How does flea medicine spread?


Topical flea medicines (flea drops) spread through dermal translocation. They incorporate into the skin’s lipid layer, and body oil eventually spreads the treatment across the animal’s entire skin in 12-24 hours.


Dermal Translocation

Flea drops spread on pets through a process called translocation (topical diffusion). The medicine is applied on a single location, or multiple locations on larger animals, but it quickly covers the animal’s entire skin. The process takes 12-24 hours.

The primary active ingredients in flea drops are imidacloprid (Advantage & Advantix) and fipronil (Frontline). Both of these compounds spread in a similar way. They don’t absorb through the skin (dermal or hypodermal layers), as they can’t pass the diffusion barrier created by the epidermis’ basal cells (basal lamina). Instead, the compounds remain on the superficial layers of the skin (stratum corneum and epidermis), and collect in the animal’s sebaceous (oil) glands and hair follicles.

The sebaceous glands and hair follicles act a reservoir for the medicine. It’s slowly and continually secreted through sebum (oil) onto the surface of the skin and hair. The animal’s oils help carry the treatment across the whole body.

Frontline Plus spreads from a single point of application, rapidly covering the entire dog and localizing in the hair, on the surface of the skin and in sebaceous glands. These glands act as a reservoir, continuously replenishing Frontline Plus onto the skin and hair coat, so it keeps working even if the dog gets wet.

Many flea drops also contain insect growth regulators, namely pyriproxyfen and methoprene. These compounds also spread through translocation, getting dispersed over the animal’s entire body.

Related Application Mistakes

Not Applying to Skin

Since flea drops spread through dermal translocation, it’s important to apply them to the dog’s or cat’s skin. Treatments may not work if applied to fur. This is why product labels instruct users to part the animal’s hair and apply the drops directly to skin, not the top of the hair coat.

Not Applying to a Dry Pet

Since flea drops work by incorporating into the animal’s body oils, it’s best to apply the drops when the pet’s fur and skin is completely dry. Efficacy could be diminished if applied to a wet animal.

Excessive Water Immersion

After properly applied, the medicine will be included into the dog’s or cat’s body oil and hair. Thus, the treatments are often labeled as water-resistant. They aren’t easily removed by rain, swimming, or bathing, because the lipid layer is always present. Efficacy remains high for four weeks, despite contact with water.

While fairly water-proof, excessive shampooing or frequent water immersion can reduce the efficacy of topical flea medication. Even when products explicitly state water resistance, weekly or daily contact with water could result in a decrease of efficacy. Shampooing should be limited to once a month. This is especially true with harsh oil-removing detergents that aren’t formulated for pets, such as Dawn.

Spreading to the Environment

When pets are treated with flea drops, trace amounts of the insecticides will transfer into the environment, especially where the animals rest. This can help reduce the number of developing fleas in the environment. For example, imidacloprid (IMI) kills flea larvae almost immediately upon contact. Even hair and skin debris from IMI-treated dogs has a profound larvicidal effect. Likewise, trace amounts of insect growth regulator (pyriproxyfen) will also transfer to environmental hot-spots and cause a decline in immature stages.

The insect growth regulators (IGR) in flea drops can also spread through flea feces (‘flea dirt’). Fleas defecate on their host, but it falls into the environment when the animal grooms itself. Flea larvae in the environment rely on the flea dirt for food. If the feces is tainted with IGR, then the larvae will be unable to reach adulthood upon consuming it.


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