Our analysis supports Filippova's hypothesis that the vectors and agents of Lyme disease comprise a monophyletic clade indicating evolution by descent within this group (11
). The one exception is B. miyamotoi
, originally isolated from Ixodes persulcatus
), the main Eurasian vector of B. burgdorferi
sensu lato. Although this association may reflect a secondary host shift from the metastriate-associated borreliae, it may be that I. persulcatus
is not the main vector but that sympatric Japanese ticks such as the metastriate Haemaphysalis longicornis
serve in this capacity. Further studies on the epizootiology of B. miyamotoi
may resolve this interesting exception to the otherwise strongly supported congruence between Ixodes
ticks and Lyme disease-like borreliae.
The metastriate tick-transmitted B. theileri
and the Amblyomma
spirochete clearly group outside the other ixodid-borne borreliae, and certain features of their biology corroborate this finding. Bovine borreliosis due to B. theileri
is characterized by prominent spirochetemias in the peripheral blood (7
), as is relapsing fever. Both B. theileri
and many of the argasid-transmitted spirochetes may be efficiently maintained by transovarial transmission. In the vector, these spirochetes may easily be found in the hemolymph (25
), in sharp contrast to the Ixodes
-transmitted borreliae, which less frequently produce disseminated infections in the tick hosts (6
). The Amblyomma
agent is detectable within ticks as frequently by the hemolymph test as it is by indirect immunofluorescence of gut contents (Fisher exact test, P
= 0.376) (our unpublished results), suggesting that other features of B. theileri
's biology may be shared, such as the presence of spirochetemia in the as-yet-undescribed reservoir and its possible maintenance in ticks by transovarial transmission.
The degree of sequence similarity between the Amblyomma
agent and B. theileri
is similar to that between different isolates of B. hermsii
and may reflect a very recent divergence of the two metastriate-transmitted spirochetes. Because A. americanum
feeds mainly upon deer in all three instars, we previously reasoned that any spirochete maintained by this tick might be closely related to that maintained by the cattle tick Boophilus
). Indeed, deer served as important enzootic hosts for the tick B. microplus
, and the eradication program for this economically important ectoparasite of cattle (because of its role as a vector of Texas cattle fever or bovine babesiosis) targeted cervids throughout the southern United States (19
). Because cattle and deer are sympatric in many sites, Boophilus
and the agents that it maintained could have established parallel transmission cycles in deer.
Although specific names have been applied to the Amblyomma agent (B. lonestari, B. barbouri, and a Borrelia sp. similar to B. nr. miyamotoi), given its genetic similarity to B. theileri, any formal description would require justifying the application of a new name. More importantly, because the Amblyomma agent is as yet uncultivatable, determining its role in the etiology of southern tick-associated rash infection (Masters' disease) may be facilitated by the use of B. theileri as a surrogate. These spirochetes attain a great density within the blood of cattle and may be purified for use as an antigen. In addition, the knowledge that B. theileri is maintained transovarially and may be found as disseminated infections within the vector, as well as producing transient spirochetemia in the ungulate host, may help advance studies that support or refute the role of borreliae in the epidemiology of Lone Star tick-associated, Lyme disease-like infections of humans.