Lyme borreliosis: of ticks and spirochetes 
L YME BORRELIOSIS: 
OF TICKS AND SPIROCHETES 
W. Burgdorfer 
ABSTRACT 
Lyme disease is now recognized as the most prevalent tick-borne spirochetosis in North America, Europe 
and in many Asien countries, especially Japan. Its global occurrence coincides with the geographic distribution 
of ticks belonging to the Ixodes ricinus!persulcatus complex. In this complex, four species, Ixodes ricinus, Ixodes 
persulcatus, Ixodes scapularis, and Ixodes pacificus are recognized as efficient vectors of the Lyme disease 
spirochete, Borrelia burgdorferi. At least ten additional species of ixodid ticks have been found occasionally 
infected, but of these only three, namely Ixodes ovatus from Japan, Ixodes holocyclus from Australia and 
Amblyomma americanum from the U.S.A., appear to be associated with Lyme disease in humans; the 
remaining seven ticks usually do not attack humans, but are important in maintaining natural foci in which 
Lyme disease spirochetes persist. Special attention is given to Ixodes uriae, a bird tick that maintains Borrelia 
burgdorferi among seabirds in Sweden. 
On the basis of DNA homology, rRNA gene restriction patterns, and immunological reactivities, Borrelia 
burgdorferi has been classified into several gene species. Their distribution and possible relationship to clincial 
manifestations of Lyme disease will be discussed. 
It is speculated that ongoing and future tick/spirochete surveys will discover additional distinct gene types 
of Borrelia burgdorferi as well as hitherto undescribed spirochetes. 
KEY WORDS 
Lyme disease, tick vectors, spirochetes, taxonomy 
Erythema ( chronicum) migrans (EM) was initially 
described as a rare skin disease in the Scandinavian 
countries (1). Today, it is recognized in various 
parts of the world, particularly in Europe, North 
America, and Asia as the most prevalent tick-borne 
spirochetosis. It is a complex illness that may affect 
not only the skin but also the skeleton, muscles, 
heart, eye, and nervous system of children and 
adults alike (2). 
acta dermatovenerologica A.P.A. Vol 5, 96, No 3-4 
Ever since Afzelius described EM in 1910 (3), 
ticks had been suspected as vectors of a then 
unknown causal agent. Once the spirochetal etiology 
was established in late 1981 ( 4), intensive tick/ 
spirochete surveys led to the realization that this 
disease - now known as Lyme disease (LD) or 
Lyme Borreliosis (LB) - occurs around the globe 
where it coincides with the geographic distribution 
of ixodid ticks of the Ixodes ricinus!persulcatus complex 
87 
Lyme borreliosis: of ticks and spirochetes 
- a group of hard ticks whose distribution is limited 
to the temperate zone, i.e., to regions between the 
33rd and 65th degrees latitude (see Fig. 1) (5). 
In addition to the availability of suitable hosts 
these ticks require a constant relative humidity of at 
least 80% and above - a condition found in deciduous 
forests with damp soil and rich undergrowth and 
brush. Of the 7 species listed in the ricinus/persulcatus 
complex (see Table 1), 4 are recognized as efficient 
vectors of LD spirochetes to humans; 
(1) the European castor bean or sheep tick, Ixodes 
ricinus, - the most common tick of western and 
central Europe, 
(2) the Taiga tick, Ixodes persulcatus, the main 
vector in Russia, China, and J apan, 
(3) the black-legged deer tick, Ixodes scapularis that 
occurs in eastern, southeastern, and midwestern parts 
of the United States, 
(4) the black-legged western deer tick, Ixodes pacificus 
found abundantly in California, the north western 
states, and in western Canada. 
The eastern deer tick, Ixodes dammini, is no 
longer recognized as a separate species. Extensive 
studies have shown that it is morphologically and 
genetically indistinguishable from Ixodes scapularis 
and therefore, should be considered a synonym of 
the latter (6). 
All three parasitic stages of these ticks - larvae, 
nymphs, and adults, have a wide range of vertebrate 
hosts. The immature larvae and nymphs attack 
predominantly rodents, lizards, birds, and humans, 
whereas the adults parasitize larger animals, such as 
deer, dogs, cattle, and humans. 
Intensive surveys around the globe have identified 
several additional tick/spirochete associations that 
are less significant from a public health point of 
view, because the ticks involved do not, or only 
rarely, come into contact with humans. Yet such 
ticks play a significant role in maintaining and 
distributing terrestrial and maritime enzootic foci of 
LD spirochetes. 
In Europe for instance, the hedgehog tick, Ixodes 
hexagonus, is said to be a competent vector of 
Borrelia burgdorferi (Bb) among its hosts, primarily 
hedgehogs, foxes, martens, dogs and cats (7,8). 
Certain terrestrial species of birds have long been 
suspected of playing an important role in the ecology 
of LD spirochetes; they not only are the means by 
which ticks are spread over large geographic regions, 
but also are sources for infecting ticks (9). In a 
recent study in northern Sweden, a marine enzootic 
88 
cycle of LD was discovered (10). Its major components 
are said to be seabirds (Razorbills and Guillemots) 
and their tick parasite, Ixodes uriae. This tick is 
found abundantly on islands of the Baltic Sea where 
terrestrial mammals are absent. It is associated with 
seabird colonies in both the northern and southern 
hemispheres. Spirochetes similar to those isolated 
from Ixodes uriae in Sweden were recovered from 
the same species of ticks collected off seabirds in 
New Zealand, Crozet Islands, Falkland Islands, Alaska, 
Labrador, Iceland, Faeroer, and France. The study 
confirms the role of birds, not only in the dispersal 
of infected ticks, but also in the multiplication and 
survival of Borrelia spirochetes where rodent reservoirs 
are absent (11). 
Involvement of ticks in enzootic cycles of LD 
spirochetes has been reported also from California 
where the woodrat tick, Ixodes neotomae, maintains 
a primary cycle among its hosts (woodrats). They 
serve as sources of spirochetal infections for the 
simultaneously parasitizing Ixodes pacificus - the primary 
vector to humans in California (12). 
Similarly, the rabbit tick, Ixodes dentatus has been 
found to maintain spirochetal infections among its 
host, the cottontail rabbit (Sylvilagus floridanus) . 
Isolations of spirochetes from this tick, as well as 
rabbits within New York City, New York, suggested 
the presence of enzootic foci that could become the 
source of infection for other ticks, such as the 
human-biting Ixodes scapularis (13). Molecular and 
genetic analyses of isolates from Ixodes dentatus and 
cottontail rabbits indicated a previously undescribed 
spirochete, for which the name Borrelia andersonii 
sp. nov. was proposed in honor of Dr. J.E. Anderson 
who first isolated and characterized this species 
(14) . 
Other species of rabbit ticks that do not attack 
humans, but are important in maintaining natural 
foci of spirochetes belong to the genus Haemaphysalis. 
They have been found infected with spirochetes in 
the United States (15), China (16), Japan (17), and 
Australia (18). 
Early molecular investigations of North American 
and European isolates of spirochetes from patients 
and ticks have indicated those from North America 
are more homogenous than those from Europe. 
Subsequently on the basis of DNA relatedness, Bb 
was classified into three genospecies: Bb sensu stricto 
(s.s.), B. garinii, and B. afzelii (19). 
Bb s.s. was found on the North American, European, 
and Asian continents, whereas B. garinii and B. 
afzelii have been reported so far only from Europe 
acta dermatovenerologica A.P A . Vol 5, 96, No 3-4 
Lyme borreliosis: of ticks and spirochetes 
.. 
o 
.... 
<:;,'," 
c:>b 
' 
Fig. l. Globa! distribution of Ixodes ticks able to transmit Lyme disease spirochetes. 
Modified from Filippova NA, editor: Taiga tick, Ixodes persulcatus Schulze (Acarina, Ixodidae). Leningrad, 1985, 
Nauka Publishers. 
and Asia. Unidentified isolates are referred to as 
Bb sensu lato. 
Several studies conducted in Europe have shown 
that the specific genospecies are associated with 
distinct clinical manifestations of LD (20). Thus, Bb 
s.s. tends to lead to arthritic symptoms, whereas B. 
garinii and B. afzelii cause neurological complications 
and cutaneous manifestations, respectively. 
Of particular interest is a recent report from 
Belgium on the simultaneous presence of different 
Bb genotypes in the same patient (21). Whereas 10 
patients were infected with a single genospecies, 8 
were infected with more than one. Such multi-
infections may be due to consecutive infections by 
separate individual ticks, or by a single tick infected 
with more than one genospecies. Obviously multi-
infections are likely to occur in small geographic 
regions where more than one genospecies is found. 
This has been demonstrated in a recent tick/ 
spirochete survey in the southern part of Switzerland. 
Of 51 isolates from Ixodes ricinus, 26 were typed as 
Bb s.s., 20 as B. garinii, 3 as B. afzelii, and 2 
(referred to as VS 116) as a hitherto unclassified 
spirochete (22). 
acta dermatovenerologica A.P.A. Vol 5, 96, No 3-4 
Intensive studies have been conducted also in 
Japan where the first human case with characteristic 
EM was diagnosed and serologically confirmed in 
1986 (23). The ticks, Ixodes persulcatus and lxodes 
ovatus, are considered the main vectors, although 
no human case has ever been attributed to spirochetes 
from Ixodes ovatus, even though this tick is said to 
attack humans more frequently than does the 
recognized main vector, Ixodes persulcatus (24). It 
has been speculated that the spirochete in Ixodes 
ovatus represents a low virulent or even a 
nonpathogenic variant of Bb. Gena- and phenotypic 
analyses (restriction fragment length polymorphism 
ribotyping) indicated that the strains from Ixodes 
persulcatus represent the genospecies B. garinii and 
B. afzelii, whereas those from Ixodes ovatus differed 
from all available genospecies, and therefore were 
named Borrelia japonica sp. nov. (25). The genospecies, 
Bb s.s., does not occur in J apan. Recent genomic 
analyses of spirochetes from the main vector, Ixodes 
persulcatus also showed that certain isolates were 
unrelated to any of the known genospecies of Bb; 
the name Borrelia miyamotoi sp. nov. was proposed 
for these isolates (26). 
89 
Lyme borreliosis: of ticks and spirochetes 
LD is supposed to occur also on the Australian 
continent, where severa! cases of EM, arthritis, and 
radiculopathy have been diagnosed since 1986 (27). 
The paralysis tick, Ixodes holocyclus, was incrirninated 
as the most logical vector, although none of the 
patients claimed having been bitten by this tick 
which in the laboratory has been shown to be an 
incompetent vector of LD spirochetes (28). 
Nevertheless, large percentages (up to 42%) of 
field-collected Ixodes holocyclus and Haemaphysalis 
spp., were found to harbor spirochetes that could 
not be isolated and established in BSK II medium 
because of fast-growing bacterial contamination (29). 
In a few instances, sufficient quantities of spirochetes 
could be used for limited molecular and 
immunochemical characterization. The results of these 
tests (PAGE, monoclonal antibody reactions, PCR), 
along with clinical case reports, suggested the existence 
of a genuine, indigenous form of LB in Australia 
(30). 
In spite of these findings, the occurrence of LB 
in Australia is being questioned, at least until 
spirochetes are isolated from either ticks or patients 
(31). The fact that LD has been contracted in a 
region where Ixodes holocyclus does not occur, 
suggests that other species of ticks may be involved. 
The behavior of the Australian spirochete in Ixodes 
holocyclus is reminiscent of the behavior of spirochetes 
in the Ione star tick, Amblyomma americanum, from 
the Southeastern United States (32,33). Also, this 
spirochete could not be recovered either from ticks 
Table l. Ixodes ricinus/persulcatus comple,x. 
P ALEARCTIC REGION 
Ixodes persulcatus Schulze, 1930 
Ixodes ricinus Nuttall and Warburton, 1911 
NEARCTIC REGION 
Ixodes (Ixodes) scapularis Say, 1821 
Ixodes (lxodes) pacificus Cooley & Kohls, 1943 
Ixodes (Ixodes) jellisoni Cooley & Kohls, 1938 
Ixodes (Ixodes) affinis Neumann, 1899 
NEOTROPICS 
Ixodes (Ixodes) pararicinus 
90 
or from hundreds of patients who had developed 
expanding annular lesions similar to those of EM. 
Surprisingly, both ticks have been shown to be 
incompetent in maintaining and distributing Bb s.s. 
in the laboratory. Yet both species appear to be 
associated with LD- like illness. Failure to isolate 
the causative spirochete in BSK II medium does 
not rule out the possibility of Ixodes holocyclus and 
Amblyomma americanum being vectors of closely 
related yet distinct spirochetes incapable of survival 
in cultures. 
lndeed, PCR technology applied to infected 
Amblyomma americanum tissues led to the description 
of Borrelia lonestari as a new species of borrelia 
capable of infecting humans but so far incapable of 
being cultivated in BSK II medium (34). 
LD-like manifestations (EM rash, mild constitutional 
symptoms) after bites by Amblyomma americanum 
have also been reported from the state of Maryland 
(35). Five of 296 Amblyomma americanum were 
found infected with spirochetes that could not be 
cultivated. 
It is safe to speculate that current and future tick/ 
spirochete surveys around the globe will result in 
identifying additional Bb-like, yet distinct, spirochetes. 
Many isolates distinct from those already described 
have been placed into groups that await final 
characterization and nomenclature. It is also becoming 
apparent that development and behavior of newly 
described genospecies may differ from those established 
for Bb in its tick vectors, Ixodes scapularis or Ixodes 
ricinus. For instance, according to Russian investigators, 
the percentage of field-collected Ixodes persulcatus 
with systemic infections, including salivary gland 
tissues, is much higher than that seen in Ixodes 
scapularis (36). Thus, the migration of spirochetes 
from the midgut into the salivary glands during 
early feeding, as has been postulated (37) for Bb 
s.s. in Ixodes scapularis, is no longer a pr~requisite 
for early transmission via saliva. 
For many of the newly-described spirochete/tick 
associations, the relationship of the spirochetes to 
their tick vector(s) is still unknown and awaits 
intensive investigations as to the ticks'ability to maintain 
and transmit their spirochetes horizontally as well 
as vertically. In many instances the demonstration 
of spirochetes in ticks is merely an indication that 
these ticks did feed on an infective host; it does not 
reflect the ticks ability to maintain and distribute 
the spirochetes in nature. In Europe, for mstance, 
Ixodes trianguliceps and Ixodes acuminatus from westem 
France have been found infected (38), as have 11 
acta dennatovenerologica A.P A. Vol 5, 96, No 3-4 
Lyme borreliosis: of ticks and spirochetes 
of 97 Dennacentor reticulatus from eastern Germany 
(39). In North America, both the Amercian dog 
tick, Dennacentor variabilis, and the brown dog tick, 
Rhipicephalus sanguineus, have been recorded as 
being infected occasionally with Bb (40,41). Under 
experimental conditions, Dennacentor variabilis like 
the above discussed Amblyomma americanum and 
Ixodes holocyclus is an incompetent vector of this 
spirochete ( 42). 
Finally, reports from China have indicated the 
presence of Bb in Ixodes granulatus and in Ixodes 
rangtangensis (16). It is not clear whethei these 
species are efficient vectors to humans. 
Considering the complexity of the clinical, ecological, 
and bacteriological problems in LB it has to be 
stated that there is still a great need for additional 
research in the field of this tick-borne spirochetosis. 
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AUTHORS' ADDRESS 
92 
Willy Burgdorfer, PhD, MD (hon.), scientist emeritus, National Institutes of Health 
N ational Institute of Allergy and Infectious Diseases, Rocky Mountain Laboratories, 
Hamilton, USA 
acta dermatovenerologica A.P A. Vol 5, 96, No 3-4