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: http://www.contagionlive.com/link/7
- 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.
- Centers for Disease Control and Prevention. Lyme disease data tables. Reported cases of Lyme disease by state or locality, 2005-2015.CDC website. https://www.cdc.gov/lyme/stats/tables.html. Published November 21, 2016. Accessed July 31, 2017.
- 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. https://www.cdc.gov/mmwr/pdf/wk/mm6053.pdf. Published July 5, 2013. Accessed July 31, 2017.
- Lou Y, Wu J. Modeling Lyme disease transmission. Infect Dis Mod. 2017;, 2(2):229-243. doi.org/10.1016/j.idm.2017.05.002.
- 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.
- Brisson D, Dykhuizen DE, Ostfeld RS. Conspicuous impacts of inconspicuous hosts on the Lyme disease epidemic. Proc Biol Sci. 2008;275(1631):227-235.