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Lyme Disease

Introduction

Prevention and Control

Clinical Aspects

The Bacterium and Vector Ecology

 

Introduction

Lyme disease is the most common vector-borne disease in the United States. The infection is caused by Borrelia burgdorferi, a bacteria-like organism called a spirochete which is endemic in at least 15 states. Transmission to humans occurs by the bite of an infected deer tick. The tick responsible for transmitting Lyme disease in humans in the Northeast and North Central regions is Ixodes scapularis and in the west is the Ixodes pacificus. Deer ticks are much smaller than dog ticks. The nymph and larvae stages of Ixodes scapularis are about 1 mm in size.  This is about the size of a pin head. Ticks feed by inserting their mouth parts into the host and taking a blood meal. The longer a tick is attached, the more likely it is to transmit Lyme disease.

The risk of Lyme disease in the United States is well localized to three areas: the Northeast from Maine to Maryland, the Midwest, specifically, Wisconsin and Minnesota and the Northwest, specifically Northern California and parts of Oregon. Lyme disease is the leading vector-borne illness in the United States, with approximately 15,000 cases reported every year.  Although cases have been reported in 49 states and the District of Columbia, 90% of cases occurred in ten states. 

Fire fighters who work and live in these areas are at increased risk of getting Lyme disease. Fire fighters who are surrounded by woods or overgrown brush, or who participate in outdoor recreational activities such as hunting, camping, fishing, or hiking are at increased risk of Lyme disease. Preventive measures can reduce the chance of being bitten by a tick.
 

                 Borrelia burgdorferi               


The deer tick, Ixodes scapularis
at different stages of development
next to a centimeter ruler. From left to right:
Adult female, adult male, nymph, and larvae

 

Prevention and Control

Tick Habitat

Ticks favor a shaded, moist environment with low-lying vegetation in a heavily wooded or grassy area. Tick infestation is more likely to occur in the spring and summer and requires the presence of both deer and rodent hosts. Fire fighters exposed to these areas should be covered up and if possible, wear light colored clothing so ticks can be easily seen. High boots and socks tucked into pants may provide additional protection since ticks are usually located close to the ground. Insect repellents such as DEET (n,n-diethyl-m toluamide)or Permethrin on clothing or exposed skin may also decrease the risk of a tick bite. Application of insect repellents should be done in accordance with the Environmental Protection Agency guidelines.
 

Tick Removal

Removal of ticks promptly after they attach themselves decreases the likelihood of transmission of Lyme disease. Therefore, daily tick checks will help to minimize the risk of acquiring Lyme disease. Once attached, ticks should be removed with a tweezers. Products such as a hot matches or petroleum jelly should not be used.  Even if the tick's mouthparts remain in the skin, there is no longer a risk of acquiring Lyme disease since the tick's mid-gut is the site of the bacteria that causes Lyme disease.
 

Prophylactic Treatment Following a Tick Bite

Though controversial, currently treatment for individuals who have tick bites with no evidence of Lyme disease is not recommended according to the Practice Guidelines for the Treatment of Lyme Disease which was published in Clinical Infectious Disease in 2000. The chance of acquiring Lyme disease after a tick bite depends on the prevalence of ticks infected with Borrelia burgdorferi in the community, the amount of time the tick is attached and the stage of the tick. Individuals who have sustained a tick bite should watch for any signs or symptoms suggestive of Lyme disease or other tick borne diseases and if present, should seek medical attention. If an individual sustains a prolonged tick bite in an endemic area, then the pros and cons of prophylactic treatment may be discussed with a healthcare provider. 
 

Ways to Decrease the Tick Population

Altering the habitat best suited for the tick population is one method of decreasing the tick population. This entails removing wood piles, brush and leaves as well as clearing trees. Other methods of decreasing the tick population involves application of pesticides on properties, reducing the deer and rodent populations and killing ticks on the deers and rodents.
 

Vaccine

The vaccine, LYMErix, is a newly developed vaccine for individuals between the ages of 15-70 in the United States. It is administered by an intramuscular injection and consists of three doses. The second dose is administered one month after the first dose and the third dose 12 months after the first dose. Boosters might be required but additional data are needed before specific recommendations can be made.  The Advisory Committee on Immunization Practices recommends that the decision to use the vaccine be made based on the person's risk of acquiring Lyme disease.  This takes into consideration an individual's activities, behaviors, occupation and geographic location. Individuals whose exposure to tick infested habitat is frequent or prolonged should consider vaccination. A full set of recommendations are published by the advisory committee on Immunization Practices. 

 

Clinical Aspects of Lyme Disease

Clinical Description

The presentation of Lyme disease can vary. Approximately 80% of individuals who get Lyme disease have a characteristic rash called Erythema migrans. This rash occurs at the site of the tick bite. It starts out as a round circular lesion which increases in size over time and has a central clearing. The lesion can be present as early as three days after a tick bite and as late as 30 days. More commonly, it occurs one to two weeks after a tick bite. Erythema migrans is often accompanied by non-specific flu-like symptoms such as a fever, fatigue, muscle and joint pains and a headache. Other individuals may have no symptoms and still others may have non-specific flu-like symptoms but no skin rash. Once the infection has occurred, days to weeks after the initial skin lesion spirochetes spread from the site of the initial lesion and result in early-disseminated infection. If untreated, disseminated Lyme disease can involve the nervous system, the musculoskeletal system and the cardiac system. Neurologic involvement can include meningitis, encephalitis, cranial neuropathies, especially facial nerve palsies, inflammation of the motor and sensory nerves (radiculoneuritis) and other symptoms. The musculoskeletal system typically involves large joints  with or without evidence of swelling. Occasionally, if the heart is involved, conduction problems (atrioventricular block), inflammation of the tissue around the heart or an enlarged heart can result.
 

Diagnosis

The diagnosis of Lyme disease is based on clinical presentation coupled with a known tick bite. Additional supportive evidence of Lyme disease can be obtained from blood work called serologic testing. Serologic testing alone should not be relied upon since it is often negative early in the course of Lyme disease and positive results may persist for months to years following treatment. Serologic testing involves enzyme-linked immunosorbent assay (ELISA) test followed by a more specific Western immunoblot test for positive or borderline results. Neither a prior history of Lyme disease nor positive serology assures that individuals will not acquire Lyme disease again.
 

Treatment

Lyme disease can be treated with 14 to 21 days of antibiotics.  Doxycycline or amoxicillin are generally considered effective.  The benefit of doxycycline is that it is effective against human granulocytic ehrlichiosis, another tick borne disease which can be a coinfecter. Amoxicillin should be reserved for pregnant women or children under the age of 8.  For individuals allergic to these antibiotics other possibilities exist, including Cefuroxime and erythromycin.  For later stage disease, where an individual has objective evidence of neurologic involvement, 2 to 4 weeks of IV ceftriaxone is most commonly used. 

                          Erythema migrans

 

 

The Bacterium and the Vector Ecology

The Tick Life Cycle

The organism that causes Lyme disease is Borrelia burgdorferi, a bacteria-like organism called a spirochete.  In order for Lyme disease to exist, three factors must be present.  There must be a vector, a host and the spirochete. The vector is the tick, Ixodes scapularis, which transmits the spirochete that causes Lyme disease.  The host is the mammal such as a mouse or deer which provides a blood meal.  The bacteria-like organism called a spirochete is Borrelia burgdorferi.

To better understand Lyme disease, it is helpful to understand the life cycle of the Ixodes scapularis tick.  Life cycles vary from region to region and for specific ticks.  Typically, the life cycle for the Ixodes scapularis tick in the Northeast requires two years to complete.  In early spring, the adult female ticks lay eggs on the ground.  The eggs hatch into larvae in the summer.  The larvae feed on mice, deer and birds.  In the late summer and early fall the larvae molts into nymphs and remains dormant until spring.  In the late spring and summer, these nymphs then feed on rodents, small mammals, birds and humans.  They again molt into adults in the fall.  At this time, the adult ticks mate and feed on large mammals such as deer and bite humans.  The adult female ticks then falls off of the large mammals and lays eggs in the spring.  This completes a two year cycle.  The reservoir for Borrelia burgdorferi is certain non-human vertebrate such as small rodents and ticks.  A reservoir is an animal where Borrelia burdorferi can live and grow without causing the animal to die.  It is the reservoir where the feeding tick can acquire the bacteria for subsequent transmission to the next host.  In the course of normal feeding, larvae and nymph ticks usually become infected with Borrelia burgdorferi.  The bacteria remains in the tick as it changes from larva to nymph to adult.  The infected ticks bite other rodents and humans and transmit the bacteria. 
 

Human Transmission

Research has indicated that larvae rarely carry Borrelia burgdorferi.  Nymphs, on the other hand, do carry the bacteria and because the feeding nymphs are so small and are rarely noticed, they tend to remain attached for extended periods of time.  The longer the attachment, the more likely the transmission of Lyme disease from ticks to humans.  Adult ticks also carry the bacteria and can therefore transmit Lyme disease, however, because they are larger, they are usually noticed and more likely to be removed prior to transmission of Lyme disease.  Ticks tend to be found on shrubs, leaf litter and the tips of grasses.  They do not jump, they crawl and attach to host animals or persons.  Though they can attach to any part of the human body, they tend to attach to areas that are more hidden such as under the arm, in the groin, or in areas of tight clothing.  As they feed over several days, they enlarge.  Those fire fighters that frequent wooded areas containing tall grasses, brush for either work or recreation are at risk for Lyme disease.  Likewise, houses or gardens that are on the fringe of wooded areas may also be subject to Lyme infected ticks. 

                   Tick Life Cycle


All graphics were obtained from the Center for Disease Control (CDC)

References:

1)The Center for Disease Control.

2)Steere AC. Lyme Disease. New England Journal of Medicine.  2001;345: 115-125.

3)Wormser GP, Nadelman RB, Dattwyler RJ, et al.  Practice guidelines for the treatment of Lyme disease.  Con Infect Dis 2000;31:Suppl1:1-14.
 

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