Differential distribution of immature Ixodes dammini (Acari: Ixodidae) on rodent hosts

Ectoparasites such as ixodid ticks that remain attached to hosts for several days while feeding on blood are able to overcome the inflammatory and immune responses of some hosts and not others. The immature stages of the deer tick Ixodes dammini are found more frequently on the white-footed mouse, Peromyscus leucopus, than on other rodents. We propose that P. leucopus is more tolerant to I. dammini than is a less common host, the meadow vole, Microtus pennsylvanicus. To test this hypothesis, the distribution patterns and engorgement indices were determined for larval and nymphal I. dammini collected from wild-caught P. leucopus and M. pennsylvanicus. There were more immature ticks, which were more fully engorged, on P. leucopus than on M. pennsylvanicus. There were more and better engorged ticks on male than on female hosts. Laboratory studies on the number and weights of larval I. dammini collected off naive and previously exposed P. leucopus and M. pennsylvanicus support the results of the field study. Fewer larval ticks were recovered from previously exposed M. pennsylvanicus than P. leucopus, and the ticks weighed less. Larval and nymphal ticks aggregated among hosts in the study grid, and higher densities per male P. leucopus were correlated with higher engorgement indices, suggesting that immature I. dammini feed better at higher densities. The feeding success of I. dammini on its preferred host species might be due to its adaptation to the immune and inflammatory reactions of the host.     

Variation in the density of questing Ixodes pacificus (Acari:Ixodidae) nymphs infected with Borrelia burgdorferi at different spatial scales in California

The density of, and prevalence of infection with Borrelia burgdorferi in, Ixodes pacificus nymphs as well as the density of infected nymphs were compared at 12 properties at a small rural community at high risk for Lyme disease (CHR) and at 12 areas at the University of California Hopland Research and Extension Center (HREC), Mendocino County, California. The mean infection prevalence and density of infected nymphs were 1.7% (range, 0-4.2%) and 0.10 infected nymphs per 100 m2 (range, 0-0.23 per 100 m2) at the HREC, and 12.4% (range, 3.9-41.3%) and 1.83 infected nymphs per 100 m2 (range, 0.29-22.17 per 100 m2) at the CHR. Thus, the mean density of infected nymphs differed 18-fold between CHR and HREC and 76-fold between properties at the CHR. Also, there was up to 10-fold variation in infection prevalence and 16-fold variation in density of infected nymphs between discrete areas within properties at the CHR. The high densities of infected nymphs recorded at the CHR suggest that, despite the low statewide incidence of Lyme disease, the medical community should be alerted that Lyme disease can be highly endemic in rural areas of northwestern California. The prevalence of spirochetal infection was higher for nymphs collected in southern/western, as compared to northern/eastern, exposures at both HREC and CHR. Infection prevalence and nymphal density were negatively associated at the HREC, whereas they tended to be associated positively at the CHR. A positive association was observed between nymphal density and density of infected nymphs when data from CHR and HREC were combined, and when data from the CHR were considered alone, but not for data from the HREC alone.     

Prevalence and abundance of Ixodes pacificus immatures (Acari: Ixodidae) infesting western fence lizards (Sceloporus occidentalis) in northern California: temporal trends and environmental correlates.

The prevalence and abundance of immature Ixodes pacificus ticks on western fence lizards (Sceloporus occidentalis) were examined in relation to time of year, host attributes (i.e., age, gender, and presence or absence of blood parasites), and 5 environmental characteristics, including topographic exposure and ground cover substrate, over a 2-year period in northern California. Lizards were infested with subadult ticks from early March until late July or early August, with peak median numbers of larvae and nymphs recorded in late April and early May of both years. Peak larval and nymphal abundances differed between years. The overall ratio of larvae to nymphs on adult male lizards was low, ranging from 0.80 in 1999 to 2.41 in 2000. Such intensive feeding of nymphs versus larvae on these lizards, which are reservoir-incompetent for Borrelia burgdorferi spirochetes, may explain previous observations of decreasing spirochetal infection prevalence from the nymphal to adult stage in northwestern California. Adult male lizards were more likely to be infested with nymphs and harbored greater abundances of larvae and nymphs than adult females. Lizards uninfected with blood parasites had more nymphs than infected lizards. The measured environmental characteristics could explain only a small percentage of the total variation observed in larval prevalence (22%) and in larval and nymphal abundance (12 and 3%, respectively).     

Survival rates of immature Ixodes pacificus (Acari: Ixodidae) ticks estimated using field‐placed enclosures

Granulocytic anaplasmosis (GA) and Lyme borreliosis are emerging tick‐borne diseases caused by infection with Anaplasma phagocytophilum and Borrelia burgdorferi, respectively, and maintained in rodent‐Ixodes spp. tick cycles, including I. pacificus in the western U.S. Ixodes pacificus has a multiple‐year life cycle and B. burgdorferi and A. phagocytophilum are transstadially, but not transovarially, transmitted within ticks, thus ticks function importantly in maintaining infection in nature. In this study, the survival of larval and nymphal I. pacificus was determined using ticks placed in tubes in leaf litter from June 2005 to September 2006 at two field sites in the California northern coast range mountains and a laboratory control. In all three sites, nymphal and larval survival ranged from 90–400 d, with differences in mean survival among sites. Fewer ticks died in the autumn in the moister field sites compared with the drier incubator control treatment. The first large die‐off event in late autumn occurred at all sites shortly before relative humidity increased from 80–100% and temperature declined from approximately 22–15° C. The concurrent die‐off in the incubator population, subject to relative humidity and temperature regimes that were invariant, suggests that survival time was dependent on other factors in addition to environmental conditions. These results suggested that many ticks exhausted resources or tolerance for relatively low humidity within six months of questing, and that higher humidity prolonged survival. Based on observed longevity, humans and other animals could acquire A. phagocytophilum infection from adult I. pacificus that were infected up to three years earlier.     

Host-seeking behavior of Ixodes pacificus (Acari: Ixodidae) nymphs in relation to environmental parameters in dense-woodland and woodland-grass habitats

In the far-western United States, the bacteria that cause Lyme disease (Borrelia burgdorferi, Bb) and human granulocytic anaplasmosis (Anaplasma phagocytophilum, Ap) are transmitted by the western black-legged tick (Ixodes pacificus). In a dense woodland, human behaviors involving contact with wood were recently found to pose greater risk for encountering I. pacificus nymphs than behaviors entailing exclusive exposure to leaf litter. A four-year follow-up study was undertaken in the same woodland and, as a comparison area in one year only, in a nearby woodland-grass habitat to explore the biotic and abiotic factors that might elevate human exposure to host-seeking nymphs. Nymphs were active in the dense woodland throughout the daytime, but no consistent pattern of activity was observed with respect to time of day, temperature, or relative humidity. Significantly more nymphs were collected from the southern aspects of dense-woodland trunks than from other aspects, and more nymphs quested at a height of < or = 1 m vs 1-2 m aboveground. The prevalence of bacterial infection in ticks from the dense woodland was highly variable among years, with maxima of 22.6% and 42.9% for Bb, and 15.6% and 1.8% for Ap, in nymphs from logs and trunks, respectively. The mean densities of nymphs, and of Bb- or Ap-infected nymphs, were typically higher on logs and trunks than in adjoining leaf litter or grass in both habitats. The acarologic risk of encountering an infected nymph on dense-woodland logs or trunks was 2.8 to 11 times higher for Bb than for Ap in two of three years, and it was usually higher in dense woodland than in woodland-grass for both agents. Coinfections were rare (0.27%, n = 369 nymphs from both habitats). Individuals having prolonged contact with logs or trunks in spring would be well advised to employ personal protective measures to minimize exposure to I. pacificus nymphs and their attendant bacterial zoonotic agents.     

Optimal sampling and spatial distribution of Ixodes pacificus, Dermacentor occidentalis and Dermacentor variabilis ticks (Acari: Ixodidae)

A common method for sampling tick populations is flagging, which is a method of dragging a white cloth over a plant substrate for a fixed distance along a transect. Flagging over rough physical surfaces or using long subtransect lengths could lead to the underestimation of tick densities. Using estimates of the drop-off rates of adult Ixodes pacificus in flag sampling, optimal sampling schemes (the length and number of subtransects) were examined using the relationships between the tick drop-off rates (c), the tick density and distribution patterns and the roughness of the sampled plant substrate. It was found that the optimal number of subtransects and Lmax, the longest subtransect length which did not significantly underestimate the tick density from c, were affected by the tick density, substrate roughness and tick distribution pattern. This study also showed that the density and distribution of I. pacificus and Dermacentor occidentalis varied greatly over time in the populations sampled, while the Dermacentor variabilis densities were low and showed no significant changes over time. Both I. pacificus and D. occidentalis had clumped distributions along trails and these clumps were aggregated. However, the clump sizes (or individual clump areas) changed significantly over time because of density fluctuation or the movement of ticks. Finally, a positive association between the number of I. pacificus and D. occidentalis adults was observed from 2 m subtransect flag collections in March 1995; no relationship was found in 1994. © Rapid Science Ltd. 1998     

Questing behaviour of Ixodes ricinus ticks (Acari: Ixodidae)

The vertical distribution in the vegetation of questing Ixodes ricinus ticks was investigated in two different vegetation types (high and low vegetation) at two localities in south-central Sweden during 1992-1993 (Toro) and 1995 (Bogesund). Significant correlations were found between the vertical distribution of immature ticks and the height of the vegetation. The greatest mean availabilities of the larvae and nymphs in low vegetation were in the intervals 0-9 and 30-39 cm, respectively. The larval numbers were greatest close to the ground (0-29 cm) in both high and low vegetation. The larval : nymphal ratio, at ground level at localities free of ground vegetation, varied between 8 : 1 and 32 : 1. In high vegetation, the greatest mean numbers of nymphal and adult ticks were at height intervals of 50-59 and 60-79 cm, respectively. These ranges are within the estimated height interval (40-100 cm) of the main part of the body surface of their preferred host, the roe deer (Capreolus capreolus). The presence of most questing I. ricinus larvae at ground level would favour the transmission of Borrelia burgdorferi s.l., since this is where the highly reservoir-competent rodents and shrews usually occur.     

Horizontal and vertical movements of host‐seeking Ixodes pacificus (Acari: Ixodidae) nymphs in a hardwood forest

The nymph of the western black‐legged tick (Ixodes pacificus) is an important bridging vector of the Lyme disease spirochete (Borrelia burgdorferi) to humans in the far‐western United States. The previously unknown dispersal capabilities of this life stage were studied in relation to logs, tree trunks, and adjacent leaf‐litter areas in a mixed hardwood forest using mark‐release‐recapture methods. In two spatially and temporally well‐spaced trials involving logs, the estimated mean distances that nymphs dispersed ranged from ≈0.04 to 0.20 m/day on logs vs 0.11 to 0.72 m/day in litter. Prior to recapture in either trial and within the confines of the sampling grids, the greatest estimated dispersal distances by individual nymphs released on logs, and in litter 0.5 m or 1.5 m from logs, were 2.4, 3.0, and 3.0 m, respectively. Nymphs released on logs or litter tended to remain within the same biotopes in which they were freed while host‐seeking. In two simultaneous trials involving trunks spaced close‐at‐hand, nymphs released at the trunk/litter interface on all four aspects collectively dispersed a mean of 0.353 m/day on trunks vs 0.175 m/day in litter. In either trial, the greatest distances that recaptured nymphs climbed trunks, or dispersed in litter in an encircling 3‐m grid, were 1.55 m and 2.97 m, respectively. Nymphs ascending trunks did not exhibit a preference for any one aspect, and the B. burgdorferi‐infection prevalences in nymphs that climbed trunks (3.2–4.0%) did not differ significantly from those that moved horizontally into litter (10.5–17.6%). We conclude that I. pacificus nymphs use an ambush host‐seeking strategy; that they disperse slowly in all biotopes studied; that they usually continue to host‐seek in or on whatever substratum they access initially; and that B. burgdorferi‐infected nymphs are as likely to move horizontally as vertically when offered a choice.     

The midgut hemolysin of Ixodes dammini (Acari:Ixodidae)

Midgut homogenates of the hard tick, Ixodes dammini, lyse erythrocytes from rabbits, rats, hamsters, and guinea pigs. The activity displays sigmoidal kinetics, has an alkaline pH optimum, and is activated by temperature. Hemolytic activity is lost when homogenates are incubated with trypsin or heated for 1 hr at 60 C. Activity is not detectable in nonfed ticks as well as ticks attached for up to 2 days to a host, but increases during the growth phase of feeding. Such activity is postulated to help the initial process of the blood meal digestion by releasing the contents of erythrocytes for further enzymatic hydrolysis.     

Sigmodontinae rodents as hosts for larvae and nymphs of Ixodes loricatus Neumann, 1899 (Acari: Ixodidae)

Larvae and nymphs of Ixodes loricatus Neumann, 1899 ticks (confirmed by morphological characters and by comparison of 16S mitochondrial rDNA sequences) were collected from Sigmodontinae Wagner, rodents in central and northern Argentina and Uruguay. A total of 100 larvae and 38 nymphs of I. loricatus were collected on the genera Akodon Meyen (n = 36 individuals), Calomys Waterhouse (n = 2), Oligoryzomys Bang in = 12), Oxymycterus Waterhouse (n = 9), and Scapteromys Waterhouse (n = 13). 72 larvae and 18 nymphs were collected on Akodon. Adults of I. loricatus were found in central Argentina and Uruguay on Didelphimorphia of the genera Didelphis Linnaeus and Lutreolina Thomas. Ixodes loricatus has been considered a species with strict total specificity to Didelphimorphia. Our results show that this statement may not be justified. Sigmodontinae rodents are sympatric and share habitats with the phylogenetically distant Didelphimorphia; infestation with I. loricatus seems to be its consequence. We tentatively consider I. loricatus moderately specific to Didelphimorphia.     

Reproductive strategies of the seabird tick Ixodes uriae (Acari: Ixodidae)

Ixodes uriae is a common ectoparasite of colonial seabirds in the circumpolar areas of both hemispheres. Despite its potential effects on host population dynamics and its role as a vector of pathogens, little is known about the reproductive strategies of this tick under natural conditions. Multiple mating of engorged females has been recorded in several instances, but the resulting paternity of offspring and its potential evolutionary significance have never been evaluated. Here, the paternity of offspring produced by females collected in the field was determined by using 5 polymorphic microsatellite markers. The results indicate that multiple mating in I. uriae can lead to multiple paternity in broods and, given the life history of this parasite, may be related to the adaptive benefits of producing genetically diverse offspring. Copulations took place both before and after the female's blood meal, but most successful fertilizations seemed to occur before engorgement. This suggests that the mating strategies of this tick may have evolved in response to local environmental constraints and, in particular, to the availability of conspecific ticks.     

Treatment of clothing with a permethrin spray for personal protection against the western black-legged tick,Ixodes pacificus (Acari: Ixodidae)

The synthetic pyrethroid, permethrin, when applied to clothing with a pressurized spray at an application rate estimated previously to be 4 g a.i./cm², was found to be 100% effective for personal protection against all three parasitic stages of the western black-legged tick,Ixodes pacificus Cooley and Kohls. This tick has been implicated as the primary vector of the Lyme disease spirochete (Borrelia burgdorferi) to humans in the far-western United States. Periods of exposure to permethrin-treated cloth as brief as 10 and 45 seconds incapacitated 100% of subadult and adult ticks, respectively, within 1–3 h post-treatment. Under field conditions, permethrin appareatly does not repel questing adult ticks, though 100% of ticks recovered from treated clothing after exposures as brief as 15 seconds were moribund 1 h later.     

Seasonal and geographical distribution of adult Ixodes scapularis say (Acari: Ixodidae) in Louisiana.

The distribution and seasonality of adult black-legged ticks (Ixodes scapularis Say) in Louisiana was measured. The presence of adult ticks was determined by flagging at 106 sites throughout Louisiana. It was concluded that Ixodes scapularis is widely distributed throughout Louisiana. Ticks were also collected twice per month at one site over a 15-month period by flagging and use of CO2 traps to establish the relative seasonal abundance pattern of free-living adult ticks. Host-seeking, black-legged adult ticks were collected from October to May. Peak adult abundance was observed in December. More ticks were collected by the use of CO2 traps compared to flagging in October, November, and February. No black-legged tick larvae or nymphs were collected in this study using either collection method.     

Field and laboratory studies on the timing of oviposition and hatching of the western black-legged tick, Ixodes pacificus (Acari: Ixodidae)

The timing of oviposition and hatching of Ixodes pacificus was investigated in the field and at constant temperatures in the laboratory. Replete females held at temperatures between 9 and 29°C began depositing eggs a mean of 9–70 days after drop off. Egg masses held between 12 and 25°C commenced hatching 25–178 days after the onset of oviposition. Eggs held at 9 or 29°C did not hatch. The lower temperature thresholds for development (LTD) for oviposition and hatching were 6.5 and 9°C, respectively. The number of degree days required for oviposition and hatching was 173 and 588, respectively. Replete females placed in the field on 2 December through to 8 March deposited eggs from 2 February through to 24 April; the eggs commenced hatching between 2 July and 21 August. Unfed larvae from two of 20 egg masses survived through the winter and fed readily when exposed to deer mice (Peromyscus maniculatus) on 22 April. Replete larvae were returned to the field and moulted between 9 and 21 August. Larvae exposed to deer mice in August, 4 weeks after hatching, also fed readily. Although further studies are needed to clarify the timing of nymphal development, the present study suggests that I. pacificus requires more than 1 year to complete its life cycle.     

A new species of Ixodes (Acari: Ixodidae) from South African mammals

A new tick species belonging to the African subgenus Afrixodes Morel, 1966, namely, Ixodes ( Afrixodes ) fynbosensis n. sp., is described. The female of I. fynbosensis is easily differentiated from the other African Ixodes species by a large, tapering triangular ventrolateral spur on palpal segment I. Nymph and larva of I. fynbosensis can be distinguished from those of other members of Afrixodes by a combination of the following characters: pointed hypostome, long auriculae, long and acute ventrolateral projections of basis capituli of nymph, only 2 pairs of central dorsal setae, and a straight posterior margin of scutum of the larva. Cytochrome oxidase I mtDNA sequence comparisons between I. fynbosensis and 10 other Ixodes Latreille, 1795, species support the recognition of this taxon as genetically distinct (>13% corrected sequence divergences separate it from the remainder of the 10 recognized species used in this study), and preliminary phylogenetic analyses reveal that this taxon is most closely related to the southern African Ixodes pilosus Koch, 1844, and Ixodes rubicundus Neumann, 1904. Ixodes fynbosensis is known only from South Africa, where females have been collected from a domestic dog and a rodent, Rhabdomys pumilio (Sparrman), and nymphs and larvae have been collected from R. pumilio and unidentified shrews belonging to the Soricidae. Sequences generated for both nymphs and adult individuals were identical, confirming the correlation between the described life stages.     

Development of the salivary glands in embryos of Ixodes ricinus (Acari: Ixodidae)

We examined Ixodes ricinus embryos between 18 and 28 days of development with light, scanning and transmission electron microscopy. The differences in inner structure attested to establish three successive developmental stages: days 18–20, day 23, and days 26–28. Between 18 and 20 days the embryos are at early stages of organogenesis. Salivary glands cannot be identified at that stage. In 23-day-old embryos salivary glands are already outlined but the structure of alveoles is still different from that in larvae in which the embryonic development has been completed. Gland cells start to form alveoles and become active between 26 and 28 days of the development. This revised version was published online in July 2006 with corrections to the Cover Date.     

Development of the salivary glands in embryos of Ixodes ricinus (Acari: Ixodidae)

We examined Ixodes ricinus embryos between 18 and 28 days of development with light, scanning and transmission electron microscopy. The differences in inner structure attested to establish three successive developmental stages: days 18-20, day 23, and days 26-28. Between 18 and 20 days the embryos are at early stages of organogenesis. Salivary glands cannot be identified at that stage. In 23-day-old embryos salivary glands are already outlined but the structure of alveoles is still different from that in larvae in which the embryonic development has been completed. Gland cells start to form alveoles and become active between 26 and 28 days of the development.