Content Reviewers:Yifan Xiao, MD
Ectoparasiticides are topical medications used to treat these infestations.
Endectocides on the other hand, are multi-purpose antiparasitics that work on both external parasites and some internal parasites, like roundworms.
Now, lice are wingless insects that live off of the blood of their hosts.
There are 3 types of lice that infest humans: head louse that lives in the hair of the scalp; body louse that lives in the hair of the axilla, back, and pubic region; and pubic louse, also called crab louse, that lives in pubic and axillary hair, but also the eyebrows.
Lice infestation is called pediculosis and the main symptom is pruritus, (or itchiness), which is caused by an allergic reaction to louse’s saliva.
These tiny critters are eight-legged arthropods that are related to spiders.
Unlike spiders, scabies mites burrow into the skin and cause a pimple like rash locally.
Even worse, they tunnel around under the skin, forming elevated tracks that are visible on the skin surface.
The burrowing causes intense itching that’s more frequent at night when the mites are more active.
The most common sites for scabies are between the fingers, on the flexor surface of the wrist, and the extensor surface of the elbow and knees.
They can also be found around the areola of the breast and the pubic region.
Permethrin is an ectoparasiticide and an insecticide. It’s the first line medication for both scabies and pediculosis.
This medication is used topically in the form of creams, lotions, or shampoos.
Once taken in by the parasite, permethrin binds to sodium channels on the membrane of neurons, and prolongs their inactive state. This results in delayed repolarization, leading to respiratory muscle paralysis and death.
One benefit of permethrin is that it’s ovicidal, meaning it also kills the eggs of lice and mites.
This medication is slowly absorbed into the skin and has a residual effect even after it’s rinsed off.
Another advantage of this medication is the relatively mild profile of its side effects, like local skin irritation and burning sensation.
It’s generally considered safe to use with younger children and during pregnancy.
One down side for pet owners is that even a small dose is extremely toxic for felines, so it’s not a good idea to handle cats or kittens while using permethrin.
Another topical ectoparasiticide is lindane, which is an organochlorine that was formerly used as an insecticide until it was banned.
It’s less effective than permethrin for treating scabies and pediculosis, and it has more side effects as well, so it’s only used as a second line medication.
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