Lyme disease, also called Lyme borreliosis, is a disorder caused by a bite from a deer tick, which results in the transmission of the bacteria Borrelia burgdorferi (or one of its related species) to the host. There are two forms of the disease: acute and chronic. The chronic form may elicit a destructive autoimmune response in the host, in which the host's own immune system will attack its own tissue causing an abnormal response. The acute form of the disease presents with fever and a migratory “bull's eye” skin rash. The rash is called erythema migrans, which is Latin for “migrating redness,” and is commonly accompanied with pain in the muscles and joints. The chronic form usually takes years to evolve and can result in painful arthritis, myocarditis (inflammation of the muscular walls of the heart), and meningoencephalitis (inflammation of the brain and the membranes covering the brain and spinal cord).
Although humans have received tick bites since coming to North America in the 1600s, neurological systems were not officially associated with tick bites until the 1900s. It took until 1975 for the full syndrome to be identified as what is now known as Lyme disease. The disease was named after the township of Old Lyme, Connecticut, where the first cases of the disease were observed. About six years after the first diagnoses, the bacterium that causes Lyme disease was first identified in 1982 by medical entomologist Willy Burgdorfer (1932–). The bacterium bears his name, Borrelia burgdorferi. Today in the United States, Lyme disease is a relatively common tick-borne disease found in the states of the upper midwest (Illinois, Michigan, Minnesota, North Dakota, Ohio, and Wisconsin), northeast (Connecticut, Maine, Massachusetts, New Hampshire, Rhode Island, and Vermont), and mid-Atlantic seaboard (New Jersey, New York, and Pennsylvania). Each year, the United States reports more than 300,000 new cases, which is 10 times higher than what was originally reported (CDC, 2013a). Lyme disease is a condition that has now been identified around the world.
B. burgdorferi is transmitted to ticks during their early stage of development, specifically when the immature tick (nymph stage) is feeding on the blood of mice that harbor the bacteria. The ticks then feed on the blood of deer and other animals as they mature into adult ticks, carrying the bacteria in their system. The deer are a necessary part of the tick life cycle. During the spring, female ticks feed on the deer and then fall to the ground to deposit their eggs. As summer approaches, the tick larvae feed on the blood of mice that harbor the bacteria as they mature into their nymph stage. Nymphs feed on any available vertebrate host and once satiated, fall to the ground where they can move on to their next host, such as a deer, to complete their maturation cycle. Because this nymph stage of development occurs during a time of year when humans are enjoying time outdoors in wooded areas, humans can easily serve as a host to nymphs and become infected with B. burgdorferi.
The pathogenesis (the development of the illness) of Lyme disease is not well understood but appears to be complex and involves multiple organ systems in the body. The bacteria begin their journey in a human by infecting a localized region where the tick has fed before spreading to other tissues of the body via the bloodstream. The initial bull's eye rash can take up to 30 days to appear before the infection becomes systemic (CDC, 2013b). Failure to treat the condition increases the likelihood that the patient will develop chronic and sometimes disabling symptoms as the disease progresses. In fact, the chronic symptoms can be so severe that it is often misdiagnosed as an autoimmune or neurodegenerative disease, such as lupus or multiple sclerosis. The outer surfaces of the bacteria express antigens (bacteria surface proteins), which are substances that cause a person's immune system to naturally produce antibodies against the antigens as they are not recognized as “self.” In turn, the antigens induce inflammatory properties in the host and then start a molecular mimicry process. This means the host's immune system does not kill the bacterium; instead it causes a down regulation (reduce) of the immune response in the host animal. In fact, the arthritis that occurs in chronic Lyme disease has been linked to this property of the bacteria and its ability to trigger an autoimmune response in the host.
Diagnosing Lyme disease includes determining appropriate symptoms and clinical signs, possible exposure to ticks in certain regions, and blood tests. Clinical signs include an erythema migrans rash, facial palsy (specifically Bell's palsy), and/or arthritis. If a patient does not have the typical erythema migrans rash, a physician can measure the level of antibodies against B. burgdorferi in the patient's blood. Early in an infection, however, the patient may not have produced enough antibodies to be measured and a false-negative test may result. Thus, blood tests are often used in diagnosing later stages of the disease, as they become more reliable over time. Another initial sign that is often observed in European patients but not in patients from the United States is a purplish lump called borrelial lymphocytoma. It may be found on the ear lobe, nipple, or scrotum and can be associated with painful joints, abnormal heart rates, and dizziness.
As previously stated, arthritis is common with chronic Lyme disease. However in acute Lyme disease, Bell's palsy may occur in 10–15 percent of patients (CDC, 2013b). This is a result of the facial nerve becoming inflamed by the bacterial infection. The facial nerve is one of 12 paired cranial nerves that supply motor, sensory, and autonomic functions to the head, neck, and face. Thus, if this nerve is damaged, it results in paralysis of all muscles used for facial expression on the same side of the infection. This condition is called Bell's palsy, which can be an acute or chronic disorder depending on the location of the lesion. In general, only one of the facial nerves is affected; it is rare to have both nerves affected. Symptoms of Bell's palsy are facial asymmetry in which the eyebrow, eyelid, and lip droop, the forehead and nasolabial folds (around the mouth) smooth out, and the tears of the affected eye will not properly drain into the nasolacrimal duct. Bell's palsy is named for the Scottish neurologist Charles Bell (1774–1842) who was the first to describe the asymmetrical facial paralysis.
Treatment of humans diagnosed with Lyme disease involves a class of drugs called antimicrobics or antibiotics such as doxycycline (part of the tetracycline class of drugs), amoxicillin (from the class of penicillin), or cefuroxime (a cephalosporin antibiotic). The medicine is given orally and is often curative. Response to treatment can be slow, however, and require treatment for up to 30–60 days. More advanced stages of the disease offer a poorer prognosis since the damage caused by the bacteria to the joints, the heart, and the nervous system can remain even after the infection has been successfully treated.
The most successful measures for preventing Lyme disease involve covering up the body with protective clothing, especially the parts of the body that are likely to come into contact with ticks. It is also important for people who have traveled into wooded areas during tick season to inspect their bodies and those of their pets for ticks after returning home. If a tick is discovered on the body, it should be removed promptly using tweezers to grasp the tick as close to the skin as possible. Nymph forms of the ticks are tiny and harder to detect than adults. Therefore, it is advised that hikers wash their bodies well as soon as they can. Although vaccines against bacterial surface proteins have been tested in humans and found to be moderately effective in preventing the disease, they are very expensive and somewhat controversial. This is because it is estimated that only about 1 percent of tick bites actually transmit the bacteria (Ryan et al., 2010). Several vaccines have been widely approved for use in dogs, by killing the bacteria in the tick during feeding before it can be transmitted to the dog or to a neighboring host such as the dog's owner.
See also Bell's Palsy (Bell Palsy); Dizziness and Vertigo; Facial Nerve; Multiple Sclerosis
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