Tick-Borne Relapsing Fever Spirochetes in the Americas

Abstract

Relapsing fever spirochetes are tick- and louse-borne pathogens that primarily afflict those in impoverished countries. Historically the pathogens have had a significant impact on public health, yet currently they are often overlooked because of the nonspecific display of disease. In this review, we discuss aspects of relapsing fever (RF) spirochete pathogenesis including the: (1) clinical manifestation of disease; (2) ability to diagnose pathogen exposure; (3) the pathogen's life cycle in the tick and mammal; and (4) ecological factors contributing to the maintenance of RF spirochetes in nature.

Keywords: Borrelia; Ornithodoros; argasid; ixodid; relapsing fever spirochetes.

Figure 1

Giemsa-stained peripheral blood smear of a mouse infected by tick bite with Borrelia turicatae. The black bar represents 10 µm.

Figure 2

The tick-mammalian transmission cycle of ABRF spirochetes. In the tick, the salivary gland population of RF spirochetes is essential for mammalian infection because of the rapid feeding behavior of the tick. Entry into the mammal is characterized by early infection, and the pathogens are likely preadapted to evade innate immunity. During the following three to 10 days, RF spirochetes subvert the host antibody response leading to systemic infections. This phase of the pathogen’s life cycle is characterized by evasion of the host antibody response through antigenic variation, and replication to densities upwards of 1 × 10⁷ spirochetes per milliliter of blood. During an acquisition bloodmeal, RF spirochetes enter and colonize the midgut. Within 10–14 days a population exits the midgut and migrates to colonize the salivary glands, completing the life cycle of the spirochetes in the argasid tick vector.

Figure 3

Distribution of Ornithodoros parkeri in North America.

Figure 4

Distribution of Ornithodoros hermsi in North America.

Figure 5

Distribution of Ornithodoros turicata and Ornithodoros talaje in North America.

Figure 6

Morphological characteristics between Ornithodoros turicata and Ornithodoros talaje. O. turicata (A) and O. talaje (B) that were collected from a burrowing owl nest in Southern Texas. The mouthparts of O. turicata are exposed (A, black arrow) while “cheeks” cover the mouthparts of O. talaje (B, black arrow).

Figure 7

Adult Ornithodoros puertoricensis collected in a human dwelling in Ancon, Panama City, Panama (A,B); Immature and adult O. puertoricensis in a reptile terrarium in Escobar, Colon, Panama (C).