Tick Borne Relapsing Biology
Organisms, Vectors, and Reservoirs
In the United States, TBRF is caused by one of three Borrelia species: B. hermsii, B. parkerii, and B. turicatae. Most human illness is caused by B. hermsii.
The relapsing fever Borrelia spp are gram negative helical bacteria normally 0.2 to 0.5 microns in width and 5 to 20 microns in length. They are visible with light microscopy and have the cork-screw shape typical of all spirochetes (see picture). They have a unique process of DNA rearrangement in their linear DNA. Each time the DNA is read a different antigenic marker, also known as a variable major protein, is created, which allows the organism to evade the immune system and therefore cause recurrent patterns of fever and other symptoms (Barbour 1990).
Borrelia is transmitted to humans through the bite of infected soft ticks of the genus Ornithodoros. Soft ticks (family Argasidae) differ in many ways from the so-called hard ticks (family Ixodidae), including the more familiar dog tick and deer tick.
Ornithodoros hermsi
In contrast to hard ticks, soft ticks take brief blood meals lasting less than a half hour, usually at night. Between meals the ticks live in the nesting materials in their host burrows. Individual ticks will take many such blood meal during each stage of their life cycles, including the development of eggs by adult females. The bites of soft ticks are usually painless and the persons who are bitten while asleep are usually unaware that they were bitten.
The individual Borrelia species that cause TBRF are usually associated with specific tick vectors. For instance, B. hermsii is transmitted to humans by O. hermsi ticks, while B. parkerii is transmitted by O. parkeri and B. turicatae is transmitted by O. turicata. Each tick has a preferred environment and preferred set of hosts. O. hermsi tends to be found at higher altitudes (1500 – 8000 feet) where it is associated primarily with ground or tree squirrels and chipmunks. O. parkeri occurs at lower altitudes, where they inhabit caves and the burrows of ground squirrels and prairie dogs, as well as those of burrowing owls. O. turicata occurs in caves and ground squirrel or prairie dog burrows in the plains regions of the Southwest, feeding off these animals and occasionally burrowing owls or other burrow- or cave-dwelling animals.
In the tick, Borrelia can be found in all the tissues including salivary glands and ovaries of certain subspecies of ticks (Schwan and Piesman 2002). Infected Ornithodoros ticks can transmit relapsing fever spirochetes to humans through their saliva while feeding. O. turicatae and O. parkeri ticks also can transmit spirochetes through secretion of infectious fluids from their coxal glands, which are excretory organs located at the base of the ticks' legs. O. hermsi also secretes infectious coxal fluid but in such small amounts that it dries very quickly after being secreted and, therefore, poses little or no threat to its hosts. Since the infection is also in the ovaries of certain ticks, such as O. hermsi, they can transmit their infections over many generations from female ticks to their offspring. Soft ticks can live up to 10 years. In certain parts of the Russia the same tick has been found to live almost 20 years.
Content Source:
Division of Vector Borne Infectious Diseases
National Center for Zoonotic, Vector-Borne, and Enteric Diseases