Lyme disease

From MicrobeWiki, the student-edited microbiology resource
This is a curated page. Report corrections to Microbewiki.

Etiology/Bacteriology

Taxonomy

| Domain = Bacteria | Phylum = Spirochaetes | Class = Spirochaetia | Order = Spirochaetales | Family = Spirochaetaceae | Genus = Borrelia | species = burgdorferi

Description

Lyme disease is caused by the bacteria Borrelia burgdorferi sensu lato. Because the bacteria has a number of hybrid strains that also cause Lyme disease, Borrelia burgdorferi sensu lato is used to refer to all Lyme disease causing strains. Different strains may cause slightly different symptoms in the host. B. garinii and B. afzelii are Lyme disease causing strains that inhabit Eurasia while B. burgdorferi typically inhabits North America. This disease was first isolated in 1975 by Allen Steere in Old Lyme, Connecticut. Willy Burgdorfer isolated the bacteria from Ixodes ticks (deer ticks) which caused an immune response in the immune serum of diagnosed patients [1]. B. burgdorferi are microaerophilic, spirochaetes. A key to the pathogen’s virulence is that it does not require iron to survive. Instead, the bacteria has evolved to use magnesium. If detected early, antibiotics can be administered and the infection will cease. But if left untreated, serious symptoms may arise such as joint pain, sleep disturbance, and neurological problems. The disease is the most common in summer months and early fall.

Pathogenesis

Transmission

Depiction of the lyme disease life cycle From: Nature [1]

Lyme disease is a zoonosis that is typically transmitted via ticks of the Ixodes genus such as the deer tick [3]. To initiate infection, the tick must bite the host and the saliva of the tick containing the spirochaetes will enter the host. Transmission typically occurs via nymph ticks, but may occur with ticks of any life stage. Nymph ticks are the most likely transmitter because they are small and not easily detectable by humans. Once attached to the host, the tick must remain there for 36-48 hours to transmit B. burgdorferi to the host. Infection usually occurs during spring and summer months in wooded areas where Ixodes nymph ticks are prominent. White footed mice are the natural reservoirs of this disease. A nymph tick is uninfected with the spirochaete until it feeds on an infected animal. Once infected the tick will more on to infect more hosts.

Infectious Dose, Incubation Period, and Colonization

Unfortunately, the infectious dose for Lyme disease is unknown. The incubation period for B. burgdorferi is 3-32 days[2]. The first symptom shown after the incubation period is usually the bull's eye rash, but this is not always the case. Other, less recognizable, symptoms can occur such as fever and fatigue. The bacteria typically colonizes skin tissue first, but can move on to colonize the heart, peripheral and central nervous system and large joints. Colonization is directed by the detection of required nutrients such as glucose, amino acids, N-acetyl-glucosamine, and long chain fatty acids. After prolonged exposure, the spirochaetes usually begin to infect the extracellular matrix of the host which provides the bacteria extra protection from the host immune response.

Epidemiology

Map of lyme disease outbreaks in United States From: CDC [2]

Lyme disease is the most common vector-borne illness in the world [4]. Most outbreaks of this disease in the United States occur in the Northeast usually from Maine to Virginia. Outbreaks have occurred in other states such as Wisconsin, Minnesota, California, and Oregon. Nearly 30,000 cases were confirmed in 2008 in the United States with 60,000 cases occurring worldwide [2]. The most commonly effected age group is 5 to 9 years old.

Virulence Factors

Structure of Borrelia burgdorferi From: Nature [3]

Clinical features

Symptoms

Symptoms of Lyme disease can develop in three stages. Stage 1 is the early localized stage which can occur 3-32 days after a tick bite (CDC, symptoms). The characteristic symptom of this stage is the bull's eye rash (erythema migrans) on the skin where the bite occurred. Only 70-80% of infected people get this rash and the rash can be delayed up to 32 days after the tick bite has occurred. These factors can lead to the disease being undiagnosed. Other symptoms of this stage typically include fatigue, chills, fever, headache, muscle and joint aches, and swollen lymph nodes.

Stage 2 is the early disseminated stage which can occur days to weeks after the tick bite. After time has passed the bacteria can spread to various sites in the body which produces different symptoms in the host. Some of these symptoms include additional bull's eye rashes, facial or bell's palsy, severe headaches, meningitis, pain and swelling in joints, radiculopathy, arthralgia, heart palpitations and dizziness. Meningitis could result from the bacteria spreading to the spinal cord and causing inflammation of the meninges. This is generally not fatal.

Stage 3 is the late disseminated stage which can occur months to years after the tick bite. Patients that go without treatment may begin to have arthritis, with severe joint pain and swelling. Large joints are often effected in this type of arthritis. This stage occurs in approximately 60% of untreated patients. Up to 5% of untreated patients may develop chronic neurological complaints such as shooting pains, numbness or tingling in the hands or feet, and problems with short-term memory.

About 10% of patients that undergo antibiotic treatment develop post-treatment Lyme disease syndrome (PTLDS). Symptoms include muscle and joint pain, cognitive defects, sleep disturbance, and fatigue. This disease may be due to the host's immune response continuing after the infection has been cleared.

Morbidity and Mortality

Diagnosis

If the characteristic bull’s eye rash is detected, the diagnosis of Lyme disease may be made [4]. If no rash is present, other laboratory tests must be carried out to determine if the disease is present. Three to six weeks after the bite occurred, an enzyme immunoassay or immunofluorescent test may be done to test the level of Lyme IgM or IgG in the blood. If this is positive, a Western blot can be ordered to detect IgM or IgG antibody levels. Both of these positive test would indicate Lyme disease. If the disease is in the last stage and involving the central nervous system, clinical presentation is the only indicator of disease.

Treatment

Prevention

Risk Avoidance

To avoid contracting Lyme disease, the central place to be cautious would be in wooded areas with high grass. Repelling ticks with DEET (N, N-diethyl-m-toluamide) is a good option if a wooded area is nearby or one has been entered recently. Always check children for ticks paying special attention to the hairline and hard to see places. Use a mirror to check yourself for ticks. Permethrin treated clothing is also available to repel ticks.

Immunization

Host Immune Response

References

1 "Lyme Disease Bacteria." "A Practical Guide to Lyme Disease."

2 Radolf, Justin D. Melissa J. Caimano, Brian Stevenson, Linden T. Hu. "Of ticks, mice and men: understanding the dual-host lifestyle of Lyme disease spirochaetes." "Nature."

3 "Transmission" "CDC"

4 "Reviewing Lyme Disease with Facial Palsy and Meningitis" "Healio"
,br />Created by {Halen Borron}, student of Tyrrell Conway at the University of Oklahoma.