Geographic Expansion of Lyme Disease Emphasizes Importance of Recognizing Modes of Transmission

ContagionAugust 2017
Volume 2
Issue 3

Although focused in the northeastern United States, Lyme disease has ex­panded geographically over the past few decades, with a >320% increase in the number of high-incidence counties and reported cases in 49 states.

Lyme disease, although focused in the northeastern United States, has ex­panded geographically over the past few decades, with a >320% increase in the number of high-incidence counties1 and reported cases in 49 states.2 Recognition of Lyme disease has improved in endemic areas; however, health care practitioners in other areas of the United States may be unfamiliar with the clinical presentation, according to experts who participated in a Contagion® Peer Exchange panel.

The geographic spread of Lyme disease is due to multiple factors. The panelists high­lighted an increase in development leading to a greater density of deer populations and few­er small-animal predators as key contributors to the spread and transmission of the disease.

Epidemiology of Lyme Disease

Lyme disease, the most common tick-borne ill­ness in the United States and Europe, is caused by spirochetes within the Borrelia burgdorferi sensulato species complex. Approximately 95% of the confirmed cases of Lyme disease were reported from 14 states in 2015, with the highest incidence rates in Vermont, Maine, Connecticut, and Massachusetts.2

“In the early days when Lyme disease was first being described, it was found in Old Lyme, Connecticut, which is in the northeast region of the United States,” said Peter L. Salgo, MD. “And early on, everyone thought, ‘Oh, it’s a [northeastern] United States problem.’”

However, the causative organism, the spirochete Borrelia burgdorferi, trans­mitted through the bite of an infected tick, like­ly dates back several thousand years. Samuel Shor, MD, FACP, pointed out that a 5300-year-old mummy, discovered in Italy some 25 years ago, was found with evidence of knee arthritis and genetic evidence of Borrelia.

Alfred Buchwald, a Ger­man physician, was the first to describe the erythema migrans rash often seen with Lyme disease more than 130 years ago, although he did not associate the rash with a tick-borne pathogen. In the 1960s and 1970s, a group of children and adults in Lyme, Connecticut, and surrounding areas began experiencing unexplained symptoms that included arthritis, paralysis, skin rashes, headaches, and chronic fatigue. Researchers began to describe the signs and symptoms of this condition as Lyme disease, and the children reported having a tick bite before the onset of symptoms; however, the cause was still unclear. Leonard Sigal, MD, noted that many of these patients were mistakenly diagnosed with juvenile rheumatoid arthritis until researchers noticed the geographic clustering of cases.

The causative bacteria Borrelia burgdorferi was not officially classified un­til 1981, when scientist Wilhelm Burgdorfer discovered that a spirochete carried by ticks was causing Lyme disease. Since then, the number of reports and geographic expansion of Lyme disease have increased dramatically in the United States, and the Centers for Disease Control and Prevention (CDC) included Lyme dis­ease in their top 10 notifi­able diseases in 2012.3 How­ever, Patricia V. Smith, president of the Lyme Disease Association, noted that the extent of Lyme disease in the United States may be underesti­mated in many states be­cause they do not have the money to perform surveillance and identify the extent of the infected tick population.

Lyme disease has also been reported in about 80 countries worldwide, although Robert C. Bransfield, MD, DLFAPA, said that it is unclear wheth­er Lyme disease is endemic in those countries or wheth­er it spread from the north­eastern United States. “It seems like we’re more aware of it, and now that we’re more aware of it, we identify it,” he said. He pointed out that the increased rates of diagnosis may be due in part to greater aware­ness. “In years past, it was called many other things,” he said.

However, Dr. Bransfield noted that there is debate among experts about whether to use a narrow or broad definition for identifying and treating Lyme disease. The diagnostic crite­ria from the CDC are narrow, as their prima­ry purpose is to monitor the national rate of Lyme disease with a high degree of specificity in the diagnosis. For example, the CDC crite­ria do not recognize Lyme disease in patients with late-onset encephalopathy and a positive blood test, and physicians who use the CDC criteria may fail to diagnose Lyme disease in some patients. By contrast, physicians who use more inclusive clinical criteria for probable cases of Lyme disease may initiate treatment in some patients who do not have the disease, with the rationale that long-term risks with Lyme disease outweigh the risks associated with antibiotic therapy.

Experts generally agree about the signs and symptoms in acute phases of Lyme disease, which may include fever, chills, head­ache, fatigue, muscle and joint aches, swollen lymph nodes, and an erythema migrans rash that begins at the site of the tick bite, 3 to 30 days after the bite and spreads gradually. However, the concept of chronic Lyme disease is contro­versial among experts, according to Dr. Shor, and no systematic evidence supports the pres­ence of Borrelia burgdorferi in patients with chronic symptoms after treated Lyme disease.

Geographic Spread and Transmission of Lyme Disease

Multiple tick-borne disease modeling efforts have been dedicated to analyzing the geo­graphic spread and epidemiological charac­teristics of Lyme disease. According to a recent review,4 tick population ecology, effect of dis­ease hosts on persistence of tick-borne diseas­es, dynamics of the seasonal tick population and disease transmission, climatic effects, and spatial invasion of ticks and spreading of the disease have been modeled mathematically to further clarify factors that explain disease risk and transmission.

According to Dr. Salgo, the density of deer populations is commonly associated with pre­sence of infected ticks. “The lifecycle of these ticks involves deer, and where deer are ende­mic, ticks are endemic—that’s part of the pro­blem,” he said.

Dr. Shor identified that the increased development of wilderness has reduced the po­pulation of deer predators and increased the population density of deer, which are vehicles for in­fected adult ticks to spread across wider areas and breed. “The ticks jump on the deer and it takes them from point A to point B,” he said. “It also provides proximity so that the ticks can mate.”

However, deer are not competent hosts for imma­ture ticks (larva or nym­ph), the primary sources of transmission of Borrelia burgdorferi. Thus, management objectives to stabilize or reduce deer populations have not reduced the inci­dence of Lyme disease, and an analysis showed no relationship between spatial distribution of Lyme disease and abundance of deer in New York, Wisconsin, Pennsylvania, and Virginia.5 Thus, other ecological factors likely contribu­ted to the increased spread of Lyme disease in recent years.

Dr. Shor pointed out that small mammals, such as the white fruited mouse, act as a “cul­ture medium” on which the larva or nymph feed. An infected larva or nymph transmits the Borrelia pathogen to the small mammals, which are usually asymptomatic, and the pa­thogen can be transmitted to another tick that feeds on them. Some evidence suggests that the density of small mammals in a gi­ven area is a key determinant of the infected nymph population, the most common source of Lyme disease, and one study showed that the white footed mouse Peromyscus leucopus, Eastern chipmunk Tamias striatus, short-tailed shrew Sorex brevicauda, and masked shrew Sorexcinereus are responsible for infecting 80% to 90% of ticks in the north-eastern United States.6 Furthermore, a shift in the population of small-mammal predators, namely an increase in coyotes and decrease in red foxes, which rely more heavily on small mammals for food than do coyotes, stron­gly predicted spatial distribution of Lyme disease in New York.5

The panelists concluded that ecological factors affecting the spread of Lyme disease are complex, and regardless of how it is spread, the Borrelia spirochete is not going anywhere, according to Dr. Bransfield. “Bor­relia has been around much longer than people,” he said. “It has great adaptive capability. It’s been on this planet longer than we’ve been here, and it may still be here long after we’re gone.”

The video clip of the Peer Exchange panel is available here:


  1. Kugeler KJ, Farley GM, Forrester JD, Mead PS. Geographic distribution and expansion of human Lyme disease, United States. Emerg Infect Dis. 2015;21(8):1455-1457. doi: 10.3201/eid2108.141878.
  2. Centers for Disease Control and Prevention. Lyme disease data tables. Reported cases of Lyme disease by state or locality, 2005-2015.CDC website. Published November 21, 2016. Accessed July 31, 2017.
  3. Centers for Disease Control and Prevention. Summary of notifiable diseases — United States, 2011 [Erratum appears in MMWR. 2014;63(1):24]. MMWR.2012;61(53):1-120. Published July 5, 2013. Accessed July 31, 2017.
  4. Lou Y, Wu J. Modeling Lyme disease transmission. Infect Dis Mod. 2017;, 2(2):229-243.
  5. Levi T, Kilpatrick AM, Mangel M, Wilmers CC. Deer, predators, and the emergence of Lyme disease. Proc Natl Acad Sci U S A. 2012;109(27):10942-10947. doi: 10.1073/pnas.1204536109.
  6. Brisson D, Dykhuizen DE, Ostfeld RS. Conspicuous impacts of inconspicuous hosts on the Lyme disease epidemic. Proc Biol Sci. 2008;275(1631):227-235.
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