Mapping Lyme disease enzootic risk and linking to disease outcomes in the United States

Lyme Disease Human Risk Map
Lyme disease is the most commonly reported vector-borne disease in the US, with approximately 30,000 cases reported annually. Human Lyme disease risk is determined by the density of I. scapularis nymphs infected with Borrelia burgdorferi. We have developed the first standardized spatial risk model for human infection with B. burgdorferi in the eastern United States. This map provides the public, health agencies and clinicians with critical information on the distribution of this pathogen. Research in my lab seeks to understand the processes driving the epidemics of Lyme disease and other tick borne diseases at multiple spatial scales. We have identified climatic, landscape and tick activity correlates of Lyme disease risk, including the distribution of highly pathogenic strains of Borrelia burgdorferi. By integrating field-derived data with laboratory experiments, we are developing mechanistic transmission models to identify key determinants of pathogen establishment and enzootic prevalence. We also examine the association between the distribution of infected ticks and nationally reported Lyme disease incidence rates and examine factors modifying this association.

Babesiosa-Lyme disease co-infection

Tick-borne diseases are increasingly becoming an important public health issue as is evidenced by the current national epidemic of Lyme disease and the increasing prevalence of human babesiosis, a more serious malaria-like illness caused by the same tick vector. The emergence of these diseases in recent decades is due the rapid expansion in the range of the vector tick, Ixodes scapularis, but the expansion of the agent of human babesiosis (Babesia microti) has been much slower than that of the agent of Lyme disease (Borrelia burgdorferi). 

This project explored the interactions between these pathogens in a series of laboratory experiments involving a natural reservoir host (Peromyscus leucopus mice). What was found was that coinfection with B. burgdorferi enhances the transmission of B. microti from reservoir host to the Ixodes scapularis tick vector. 

The finding suggests that the geographic expansion of human babesiosis may be dependent upon the presence of B. burgdorferi and that this disease will continue to emerge throughout the eastern US wherever Lyme disease is epidemic.

Babesiosis emergence in the United States

Dragging for Ticks
Human babesiosis is a rapidly emerging tick-borne zoonosis in the United States caused by the protozoan Babesia microti and transmitted by Ixodes scapularis, the Lyme disease vector. Unlike Lyme disease, babesiosis is a potentially fatal disease and poses a threat to blood transfusion recipients. In the Northeast, both Lyme disease and babesiosis were originally restricted to coastal New England. While Lyme disease has expanded dramatically over the past thirty years and now is endemic from Maine to Virginia, babesiosis cases appears to be expanding at a slower rate for reasons that are not understood. This differential spread provides a unique opportunity to examine factors driving pathogen emergence by comparing these species’ natural history and transmission dynamics. Furthermore, because B. microti encounters a large proportion of host populations already infected with B. burgdorferi throughout its geographic expansion, the goal of this study is to investigate how pathogen interactions at the individual, population, and community levels influence B. microti invasion patterns and enzootic prevalence.

Human Borrelia miyamotoi Infection in the United States

Borrelia miyamotoi, a spirochete that is genetically related to the species of borrelia that cause relapsing fever, has been detected in all tick species that are vectors of Lyme disease. It was detected in Ixodes scapularis ticks from Connecticut in 2001 and subsequently has been detected in all areas of the United States where Lyme disease is endemic. The first human cases of B. miyamotoi infection were reported in Russia in 2011. We now provide evidence of B. miyamotoi infection and the prevalence of this infection among people in the United States. New England Journal of Medicine paper.

PI: Peter Krause

EcoEpidemiology of Lyme disease on Block Island, RI

Block Island

The number of cases of Lyme disease and Babesiosis have increased dramatically in recent years. Current strategies to reduce the incidence of tick-borne diseases rely mostly on the use of area-wide applications of acaricides, which have been reported to be highly effective in reducing the density of infected ticks, although a recent study did not find them effective in reducing Lyme disease incidence. Less than 25% of residents of hyperendemic communities report using acaricides due to their potential toxicity to humans, pets or the environment. Personal protective measures have been found to be effective in reducing tick-borne disease incidence. Comparisons between environmental and behavioral risk factors have been hindered by the paucity of studies linking risk to disease incidence. 

Block Island

We are currently evaluating the relative importance of environmental and behavioral approaches for reducing the incidence of Lyme disease and Babesiosis in two contrasting ecological settings: an island and a mainland site. The project will take advantage of ongoing human cohort studies on the epidemiology of Lyme disease and Babesiosis at both sites. We shall couple these studies with assessments of ecological and behavioral risk factors in peridomestic and community setting. We will also develop an acarological risk map for Block Island, RI and Mansfield, CT that will describe spatial heterogeneity in acarological risk due to land use and landscape patterns.

Role of host community composition on the prevalence and genetic makeup of tick-borne pathogens

Tick ID Microscope

A research team studying vector-borne diseases places ticks in the spotlight – and under a microscope – as part of research that seeks to understand and prevent maladies such as Lyme disease.

The composition of the vertebrate host community has been proposed to influence both the prevalence and genetic makeup of B. burgdorferi. We are examining this effect by comparing three host communities with contrasting host diversities. Block Island, RI, which is highly species-poor, almost exclusively composed of white-footed mice; Nantucket, with intermediate diversity and a mainland site in Mansfield, CT, where all species are present.
Babesia life cycle

The Babesia microti life cycle involves two hosts, which includes a rodent, primarily the white-footed mouse, Peromyscus leucopus, and a tick in the genus, Ixodes.  During a blood meal, a Babesia-infected tick introduces sporozoites into the mouse host  .  Sporozoites enter erythrocytes and undergo asexual reproduction (budding)  .  In the blood, some parasites differentiate into male and female gametes although these cannot be distinguished at the light microscope level  .  The definitive host is the tick.  Once ingested by an appropriate tick  , gametes unite and undergo a sporogonic cycle resulting in sporozoites  .  Transovarial transmission (also known as vertical, or hereditary, transmission) has been documented for “large” Babesia spp. but not for the “small” babesiae, such as B. microti  .

Humans enter the cycle when bitten by infected ticks.  During a blood meal, a Babesia-infected tick introduces sporozoites into the human host  .  Sporozoites enter erythrocytes   and undergo asexual replication (budding)  .  Multiplication of the blood stage parasites is responsible for the clinical manifestations of the disease.  Humans are, for all practical purposes, dead-end hosts and there is probably little, if any, subsequent transmission that occurs from ticks feeding on infected persons.  However, human to human transmission is well recognized to occur through blood transfusions  .

"An "emerging" disease grows stronger

babesiosis article
White-tailed deer are plentiful on Block Island and in coastal New England and serve as hosts to the ticks that carry the microorgnaism responsible for babesiosis. Article starts on page 16 of the Fall 2011 edition of Yale Public Health magazine.

Victory Over the Vector

Victory over the vector magazine art
Yale public health researchers are pursuing widely different approaches to stem or stop insect-borne diseases that plague different areas of the world. article starts on page 10 of the Fall 2011 edition of Yale Public Health magazine.