In this lesson, you will learn about a disease with great notoriety in the eastern United States, Lyme disease. Learn about what causes this disease, the epidemiology, and prevention.
Conspiracy on Plum Island
Everyone loves a good conspiracy theory, and today, I've got a great one for you. About 10 miles off the coast of Connecticut is Plum Island, a small island with a government lab facility on it. Rumor has it, after World War II ended, a German scientist started working at the Plum Island lab. His area of expertise was diseases carried by arthropods, specifically ticks. He allegedly worked on developing tick-borne pathogens for biological warfare.
Over the years, several safety and security breaches led to the release of many of these deadly ticks. Then, in 1975, a strange disease started popping up in Old Lyme, CT, just 10 miles north of Plum Island. In the 1980s, bacteria found in the gut of deer ticks was identified as the cause of this disease, now commonly known as Lyme disease, after the town it first infected. Since then, Lyme disease has been slowly spreading outward from Connecticut, infecting people throughout the northeastern United States.
So, is this a true story? Was our government working with a German scientist, and together, they are responsible for unleashing Lyme disease on the world? There are not really any confirmed cases of Lyme disease in North America before 1975. Did the bacteria even exist before then? Realistically, it probably did, and any number of factors could explain why it wasn't documented until the '70s.
Recent studies of the genomes of bacteria that cause Lyme disease have suggested that the bacteria have been in North America for thousands of years, just not very prevalent or active. But, this is still a good story, worthy of its own Hollywood treatment. Regardless of where it came from, Lyme disease today is the most commonly reported arthropod-transmitted disease in the U.S., so let's move away from the conspiracy theories and learn the facts behind this disease.
Lyme disease is caused by organisms in the genus Borrelia, which are spirochete bacteria commonly found in the gut of infected deer ticks. A spirochete is a spiral-shaped bacterium. Several species of Borrelia can cause Lyme disease; which one depends on where you are in the world. B. afzelii is common in Asian countries and part of Europe. B. garinii is common throughout Europe. The major species that causes Lyme disease in the United States is Borrelia burgdorferi. These species all have very similar characteristics, but we will focus on Borrelia burgdorferi, the one you are most likely to run into.
In the northeastern United States, Borrelia burgdorferi can be found in the gut of the deer tick, Ixodes scapularis. The deer tick serves as an arthropod vector, or an insect or arachnid that transmits a disease-causing organism from one host to another. When a tick vector bites another animal or human to get a blood meal, it can inject the bacteria into the bloodstream. Any tick that feeds from an infected human or animal can pick up and then spread the bacteria to others. It is estimated that up to 50% of deer ticks in the northeast harbor Borrelia burgdorferi.
Deer and mice are the primary reservoir hosts for Borrelia. A reservoir host is a nonhuman host that serves as a means of sustaining an infectious organism as a potential source of human infection. The bacteria survive in the mice and deer, infect ticks when they feed on these animals and then later can infect a human via a tick bite. The high populations of deer and mice in the United States have helped Borrelia spread and become the most commonly reported tick-borne disease in the United States.
The key distinction between the vector and the reservoir host is that a human cannot acquire Lyme disease through contact with a reservoir host, like a deer or a mouse. The tick vector is required to transmit the bacteria between the reservoir host and the human.
Map showing where the majority of Lyme disease occurs
So, how many cases are we talking about? The United States averages between 20,000 and 30,000 cases of Lyme disease every year. The vast majority of Lyme disease cases, 96%, occur in only the 13 states seen above. In fact, if you look at the Centers for Disease Control and Prevention map below, you can see every reported case of Lyme disease from 2011.
Centers for Disease Control map showing 2011 Lyme disease cases
Each blue dot represents a confirmed case. There are isolated incidents in nearly every state, but several states in the northeast are almost completely blue. And, it does seem to cluster around the Plum Island area. Coincidence?
Well, coincidence or not, it is best to take proactive steps to prevent getting Lyme disease in the first place. First of all, there is not currently a human vaccine for Lyme disease. There used to be a vaccine, but in 2002, the company that produced it discontinued it due to low demand. Not to worry; there are still many things you can do to prevent Borrelia infection.
Not surprisingly, prevention focuses on eliminating tick bites. The deer tick is most active during the warmer months, April to September. During these times, if possible, it is best to avoid tick habitats like forests, bushy areas, high grass and leaf litter. But, don't ruin your summer fun. If camping and hiking are your favorite summer hobbies, or if you work in these environments, there are other preventative measures. Insect repellants with at least 20% DEET can be your best friend. Applying this to exposed skin should provide you with several hours of protection.
For added protection, clothing and gear treated with the insecticide permethrin will inhibit or kill ticks and usually stays active over several washes. If chemicals aren't your cup of tea, you should cover as much skin as possible, and even go so far as to tuck your pant legs into thick socks to prevent ticks from crawling in.
After spending any time in tick-infested areas, it is best to shower immediately. Deer ticks are very small and can be overlooked, so the running water can help rinse off those that haven't attached yet. You also want to thoroughly check yourself, your pets and your gear for ticks that might have hitched a ride. If you find any ticks that have already attached and are feeding on you, remove them as soon as possible. A tick can feed from you for 24-36 hours, with the chance of Borrelia infection going up every hour the tick is attached.
The only recommended method for removing an attached tick is to grasp it with thin tweezers as close to the skin as possible and pull straight up. Any home remedies or even twisting the tick while removing can prompt it to inject its stomach contents and bacteria directly into your bloodstream. For those of you interested in learning more, in another lesson, we will discuss the symptoms, diagnosis and treatment of Lyme disease.
Let's take a minute and review Lyme disease. Lyme disease is a bacterial infection caused by Borrelia burgdorferi, a spirochete bacterium commonly found in the gut of deer ticks. There are several additional species of Borrelia that cause Lyme disease worldwide, but Borrelia burgdorferi is the most prevalent in the United States.
The deer tick, Ixodes scapularis, serves as an arthropod vector, which is an insect or arachnid that transmits a disease-causing organism from one host to another. Frequently, deer and mice serve as reservoir hosts, nonhuman hosts that serve as a means of sustaining an infectious organism as a potential source of human infection. The deer and mice reservoir hosts infected with Borrelia are bitten by the tick vectors, which can transmit the bacteria to humans or other animal hosts.
Most cases of Lyme disease occur in the Northeastern United States. Every year, the U.S. sees between 20,000 and 30,000 Lyme disease cases, making it the most prevalent arthropod-borne disease. Preventing Lyme disease focuses on reducing tick bites. Applying insect repellants with at least 20% DEET, wearing clothing impregnated with permethrin and avoiding tick habitats should reduce exposure. Immediately after spending time in a tick-infested area, you should shower to wash off any ticks and perform a thorough tick-check of your body, gear and pets. If you find an attached tick, simply grasp it with thin tweezers and pull straight out.
After you've completed this lesson, you will be able to:
- Describe the structure of Borrelia
- Define arthropod vector and reservoir host
- Identify the species of Borrelia that causes Lyme disease and where in the U.S. this disease is prevalent
- Explain how Lyme disease is transmitted
- Summarize ways to prevent Lyme disease