Lyme disease is an infectious tick-borne disease, caused by the Borrelia spirochete, a gram-negative microorganism. It is transmitted to humans by the bite of infected ticks.
Adult deer ticks can be carriers of Lyme disease.
History
Lyme Disease is so named because it is generally believed to have first been observed in and around Lyme, Connecticut in 1975. Before 1975 the Borrelia infection was also called tick-borne meningopolyneuritis, Garin-Bujadoux, Bannwarth Syndrome or sheep tick fever.
The disease was first documented as a skin rash in Europe in 1883. Over the years, researchers there identified additional features of the disease, including an unidentified bacterium that was treatable with penicillin, the role of the Ixodes, or wood, tick as its carrier, and symptoms that included not only the rash but additional ones that affected the nervous system.
Researchers in the US have been aware of tick infections since the early 1900s. For example, an infection called tick relapsing fever was reported in 1905, and the wood tick, which carries an agent that causes Rocky Mountain spotted fever, was identified soon after.
The full syndrome now known as Lyme disease, however, was not identified until a cluster of cases thought to be juvenile rheumatoid arthritis occurred in three towns in southeastern Connecticut, in the United States. Two of these towns, Lyme and Old Lyme, gave the disease its popular name.
Microbiology
In the 1982 a novel spirochete was isolated and cultured from the mid-gut of Ixodes ticks, and subsequently from patients with Lyme disease. The infecting agent was first isolated by Willy Burgdorfer, a scientist at the National Institute of Health who specialized in the study of spirochete microorganisms. This gave this spirochete its name, Borrelia burgdorferi.
Borrelia burgdorferi resembles other spirochetes in that it is a highly specialized, motile, two-membrane, spiral-shaped bacterium which lives primarily as an extracellular pathogen. One of the most striking features of Borrelia burgdorferi as compared with other eubacteria is its unusual genome, which includes a linear chromosome approximately one megabase in size and numerous linear and circular plasmids.
Long-term culture of Borrelia burgdorferi results in a loss of some plasmids and changes in expressed protein profiles. Associated with the loss of plasmids is a loss in the ability of the organism to infect laboratory animals, suggesting that the plasmids encode key genes involved in virulence.
Borrelia burgdorferi may persist in humans and animals for months or years following initial infection, despite a robust humoral immune response.
Borrelia burgdorferi is susceptible to antibiotics in vitro, however, there are contradictory reports as to the efficacy of antibiotics in vivo in regard to complete eradication of the bacterium from the host.
Borrelia burgdoferi have been found in skin specimens of white footed mice in museum specimens as far back as the 1870's in Massachusetts, USA.
Consequent to the apparent long term infection, considerable attention has focused on the development of a vaccine for Lyme disease.
Current evidence suggests humoral immunity plays an important role in prevention of infection and resolution of disease; however, one of the difficulties in developing a meaningful strategy for immunization is that it is not understood what aspects of humoral and cell-mediated immunity are required to counter established infection.
Cause
It is caused by the bacterium Borrelia, which has well over a hundred known genomic strains but is usually cultured as Borrelia burgdorferi, Borrelia afzelii and Borellia garinii.
The disease has been found to be transmitted to humans by the bite of infected ticks. Not all ticks carry or can transmit the disease.
Lyme disease also incoporates the transmissions of tick borne co-infections including Bartonella, Babesiosis, Ehrlichiosis, and Rickettsia.
Vector
Borrelia is found in mammal blood upon infection and is transmitted by the tick "spitome" or saliva. The spirochete is transfered when the tick feeds on a desirable host. Roughly 17,000 infections are reported in the United States each year. The illness often goes unreported and the real numbers may be ten-fold higher.
The wood or black-legged deer tick (Ixodes rinicus) has been identified as the key to the disease's spread. This condition had been described in medical literature dating back to the early twentieth century but little to no research had been done until Lyme Disease was reintroduced to the medical field in the late 1970's.
The number of cases, as well as endemic regions in the United States, have been increasing. Lyme disease is reported in nearly every state in the U.S. There are concentrated areas in the northeast, mid-Atlantic states, Wisconsin, Minnesota, and northern California. Lyme disease is endemic to Europe and Asia.
Lyme disease has been proven to be congenitally passed from an infected mother to fetus through the placenta during pregancy. There is some anecdotal, largely unconfirmed evidence of Censored page.
Tick Life Cycle
The deer (or black-legged) tick, and the related western black-legged tick, are the primary known transmitters of Lyme disease in the United States. Both are hard-bodied ticks with a two-year life cycle. Like all species of ticks, deer ticks and their relatives require a blood meal to progress to each successive stage in their life cycles.
The life cycle of the deer tick comprises three growth stages: the larva, nymph and adult. In both the northeastern and mid-western U.S., where Lyme disease has become prevalent, it takes about two years for the tick to hatch from the egg, go through all three stages, reproduce, and then die. A detailed description of this life cycle and the seasonal timing of peak activity, as they occur in these regions, is provided below.
Stage 1
Larva - Eggs laid by an adult female deer tick in the spring hatch into larvae later in the summer. These larvae reach their peak activity in August. No bigger than a newsprinted period, a larva will wait on the ground until a small mammal or bird brushes up against it. The larva then attaches itself to its host, begins feeding, and over a few days, engorges (swells up) with blood.
If the host is already infected with the Lyme disease spirochete from previous tick bites, the larva will likely become infected as well. In this way, infected hosts in the wild (primarily white-footed mice, which exist in large numbers in Lyme-endemic areas of the northeast and upper mid-west) serve as spirochete reservoirs, infecting ticks that feed upon them. Other mammals and ground-feeding birds may also serve as reservoirs.
Because deer tick larvae are not born infected, it is believed that they cannot transmit Lyme disease to their human hosts. Instead, "reservoir" hosts, as mentioned above, can infect the larvae. Having already fed, an infected larva will not seek another host, human or otherwise, until after it reaches the next stage in its life cycle. It is not completely known whether larvae, in themselves, pose a threat to humans or their pets.
Stage 2
Nymph - Most larvae, after feeding, drop off their hosts and molt, or transform, into nymphs in the fall. The nymphs can remain remain active throughout the winter and early spring.
In May, nymphal activity begins. Host-seeking nymphs wait on vegetation near the ground for a small mammal or bird to approach. The nymph will then latch on to its host and feed for 4 or 5 days, engorging with blood and swelling to many times its original size. If previously infected during its larval stage, the nymph may transmit the Lyme disease spirochete to its host. If not previously infected, the nymph may become infected if its host carries the Lyme disease spirochete from previous infectious tick bites. In highly endemic areas of the northeast, at least 25% of nymphs have been found to harbor the Lyme disease spirochete.
Too often, humans are the hosts that come into contact with infected nymphs during their peak spring and summer activity. Although the nymphs' preferred hosts are small mammals and birds, humans and their pets are suitable substitutes. Because nymphs are about the size of a poppy seed, they often go unnoticed until fully engorged, and are therefore responsible for the majority of human Lyme disease cases.
Stage 3
Adult - Once engorged, the nymph drops off its host into the leaf litter and molts into an adult. These adults actively seek new hosts throughout the fall, waiting up to 3 feet above the ground on stalks of grass or leaf tips to latch onto deer (its preferred host) or other larger mammals (including humans, dogs, cats, horses, and other domestic animals). Peak activity for adult deer ticks occurs in late October and early November. Of adults sampled in highly endemic areas of the northeast, at least 50% have been found to carry the Lyme disease spirochete.
As winter closes in, adult ticks unsuccessful in finding hosts take cover under leaf litter or other surface vegetation, becoming inactive when covered by ice and snow. Generally, winters in the northeast and upper mid-west are cold enough to keep adult ticks at bay until late February or early March but not when temperatures begin to rise. At this time, they resume the quest for hosts in a last-ditch effort to obtain a blood meal allowing them to mate and reproduce. This second activity peak typically occurs in March and early April.
Adult female ticks that attach to deer, whether in the fall or spring, feed for approximately one week. Males feed only intermittently. Mating may take place on or off the host, and is required for the female's successful completion of the blood meal. The females then drop off the host, become gravid, lay their eggs underneath leaf litter in early spring, and die. Each female lays approximately 3,000 eggs. The eggs hatch later in the summer, beginning the two-year cycle anew.
Symptoms
Lyme Disease has many symptoms, but skin symptoms, arthritis and various neurological symptoms are usually present. Coventional therapy is with antibiotics.
Acute (early) symptoms
The incubation period from infection to the onset of symptoms is usually 1-2 weeks, but can be as long as one month. However, it is possible for an infected person to display no symptoms, or display only one or two symptoms, which can make diagnosis difficult.
Chronic (late) symptoms
The late symptoms of Lyme disease can appear months to years from infection. Left untreated, Lyme disease can cause chronic disability, but is rarely fatal. Chronic cases have been known to linger for years before a definitive diagnosis.
Lyme Disease has been misdiagnosed as Chronic-Fatigue Syndrome, Multiple Sclerosis, Rheumatoid arthritis and a host of other autoimmune and neurological diseases.
Diagnosis
The most reliable method of diagnosing Lyme disease is a clinical exam supported by laboratory tests. In cases where the "bull's eye" rash is present in conjunction with a fever or the patient saw the tick, treatment can begin without any further tests.
The laboratory tests available are the Western blot and ELISA, but neither is a reliable indicator: test results vary between labs and within the same lab, sero-negative results are frequent.
Polymerase chain reaction (PCR) tests for Lyme disease may also be available to the patient. A PCR test attempts to detect the genetic material (DNA) of the Lyme disease spirochete, where as the Western blot and ELISA tests look for antibodies to the organism. PCR tests are also susceptible to false-positive results.
In cases of chronic Lyme disease, diagnosis is often clinical and must take all factors into account (tick bite exposure, symptom history, etcetera). Positive diagnosis will continue to be problematic until a more reliable test is developed.
Prognosis
The severity and treatment of Lyme disease can be complicated by simultaneous infection with other tick-borne diseases, also known as coinfections, bacterial load and immune suppression in the patient.
The disease is rarely fatal in and of itself. Chronic Lyme can cause severe disability and morbidity.
Prevention
The probability of contracting Lyme disease can be reduced by avoiding areas in which ticks are found. If such places cannot be avoided, exposure to Lyme disease can be reduced by:
- applying insect repellent to exposed skin, especially those containing DEET. Permethrin can also be applied to clothing,
- wearing light-coloured clothing so that ticks can be located easily and removed,
- wearing long sleeves and pants and tucking pant bottoms into the tops of socks.
In addition, tick removal immediately when found may prevent infection. It is an excellent idea to preserve the tick and have it tested for Lyme disease if the bite occurred in an endemic area.
Carefully remove the tick with a pair of tweezers. Take extra care to preserve as much of the tick as you can for identification and laboratory testing.
Treatment
Treatment of acute Lyme disease usually consists of a one month course of antibiotics. Patients with coinfections may need prolonged treatment. With the chronic late-stage form of the disease, it may be necessary to continue antibiotic treatment for months or years. In some cases immunomodulating drugs are necessary.
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Last updated: 05-06-2005 14:35:24
