Scabies I

Figure 1: Scabies mite 

(Source)

What did you do on your vacation this summer? Laid on the beach or hiked in the mountains maybe. Me? I got scabies (Figure 1). I'm sure at least a few of you saw this blog post coming. Since I found out that all these little bumps on my skin were scabies and not bed bugs, I've been reading all about the little fu-, um, critters. My conclusion is that they're disgusting.

Symptoms of a scabies infestation/ infection*

Figure 2: Scabies burrow 

(Source: my leg)

• Rash
• Red bumps or papules, they can look like mosquito bites or little tiny bulbous blisters that ooze clearish liquid if you scratch them... and you will because...
• Itchiness. Intense itchiness. These guys aren't messing around. 
• Sleep loss to due itching. (The itch tends to be worse at night.)
• Burrows, sometimes difficult to find but often in between fingers or toes (Figure 2)

Scabies are tiny parasitic arachnids, called the Sarcoptes scabiei mite. They've been bugging** humans for at least 2,500 years. The male mite bites your skin and causes pimple-like bumps (Figure 3). Males aren't too terrible because they just hang out on top of your skin, bite a little, do the nasty with the females, and then die. 

The females, on the other hand, get preggers and use enzymes to dissolve the outermost layer of skin, called the stratum corneum, and then eat their way underneath it.  They like to lay their eggs there, presumably because it's a nice safe place away from danger. While under the skin, the females crawl a little and lay more eggs, then crawl a little and lay more eggs, pooping along the way. This burrowing behavior is what causes the characteristic curvy lines of bumps (Figure 2). 

Mites crawling under your skin sounds itchy, huh? It can be but the real deep-down-drive-you-crazy itch actually comes from an allergic reaction to the eggs and feces. Allergies take awhile to develop so you can have scabies mites on and in your skin for 2-4 weeks without showing any bumps or itchiness. Unfortunately, they are there and you're very contagious during that time. This means I probably didn't get scabies on my vacation. It just seemed like it because the symptoms began during my vacation. It can be difficult to pin down where (or, more likely, who) your scabies came from because of this. As with other allergies, re-exposure to the allergen will produce a reaction much faster, usually within 24-48 hours with scabies because your immune system is already on the lookout for the antigens.

Figure 3: Scabies bump from a male bite maybe 

(the literature is unclear on this). (Source: my leg again)

If a scabies infection isn't caught quickly, the eggs hatch and the little baby scabies mites come out of the skin just long enough to mate. The impregnated females dive back under the skin to repeat the cycle, which takes about two weeks. Thankfully, only about 1% of the immature mites, called nymphs, live to adulthood. It's surprising, but in a classic case of scabies there are only 10-15 mites on any one person. How such a small number of mites can cause so much frustration and itching is impressive but what you really want to know is how to kill them.

There are two options for treatment: a cream and a pill. The cream contains permethrin (5%), which is used at a lower concentration (1%) over-the-counter called Nix for lice. The cream is slathered all over the body from the neck down usually before bedtime. The cream has to stay on for 8-14 hours in order to kill off all the adult mites. Permethrin works by disrupting neuronal transmission.*** It's 2550 times more effective at disrupting transmission in invertebrates, which are animals without a spinal column, than in humans. It's also poorly absorbed by the skin and metabolized quickly by the liver to non-toxic compounds. This makes short-term permethrin treatment safe for humans at low doses.

The pill is called Ivermectin and is usually reserved for more severe cases of infestation, most notably Norwegian crusted scabies which is something you probably don't want to google for images. I cannot imagine how horrible it must be to have an infestation progress to that stage. Thankfully, crusted scabies are less itchy but unfortunately, are much harder to get rid of. Ivermectin also works by disrupting neurotransmission but in a slightly different way.****

Figure 4: Contact dermatitis. 

(Source: you guessed it, same leg)

Which is better: permethrin or ivermectin? At first glance, the scientific literature favors permethrin but several weeks after treatment with either, the cure rates were not significantly different in two studies. A pill seems easier but the cream may work faster, possibly because Ivermectin acts on the adult mite's central nervous system and can't kill the eggs or the newly hatched nymphs. The cream, I found out firsthand, also has the unfortunate side effect of causing contact dermatitis, aka an itchy rash (Figure 4). No matter what the treatment to actually kill the scabies, antihistamines (benadryl) and topical corticosteroids (cortizone) help immensely to reduce the itch. Benadryl has the added side effect of knocking you out so you can sleep your way through the nightmare that is scabies infestation. Using topical corticosteroids for longer than a week or on a large surface area of the body is not recommended though because it can thin the skin, a condition called steroid atrophy.

So far, I've described the standard information out there about scabies albeit hopefully in a more entertaining format. But it's not enough information for me because I always want to know WHY. In my next post, I'll attempt to explain some of the whys, such as: Why do I need to use permethrin cream at night? Why do scabies itch more at night? Am I going to get scabies from my friend who I ate dinner with the other night who had them? Inquiring minds want to know.

*The term infection seems more benign but I think the technically correct term in this case is infestation. Ew.

**It's funny because some pedant is going to want to correct me and say that arachnids aren't bugs. To that I say, hah! 

***More detail? It causes prolonged depolarization of sodium channels. 

****Ivermectin probably works through ligand-gated chloride channels as in the nematode but the mechanism hasn't been studied in the scabies mite yet.

Please see my second post on scabies for some FAQs.

I am not a medical doctor, none of this is intended as medical advice, and I cannot diagnose you in the comments. If you are concerned about your health, please see your physician.