BUG WEEK: Day 3- Tick talk

Often a fair amount of time elapses before people realize they have a tick on them. This goes back to yesterday's post, where I listed the most common bug bites that are painless. (Technically, ticks aren't even bugs. They have eight legs, making them arachnids. Bugs have 6 legs.) Unlike bed bugs, ticks do not usually elicit an allergic response, so there's no itchiness to alert one to the presence of the feeding tick. There are a few things I want to talk about with ticks...
1. The different types of skin reactions they elicit
2. The different diseases that they carry and transmit
3. The regions of the country where you will find certain ticks
4. How long they have to be attached to transmit disease
5. When to give antibiotic prophylaxis
Hopefully I'll get through everything tonight.

There are two kinds of ticks: hard ticks (Ixodae) and soft ticks (Argasidae). Mostly hard ticks are responsible for disease transmission. I'm going to try to minimize the tick pictures I post because I think they're gross.


Erythema chronicum migrans is associated with Lyme disease. You see multiple large red patches that are clear in the middle. The skin reaction is in the early stages of the disease and resolves on its own. So you may see a patient within the first few weeks of infection that has the skin findings, but after that, you'll have to elicit the history about the skin findings. Judging by how long it takes to get into see a dermatologist these days, I bet you're more likely to see patients who had the rash when they made the appointment, but it resolved before they made it into the office. I got the picture from DermAtlas

LYME DISEASE (Borreliae burgdoferi)- The most well known of all the tick-borne diseases. So, I didn't realize that there was a commercially-available Lyme disease vaccine between 1998 and 2002 (LYMErix). It contained a surface antigen of the bacteria that caused antibody production in humans. The neat part about the vaccine is that the antibodies actually fight the bacteria in the tick instead of in the human. When the tick attaches to the human, it does not transmit the bacteria until it regurgitates some blood and saliva. This doesn't happen until 18-24 hours after the original bite. During this time, the antibody immune response is going on inside the tick, so that when it regurgitates, it should not have active bacteria in the regurgitant. I guess that living in Florida and Texas aren't close enough to the tick belt to get you the vaccine. But it looks like I didn't miss much, since they took the vaccine off the market because of reports of a vaccine-induced, treatment-resistant, chronic arthritis.


The ticks responsible for Lyme disease are from the Ixodes genus and are commonly known as black-legged ticks. They are merely the vector for transport, as they acquire the disease from white-footed mice, deer, and other mammals. When the infected tick bites humans, it transmits the disease 18-24 hours after biting. You have almost a full day to find the tick before it transmits disease! This disease is most common in the northeast and Wisconsin, but it can be found in the Northwest as well.

It can present clinically as a relapsing fever with general symptoms of malaise and muscles aches or it can present as classic Lyme disease. This involves three stages. The first is relapsing fever with erythema migrans. The second is disseminated disease which could include arthritis, carditis, and neurologic disease. This happens after the little spirochetes screw their way down to the blood stream and spread throughout the body. The third stage is chronic arthritis. This is if you don't treat it. Treatment is with doxycycline or penicillin. Antibiotic prophylaxis after a tick bite is quite controversial.

RB Nadelman, J Nowakowski and D Fish et al., Prophylaxis with single-dose doxycycline for the prevention of Lyme disease after an Ixodes scapularis tick bite, N Engl J Med 345 (2001), pp. 79–84.

This study showed that one dose of Doxycycline 200 mg was effective in preventing Lyme disease. They did not claim to prevent all cases, but they did have statistically significant results between the placebo and doxy groups. They showed an efficacy of treatment of 87%, but the 95% confidence interval is 25-98%. Confidence intervals this wide weaken the results of the study.

Maraspin V, Lotric-Furlan S, Strle F. Development of erythema migrans in spite of treatment with antibiotics after a tick bite. 2002 Jul 31;114(13-14):616-9.

This study showed a 0.14% rate of erythema migrans after prophylaxis, which sounds pretty good to me, but it doesn't seem like everyone agrees...

D Volkman, Prophylaxis of tick bites, Lancet Infect Dis 7 (2007), pp. 370–371.

Volkman was pretty adamant that the NEJM study was flawed in how it measured its outcomes.

Anyway, it seems like the consensus is to prophylax for Lyme disease as it shows good results and is low-risk. But this can be tailored depending on where in the country you are.

OK, that's enough for today. I'll do more tick-borne diseases tomorrow.