Ixodes scapularis ticks (Acari: Ixodidae) from Louisiana are competent to transmit Borrelia burgdorferi, the agent of Lyme borreliosis

The principal vector of Borrelia burgdorferi, the Lyme borreliosis spirochete, in the Northeast and Midwestern regions of the United States is the blacklegged tick Ixodes scapularis. Because of a favorable environment, I. scapularis is also plentiful in the South; however, a correlation with Lyme borreliosis cases does not exist in this region of the United States. Concern existed that something intrinsic to ticks found in Louisiana could mitigate their ability to transmit B. burgdorferi. Therefore, we set out to assess the ability of I. scapularis ticks from Louisiana to become infected with and transmit B. burgdorferi using mice as hosts. In the laboratory, mating adult female ticks collected in southeastern Louisiana were fed on the ears of rabbits. After oviposition and egg hatching, the resulting larvae were fed on mice that had been needle-inoculated with two different strains of B. burgdorferi sensu stricto, B31 and JD1. Larvae were found to be positive for spirochetes. Additional fed larvae were allowed to molt into the nymphal stage. Flat nymphs remained infected with B. burgdorferi. Infected nymphs were allowed to feed on na?ve mice, all of which became infected as shown by culture of ear biopsy specimens. Na?ve larvae were then fed on these same mice to assess transmissibility. The resulting engorged larvae harbored spirochetes. We have demonstrated that the I. scapularis ticks found in Louisiana are fully competent to carry and transmit B. burgdorferi infection.     

Populations of Ixodes scapularis (Acari: Ixodidae) are modulated by drought at a Lyme disease focus in Illinois

From 1990 through 1997, Ixodes scapularis Say larvae and nymphs were sampled between May and October along a 400-m segment of a nature trail in a Lyme disease endemic site in northern Illinois. Ticks were removed from Peromyscus leucopus mice and collected via tick drags at approximately 3-wk intervals. Mouse population estimates along the trail varied from 2, in the spring of 1996 following a year of drought, to > 200 in 1993, the wettest year on record. During the 8-yr period, there were major droughts during the summers of 1991 and 1995. Cumulative degree-days were positively correlated with the number of ticks collected on drags in the same year and negatively correlated with larval tick populations for the following year (P < 0.05). Cumulative rainfall was positively correlated with larval tick abundance for the following year. This was most readily apparent by examination of the larval density on captured mice. In the year following each of two drought years, larval densities were significantly depressed compared with the 8-yr average at the site.     

Methodology for sampling questing nymphs of Ixodes ricinus (Acari: Ixodidae), the principal vector of Lyme disease in Europe

To assess the Lyme borreliosis vector population density we set up a methodology for sampling the Ixodes ricinus L. population host questing on the vegetation. We focused on the collection of the nymphal stage, which is the principal stage of disease transmission to humans. This study was carried out in Rambouillet forest (Yvelines, France) where seven study areas were demarcated. These areas are maximally homogeneous for plant species using a finer scale than the phytosociological classification as defined by the method of landscape diagnostics. Out of 23 collections performed from March 1997 to May 1998, 2,906 I. ricinus nymphs were collected. The sampling technique chosen was the cloth lure technique. The technical parameters were studied and fixed (cloth type, cloth size, sample size, researcher position). It appeared that toweling was the best cloth type to optimize the number of ticks collected; the position of the researcher had no effect on tick samples. To satisfy the criteria for correct sampling, we studied representativity, randomness, and nonselectivity of our methodology. The spatial distribution of nymphs in a homogeneous area was close to random and thus very few subsamples were needed to obtain a relative density which was representative. No significant differences were found between random samples and following transect samples; and nonselectivity was totally satisfied because we only worked on questing nymphs. We grouped the samples that presented no significant differences to attribute a density index, which varied from 0 to 5. This methodology, applied with the same parameters, offers potential for producing comparable results from studies in different geographical areas and at different times of the years.     

Parasitization of humans in West Virginia by Ixodes cookei (Acari: Ixodidae), a potential vector of Lyme borreliosis

In 32 collections, two larvae, 33 nymphs, and one adult female Ixodes cookei Packard were collected from humans in West Virginia from August 1987 to May 1990. Most were attached. The ticks were found in 14 counties and were the most abundant Ixodes found biting humans. One nymphal I. cookei was removed from the left axilla of a 39-yr-old woman who lives and works in Monongalia and Marion counties, W. Va. The bite was the center of an expanding erythematous lesion reaching 4 cm in diameter, clearing centrally, and typical of erythema migrans. This association and the near absence of Ixodes dammini Spielman, Clifford, Piesman & Corwin from the state suggests the possibility that I. cookei may be an important vector of Lyme borreliosis in West Virginia. In five separate collections, five nymphal Ixodes dentatus Marx were removed from humans in four counties, implicating this species as a potential minor vector of Lyme borreliosis in West Virginia.     

Competence of a rabbit-feeding Ixodes (Acari: Ixodidae) as a vector of the Lyme disease spirochete

We compared the development of the Lyme disease spirochete, Borrelia burgdorferi Johnson, Schmid, Hyde, Steigerwalt and Brenner, in subadult rabbit-feeding Ixodes dentatus Marx with that in mouse-feeding I. dammini Spielman, Clifford, Piesman and Corwin. Rabbits were infected with spirochetes by the bites of I. dammini that had been infected naturally in a zo?notic site. Larval ticks of both species were permitted to engorge simultaneously on each of these infected hosts. Spirochetes were present in the guts of about half of the resulting nymphal I. dentatus and most of the I. dammini that developed. An experimentally infected nymphal I. dentatus, in turn, infected a rabbit. Because I. dentatus feeds solely on rabbits, and these hosts may be extraordinarily abundant in nature, this tick provides potential for a hidden enzo?tic cycle of natural Lyme disease transmission.     

Defining plasmids required by Borrelia burgdorferi for colonization of tick vector Ixodes scapularis (Acari: Ixodidae)

Maintenance in nature of Borrelia burgdorferi, the pathogenic bacterium that causes Lyme disease, requires transmission through an infectious cycle that includes a tick vector and a mammalian host. The genetic requirements for persistence in these disparate environments have not been well defined. B. burgdorferi has a complex genome composed of a chromosome and >20 plasmids. Previous work has demonstrated that B. burgdorferi requires two plasmids, lp25 and lp28-1, in the mammalian host. To investigate the requirement for these same two plasmids during tick infection, we experimentally infected larval ticks with B. burgdorferi lacking either lp25 or lp28-1 and then assessed the spirochete load in ticks at different points of the infection. Whereas plasmid lp28-1 was dispensable in ticks, plasmid lp25 was essential for tick infection. Furthermore, we investigated the requirement in ticks for the lp25 gene bbe22, which encodes a nicotinamidase that is necessary and sufficient for mammalian infection by B. burgdorferi clones lacking lp25. This gene was also sufficient in ticks to restore survival of spirochetes lacking lp25. This is the first study to investigate the requirement for specific plasmids by B. burgdorferi within the tick vector, and it begins to establish the genomic components required for persistence of this pathogen throughout its natural infectious cycle.     

Deer density and the abundance of Ixodes scapularis (Acari: Ixodidae)

The abundance of Ixodes scapularis Say (Ixodes dammini Spielman, Clifford, Piesman & Corwin), the vector tick of the Lyme disease spirochete and other human pathogens, is related to the presence of its primary reproductive stage host, white-tailed deer (Odocoileus virginianus Zimmerman). However, this relationship has not been quantified in terms that would guide wildlife management in areas in which the public is, or is likely to become, exposed to infected ticks. In this study, deer density and tick abundance were measured in an emergent area for Lyme disease at three spatial scales using estimation methods appropriate for each. Simple linear regression was used to relate (1) the number of ticks found on deer at tagging stations in southern Maine to harvest-derived estimates of the density of deer within the towns in which they were killed, (2) tick densities estimated from fall flagging counts to deer densities estimated from pellet group counts made within multiple transects distributed through 5.2-km2 study sites, and (3) tick counts to pellet group counts within the individual transects. At the broadest scale, ticks on deer decreased with elevation and distance from the coast and increased with deer density, although deer and tick presence were only weakly related. Among the 5.2-km2 study sites and within individual transects, tick abundance related more strongly to deer pellet group counts. Few ticks were collected at deer densities <7/km2.     

Preprandial inhibition of re-mating in Ixodes ticks (Acari: Ixodidae)

Questing female blacklegged ticks, Ixodes scapularis, Say in Massachusetts rarely bear more than one endospermatophore in their reproductive tracts. We evaluated the cause of this nonrandom distribution by examining the stability of endospermatophore retention in females and the effect of prior insemination of females on the copulatory behavior of male ticks. Endospermatophores were retained without degradation in unfed female ticks for >1 yr at 5 degrees C and for at least 4 mo at 21 degrees C. Males were much more likely to abort preprandial (before feeding) copulations without inseminating females bearing endospermatophores. This remating inhibition activity persisted in unfed females for at least 2 mo after insemination. Perprandial (during feedng) copulations were less restrictive, particularly when females became partially engorged. Males were more likely to remain in copula with previously inseminated females that were engorged, but generally did not transfer spermatophores to them until their fifth day of attachment to a rabbit. Little inhibitory activity was evident during the final, rapid engorgement phase of feeding. Thus, remating inhibition appears to degrade as feeding progresses. In summary, an unknown factor associated with the previous insemination of females inhibits subsequent spermatophore transfer by causing mating pairs to interrupt copulation soon after initiation. Remating inhibition is strongest in unfed ticks and becomes less apparent as females become engorged.     

Ixodes scapularis (Acari: Ixodidae) distribution surveys in the Chicago metropolitan region

Considering recent studies confirming an increased risk of contracting Lyme disease near metropolitan Chicago, we surveyed a more comprehensive area to assess whether the geographical distribution and establishment of Ixodes scapularis (Say) populations across northeast Illinois are widespread or limited in occurrence. From May through October 2008 and from April through October 2009, 602 I. scapularis ticks of all three life stages (larva, nymph, adult) were collected from sites in Cook, DuPage, Lake, and McHenry counties in northeast Illinois. The surveys were conducted by drag sampling vegetation in public-access forested areas. I. scapularis comprised 56.4% of ticks collected (n = 1,067) at 17 of 32 survey sites. In addition, four other tick species were incidentally collected: Dermacentor variabilis (Say), Haemaphysalis leporispalustris (Packard), Ixodes dentatus (Marx), and Amblyomma americanum (L.). This study updates the I. scapularis distribution in northeast Illinois. Our random sampling of suitable tick habitats across a large geographic area of the Chicago metropolitan area suggests a widespread human exposure to I. scapularis, and, potentially, to their associated pathogens throughout the region. These results prompt continued monitoring and investigation of the distribution, emergence, and expansion of I. scapularis populations and Borrelia burgdorferi transmission within this heavily populated region of Illinois.     

Distribution of the Lyme disease vector, Ixodes dammini (Acari: Ixodidae) and isolation of Borrelia burgdorferi in Ontario, Canada.

Ixodes dammini Spielman, Clifford, Piesman & Corwin was confirmed at Long Point, Lake Erie, Ontario, on small mammals and white-tailed deer and by dragging for ticks. Mean intensities of up to 16.2 larvae and 2.1 nymphs were found on Peromyscus leucopus (Rafinesque), with an overall prevalence of infestation up to 92%. Adult I. dammini (101.6 +/- 77.63) (mean +/- SD) were found on 8 white-tailed deer, Odocoileus virginianus (Zimmerman). The seasonal pattern of recovery of ticks from hosts and the environment resembled that described elsewhere. I. dammini was not found on 952 small mammals trapped at 25 other localities throughout Ontario, although other ticks (Derma-centor variabilis (Packard), Ixodes angustus Neumann, I. marxi Banks, I. muris Bishopp & Smith) were encountered sporadically. I. dammini is not widespread or common in Ontario other than at Long Point. Borrelia burgdorferi was isolated from 10 of 151 P. leucopus; from larval and nymphal I. dammini; and from nymphal and adult D. variabilis, all from Long Point. B. burgdorferi was not recovered from 116 small mammals from localities other than Long Point. Seropositive P. leucopus (indirect fluorescent antibody test titer > or = 1:20) were common (up to 30% prevalence in July 1988, n = 23) on Long Point. Where I. dammini was not found, the prevalence of seroreactors among Peromyscus was 0 (15 sites), < 12% (5 sites), and 29% (1 site); seroprevalence at 1:20 could not be calculated for a further 4 sites examined in 1987. Antibody to B. burgdorferi was also detected in other small mammals at some sites. Such antibody was interpreted as possibly cross-reacting or caused by direct transmission.     

Protection against lyme disease spirochete transmission provided by prompt removal of nymphal Ixodes scapularis (Acari: Ixodidae)

Public health recommendations for Lyme disease prevention generally include daily tick checks and prompt removal of attached ticks as a means of decreasing the risk of acquiring Lyme disease in highly endemic regions. In the current study, we determined whether crushing nymphal ticks during removal with forceps increased the risk of B. burgdorferi transmission, what degree of protection from transmission of B. burgdorferi was provided by removal of nymphal Ixodes scapularis Say at specific intervals, and whether commercial devices marketed for tick removal worked when tested against nymphal I. scapularis. Both removal via gentle pressure (26% transmission) or crushing the tick (30% transmission) caused a significant decrease in transmission as compared with the sham control (70% transmission). The degree of protection provided via tick removal decreased steadily up to 60 h of attachment; between 60 and 66 h, a dramatic falloff in protection occurred to the point where no protection was observed at 66 h. Finally, commercial tick removal devices varied widely in their efficacy for the removal of attached nymphal I. scapudaris.     

Pathogenicity of Bacillus thuringiensis variety kurstaki to Ixodes scapularis (Acari: Ixodidae)

Pathogenicity of the entomopathogenic bacterium Bacillus thuringiensis variety kurstaki de Barjac & Lemille was tested against the black-legged tick, Ixodes scapularis Say. Engorged larvae dipped in a solution of 10(8) spores per milliliter showed 96% mortality 3 wk after infection. The LC50 value for engorged larve (concentration required to kill 50% of ticks) was 10(7) spores per milliliter. B. thuringiensis shows considerable potential as a microbial control agent for the management of I. scapularis.     

Lyme disease in California: interrelationship of Ixodes pacificus (Acari: Ixodidae), the western fence lizard (Sceloporus occidentalis), and Borrelia burgdorferi

The relationship of immature western black-legged ticks, Ixodes pacificus Cooley and Kohls, to the western fence lizard, Sceloporus occidentalis Baird and Girard, and to the Lyme disease spirochete, Borrelia burgdorferi, was investigated in chaparral and woodland-grass habitats in northern California from 1984 to 1986. Immature ticks were found on lizards in spring and summer, but the prevalence and abundance of ticks on this host were considerably greater in spring. The peak of larval abundance preceded that of nymphs by several weeks, but there was considerable seasonal overlap between these parasitic stages. Larvae and nymphs attached primarily to the lateral nuchal pockets of lizards in chaparral (99.5%) and woodland-grass (91.8%). The numbers of larvae infesting lizards in spring fit the negative binomial distribution in woodland-grass but not in chaparral; insufficient data precluded similar analyses for nymphs. Tick loads did not differ significantly with respect to age or gender of the lizard. Spirochetal infection rates (range, 0-3.7%) in I. pacificus immatures were comparable in both habitats and were similar to those reported previously for adults of this tick. Overall, 1 (0.9%) of 117 larvae and 10 (1.8%) of 552 nymphs were infected with spirochetes resembling B. burgdorferi. Spirochetes were not observed in blood smears prepared from 261 wild-caught lizards, including five lizards fed upon by infected ticks at the time of collection. These and other findings suggest that S. occidentalis, although an important host of I. pacificus immatures, may be less important as a source for infecting ticks with B. burgdorferi.     

Prevalence of Lyme borreliosis spirochetes in ixodid ticks of Japan, with special reference to a new potential vector, Ixodes ovatus (Acari: Ixodidae).

To determine the vectors of Lyme borreliosis in Japan, the ixodid ticks taken in Hokkaido, Fukushima, Nagano, Ehime, and Kochi Prefectures were examined individually for spirochetal infections by culturing their midgut tissues in BSK medium. The cultivable spirochetes identified as Borrelia burgdorferi were detected in Ixodes persulcatus Schulze adults (15.5% of 634), nymph (6.7% of 15) and Ixodes ovatus Neumann adult (9.6% of 188) from Hokkaido, I. ovatus adults (25.5% of 47) from Fukushima, and I. persulcatus adults (21.7% of 143) and I. ovatus adults (27.1% of 85) from Nagano. However, 72 Haemaphysalis spp. from Ehime and Kochi were free from spirochetal infection. I. persulcatus is a vector of Lyme borreliosis in the Far East, but in Japan, I. ovatus may also transmit the spirochetes to human and feral animals.     

A technique for longitudinally sampling individual adult Ixodes scapularis (Acari: Ixodidae)

The first technique for repeatedly sampling individual Ixodes scapularis adult ticks was developed and validated. Gut samples from the same individual ticks were removed and analyzed at weekly intervals. Multiple analyses were conducted on each gut sample (e.g., total protein concentration, presence of viable B. burgdorferi spirochetes, and concentration of outer surface protein A [OspA]). Female I. scapularis survived for up to 25 d after gut sampling. Seventy-five percent of females oviposited after the sampling procedure, with 14% of ticks laying >1,500 eggs. No significant differences in either fecundity or length of survival were found between B. burgdorferi-infected and uninfected I. scapularis. This technique will enable longitudinal studies on both tick-pathogen interactions and physiological studies that have hitherto not been attempted.     

Ixodes scapularis (Acari: Ixodidae) in Kansas: a new distribution record.

The blacklegged tick, Ixodes scapularis Say, previously known to occur only in the extreme southeastern corner of Kansas, has been collected in Douglas and Jefferson counties in the northeast. The new collections extend the northern range of this species in Kansas by approximately 240 km. The role of I. scapularis as a vector of the Lyme borreliosis spirochete, Borrelia burgdorferi, makes this a noteworthy extension of its known range. The proximity of these new collection sites to large centers of human population is of special significance to health-care providers in that region.     

Lack of spatial autocorrelation in fine-scale distributions of Ixodes scapularis (Acari: Ixodidae)

Spatial patterns of Ixodes scapularis Say, the vector of the Lyme disease agent, have been examined at various geographic scales, demonstrating that distributions of these ticks are spatially autocorrelated at both national and state scales. We tested the hypothesis that distributions of nymphal I scapularis ticks at the fine scale of an endemic community also are spatially autocorrelated. Nymphal tick densities were determined by collecting ticks from 51 and 47 wooded residential properties in a southern Rhode Island town in 2002 and 2003, respectively. The average tick density at residences during 2002 was 51.17 ( +/- 46.04) nymphs per hour, with a range of 3-297 and median of 40.82. In 2003, the average tick density was 44.48 (+/- 38.31) nymphs per hour, with a range of 3-153 and median of 36. Semivariance analysis revealed no spatial autocorrelation in tick densities between residences, likely due to the high variability of tick distributions at this scale. Further analysis of drag-sampling data at individual residences by using Lloyd's patchiness index (m*/m) demonstrated a patchy distribution of nymphs. High variability of nymphal I. scapularis densities may greatly affect predicting spatial patterns of ticks at a fine scale.     

Comparative vector competence of Dermacentor variabilis and Ixodes scapularis (Acari: Ixodidae) for the agent of human granulocytic ehrlichiosis

Vector competence of Ixodes scapularis Say and Dermacentor variabilis Say for the agent of human granulocytic ehrlichiosis (HGE) was compared. Five white-footed mice, Peromyscus leucopus Rafinesque, were inoculated intra-peritoneally with blood from a mouse infected with the agent of HGE. Approximately 100 I. scapularis and D. variabilis larvae were placed on each mouse and allowed to feed to repletion. Fed larvae were collected, separated according to species and allowed to molt to nymphs. Twenty-six per cent of I. scapularis (34/131) and 11% of D. variabilis (11/96) tested positive for the agent of HGE by polymerase chain reaction (PCR) after molting to nymphs. Positive I. scapularis nymphs transmitted the agent of HGE to P. leucopus mice in 5 of 5 trials. In contrast, the positive D. variabilis nymphs did not transmit the agent of agent of HGE in any of 6 trials. In a 2nd experiment, 3 P. leucopus mice were infested with I. scapularis nymphs that were infected with the agent of HGE. All 3 mice became infected with the agent of HGE and approximately 300 D. variabilis larvae were placed on each mouse and allowed to feed to repletion. Larvae were collected and allowed to molt to nymphs as before. Approximately 8% (5/60) of the nymphs became positive for the agent of HGE as determined by PCR. Twenty-five of these nymphs were then placed on each of 9 P. leucopus mice and allowed to feed to repletion. Evidence of transmission was not observed in any of 9 mice exposed to D. variabilis nymphs. These results demonstrate that although I. scapularis is a competent vector of the agent of the HGF, D. variabilis is not.