Ecology of Borrelia burgdorferi in ticks (Acari: Ixodidae), rodents, and birds in the Sierra Nevada foothills, Placer County, California

History of the geographic distribution of the western blacklegged tick, Ixodes pacificus, in the United States

Ixodes pacificus (the western blacklegged tick) occurs in the far western United States (US), where it commonly bites humans. This tick was not considered a species of medical concern until it was implicated in the 1980s as a vector of Lyme disease spirochetes. Later, it was discovered to also be the primary vector to humans in the far western US of agents causing anaplasmosis and hard tick relapsing fever. The core distribution of I. pacificus in the US includes California, western Oregon, and western Washington, with outlier populations reported in Utah and Arizona. In this review, we provide a history of the documented occurrence of I. pacificus in the US from the 1890s to present, and discuss associations of its geographic range with landscape, hosts, and climate. In contrast to Ixodes scapularis (the blacklegged tick) in the eastern US, there is no evidence for a dramatic change in the geographic distribution of I. pacificus over the last half-century. Field surveys in the 1930s and 1940s documented I. pacificus along the Pacific Coast from southern California to northern Washington, in the Sierra Nevada foothills, and in western Utah. County level collection records often included both immatures and adults of I. pacificus, recovered by drag sampling or from humans, domestic animals, and wildlife. The estimated geographic distribution presented for I. pacificus in 1945 by Bishopp and Trembley is similar to that presented in 2022 by the Centers for Disease Control and Prevention. There is no clear evidence of range expansion for I. pacificus, separate from tick records in new areas that could have resulted from newly initiated or intensified surveillance efforts. Moreover, there is no evidence from long-term studies that the density of questing I. pacificus ticks has increased over time in specific areas. It therefore is not surprising that the incidence of Lyme disease has remained stable in the Pacific Coast states from the early 1990s, when it became a notifiable condition, to present. We note that deforestation and deer depredation were less severe in the far western US during the 1800s and early 1900s compared to the eastern US. This likely contributed to I. pacificus maintaining stable, widespread populations across its geographic range in the far western US in the early 1900s, while I. scapularis during the same time period appears to have been restricted to a small number of geographically isolated refugia sites within its present range in the eastern US. The impact that a warming climate may have had on the geographic distribution and local abundance of I. pacificus in recent decades remains unclear.

A Comparison of Questing Substrates and Environmental Factors That Influence Nymphal Ixodes pacificus (Acari: Ixodidae) Abundance and Seasonality in the Sierra Nevada Foothills of California

In California, the western blacklegged tick, Ixodes pacificus Cooley and Kohls, is the principal vector of the Borrelia burgdorferi sensu lato (sl) complex (Spirochaetales: Spirochaetaceae, Johnson et al.), which includes the causative agent of Lyme disease (B. burgdorferi sensu stricto). Ixodes pacificus nymphs were sampled from 2015 to 2017 at one Sierra Nevada foothill site to evaluate our efficiency in collecting this life stage, characterize nymphal seasonality, and identify environmental factors affecting their abundance and infection with B. burgdorferi sl. To assess sampling success, we compared the density and prevalence of I. pacificus nymphs flagged from four questing substrates (logs, rocks, tree trunks, leaf litter). Habitat characteristics (e.g., canopy cover, tree species) were recorded for each sample, and temperature and relative humidity were measured hourly at one location. Generalized linear mixed models were used to assess environmental factors associated with I. pacificus abundance and B. burgdorferi sl infection. In total, 2,033 substrates were sampled, resulting in the collection of 742 I. pacificus nymphs. Seasonal abundance of nymphs was bimodal with peak activity occurring from late March through April and a secondary peak in June. Substrate type, collection year, month, and canopy cover were all significant predictors of nymphal density and prevalence. Logs, rocks, and tree trunks had significantly greater nymphal densities and prevalences than leaf litter. Cumulative annual vapor pressure deficit was the only significant climatic predictor of overall nymphal I. pacificus density and prevalence. No associations were observed between the presence of B. burgdorferi sl in nymphs and environmental variables.

Perpetuation of Borreliae

With one exception (epidemic relapsing fever), borreliae are obligately maintained in nature by ticks. Although some Borrelia spp. may be vertically transmitted to subsequent generations of ticks, most require amplification by a vertebrate host because inheritance is not stable. Enzootic cycles of borreliae have been found globally; those receiving the most attention from researchers are those whose vectors have some degree of anthropophily and, thus, cause zoonoses such as Lyme disease or relapsing fever. To some extent, our views on the synecology of the borreliae has been dominated by an applied focus, viz., analyses that seek to understand the elements of human risk for borreliosis. But, the elements of borrelial perpetuation do not necessarily bear upon risk, nor do our concepts of risk provide the best structure for analyzing perpetuation. We identify the major global themes for the perpetuation of borreliae, and summarize local variations on those themes, focusing on key literature to outline the factors that serve as the basis for the distribution and abundance of borreliae.

The Unexpected Holiday Souvenir: The Public Health Risk to UK Travellers from Ticks Acquired Overseas

Overseas travel to regions where ticks are found can increase travellers' exposure to ticks and pathogens that may be unfamiliar to medical professionals in their home countries. Previous studies have detailed non-native tick species removed from recently returned travellers, occasionally leading to travel-associated human cases of exotic tick-borne disease. There are 20 species of tick endemic to the UK, yet UK travellers can be exposed to many other non-native species whilst overseas. Here, we report ticks received by Public Health England's Tick Surveillance Scheme from humans with recent travel history between January 2006 and December 2018. Altogether, 16 tick species were received from people who had recently travelled overseas. Confirmed imports (acquired outside of the UK) were received from people who recently travelled to 22 countries. Possible imports (acquired abroad or within the UK) were received from people who had recently travelled to eight European countries. Species-specific literature reviews highlighted nine of the sixteen tick species are known to vector at least one tick-borne pathogen to humans in the country of acquisition, suggesting travellers exposed to ticks may be at risk of being bitten by a species that is a known vector, with implications for novel tick-borne disease transmission to travellers.

A quantitative synthesis of the role of birds in carrying ticks and tick-borne pathogens in North America

Birds play a central role in the ecology of tick-borne pathogens. They expand tick populations and pathogens across vast distances and serve as reservoirs that maintain and amplify transmission locally. Research into the role of birds for supporting ticks and tick-borne pathogens has largely been descriptive and focused in small areas. To expand inference beyond these studies, we conducted a quantitative review at the scale of North America to identify avian life history correlates of tick infestation and pathogen prevalence, calculate species-level indices of importance for carrying ticks, and identify research gaps limiting understanding of tick-borne pathogen transmission. Across studies, 78 of 162 bird species harbored ticks, yielding an infestation prevalence of 1981 of 38,929 birds (5.1 %). Avian foraging and migratory strategies interacted to influence infestation. Ground-foraging species, especially non-migratory ground foragers, were disproportionately likely to have high prevalence and intensity of tick infestation. Studies largely focused on Borrelia burgdorferi, the agent of Lyme disease, and non-migratory ground foragers were especially likely to carry B. burgdorferi-infected ticks, a finding that highlights the potential importance of resident birds in local pathogen transmission. Based on infestation indices, all "super-carrier" bird species were passerines. Vast interior areas of North America, many bird and tick species, and most tick-borne pathogens, remain understudied, and research is needed to address these gaps. More studies are needed that quantify tick host preferences, host competence, and spatiotemporal variation in pathogen prevalence and vector and host abundance. This information is crucial for predicting pathogen transmission dynamics under future global change.

Parallelisms and Contrasts in the Diverse Ecologies of the Anaplasma phagocytophilum and Borrelia burgdorferi Complexes of Bacteria in the Far Western United States

Anaplasma phagocytophilum and Borrelia burgdorferi are two tick-borne bacteria that cause disease in people and animals. For each of these bacteria, there is a complex of closely related genospecies and/or strains that are genetically distinct and have been shown through both observational and experimental studies to have different host tropisms. In this review we compare the known ecologies of these two bacterial complexes in the far western USA and find remarkable similarities, which will help us understand evolutionary histories and coadaptation among vertebrate host, tick vector, and bacteria. For both complexes, sensu stricto genospecies (those that infect humans) share a similar geographic range, are vectored mainly by ticks in the Ixodes ricinus-complex, utilize mainly white-footed mice (Peromyscus leucopus) as a reservoir in the eastern USA and tree squirrels in the far west, and tend to be generalists, infecting a wider variety of vertebrate host species. Other sensu lato genospecies within each complex are generally more specialized, occurring often in local enzootic cycles within a narrow range of vertebrate hosts and specialized vector species. We suggest that these similar ecologies may have arisen through utilization of a generalist tick species as a vector, resulting in a potentially more virulent generalist pathogen that spills over into humans, vs. utilization of a specialized tick vector on a particular vertebrate host species, promoting microbe specialization. Such tight host-vector-pathogen coupling could also facilitate high enzootic prevalence and the evolution of host immune-tolerance and bacterial avirulence.

Borrelia burgdorferi sensu lato spirochetes in wild birds in northwestern California: associations with ecological factors, bird behavior and tick infestation

Although Borrelia burgdorferi sensu lato (s.l.) are found in a great diversity of vertebrates, most studies in North America have focused on the role of mammals as spirochete reservoir hosts. We investigated the roles of birds as hosts for subadult Ixodes pacificus ticks and potential reservoirs of the Lyme disease spirochete B. burgdorferi sensu stricto (s.s.) in northwestern California. Overall, 623 birds representing 53 species yielded 284 I. pacificus larvae and nymphs. We used generalized linear models and zero-inflated negative binomial models to determine associations of bird behaviors, taxonomic relationships and infestation by I. pacificus with borrelial infection in the birds. Infection status in birds was best explained by taxonomic order, number of infesting nymphs, sampling year, and log-transformed average body weight. Presence and counts of larvae and nymphs could be predicted by ground- or bark-foraging behavior and contact with dense oak woodland. Molecular analysis yielded the first reported detection of Borrelia bissettii in birds. Moreover, our data suggest that the Golden-crowned Sparrow (Zonotrichia atricapilla), a non-resident species, could be an important reservoir for B. burgdorferi s.s. Of 12 individual birds (9 species) that carried B. burgdorferi s.l.-infected larvae, no birds carried the same genospecies of B. burgdorferi s.l. in their blood as were present in the infected larvae removed from them. Possible reasons for this discrepancy are discussed. Our study is the first to explicitly incorporate both taxonomic relationships and behaviors as predictor variables to identify putative avian reservoirs of B. burgdorferi s.l. Our findings underscore the importance of bird behavior to explain local tick infestation and Borrelia infection in these animals, and suggest the potential for bird-mediated geographic spread of vector ticks and spirochetes in the far-western United States.

Surveillance for Ixodes pacificus and the tick-borne pathogens Anaplasma phagocytophilum and Borrelia burgdorferi in birds from California's Inner Coast Range

We investigated the involvement of birds in the ecology of the western black-legged tick, Ixodes pacificus, and its associated zoonotic bacteria, Borrelia burgdorferi and Anaplasma phagocytophilum, at two interior coast-range study sites in northern California. Anaplasma phagocytophilum, the agent of granulocytic anaplasmosis (GA), and B. burgdorferi s.s., the agent of Lyme disease (LD), are tick-borne pathogens that are well established in California. We screened blood and ticks from 349 individual birds in 48 species collected in 2011 and 2012 using pathogen-specific PCR. A total of 617 immature I. pacificus was collected with almost three times as many larvae than nymphs. There were 7.5 times more I. pacificus at the Napa County site compared to the Yolo County site. Two of 74 (3%) nymphal pools from an Oregon junco (Junco hyemalis) and a hermit thrush (Catharus guttatus) and 4 individual larvae (all from Oregon juncos) were PCR-positive for B. burgdorferi. Blood samples from a golden-crowned sparrow (Zonotrichia atricapilla) and a European starling (Sturnus vulgaris) were positive for A. phagocytophilum DNA at very low levels. Birds that forage on ground or bark and nest on the ground, as well as some migratory species, are at an increased risk for acquiring I. pacificus. Our findings show that birds contribute to the ecologies of LD and GA in California by serving as a blood-meal source, feeding and transporting immature I. pacificus, and sometimes as a source of Borrelia infection.

Multilocus sequence analysis of Borrelia bissettii strains from North America reveals a new Borrelia species, Borrelia kurtenbachii

Using multilocus sequence analyses (MLSA), we investigated the phylogenetic relationship of spirochaete strains from North America previously assigned to the genospecies Borrelia bissettii. We amplified internal fragments of 8 housekeeping genes (clpA, clpX, nifS, pepX, pyrG, recG, rplB, and uvrA) located on the main linear chromosome by polymerase chain reaction. Phylogenetic analysis of concatenated sequences of the 8 loci showed that the B. bissettii clade consisted of 4 closely related clusters which included strains from California (including the type strain DN127-Cl9-2/p7) and Colorado that were isolated from Ixodes pacificus, I. spinipalpis, or infected reservoir hosts. Several strains isolated from I. scapularis clustered distantly from B. bissettii. Genetic distance analyses confirmed that these strains are more distant to B. bissettii than they are to B. carolinensis, a recently described Borrelia species, which suggests that they constitute a new Borrelia genospecies. We propose that it be named Borrelia kurtenbachii sp. nov. in honour of the late Klaus Kurtenbach. The data suggest that ecological differences between B. bissettii and the new Borrelia genospecies reflect different transmission cycles. In view of these findings, the distinct vertebrate host-tick vector associations and the distributions of B. bissettii and B. kurtenbachii require further investigation.

Avian hosts of Ixodes pacificus (Acari: Ixodidae) and the detection of Borrelia burgdorferi in larvae feeding on the Oregon junco

Larval and nymphal western blacklegged tick, Ixodes pacificus Cooley & Kohls (Acari: Ixodidae), were collected from birds, rodents, and lizards at Quail Ridge Reserve located in Napa County in northwestern California. Species from three vertebrate classes were sampled simultaneously from two transects during two consecutive spring seasons. Feeding larval and nymphal ticks were removed and preserved for counting, examination and testing for the presence of Borrelia burgdorferi Johnson, Schmid, Hyde, Steigerwalt & Brenner. Mean infestations with I. pacificus subadults on lizards were 10.0, on birds 2.9, and on rodents 1.3. I. pacificus larvae (204) collected from 10 avian species and (215) collected from two rodent species were tested for the presence of B. burgdorferi s.s. via real-time polymerase chain reaction. Three B. burgdorferi-infected larvae were taken from two Junco hyemalis and two infected larvae from one Neotoma fuscipes Baird. This is the detection of B. burgdorferi ss in an Ixodes pacificus larvae feeding on a Junco hyemalis L., [corrected] in western North America.

Landscape epidemiology of vector-borne diseases

Landscape epidemiology describes how the temporal dynamics of host, vector, and pathogen populations interact spatially within a permissive environment to enable transmission. The spatially defined focus, or nidus, of transmission may be characterized by vegetation as well as by climate, latitude, elevation, and geology. The ecological complexity, dimensions, and temporal stability of the nidus are determined largely by pathogen natural history and vector bionomics. Host populations, transmission efficiency, and therefore pathogen amplification vary spatially, thereby creating a heterogeneous surface that may be defined by remote sensing and statistical tools. The current review describes the evolution of landscape epidemiology as a science and exemplifies selected aspects by contrasting the ecology of two different recent disease outbreaks in North America caused by West Nile virus, an explosive, highly virulent mosquito-borne virus producing ephemeral nidi, and Borrelia burgdorferi, a slowly amplifying chronic pathogen producing semipermanent nidi.

Feeding preferences of the immature stages of three western north American ixodid ticks (Acari) for avian, reptilian, or rodent hosts

Larval and nymphal Ixodes pacificus Cooley and Kohls, I. (Ixodes) jellisoni Cooley and Kohls, and Dermacentor occidentalis Marx were tested for host preference when simultaneously presented with a deer mouse (Peromyscus maniculatus Wagner), California kangaroo rat (Dipodomys californicus Merriam), western fence lizard (Sceloporus occidentalis Baird and Girard), and California towhee (Pipilo crissalis Vigors) in an experimental apparatus. Differences were observed in the preferences among the three species and between life stages. More larvae of all species approached and contacted hosts than did nymphs. Subadult I. pacificus entered all host-containing chambers in the highest numbers and remained on lizards most often after contact. Subadult I. jellisoni entered and remained in the chambers containing kangaroo rats, while rejecting mice, lizards, and birds as hosts. Subadult D. occidentalis most frequently entered rodent-containing chambers and contacted these hosts. After overnight exposure to all nonavian hosts, only I. pacificus parasitized and fed successfully on all three animals. I. jellisoni fed only on kangaroo rats and D. occidentalis fed only on rodents. Molting success ranged from approximately 66 to 95% among tick species and stages. We concluded that, under laboratory conditions, I. pacificus larvae and nymphs prefer western fence lizards, but also will parasitize rodents. Dermacentor occidentalis immatures use deer mice and kangaroo rats similarly, whereas I. jellisoni subadults exclusively parasitize kangaroo rats. California towhees are considerably less attractive as hosts for these three ticks. These host preferences are consistent with what is known about the natural feeding habits of all three ticks.

Detection of Borrelia burgdorferi and Borrelia lonestari in birds in Tennessee

Lyme disease in the United States is caused by the bacterial spirochete Borrelia burgdorferi s.s. (Johnson, Schmid, Hyde, Steigerwalt, and Brenner), which is transmitted by tick vectors Ixodes scapularis (Say) and I. pacificus (Cooley and Kohls). Borrelia lonestari, transmitted by the tick Amblyomma americanum L., may be associated with a related syndrome, southern tick-associated rash illness (STARI). Borrelia lonestari sequences, reported primarily in the southeastern states, have also been detected in ticks in northern states. It has been suggested that migratory birds may have a role in the spread of Lyme disease spirochetes. This study evaluated both migratory waterfowl and nonmigratory wild turkeys (Meleagris gallopavo silvestris, Eastern wild turkey) for B. burgdorferi and B. lonestari DNA sequences. A total of 389 avian blood samples (163 migratory birds representing six species, 125 wild turkeys harvested in habitats shared with migratory birds, 101 wild turkeys residing more distant from migratory flyways) were extracted, amplified, and probed to determine Borrelia presence and species identity. Ninety-one samples were positive for Borrelia spp. Among migratory birds and turkeys collected near migration routes, B. burgdorferi predominated. Among turkeys residing further away from flyways, detection of B. lonestari was more common. All A. americanum ticks collected from these areas were negative for Borrelia DNA; no I. scapularis were found. To our knowledge, this represents the first documentation of B. lonestari among any birds.

Ability of transstadially infected Ixodes pacificus (Acari: Ixodidae) to transmit West Nile virus to song sparrows or western fence lizards

The hypothesis that Ixodes pacificus Cooley & Kohls (Acari: Ixodidae) may serve as a reservoir and vector of West Nile virus (family Flaviviridae, genus Flavivirus, WNV) in California was tested by determining the ability of this tick species to become infected with the NY99 strain of WNV while feeding on viremic song sparrows, to maintain the infection transstadially, and then to transmit WNV to recipient naive song sparrows and western fence lizards during the nymphal stage. The percentage of ticks testing positive by reverse transcription-polymerase chain reaction (RT-PCR) decreased from 77% of 35 larvae at day 6 after ticks were transferred to donor song sparrows (day of detachment) to 23% of 35 nymphs at 59 d postinfestation (approximately 19 d after molting to the nymphal stage). However, the percentage of ticks positive by RT-PCR from which infectious virus was recovered by Vero cell assay decreased from 59% on day 6 to 12% on day 59, even though there was no statistically significant decrease in the quantity of RNA within positive ticks. Attempts to improve the sensitivity of plaque assays by blind passage through C6/36 cell cultures were unsuccessful. These data indicated that ticks maintained viral RNA but not necessarily infectious virus over time. Nymphs from larvae that fed on song sparrows with peak viremias ranging from 7.2 to 8.5 log10 plaque-forming units (PFU) per ml were used in transmission attempts. From one to seven RNA-positive nymphal ticks engorged and detached from each of four recipient song sparrows or western fence lizards. Blood samples from sparrows and lizards remained negative, indicating that transmission did not occur. An additional four lizards inoculated with 1,500 PFU of WNV developed moderate viremias, ranging from 4.2 to 5.6 log10 PFU/ml. Our data and data from previous studies collectively indicated that ixodid ticks were not able to experimentally transmit WNV and therefore most likely would not be important vectors in WNV transmission cycles.

An avian contribution to the presence of Ixodes pacificus (Acari: Ixodidae) and Borrelia burgdorferi on the Sutter Buttes of California

Birds from 45 species were sampled during three spring seasons from an isolated canyon on the Sutter Buttes in California for the presence of subadult stages of Ixodes pacificus Cooley & Kohls, and for infection with Borrelia burgdorferi Johnson, Schmid, Hyde, Steigerwalt & Brenner. These birds were found to have an infestation prevalence of 45%, a density of 1.7 ticks per bird, and an intensity of 3.8 ticks per infested bird. There was a significant difference in the I. pacificus infestations between canopy and ground-dwelling birds. Birds also demonstrated an overall infection with B. burgdorferi of 6.4% with significant difference between bird species. Amplification and subsequent sequencing of the 23s-5s rRNA intergenic spacer region of the Borrelia genome from one bird, a hermit thrush, Catharus guttatus (Nuttall), showed that the infection in this bird was caused by B. burgdorferi sensu stricto; the first such finding in a bird from the far west. Our results suggest that birds play a role in the distribution and maintenance of I. pacificus, and possibly of B. burgdoferi, at the Sutter Buttes, CA.

MIGRATORY SONGBIRDS DISPERSE TICKS ACROSS CANADA, AND FIRST ISOLATION OF THE LYME DISEASE SPIROCHETE, BORRELIA BURGDORFERI, FROM THE AVIAN TICK, IXODES AURITULUS

During a 3-yr comprehensive study, 196 ixodid ticks (9 species) were collected from 89 passerine birds (32 species) from 25 localities across Canada to determine the distribution of avian-associated tick species and endogenous Lyme disease spirochetes, Borrelia burgdorferi Johnson, Schmid, Hyde, Steigerwalt, and Brenner. We report the following first records of tick parasitism on avian hosts: the rabbit-associated tick, Ixodes dentatus Marx, from Manitoba and Ontario; the mouse tick, Ixodes muris Bishopp and Smith, from British Columbia; and the blacklegged tick, Ixodes scapularis Say, from New Brunswick. Moreover, we provide the first record of the Neotropical tick, Amblyomma humerale Koch (1 nymph), in Canada and its parasitism of any bird. This tick was compared morphologically with nymphs of other Neotropical Amblyomma spp., and genetically, using a 344-bp fragment of the 12S rDNA sequence of 41 New World Amblyomma species. The first collections of the western blacklegged tick, Ixodes pacificus Cooley and Kohls, from passerine species in Alberta and British Columbia, are also reported. Notably, we further report the first isolation of B. burgdorferi from the bird tick, Ixodes auritulus Neumann, collected from an American robin, Turdus migratorius L., on Vancouver Island. Furthermore, B. burgdorferi-positive I. auritulus larvae were collected from a reservoir-competent fox sparrow, Passerella iliaca (Merrem). Our findings indicate that ground-dwelling passerines, in particular, are parasitized by certain ixodid ticks and play an important role across Canada in the wide dispersal of B. burgdorferi-infected ticks and increased risk of Lyme disease exposure.

Western gray squirrel (Rodentia: Sciuridae): a primary reservoir host of Borrelia burgdorferi in Californian oak woodlands?

In California, dense woodlands have been recognized as important biotopes where humans are exposed to the nymphal stage of the western blacklegged tick, Ixodes pacificus Cooley & Kohls, the primary vector of the Lyme disease spirochete Borrelia burgdorferi sensu stricto (s.s.), in the far-western United States. To identify the principal reservoir host(s) of this spirochete, and of closely related spirochetes in the B. burgdorferi sensu lato (s.l.) complex, in dense woodlands in Mendocino County, California, approximately 50 species of birds and mammals, including wood rats and kangaroo rats, were evaluated as potential hosts for vector ticks and borreliae in 2002 and 2003. Although polymerase chain reaction (PCR) and sequencing analyses revealed that many vertebrate species had been exposed to one or more members of the B. burgdorferi s.l. spirochetal complex, only the western gray squirrel, Sciurus griseus, fulfilled the major criteria for a reservoir host of B. burgdorferi s.s. Ear-punch biopsies from eight of 10 squirrels collected from five separate woodlands were PCR-positive for B. burgdorferi s.s., 47% of I. pacificus larvae (n = 64) and 31% of nymphs (n = 49) removed from squirrels contained B. burgdorferi s.l., and the engorgement status of I. pacificus larvae was associated positively with acquisition of spirochetes. Overall, 83 and 100% of the amplicons sequenced from PCR-positive I. pacificus larvae and nymphs, respectively, were identified as B. burgdorferi s.s, Among the five remaining positive I. pacificus larvae, three contained B. bissettii and two had uncharacterized B. burgdorferi s.l. Borrelia burgdorferi s.s. was detected in one of five larvae and zero of two nymphs of the Pacific Coast tick, Dermacentor occidentalis Marx, that likewise had been removed from squirrels. The rickettsial agent of human anaplasmosis, Anaplasma phagocytophilum, was detected in the blood or ear biopsies of two squirrels and in one (1.6%) of 64 I. pacificus larvae and two (4.1%) of 49 nymphs obtained from squirrels. The one rickettsial-positive larva was coinfected with B. burgdorferi s.s. The apparently high reservoir potential of S. griseus for B. burgdorferi s.s., plus the fact that the geographic distribution of this squirrel coincides well with that of most reported human cases of Lyme disease in this region, indicated that it may be essential for maintaining foci of B. burgdorferi s.s. in certain types of woodlands. The findings with respect to A. phagocytophilum, although of less certain significance, suggest that S. griseus could serve as a secondary host of this rickettsia.

Lyme borreliosis in Europe and North America

Since the discovery of the Lyme disease spirochete in North America in 1982 and in Europe in 1983, a plethora of studies on this unique group of spirochetes that compriseBorrelia burgdorferisensu lato has been accumulated. In an attempt to compare and contrast Lyme borreliosis in Europe and North America we have reviewed the biology of the aetiologic agents, as well as the clinical aspects, diagnosis and treatment of this disease on both continents. Moreover, we have detailed the ecology of theIxodesticks that transmit this infection and the reservoir hosts that maintain the spirochete cycle in nature. Finally, we have examined the transmission dynamics of the spirochete on both continents, as well as the available prevention strategies. Although it has been over two decades since the discovery of the Lyme disease spirochete, Lyme borreliosis is an expanding public health problem that has defied our attempts to control it. By comparing the accumulated experience of investigators in North America and Europe, where the disease is most frequently reported, we hope to advance the cause of developing novel approaches to combat Lyme borreliosis.

Detection and characterization of Borrelia bissettii in rodents from the central California coast

This is the first report of Borrelia burgdorferi sensu lato in rodents from San Luis Obispo county, with most isolates obtained from a previously unreported host, Neotoma lepida Thomas. B. burgdorferi sensu lato was identified in seven rodent species, including the California vole, Microtus californicus Peale; dusky-footed woodrat, Neotoma fuscipes Baird; desert woodrat, Neotoma lepida Thomas; brush mouse, Peromyscus boylii Baird; California mouse, Peromyscus californicus Gambel; deer mouse, Peromyscus maniculatus Wagner; and western harvest mouse, Reithrodontomys megalotis Baird. Ear punch biopsies were cultured in BSK-H medium from 179 rodents trapped at six different study sites. Overall, prevalence of rodent infection was 44/179 (24.6%), with 34 of these isolates from N. lepida. Spirochete isolates were obtained from rodents at all study sites, indicating widespread prevalence of B. burgdorferi sensu lato across rodent species and habitats. Nucleotide sequences for 14 of these isolates have been submitted to GenBank. Isolates from three N. lepida and one P. boylii had identical flagellin gene sequences, and phylogenetic analysis placed these spirochetes in B. burgdorferi sensu lato group DN127, now known as B. bissettii Postic, Marti Ras, Lane, Hendson & Baranton. Additional sequencing of the intergenic spacer regions between the 5S and 23S ribosomal genes was performed on three of these isolates. Phylogenetic analysis separated these isolates into two clusters that grouped with Colorado or California isolates. The role of B. bissettii and related species other than B. burgdorferi sensu stricto Johnson, Schmid, Hyde, Steigerwalt & Brenner as human pathogens in the United States warrants further investigation.

Acarologic risk of exposure to Borrelia burgdorferi spirochaetes: long-term evaluations in north-western California, with implications for Lyme borreliosis risk-assessment models

Over a 5-year period (1997-2001) the population densities of Ixodes pacificus Cooley & Kohls (Acari: Ixodidae) nymphs infected with spirochaetes of Borrelia burgdorferi sensu lato (s.l.) were evaluated in areas of 2000 ha at two localities (CHR, nine sites; HREC, seven sites) 25 km apart in Mendocino County, north-western California. The 5-year median density of infected nymphs was significantly higher at CHR than at HREC (0.51 vs. 0.09 per 100 m(2) and site-specific yearly densities exceeding one infected nymph per 100 m2 were 10-fold more likely to occur at CHR than at HREC. The importance of long-term data in acarologic risk assessment was demonstrated by significantly higher median yearly densities of infected nymphs at CHR from 1997 to 1999, whereas both areas had similar densities during 2000-2001. Overall, the causative agent of Lyme borreliosis in North America, B. burgdorferi Johnson et al. sensu stricto (s.s.) accounted for 76% of 46 genetically characterized B. burgdorferi s.l. infections from I. pacificus nymphs. Tremendous variability in acarologic risk was recorded within both areas: yearly densities of infected nymphs varied 11-97-fold between sites at CHR and 8-30-fold at HREC. Part of this variation could be explained by environmental traits, most notably deer usage. However, correlations between environmental factors and density of infected nymphs (for CHR and HREC combined) did not necessarily apply when these areas were considered separately. Thus, a Lyme borreliosis ecology model developed in one of these areas needs testing in the other area.

Lyme borreliosis

Despite more than 25 years' experience with Lyme borreliosis, much remains to be learned about this complex zoonosis. Practicing veterinarians, particularly those in the northeastern and upper midwestern states, where Lyme borreliosis is highly endemic, should be familiar with the ecologic features and typical clinical signs of Lyme borreliosis. Interpretation of signs and serologic test results should be made with consideration of the regional prevalence of Lyme borreliosis and the animal's opportunity for exposure to infected Ixodes spp. The availability of recently marketed topical acaracides is a valuable adjunctive measure in prevention of Lyme borreliosis. A maximally effective prevention strategy should include consideration of environmental modification, activity restrictions, routine examinations for ticks, prompt removal of attached ticks, and vaccination. Technologic advances, such as the C6 EIA and the Osp A recombinant vaccine, offer the promise of additional tools for the clinical management and prevention of this tick-borne zoonosis.

Acarologic risk of exposure to emerging tick-borne bacterial pathogens in a semirural community in northern California

An acarologic study was conducted in a semirural community in northern California to determine the relative abundance of, and the prevalence of infection with, three emerging bacterial pathogens in the western black-legged tick (Ixodes pacificus). These included the agents causing Lyme disease (Borrelia burgdorferi), human granulocytic ehrlichiosis [Ehrlichia phagocytophila (formerly Ehrlichia equi)], and human monocytic ehrlichiosis (Ehrlichia chaffeensis). The study area in Sonoma County consisted of two properties each with four residents and an uninhabited adjacent comparison area. Six of the eight residents had been either physician-diagnosed or serodiagnosed previously with Lyme disease, and, of these, one also had been serodiagnosed with human monocytic ehrlichiosis. Direct immunofluorescent/culture assays and bacterial species-specific polymerase chain reaction assays were used to test whole ticks individually for presence of B. burgdorferi and Ehrlichia spp., respectively. Overall, 6.5% of the nymphal (n = 589) and 1.6% of the adult ticks (n = 318) from the same generational cohort were found to contain B. burgdorferi. In contrast, none of 465 nymphs and 9.9% of 202 adults were infected with E. phagocytophila. Excised tissues from another 95 adult ticks yielded a comparable E. phagocytophila infection prevalence of 13.7%. E. chaffeensis was not detected in either nymphal or adult ticks. Using a combination of culture and polymerase chain reaction assays, coinfection of I. pacificus adults with B. burgdorferi and E. phagocytophila was demonstrated for the first time. The marked disparity in the infection prevalence of these pathogens in nymphal and adult ticks suggests that their maintenance cycles are inherently different.

Vector competence of Ixodes pacificus and I. spinipalpis (Acari: Ixodidae), and reservoir competence of the dusky-footed woodrat (Neotoma fuscipes) and the deer mouse (Peromyscus maniculatus), for Borrelia bissettii

We investigated the experimental vector competence of Ixodes pacificus Cooley and Kohls and Ixodes spinipalpis Hadwen and Nuttall, and the reservoir competence of the dusky-footed woodrat (Neotoma fuscipes Baird) and the deer mouse (Peromyscus maniculatus [Wagner]), for Borrelia bissettii Postic, Marti Ras, Lane, Hendson, and Baranton. Both rodent species are capable reservoirs for B. bissettii; infection rates for I. pacificus or I. spinipalpis nymphs fed as larvae on infected animals ranged from 50 to 57%. Moreover, both I. pacificus and I. spinipalpis are efficient vectors of B. bissettii. Viable infections were recorded from all rodents known to be exposed to one or more infected nymphs of I. spinipalpis (seven deer mice, two white mice) or I. pacificus (seven deer mice). In contrast, none of four New Zealand white rabbits fed upon by 90 I. pacificus nymphs with a probable B. bissettii infection rate of >50% became infected or seroconverted. The attachment and feeding success of laboratory-reared nymphs similarly confined with deer mice in muslin-covered wire-mesh cages for 24 h ranged from 0% for I. pacificus to 17-73% for I. spinipalpis. Notably, the I. pacificus nymphs were physiologically capable of feeding; nymphs failing to attach to rodents fed readily when placed in feeding capsules upon rabbits. We conclude that the dusky-footed woodrat and the deer mouse are capable experimental reservoir hosts of B. bissettii, and that both I. spinipalpis and I. pacificus are efficient experimental vectors of B. bissettii. However, the reluctance of I. pacificus nymphs to feed on certain rodents may limit its importance as an enzootic vector of B. burgdorferi sensu lato spirochetes.

Birds, migration and emerging zoonoses: west nile virus, lyme disease, influenza A and enteropathogens

Wild birds are important to public health because they carry emerging zoonotic pathogens, either as a reservoir host or by dispersing infected arthropod vectors. In addition, bird migration provides a mechanism for the establishment of new endemic foci of disease at great distances from where an infection was acquired. Birds are central to the epidemiology of West Nile virus (WNV) because they are the main amplifying host of the virus in nature. The initial spread of WNV in the U.S. along the eastern seaboard coincided with a major bird migration corridor. The subsequent rapid movement of the virus inland could have been facilitated by the elliptical migration routes used by many songbirds. A number of bird species can be infected with Borrelia burgdorferi, the etiologic agent of Lyme disease, but most are not competent to transmit the infection to Ixodes ticks. The major role birds play in the geographic expansion of Lyme disease is as dispersers of B. burgdorferi-infected ticks. Aquatic waterfowl are asymptomatic carriers of essentially all hemagglutinin and neuraminidase combinations of influenza A virus. Avian influenza strains do not usually replicate well in humans, but they can undergo genetic reassortment with human strains that co-infect pigs. This can result in new strains with a marked increase in virulence for humans. Wild birds can acquire enteropathogens, such as Salmonella and Campylobacter spp., by feeding on raw sewage and garbage, and can spread these agents to humans directly or by contaminating commercial poultry operations. Conversely, wild birds can acquire drug-resistant enteropathogens from farms and spread these strains along migration routes. Birds contribute to the global spread of emerging infectious diseases in a manner analogous to humans traveling on aircraft. A better understanding of avian migration patterns and infectious diseases of birds would be useful in helping to predict future outbreaks of infections due to emerging zoonotic pathogens.