Flea dirt is feces, appearing as dark red or black specks in a cat’s fur.


Flea dirt is the common name for flea poop. It’s the feces from adult fleas, and is largely composed of undigested host blood. The fecal blood dries and breaks apart into reddish-black specks in a pet’s fur. Eventually it falls from the animal during grooming and lands in the environment where it’s consumed by larvae.


What Flea Dirt Looks Like

General Description

Adult fleas defecate onto their hosts. When the excreta dries, it breaks apart into irregular-shaped, dark red or black specks Img 1. Flea dirt looks like black pepper or black dandruff. It’s also been described as reddish-brown debris.

Spherules & Coils

Flea excrete feces in two different forms: Spherules and coils. Spherules are round and 0.07-0.25 mm in size. Coils are long with an average length of 0.84 mm Img 4. Newly emerged adults produce spherules. By the tenth day of feeding, their poop is mostly in the form of coils.


Finding flea dirt on a dog or cat is one of the best ways to diagnose an infestation. If it’s truly flea feces, the black speck will smear red when rubbed on a wet paper towel. The crimson color results from reconstituted blood Img 2. Similarly, placing flea dirt into a drop of water will cause the water to turn red.

wet paper towel with red flea dirt smudges

Nottingham Vet School

Img 2 A wet paper towel with flea dirt recovered from a cat. The red smears are caused from the reconstituted blood.

Blood Digestion & Excretion

Adult fleas imbibe much more blood than they can use. Thus, large amounts of feces are produced while feeding. It consists of partially digested or undigested host blood.

fecal blood or flea dirt

Fleas — Michael Dryden, DVM, MS, PhD

Img 3 A cat flea feeding off a host and excreting undigested fecal blood. The feces falls off the host and into the environment. It’s primary food source of larvae.

When a flea feeds, blood flows into its sac-like midgut. There, blood-digesting enzymes are released. However, only the blood closest to functional midcut cells is retained for digestion, because it moves rapidly through the gut. Most isn’t digested. After passing through the midgut, the blood enters into a short hind-gut which ultimately ends at the anus.

Host blood is digested more completely by other parasites, such as mosquitoes and bedbugs, than it is by fleas. This makes sense, as larvae of these insects don’t consume adult feces, while flea larvae do.

Amount of Flea Dirt Produced Daily

Gut Capacity & Blood Consumption

Cat flea guts hold around 5 µl of blood. But they ingest an average 6.97 µl of blood per day. And reproducing females can imbibe 13.6 µl of blood daily, an amount equal to over 15 times their body weight.

Feces Amount

Fleas feed at random, for 2-3 minutes at a time. 8-10 droplets of feces are excreted at each feeding. The average adult flea expels 0.645-0.770 mg of feces per day. Both genders excrete protein-rich flea dirt, but females produce a greater volume. They expel around 17 times as much feces as males (0.8546 vs 0.0503 mg daily).

Why Fleas Consume Excess Blood

Low Energy Costs

Cat fleas are vessel (capillary) feeders, as opposed to pool feeders. Vessel feeding facilitates a rapid meal, because the host’s blood pressure gets exploited. Flea mouth-parts are highly adapted for piercing and sucking, and, along with anticoagulant saliva, blood flows easily into their guts. Once a flea bites, imbibing excess blood has few energy costs.

Parental Care

Flea dirt is the primary food source for larvae. It gives them their minimal nutritional requirements. They won’t reach adulthood without it. The feces provides larvae with the iron and protein needed for normal growth and sclerotisation.

Flea dirt contains 83-90% of the original blood’s protein, as well as 44-80% of the potassium and chloride ions. Coils contain 33% more protein than spherules. The spherules are thought to be preferred by young larvae, while the protein-rich coils are more suitable for 3rd instars.

larva feeding on a coil of fecal blood

Cat Flea Biology

Img 4 Cat flea larva feeding on a coil of fecal blood. See Img 1 for what a spherule looks like.

Adult fleas may purposely ingest excess blood to feed larvae, or they may be harvesting out key nutrients, such as vitamin B, for themselves. Whatever the reason, the larvae greatly benefit in the end. Even if the feces falls out of range of larvae, it could still be used by future generations. It appears to be an evolutionary step towards parental care.

Where Flea Dirt is Found

Fleas Defecate on Hosts

Adult cat fleas are permanent residents of their hosts. They’ll rarely leave of their own choosing. As a result, they feed and defecate directly on animals.

Flea Dirt Falls when Hosts Groom

Flea dirt dries in irregular shapes and gets embedded into pet fur. However, it doesn’t stay there. The dry blood dislodges when animals scratch and groom themselves. As a result, it’s most common to find feces (and eggs) in areas where pets commonly rest and groom.


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  • Anthony September 3, 2016, 10:46 pm


    My dogs and I are currently suffering from bites from possibly two different types of parasites infesting our apartment. In addition to finding what appeared to be multiple welt like bites that turned to permanent red spots on myself, I have found the oft-described “black marker soaked in stain” associated with bed bugs, on furniture and once or twice on the bedding. And there is literature stating that although humans are the filet mignon of their world, bed bugs will feed on pets as well. However, I also continue to find the “black pepper/flakes” resembling flea dirt, wherever the dogs have been laying or sitting. For example, if they jump up on the bed and lay down, within a few seconds I will find these tiny black specks ranging in size from microscopic, like the eye of a needle or smaller, to larger flakes resembling black dandruff, so to speak. With the way they scratch and chew on themselves and shake their ears out, it made sense that this infestation must include fleas as well. If so, they must be nesting fleas, because my dogs have been to the Veterinary Clinic three times in the last four weeks, and most recently had a bath at the same Clinic, and no fleas were observed whatsoever. But since we have yet to make a conclusive identification of what we are dealing with, my question to you is could it be possible for flea dirt to NOT turn red with the water test? And if the answer is no, then please advise if you know of what parasite might be feeding on or living on my dogs that fall off onto their sleeping area, resembling black pepper speck to the human eye? Please help.

    • Adam Retzer September 5, 2016, 1:17 pm

      Hello Anthony,

      The black dandruff-like flakes on your dog certainly sounds like flea dirt. However, these flecks should turn red when exposed to water, as flea feces consists of dry host blood. Adult fleas feed exclusively on blood. If the black specks aren’t dissolving into red when placed in water, or rubbed on a wet cloth, then it’s likely not fleas. (I find it confounding that other blood-feeding parasites would leave behind black specks that aren’t composed of excreted host blood.)

      The bite reaction is hard to use for identification. Most insect bites are similar.

      The flea species that infest dogs and cats aren’t “nesting fleas”. The adults of these species are permanent ectoparasites. They’ll stay on their host once acquired. Though sometimes difficult to notice, finding fleas is relatively straight-forward upon close inspection of the animal using a flea comb. Detection is easiest where fur is less dense, such as on the abdomen.

      In light flea infestations, dogs and cats sometimes self-groom the fleas off before they can be detected. And adult fleas only make up 1-5% of the infestation. The immature stages make up 95-99% of the population and live in the environment (carpeting or pet beds).

      Dark marker-like stains aren’t associated with fleas. It sounds like you’re dealing with bed bugs. Unfortunately, I’m not all that familiar with other parasites, which limits how much I can help with identification. Like you say, the best way to make a conclusive identification is by finding a live specimen.

  • Kim Barber October 30, 2018, 12:10 pm

    I found many black specks on my mattress protector. I rubbed my hand over them and it smeared red. Is this flea related?

    • Adam Retzer November 11, 2018, 5:01 pm

      It is possibly fleas. It sounds like it may be bed bugs though. It’d be a good idea to set up a flea trap or bedbug trap to see what you are dealing with.