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Appl Environ Microbiol. Dec 2006; 72(12): 7594–7601.
Published online Oct 6, 2006. doi:  10.1128/AEM.01851-06
PMCID: PMC1694262

Longitudinal Analysis of Tick Densities and Borrelia, Anaplasma, and Ehrlichia Infections of Ixodes ricinus Ticks in Different Habitat Areas in The Netherlands[down-pointing small open triangle]

Abstract

From 2000 to 2004, ticks were collected by dragging a blanket in four habitat areas in The Netherlands: dunes, heather, forest, and a city park. Tick densities were calculated, and infection with Borrelia burgdorferi and Anaplasma and Ehrlichia species was investigated by reverse line blot analysis. The lowest tick density was observed in the heather area (1 to 8/100 m2). In the oak forest and city park, the tick densities ranged from 26 to 45/100 m2. The highest tick density was found in the dune area (139 to 551/100 m2). The infection rates varied significantly for the four study areas and years, ranging from 0.8 to 11. 5% for Borrelia spp. and 1 to 16% for Ehrlichia or Anaplasma (Ehrlichia/Anaplasma) spp. Borrelia infection rates were highest in the dunes, followed by the forest, the city park, and heather area. In contrast, Ehrlichia/Anaplasma was found most often in the forest and less often in the city park. The following Borrelia species were found: Borrelia sensu lato strains not identified to the species level (2.5%), B. afzelii (2.5%), B. valaisiana (0.9%), B. burgdorferi sensu stricto (0.13%), and B. garinii (0.13%). For Ehrlichia/Anaplasma species, Ehrlichia and Anaplasma spp. not identified to the species level (2.5%), Anaplasma schotti variant (3.5%), Anaplasma phagocytophilum variant (0.3%), and Ehrlichia canis (0.19%) were found. E. canis is reported for the first time in ticks in The Netherlands in this study. Borrelia lusitaniae, Ehrlichia chaffeensis, and the human granylocytic anaplasmosis agent were not detected. About 1.6% of the ticks were infected with both Borrelia and Ehrlichia/Anaplasma, which was higher than the frequency predicted from the individual infection rates, suggesting hosts with multiple infections or a possible selective advantage of coinfection.

Blood-sucking ticks parasitizing animals and humans are found worldwide. Their involvement in zoonotic disease transmission, transmission of microorganisms (viruses, bacteria, and parasites) from animal reservoirs to humans, is well-known. Over 800 tick species have been described, but only a few of the Ixodes, Rhipicephalus, Dermacentor, Hyalomma, and Haemaphysalis tick species are known to transfer diseases to humans (10, 17). In The Netherlands and in Europe, the most common tick is Ixodes ricinus. I. ricinus ticks may transmit the spirochete Borrelia spp. causing Lyme borreliosis, as well as other diseases (33). Other well-known tick-transmitted pathogenic microorganisms are the intracellular bacteria Anaplasma and Ehrlichia (9), Rickettsia (25), the intracellular eukaryotic protozoan parasites Babesia and Theileria (9, 12) and tick-borne encephalitis virus. Several species or genomospecies of these organisms have been associated with distinct diseases. Borrelia garinii has been associated with neuroborreliosis, Borrelia burgdorferi senso stricto has been associated with arthritis, and Borrelia afzelii has been associated with acrodermatis chronica atropicans (3, 24, 34, 35, 37). Ehrlichia chaffeensis (2) may cause human monocytic anaplasmosis, and the human granylocytic anaplasmosis agent (HGA), which has been found to be Anaplasma phagocytophilum (8), affects neutrophils (5).

Environmental factors, such as climate, vegetation type, and abundance of suitable hosts, limit the geographic distribution of the ticks and the pathogens they may carry. A comparison of the Borrelia species in Europe and the United States shows that there are some clear differences: B. burgdorferi sensu stricto is the sole B. burgdorferi genomospecies in the United States, while in Europe, B. afzelii and B. garinii are the predominant species and B. burgdorferi sensu stricto is found only in a minority of the cases. Borrelia valaisiana (or VS116) and Borrelia lusitaniae (or PotiB2) are two other subspecies that are found in European ticks and may be associated with human disease. In the United States, Ixodes scapularis is the most common disease-transmitting tick, while in Europe, it is I. ricinus (26, 27). Environmental factors, such as climate (changes), (de)forestation, increases in the roe deer population, or introduction of new animal reservoirs, may lead to changing numbers of ticks and dispersal of the tick population and the pathogens they carry. Such changes may lead to a new status quo of the risk of tick bites for human and animal health (16, 23, 31). Monitoring tick distribution and the prevalence of tick-transmitted pathogens is therefore essential to describe and understand the risk of tick-borne disease of the predominant tick species and probably the sole vector for Lyme disease. Earlier studies in The Netherlands have shown that I. ricinus may carry different Borrelia, Anaplasma, and Ehrlichia species and sporadically, some Babesia species (13).

Erythema migrans (EM) is a clear clinical manifestation of Lyme disease and serves as an indicator for transmission of Borrelia sensu lato. EM is found in about 90% of the human cases of Lyme borreliosis (22). A study using questionnaires filled out by a large group of Dutch general practitioners in the period from 1994 to 2001 showed a doubling of the reports of tick-biting incidence and the diagnosis of EM (7). Recently, this study has been repeated and again showed an increase in these incidences for 2006 (11). This suggests that the number of ticks is increasing or that people are coming into contact with ticks more often. Here we report results of tick densities in the period from 2000 to 2004 in four different areas in The Netherlands that are open for recreation: a dune area with rich vegetation near the North Sea (Duin and Kruidberg), a city park near Amsterdam (Bijlmerweide), and two areas in the Koninklijke Houtvesterijen region, an oak forest with blueberries and a heather area. Using PCR and subsequent reverse line blot (RLB) hybridizations, we determined which proportion of the collected ticks was infected with various Borrelia sensu lato species and Ehrlichia or Anaplasma (Ehrlichia/Anaplasma) species. In our RLB assay, we included the species that have been found earlier in our country and some other species found elsewhere in Europe that might have been newly introduced here, such as B. lusitaniae, E. chaffeensis, and Ehrlichia canis (19, 28, 29, 32).

MATERIALS AND METHODS

Origin of the samples.

Ticks were collected by dragging a blanket in four different areas in The Netherlands open to the public: Duin and Kruidberg, a dune area rich in vegetation (2000 to 2004); Bijlmerweide, a city park near Amsterdam (2000 to 2002); and two sites in the Koninklijke Houtvesterijen separated from each other by 200 m, an oak forest rich in blueberries (2000 to 2002) and a heather area (2001 to 2002) (Fig. (Fig.1).1). In the dune area, several species of deciduous trees and shrubs were present, and 60% of the soil was covered with vegetation litter. Ninety percent of the forest area in the Koninklijke Houtvesterijen was covered with blueberries, while the heather area consisted of heather only, with a single pine tree and very little vegetation litter. Many deciduous trees and a few shrubs with a rich secondary vegetation were seen in the city park. Eighty percent of the soil in this park was covered with vegetation litter. Every month from April to October, a maximum of 50 questing ticks were collected from each habitat. The density was calculated by multiplying the number of ticks with the number of dragged m2. After the ticks were collected, they were immersed in 70% ethanol and stored at −20°C. Preparation of DNA extracts from ticks was done as described previously (32). Briefly, the ticks were taken from the 70% ethanol solution, air dried, and boiled for 20 min in 200 μl of 0.7 M ammonium hydroxide. After the vial was allowed to cool, it was left open for 10 min at 80°C to allow the ammonia to evaporate, and the lysate was stored at −20°C until further use.

FIG. 1.
Locations of the four different habitat areas that were studied. The Netherlands is positioned between the North Sea to the north and west, Belgium to the south, and Germany to the east, and the map shows the four areas that were studied: (i) a dune area, ...

PCR amplification.

PCR amplifications and reverse line blotting were performed as described before (30) with some modifications (32). Briefly, PCRs were performed in 50-μl volumes using the HotStarTaq master mix kit (QIAGEN, Westburg, The Netherlands) using the primers (Invitrogen) displayed in Table Table1.1. PCR amplification of Ehrlichia/Anaplasma DNA was done using 80 pmol of each primer and the following program: (i) 15 min at 94°C; (ii) 20 s at 94°C, 30 s at 67°C, and 30 s at 72°C, lowering the annealing temperature by 1°C each cycle until it reaches 55°C; (iii) 20 cycles of 20 s at 94°C, 30 s at 55°C, and 20 s at 72°C; (iv) 20 cycles of 20 s at 94°C, 30 s at 63°C, and 20 s at 72°C; and (v) a final step of 10 min at 72°C. For Borrelia sensu lato, 40 pmol of each primer was used with the following program: (i) 15 min at 94°C; (ii) 20 s at 94°C, 30 s at 70°C, and 30 s at 72°C, lowering the annealing temperature by 1°C each cycle until it reaches 60°C; (iii) 40 cycles of 20 s at 94°C, 30 s at 60°C, and 20 s at 72°C; and (iv) a final step of 10 min at 72°C.

TABLE 1.
Primers and RLB probes used in this studya

Reverse line blot.

The RLB technique has been described before (18, 30, 32), and the probes to detect the different species and subspecies are displayed in Table Table11 (1, 4, 6, 29, 32). Briefly, solutions with 5′-amino-linked oligonucleotide probes ranging from 100 to 1000 pmol (in 0.5 mM NaHCO3, pH 8.4) were coupled covalently to an activated Biodyne C membrane in a line pattern by using a miniblotter (Immunetics, Cambridge, MA). After binding of the oligonucleotide probes, the membrane was taken from the miniblotter, washed in 2× SSPE (1× SSPE is 0.18 M NaCl, 10 mM NaH2PO4, and 1 mM EDTA [pH 7.7]) and 0.1% sodium dodecyl sulfate (SDS) (2× SSPE-0.1% SDS) at 60°C, and again placed in the miniblotter with the oligonucleotide lines perpendicular to the slots. Ten microliters of the biotin-labeled PCR product was diluted in 150 ml of 2× SSPE-0.1% SDS, denatured for 10 min at 99°C, and cooled rapidly on ice. The slots of the miniblotter were filled with the denatured PCR product, and hybridized for 1 h at 42°C. The samples were removed from the slots by aspiration, and then the membrane was removed from the miniblotter and washed twice for 10 min with 2× SSPE-0.1% SDS at 52°C. To visualize hybridization, the membrane was incubated for 30 min at 42°C with streptavidin-peroxidase (Boehringer Mannheim GmbH, Mannheim, Germany) in 2× SSPE-0.5% SDS, washed twice for 10 min with 2× SSPE-0.5% SDS, and then incubated with enhanced chemiluminescence detection liquid (Pharmacia Biotech). Luminescence was recorded using a LAS-300 charge-coupled device camera system from Fuji film (Rotterdam, The Netherlands). To minimize cross contamination and false-positive results, positive and negative controls were included in each batch tested by the PCR and RLB assays, and DNA extraction, PCR mix preparation, sample addition, and PCR analysis were performed in specialized and separate labs.

RESULTS

Tick densities and developmental stage of the ticks.

The highest number of ticks was found in the dune area, and for this area, ticks were collected each year of this study (2000 to 2004). Table Table22 shows for each different area the number of ticks caught each year and month (April to September). The dune area had the highest tick density, followed by the forest and the city park; the heather area had a very low tick density. A comparison of the tick densities in the dune area in five consecutive years showed a slight increase over time. However, the increase is very moderate compared to the large variations between years. The average density of the ticks caught in each area is shown in Table Table2.2. A comparison of tick densities over time shows that the highest tick densities were in the months June, July, and August. Table Table33 shows the developmental stage of the ticks collected in each area. Overall, most ticks were nymphs (55%), followed by larvae (38%) and a small number of adult males and females (both 3%). Notably, relatively large numbers of larvae were found in the heather area, and relatively large numbers of nymphs were found in the forest and dune areas.

TABLE 2.
Yearly and monthly densities of ticks collected in four different habitats in The Netherlands
TABLE 3.
Percentages of the different developmental stages of the ticks collected in four different habitat areas

Prevalence of Borrelia sensu lato and Ehrlichia/Anaplasma in ticks in four Dutch habitats.

PCR and reverse line blot analyses of total DNA extracted from the ticks showed that the ticks from all four areas studied carried Borrelia and Ehrlichia/Anaplasma. Figure 2A and B show the percentage of infected ticks found in the four areas over time, showing that the infection rates per area and per year varied substantially. The overall infection rates determined for all ticks analyzed (all years and areas) was 7.6% for Borrelia and 6.8% for Anaplasma/Ehrlichia. Table Table44 shows the mean percentage of infected ticks that were collected at the different areas. Figure Figure22 and Table Table44 show that the lowest Borrelia sensu lato infection rates were found in the heather area, twofold-higher infection rates were found in the forest area and city park, and the highest rates were found in the dune area. For Ehrlichia/Anaplasma spp. not identified to the species level, the lowest infection rates were found in the city park, about fourfold-higher levels were found in the dune area, and the highest levels were found in the heather and forest areas. The percentage of ticks that was found positive for Borrelia sensu lato (Fig. (Fig.2A)2A) and for Ehrlichia/Anaplasma spp. (Fig. (Fig.2B)2B) clearly decreased in all areas in the year 2001 and increased again the following year. In the dune area, this decrease and increase in Borrelia sensu lato prevalence was seen again in 2003 and 2004 (Fig. (Fig.2A).2A). The Ehrlichia/Anaplasma infection rate for the dune area showed similar dips in 2001 and 2003 and peaks in 2002 and 2004. For the other areas, there was no clear dip in 2001. However, compared to 2001, there was a strong increase in the prevalence in 2002 in the forest and heather areas.

FIG. 2.
Borrelia sensu lato and Ehrlichia/Anaplasma genus prevalences over time. Percentage of ticks carrying (A) Borrelia sensu lato and (B) Ehrlichia/Anaplasma spp. for the four different areas, the dune area (Duin and Kruidberg [DK]), city park (Bijlmerweide ...
TABLE 4.
Comparison of the Borrelia, Anaplasma, and Ehrlichia spp. found in ticks from the four different areas

Identification of Borrelia, Ehrlichia, and Anaplasma spp. using RLB.

The infection rate of the ticks for different Borrelia, Ehrlichia, and Anaplasma species was determined by RLB analysis. The infection rates of ticks from the four areas and ticks from the dune area each year are displayed in Table Table44 and Table Table5,5, respectively. The predominant Borrelia species in all four areas were B. afzelii (overall frequency of 2.5%), B. valaisiana (overall frequency of 0.9%), and Borrelia spp. sensu lato not identified to the species level (overall frequency of 2.5%). Borrelia burgdorferi sensu stricto was detected in ticks from the dune area and city park (Table (Table4).4). Ticks from these two areas also contained a B. afzelii-like species designated Borrelia ruski. B. garinii was found only in the dune and heather areas. In the latter areas, about 1% of the ticks appeared to contain both B. afzelii and B. garinii, showing that double infection with two distinct Borrelia genomospecies does occur. One tick from the dune area carried both B. garinii and B. ruski. B. lusitaniae was not detected in any of the ticks analyzed.

TABLE 5.
Comparison of the Borrelia, Anaplasma, and Ehrlichia spp. found in ticks collected in the dune area in the period from 2000 to 2004

The Borrelia spp. sensu lato not identified to the species level in the ticks from the dune area were found at a more or less constant rate. In contrast, infection with B. afzelii dipped in 2001 and 2003 (Table (Table5).5). B. valaisiana and B. ruski were found only in the Borrelia peak years 2002 and 2004 with large variations in the B. valaisiana prevalence. B. burgdorferi sensu stricto, B. garinii, and the B. garinii/B. afzelii combination were found only sporadically.

The Anaplasma schotti variant was the most frequently identified species in the ticks collected from three of the four areas, but not in the city park. Ehrlichia/Anaplasma species not identified to the species level were found in all areas. Next in prevalence was E. canis found in the dune and forest area. A. phagocytophilum variant, detected by the A-DPhago probe, was present only in the dune area in the year 2004 but at a relatively high prevalence (2.8%). None of the ticks reacted with the HGA agent, E. chaffeensis, and Anaplasma muris T probes. Of all the ticks, five (from the dune and forest area) contained Wolbachia species, an endosymbiont found in many insects which is also amplified by the Ehrlichia/Anaplasma generic PCR and which can clearly distinguished from Anaplasma and Ehrlichia by RLB. The A. schotti and Ehrlichia/Anaplasma variants not identified to the species level were found almost every year in the ticks from the dune area at a relatively constant level, but with a strong dip in prevalence in 2001. E. canis was found only in the high-prevalence years 2002 and 2004.

Borrelia and Ehrlichia/Anaplasma double infections.

Comparison of the rates of Borrelia and Ehrlichia/Anaplasma double infection in the four areas (Table (Table4)4) showed that in the dune area and city park, about 1% of the ticks were doubly infected. For the other areas, this percentage was higher: 2.1% and 3.3% in the heather area and the forest, respectively. The theoretically predicted percentage of double infection can be calculated from the individual Ehrlichia/Anaplasma and Borrelia prevalence rates. Comparison of the actual and predicted percentages showed that in all areas the actual percentage of double infection was higher than expected (Table (Table4).4). The percentage of doubly infected ticks in the dune area was most in agreement with the predicted value; however, it was still two times higher than predicted.

Borrelia, Ehrlichia, and Anaplasma infections in the different developmental stages.

Table Table66 shows the distribution of the development stages in relation to the infection. The lowest rate of infection was found in larvae. For Ehrlichia/Anaplasma infection, the prevalence tended to increase with the development stage. For Borrelia infection, the prevalence in larvae was twice as low as that in nymphs and male adult ticks. Remarkably, female adult ticks had lower levels of Borrelia infection than male ticks did (5.2% versus 8.3%). For the double infections, the prevalence increased from larvae to nymphs and stayed the same in adult males, but in contrast to the single infections, it doubled in females. A comparison of the predicted and determined double infections (Table (Table6)6) showed that particularly in the larval and adult females, the rate of double infection was relatively high.

TABLE 6.
Comparison of the percentages of Borrelia sensu lato and Anaplasma-infected ticks for the different development stages of the ticks

DISCUSSION

We investigated the density and infection rate of ticks in four different areas in The Netherlands in the period from 2000 to 2004 and found that these varied substantially for the different areas and years studied. The tick densities peaked between June and August, and the overall tick densities tended to increase slightly over time. The increase over the 5-year period was most obvious in the dune area, which was also the area with the highest tick density. The increasing trend was less clear in areas with lower tick densities. Very low tick densities were found in the heather area, about 100 times lower than the lowest densities found in the dune area. This shows that the heather area is probably the area with the lowest risk of sustaining tick bites, whereas dune areas pose the greatest threat. Morphological examination showed that all collected ticks belonged to I. ricinus. Overall, most ticks were nymphs (55%), followed by larvae (38%), and only a minority (6%) were adult ticks. However, the distribution of larvae and nymphs varied considerably in the different areas. The highest nymph levels were found in the dune area (67%) and the blueberry-rich oak forest (80%), and the lowest levels were in the city park (46%) and heather area (28%). Conversely, most larvae were found in the city park (49%) and heather area (67%), and fewer were found in the dunes (23%) and blueberry-rich oak forest (16%). The clear difference between the dune and heather areas, with the latter having relatively high levels of larvae and low levels of nymphs, might indicate that ticks in the heather area have difficulty surviving because of the lack of vegetation litter and difficulty in development, which again might be due to the lack of vegetation litter and suitable hosts for a first blood meal. However, we cannot exclude the possibility that the method of tick collection may play a role in the observed fluctuations, because the height of the vegetation may influence the chance of a tick to come into contact with the blanket.

The infection rates in the ticks varied substantially for the four areas and over the 5-year study period; for ticks with Borrelia sensu lato, the infection rate was between 0.8 and 11.5%, and for Ehrlichia/Anaplasma species, it was between 1 and 16%. Comparison with previous studies in The Netherlands (13, 30, 32) that reported values between 5 and 20% showed that in this study ticks carry lower levels of pathogens. This is most probably due to regional differences and different methods of tick collection. In previous studies, the ticks were collected in areas different from ours (indicated in Fig. Fig.1),1), and in two of the studies, ticks were collected from infested roe deer (32) and dogs (13) and not from the vegetation by dragging a blanket as we did in our study. Similar large variations (between 3.5 and 26.7%) have also been reported for questing ticks from different regions in Ireland (20) and elsewhere in Europe with reported Borrelia infection rates between 0 and 42% (15). In the current study, the lowest infection rates of Borrelia sensu lato were found in the heather area, which was also the area with the lowest tick density and with the highest proportion of larvae. The ticks collected from the dune area had the highest Borrelia sensu lato prevalence. The dune area also had the highest tick density. This might suggest a relation between tick density and Borrelia sensu lato infection. One hypothesis is that high levels of ticks will cause more animals to be bitten by multiple ticks. This would increase the probability that the host animals become infected and transmit Borrelia to other ticks. However, such a correlation between tick density and infection rate was not found for Ehrlichia/Anaplasma spp. The levels of Ehrlichia/Anaplasma infection also varied substantially between the different areas, with the lowest infection rates for the city park and rates for the other area more than fourfold higher. The latter might be caused by the lack of large host animals, such as roe deer, which are not present in the wild in the city park area and which are present in the wild in the other three areas.

We found that approximately 1.6% of the ticks were doubly infected, which was more than three times higher than the value predicted from the observed number of single infections. Notably, double-infection levels were highest in the blueberry-rich oak forest and heather area (2 to 3%), which was relatively high compared to the predicted levels, and we also detected these double infections in larvae. The relatively high double-infection rate might indicate the relative abundance of hosts carrying multiple infections and/or interaction of the different infections. Also, these doubly infected ticks might impose an increased risk of becoming infected by a tick from these areas, considering the immunosuppressive nature of Anaplasma and Ehrlichia.

The RLB analysis showed the presence of B. afzelii, Borrelia sensu stricto, B. garinii, B. valaisiana, the B. afzelii-like species B. ruski, and Borrelia sensu lato not identified to the species level in the ticks and several ticks with double B. afzelii/B. ruski and B. garinii/B. afzelii infections. B. lusitaniae, a species reported in Portugal, Switzerland, eastern Europe, and northern Africa (14), was not detected in any of the tick analyzed in this study. A very recent study showed that migratory birds in Switzerland appeared to be the reservoir for B. lusitaniae (21), and to be able to find this species, one should probably test ticks collected from migratory birds or from migratory bird-rich areas. For the Ehrlichia and Anaplasma variants studied, the main species were A. schotti (overall 3.5%) and Ehrlichia/Anaplasma spp. not identified to the species level (overall 2.5%), followed by E. canis and A. phagocytophilum variant. The A. phagocytophilum variant was found only in ticks collected in the year 2004 from the dune area, which was also the area with the highest tick density. E. canis, which may cause a fatal disease in dogs (36), was found in the ticks from both the dune and forest areas, the first time it has been found in ticks in The Netherlands. Although the prevalence of Ehrlichia/Anaplasma infection is lower than the prevalence in ticks collected from roe deer (32), our study also showed that the A. schotti variant and the A. phagocytophilum variant were the most abundant. In none of the ticks analyzed was the HGA agent, the HGA agent variant, A. phagocytophilum (32), E. chaffeensis, or E. muris T detected. However, we cannot exclude the possibility that these species might be present at a very low prevalence below our detection limit, which was 0.1% for the area with the highest tick density.

In conclusion, we have shown that tick densities and Borrelia, Ehrlichia, and Anaplasma infection rates in these ticks vary in different areas and even between areas separated by only 200 m, such as the heather area and forest. Our data show a trend of increasing tick densities over the years and increasing infection rates in the peak years (2000, 2002, and 2004). It is not clear what causes these peak years. It may be due to favorable host animal populations or weather conditions, such as warm winters. However, this was not studied here. The peak years, however, suggest that in particular years, the risk of tick-borne diseases for humans and animals may be higher than in other years. The increasing trend in tick numbers over time is in line with the increase in reports of tick-biting incidence in The Netherlands (11). Comparison of the tick densities and infection rates, particularly of Borrelia infections, suggests that increasing infection levels are associated with high tick densities, especially with nymph densities (compare the dune and heather areas). Given the immunosuppressive nature of Ehrlichia and Anaplasma infections and the relatively high prevalence of doubly infected ticks with these pathogens and Borrelia, these infections may be particularly relevant and should be considered in patients with EM bitten in areas where there is a high percentage of doubly infected ticks. To better understand the symptoms of double infections, they should be studied in model systems and/or patients, and the risk to human health should be taken into account in patients with Lyme borreliosis bitten again by infected ticks.

Acknowledgments

This study was financially supported by the Ministry of Agriculture, Nature Reserve and Food Quality (LNV) and the Dutch Food and Consumer Product Safety Authority (VWA).

Footnotes

[down-pointing small open triangle]Published ahead of print on 6 October 2006.

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