Vector Interactions and Molecular Adaptations of Lyme Disease and Relapsing Fever Spirochetes Associated with Transmission by Ticks

Tom G. Schwan, National Institutes of Health, Hamilton, Montana, USA; Joseph Piesman, Centers for Disease Control and Prevention, Fort Collins, Colorado, USA


Emerging Infectious Diseases. 2002;8(2) 

In This Article

Abstract and Introduction

Pathogenic spirochetes in the genus Borrelia are transmitted primarily by two families of ticks. The Lyme disease spirochete, Borrelia burgdorferi, is transmitted by the slow-feeding ixodid tick Ixodes scapularis, whereas the relapsing fever spirochete, B. hermsii, is transmitted by Ornithodoros hermsi, a fast-feeding argasid tick. Lyme disease spirochetes are generally restricted to the midgut in unfed I. scapularis. When nymphal ticks feed, the bacteria pass through the hemocoel to the salivary glands and are transmitted to a new host in the saliva after 2 days. Relapsing fever spirochetes infect the midgut in unfed O. hermsi but persist in other sites including the salivary glands. Thus, relapsing fever spirochetes are efficiently transmitted in saliva by these fast-feeding ticks within minutes of their attachment to a mammalian host. We describe how B. burgdorferi and B. hermsii change their outer surface during their alternating infections in ticks and mammals, which in turn suggests biological functions for a few surface-exposed lipoproteins.

The molecular adaptations required by pathogenic spirochetes for efficient transmission by obligate, blood-feeding ticks are largely unknown. In the new era of genomics, the complete DNA sequence of two spirochetes, Borrelia burgdorferi and Treponema pallidum, have been determined1,[2]. As additional genome sequences become available for other pathogenic and free-living spirochetes, comparisons of their genomes may elucidate genes that are unique to those species of spirochetes associated with ticks. This information, along with an increased understanding of the molecular mechanisms used by tick-borne spirochetes to adapt for transmission by their tick vectors, may lead to unique disease prevention strategies.

The genus Borrelia currently contains 37 species of spirochetes, many of which cause diseases in humans and domestic animals (Table)[3]. Except for Borrelia recurrentis (which causes louse-borne relapsing fever and is transmitted by the human body louse), all known species are transmitted by ticks[4]. Two groups of spirochetes stand out among these tick-borne species because of their prevalence as human pathogens: Lyme disease spirochetes, transmitted by the relatively slow-feeding ixodid (hard) ticks of the genus Ixodes, and relapsing fever spirochetes, transmitted by the fast-feeding argasid (soft) ticks of the genus Ornithodoros (Figure 1). Major observations in recent years have increased our understanding of how one species in each group adapts while infecting ticks. B. burgdorferi, a causative agent of Lyme disease, and B. hermsii, a causative agent of tick-borne relapsing fever, have received the most attention. We describe how these two species of Borrelia change their outer surface during their alternating infections in ticks and mammals, which in turn suggests biological functions for a few surface-exposed lipoproteins. The dynamics of infection of these two bacteria in strikingly different types of ticks provide examples of possible adaptations for their transmission.

Dorsal view of a female Ixodes scapularis (family Ixodidae, hard ticks), a vector of Borrelia burgdorferi (left), and a female Ornithodoros hermsi (family Argasidae, soft ticks), the vector of B. hermsii (right).


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