A reanalysis of cancer mortality in Canadian nuclear workers (1956-1994) based on revised exposure and cohort data.

BACKGROUND: A 15-country study of nuclear workers reported significantly increased radiation-related risks of all cancers excluding leukaemia, with Canadian data a major factor behind the pooled results. We analysed mortality (1956-1994) in the updated Canadian cohort and provided revised risk estimates. METHODS: Employment records were searched to verify and revise exposure data and to restore missing socioeconomic status. Excess relative risks per sievert (ERR/Sv) of recorded radiation dose and 95% confidence intervals (CIs) were estimated using Poisson regression. RESULTS: A significant heterogeneity of the dose-response for solid cancer was identified (P=0.02), with 3088 early (1956-1964) Atomic Energy of Canada Limited (AECL) workers having a significant increase (ERR/Sv=7.87, 95% CI: 1.88, 19.5), and no evidence of radiation risk for 42,228 workers employed by three nuclear power plant companies and post-1964 AECL (ERR/Sv=-1.20, 95% CI: <-1.47, 2.39). Radiation risks of leukaemia were negative in early AECL workers and non-significantly increased in other workers. In analyses with separate terms for tritium and gamma doses, there was no evidence of increased risk from tritium exposure. All workers had mortality lower than the general population. CONCLUSION: Significantly increased risks for early AECL workers are most likely due to incomplete transfer of AECL dose records to the National Dose Registry. Analyses of the remainder of the Canadian nuclear workers (93.2%) provided no evidence of increased risk, but the risk estimate was compatible with estimates that form the basis of radiation protection standards. Study findings suggest that the revised Canadian cohort, with the exclusion of early AECL workers, would likely have an important effect on the 15-country pooled risk estimate of radiation-related risks of all cancer excluding leukaemia by substantially reducing the size of the point estimate and its significance.

Lyme disease in California: a novel enzootic transmission cycle of Borrelia burgdorferi.

Knowledge of zoonotic transmission cycles is essential for the development of effective strategies for disease prevention. The enzootiology of Lyme disease in California differs fundamentally from that reported from the eastern United States. Woodrats, not mice, serve as reservoir hosts, and Ixodes neotomae, a nonhuman-biting tick, maintains the agent of Lyme disease, Borrelia burgdorferi, in enzootic cycles. The western black-legged tick, Ixodes pacificus, is the primary vector to humans, but it appears to be an inefficient maintenance vector. Isolates of B. burgdorferi from California exhibit considerable antigenic heterogeneity, and some isolates differ strikingly from isolates recovered from this and other geographic regions.

Isolation of a spirochete from the soft tick, Ornithodoros coriaceus: a possible agent of epizootic bovine abortion.

A Borrelia-like spirochete was detected in all three parasitic stages of Ornithodoros coriaceus, the soft tick implicated in the epizoology of epizootic bovine abortion. After the spirochete had been isolated, its distinctness from other North American tick-borne borreliae as well as from Spirochaeta aurantia, Treponema pallidum, and Leptospira interrogans serovar pomona was established on the basis of its morphology, protein components, and inability to infect mice. The spirochete is passed trans-stadially and via eggs by ticks, and it is also excreted in coxal fluid after ticks have fed and detached. Circumstantial evidence suggests that the spirochete may be causally related to epizootic bovine abortion.

Effect of prescribed fire for tick control in California chaparral.

Prescribed fire was investigated as a method for controlling ixodid and argasid ticks in chaparral habitats in northern California. Two experimental and two adjacent control plots within a wildlife preserve were monitored for 1 yr postburn. Ticks were collected by flagging vegetation, by CO2-baited pitfall trap, and by live-trapping rodents. Twice as many rodents were caught at control sites compared with burn sites and no dusky-footed woodrats, Neotoma fuscipes Baird, were found in the treatment sites postburn. This species is known to be a reservoir of the agents of Lyme disease, Borrelia burgdorferi sensu stricto Johnson, Schmid, Hyde, Steigerwalt & Brenner, and human granulocytic anaplasmosis, Anaplasma phagocytophilum Dumler, Barbet, Bekker, Dasch, Palmer, Ray, Rikihisa, Rurangirwa. Six ixodid tick species were removed from rodents (Ixodes pacificus Cooley & Kohls, Ixodes jellisoni Cooley & Kohls, Ixodes spinipalpis Hadwen & Nuttall, Ixodes woodi Bishopp, Dermacentor occidentalis Marx, and Dermacentor parumapertus Neumann), two of which transmit bacterial zoonotic agents to people in the far-western United States. There was no decrease in number of ticks per animal trapped at either burn site compared with controls; in fact, the mean number of immature I. pacificus per rodent was significantly higher at one burn site than its control site. Soil refugia may protect ticks from fire-induced mortality; the argasid tick Ornithodoros coriaceus Koch, which lives in soil, was unaffected by the prescribed fire as were I. pacificus and D. occidentalis buried in packets 2.5 cm below ground. We conclude that although prescribed fires in chaparral habitats may diminish local rodent abundance, it does not decrease tick loads on rodents. Furthermore, burning chaparral does not result in a decreased abundance of adult ixodid ticks on vegetation and apparently does not affect argasid or ixodid ticks that are sheltered within soil refugia.

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.

Sylvatic maintenance of Borrelia burgdorferi (Spirochaetales) in Northern California: untangling the web of transmission.

Lyme borreliosis is associated with several genospecies of Borrelia burgdorferi sensu lato (s.l.) (Spirochaetales), but human disease has been associated only with Borrelia burgdorferi sensu stricto (s.s.) Johnson, Schmid, Hyde, Steigerwalt & Brenner in the western United States. Restriction fragment length polymorphism (RFLP) analysis of rrf-rrl amplicons from 124 tick and mammalian isolates from various habitats yielded 13 RFLP patterns. Of these patterns, six were patterns previously associated either with Borrelia bissettii Postic, Marti Ras, Lane, Hendson & Baranton or Borrelia burgdorferi s.s., and the remaining seven patterns belonged to diverse and previously uncharacterized Borrelia spp. Uncharacterized Borrelia spp. were cultured most frequently from Ixodes spinipalpis Hadwen & Nuttall and California kangaroo rats, Dipodomys californicus Merriam, inhabiting grasslands, and B. bissettii from I. spinipalpis and dusky-footed woodrats, Neotoma fuscipes Baird, associated with oak woodlands or chaparral. B. burgdorferi s.s. typically was isolated from host-seeking Ixodes pacificus Cooley & Kohls collected in dense oak woodlands, woodland-grass, or redwood forests. Although some isolates of B. burgdorferi s.s. were cultured from woodrats, there was no clear association of this human pathogen with any vertebrate host. These findings, along with recent evidence indicating that the western gray squirrel, Sciurus griseus Ord, may be an important reservoir of B. burgdorferi s.s. in Californian oak woodlands, suggest that our earlier hypothesis implicating an enzootic cycle involving woodrats and I. spinipalpis is insufficient to account for observed patterns of infection in nature.

Geographical distribution patterns and habitat suitability models for presence of host-seeking ixodid ticks in dense woodlands of Mendocino County, California.

We used drag sampling to examine the geographical distribution patterns of ixodid ticks engaging in open (non-nidicolous) host-seeking behavior in dense woodland habitats of the climatically and ecologically diverse Mendocino County in north coastal California. The findings based on this sampling methodology reflect risk of human exposure to host-seeking ticks rather than the true distribution of the ticks. Drag sampling in 78 sites yielded 7,860 nymphal or adult Ixodes pacificus Cooley & Kohls, 220 Dermacentor occidentalis Marx, 150 Ixodes spinipalpis Hadwen & Nuttall, 15 Hemaphysalis leporispalustris (Packard), 12 Ixodes angustus Neumann, 12 Ixodes auritulus Neumann, and a single Dermacentor variabilis (Say). I. pacificus, which is the primary vector of the Lyme disease spirochete Borrelia burgdorferi to humans in California, occurred in all 78 sites examined. D. occidentalis, another tick species commonly biting humans in California, and H. leporispalustris typically were encountered in oak-associated woodlands in the central or eastern parts of the county. In contrast, three species of Ixodes ticks (I. angustus, I. auritulus, and I. spinipalpis) most commonly were found questing openly in woodlands with redwood present in the western part of the county. I. angustus and I. spinipalpis are occasional human biters and known experimental vectors of B. burgdorferi. Our study represents the first collection of large numbers of openly host-seeking I. spinipalpis ticks. Univariate tests of associations between presence of ticks (D. occidentalis, H. leporispalustris, I. angustus, I. auritulus, or I. spinipalpis) and environmental geographical information systems-remote sensing (GIS/ RS)-based data indicated that elevation, number of growing degree-days, and tasseled cap brightness, greenness, and wetness are especially useful predictors of presence of openly hostseeking ticks. Combinations of the above-mentioned GIS/RS-based data yielded significant logistic regression models for habitat suitability of host-seeking ticks for all five above-mentioned species. The model equations were used to create spatial surfaces of predicted presence of suitable habitat for openly host-seeking ticks in Mendocino County dense woodlands.

Refractoriness of the western fence lizard (Sceloporus occidentalis) to the Lyme disease group spirochete Borrelia bissettii.

The western fence lizard, Sceloporus occidentalis, is refractory to experimental infection with Borrelia burgdorferi sensu stricto, one of several Lyme disease spirochetes pathogenic for humans. Another member of the Lyme disease spirochete complex, Borrelia bissettii, is distributed widely throughout North America and a similar, if not identical, spirochete has been implicated as a human pathogen in southern Europe. To determine the susceptibility of S. occidentalis to B. bissettii, 6 naïve lizards were exposed to the feeding activities of Ixodes pacificus nymphs experimentally infected with this spirochete. None of the lizards developed spirochetemias detectable by polymerase chain reaction for up to 8 wk post-tick feeding, infected nymphs apparently lost their B. bissettii infections within 1-2 wk after engorgement, and xenodiagnostic L. pacificus larvae that co-fed alongside infected nymphs did not acquire and maintain spirochetes. In contrast, 3 of 4 naïve deer mice (Peromyscus maniculatus) exposed similarly to feeding by 1 or more B. bissettii-infected nymphs developed patent infections within 4 wk. These and previous findings suggest that the complement system of S. occidentalis typically destroys B. burgdorferi sensu lato spirochetes present in tissues of attached and feeding I. pacificus nymphs, thereby potentially reducing the probability of transmission of these bacteria to humans or other animals by the resultant adult ticks.

Sulfhydryl groups in relation to the structure and catalytic activity of 2-oxo-4-hydroxyglutarate aldolase from bovine liver.

Bovine liver 2-oxo-4-hydroxyglutarate aldolase (suggested name: 2-oxo-4-hydroxyglutarate glyoxylate-lyase catalyzing the reaction: 2-oxo-4-hydroxyglutarate in equilibrium pyruvate + glyoxylate) contains eight to ten sulfhydryl groups as determined by titration of the enzyme with either 5,5'-dithiobis(2-nitrobenzoic acid) (Nbs2) or p-mercuribenzoate in the presence of 1% sodium dodecyl sulfate. In the absence of a denaturant, all of the cysteinyl residues react with p-mercuribenzoate whereas only four are accessible to titration with Nbs2. No differences in -SH group reactivity can be detected during titration of the aldolase with p-mercuribenzoate. In contrast, two classes of sulfhydryls can be differentiated in the disulfide exchange reaction with Nbs2 in the absence of a denaturant; one -SH group (Class I) reacts rapidly whereas three additional thiols (Class II) titrate at approx. 0.1 the rate of the Class I-SH residue. Both pyruvate and glyoxylate protect one of the three -SH residues in Class II from reaction with Nbs2. Either substrate also prevents titration of one to two thiol groups by p-mercuribenzoate and decreases the rate of reaction of aldolase -SH groups with Nbs2 in 8 M urea. These ligand-induced changes in -SH reactivity provide a sensitive indication that the enzyme exists in an altered conformational state in the presence of either of its cosubstrates. Titration of the enzyme with either Nbs2 or p-mercuribenzoate results in a progressive loss of aldolase activity which is not proportional to the number of -SH groups modified. The enzyme retains 50% of the activity of the native enzyme when Class I and Class II thiols (i.e. four -SH groups total) are modified with Nbs2; 15% residual activity is still observed following titration of all of the cysteinyl residues with p-mercuribenzoate. Pyruvate and glyoxylate provide partial protection against inactivation. It is concluded that inactivation of 2-oxo-4-hydroxyglutarate aldolase by Nbs2 or p-mercuribenzoate is a consequence of alterations in protein structure which accompany modification of -SH groups. The data argue against the direct participation of an active-site thiol group in the catalytic mechanism of 2-oxo-4-hydroxyglutarate aldolase, be that aldol cleavage and condensation or beta-decarboxylation.

Production and therapeutic use of a factor XI concentrate from plasma.

Factor XI deficiency is an uncommon bleeding disorder usually manifested by excessive bleeding after surgery or trauma. Until recently the only effective therapy has been fresh-frozen plasma (FFP) infusion. We describe the efficacy and safety of a new factor XI concentrate produced from human donor plasma by a modification of the method used for antithrombin III concentrate. The mean recovery of factor XI in the circulation measured on 62 occasions was approximately 91% of the injected dose, and the mean half-disappearance-time was 52 h. The concentrate was used for 31 invasive procedures in 30 patients, including 16 patients who had a definite bleeding tendency on previous occasions, with normal haemostasis being achieved in all but 1. Only 1 patient (previously experiencing allergy to FFP) experienced adverse effects during infusion. Monitoring of liver function tests and viral antibody status in suitable patients has shown no evidence of transmission of hepatitis viruses, HIV-1 or parvovirus B19. We conclude that this concentrate provides effective treatment for patients with factor XI deficiency. Preliminary results suggest safety from virus transmission, but this needs to be established in further studies of previously untreated patients.

Mechanized blood grouping: a hospital trial using an 8-channel grouping machine.

ABO and Rh (D) groups of 6403 blood samples were assessed on an 8/9-channel autoanalyser in the Serology Department of the London Hospital; the results were independently checked at the Regional Blood Transfusion Centre, Brentwood, using the routine methods for grouping donor blood. Results of this comparative study are given and instances are described in which anomalous results or incorrect groupings occurred; the possible causes are discussed. The 8/9-channel automated blood group analyser is evaluated in terms of routine hospital laboratory practice.

Lyme borreliosis in California. Acarological, clinical, and epidemiological studies.

The relative abundance of, and spirochetal-infection rates in, adult ixodid ticks from eight Lyme borreliosis clinical-case areas and two comparison areas were investigated in northern California from late fall to early spring, 1984-87. The western black-legged tick (Ixodes pacificus) was the most abundant species at seven of nine sites yielding specimens as determined with a tick drag method. The Pacific Coast tick (Dermacentor occidentalis) was the most abundant species at two sites, and lesser numbers of this tick and the American dog tick (D. variabilis) were obtained from seven and two sites, respectively. Abundance of I. pacificus adults varied in clinical-case areas as well as in the comparison areas, and was not correlated significantly with spirochetal infection rates in this tick. Overall, spirochetes were detected in 1.4 and 1.0% of the adult I. pacificus collected from clinical case (n = 857) and comparison sites (n = 383), respectively, and in 0.8% of adult D. occidentalis (n = 253) from one comparison area. An additional 244 D. occidentalis adults from all other sites were tested with negative results. Five spirochetal isolates recovered from I. pacificus adults were identified as Borrelia burgdorferi with specific monoclonal antibodies. Seven of 10 patients interviewed reportedly contracted their infections in summer, and six presented with a history of tick bite. Nine patients owned or occasionally harbored one or more dogs, and at least one of the dogs contracted Lyme borreliosis at the same site as its owner. Clinical manifestations of the disease in human patients included erythema migrans (100%), arthritis/arthralgia (60%), neurologic abnormalities (60%), and cardiac involvement (20%). Four of nine patients whose sera were assayed by indirect immunofluorescence contained significant antibody titers to B. burgdorferi.

Risk of human exposure to vector ticks (Acari: Ixodidae) in a heavily used recreational area in northern California.

The risk of humans encountering vector ticks along hiking trails or in picnic grounds in a Lyme disease-endemic area was evaluated in a multipurpose recreational area (Tilden Regional Park) in the populous San Francisco Bay region of California. Four hillside hiking trails (two high-use, two low-use) were sampled by dragging and walking through low vegetation biweekly for one year; four heavily used picnic areas were sampled concurrently by dragging. Adults of three human-biting ticks were enumerated (n = 1,911) along all trials: Dermacentor occidentalis (63.6% of total), Ixodes pacificus (26.2%), and D. variabilis (0.2%). Subadults (n = 1,669) of D. occidentalis (0.06% of total) and two nonhuman-biters, D. albipictus (70.3%) and Haemaphysalis leporispalustris (29.7%), also were collected. Dragging yielded many more adult ticks than walking year-round for all trials. These methods were significantly correlated during periods of peak tick abundance, but the associations were not sufficiently strong or consistent to allow prediction of captures for either method based on the other. Adult ticks were distributed largely in clusters along the uphill sides of trails. Several adult ticks collected adjacent to trails were found to contain spirochetes identified with polyclonal antibodies as Borrelia burgdorferi (D. occidentalis, 0 of 861; D. variabilis, 2 of 126 [1.6%]; I. pacificus, 1 of 609 [0.2%]). Picnic areas produced low numbers of adult D. occidentalis and I. pacificus, which prohibited testing them for spirochetes. Two measures for evaluating risk were calculated, the encounter distance (= mean number of meters traveled before encountering a vector tick by either dragging or walking) and the mean number of spirochete-infected ticks encountered by these methods per kilometer of trial. These measures revealed that the risk of exposure to spirochete-infected adult ticks along trails was low year-round irrespective of usage, and risk was even lower in picnic areas. Future studies evaluating human exposure to vector ticks in recreational areas should incorporate, whenever possible, testing for multiple pathogens because most ixodid ticks that commonly bite people in the United States are capable of transmitting two or more microbial disease agents.

Susceptibility of the western fence lizard (Sceloporus occidentalis) to the Lyme borreliosis spirochete (Borrelia burgdorferi).

Attempts to infect juvenile and adult western fence lizards (Sceloporus occidentalis) with the Lyme borreliosis spirochete (Borrelia burgdorferi) were largely unsuccessful. Spirochetes could not be isolated from the blood and various tissues of 14 lizards 21-32 days after they had been inoculated ip (n = 8) or sc (n = 6) with 10(6) or 10(8) B. burgdorferi representing 3 tick isolates, although 1 lizard apparently developed a transitory spirochetemia lasting 2 days. Similarly, spirochetes could not be detected in the blood or tissues of 5 lizards fed upon by 2- greater than 8 infected larvae or nymphs of the western black-legged tick (Ixodes pacificus). Sixty-five blood samples from 59 lizards in an endemic area and various tissues from 20 of the same lizards were also assayed for B. burgdorferi with negative results. The implications of these findings for the maintenance of this spirochete in natural foci are discussed.

Reservoir competence of four chaparral-dwelling rodents for Borrelia burgdorferi in California.

Aspects of the reservoir competence of four rodents for the Lyme disease spirochete, Borrelia burgdorferi, were evaluated in California. Rodents were live-trapped and ear-punch biopsies were cultured during each season. A second set of biopsies was cultured from representative individuals after 2-3 weeks of captivity and the results of culturing biopsies taken on both dates were compared with the results of feeding Ixodes pacificus larvae on hosts xenodiagnostically. The prevalence of infections did not differ significantly between dusky-footed woodrats (Neotoma fuscipes) and California kangaroo rats (Dipodomys californicus) nor among seasons. Combined results of the three tests showed that 85.7% of dusky-footed woodrats (n = 21) and 78.6% of California kangaroo rats (n = 14) were infected with B. burgdorferi. In contrast, only 22.2% of brush mice (Peromyscus boylii) (n = 14) and 7.1% of pinyon mice (P. truei) (n = 9) were infected. The sensitivity of culturing ear-punch biopsies as an assay for borrelial infection was significantly greater when biopsies were taken after a short period of captivity (0.89) rather than on the day of capture (0.52). Tick xenodiagnosis, in which I. pacificus was used as the vector, revealed borrelial infections in 90.3% of infected rodents. Spirochetes were observed in 37.7% of 239, 45.2% of 155, 60.0% of 10, and 7.1% of 14 cultures of nymphal I. pacificus fed as larvae on naturally infected woodrats, kangaroo rats, brush mice, and a pinyon mouse, respectively. The mean prevalence of infection in xenodiagnostic ticks varied significantly among host species with a greater proportion of ticks infected while feeding on woodrats and kangaroo rats than on mice. This study reconfirms previous reports that implicated woodrats and kangaroo rats as reservoirs of B. burgdorferi in California.

Transovarial and transstadial passage of Borrelia burgdorferi in the western black-legged tick, Ixodes pacificus (Acari: Ixodidae).

Transovarial and transstadial passage of Borrelia burgdorferi was demonstrated for the first time in the western black-legged tick, Ixodes pacificus. One of three field-collected females with spirochetes in ovarial tissues produced 100% infected progeny that maintained the spirochetes transstadially and in 4 of 5 cases passed them via eggs to as many as 97% of F2 filial ticks. The progeny infected ovarially and by subsequent transstadial passage had generalized tissue infections that exhibited reduced immunofluorescence staining reactivity with a fluorescein isothiocyanate-labeled polyclonal antibody. Attempts to isolate the spirochete from ticks in BSK medium or various modifications of it were unsuccessful. Spirochetes in tissue smears of all three parasitic stages of the F1 generation were nonreactive with a monoclonal antibody (H5332) specific for B. burgdorferi, whereas those present in tissue smears of F2 larvae bound with it.

Evidence implicating nymphal Ixodes pacificus (Acari: ixodidae) in the epidemiology of Lyme disease in California.

To clarify the role of nymphal versus adult western black-legged ticks (Ixodes pacificus) in the epidemiology of Lyme disease, the seasonal distribution, abundance, and spirochetal infection rates in these stages, and the seasonal occurrence of ticks biting humans and of incident cases of Lyme disease were determined in northern California. Although their seasonal activity periods overlapped for about one-third of the year, nymphs and adults predominated in different seasons, the former from late spring to summer and the latter from fall to early spring. At one site, four (4%) of 100 adults from low vegetation bordering a hardwood forest and 44 (13.6%) of 324 nymphs from leaf litter in the forest were found to contain Borrelia burgdorferi. Biting-collection records revealed that nymphs attach to people more commonly than recognized previously; I. pacificus nymphs comprised 12.5% of 967 ticks of various species and stages and 42% of all nymphs submitted for identification. Attachments by nymphs occurred primarily between April and August, which coincided with the seasonal occurrence of most incident cases of Lyme disease. Collectively, these findings strongly implicate the nymphal stage of I. pacificus as the primary vector of B. burgdorferi to humans in this region.

Density and spatial distribution of Ixodes pacificus (Acari: Ixodidae) in two recreational areas in north coastal California.

The density and distribution of Ixodes pacificus was assessed at 2 parks in north coastal California. The density of I. pacificus adults and nymphs varied significantly between years, trails, and sides of trails. Adult ticks occurred on vegetation along sun-exposed trails in January through March, their density (0-1.93 per 20 m) correlated with brush density, trail width, and presence of an uphill slope. Nymphs (0.06-5.10 per 20 m) occurred in leaf litter along shaded trails in May-July. Adult I. pacificus were rare at picnic sites (0.00-0.24 per 20 m), but nymphal densities (0.93-2.37 per 20 m) were comparable with those along some shaded trails. The prevalence of Borrelia burgdorferi in ticks (2.8% overall) did not differ significantly between locations, years, or stages. We conclude that the risk of acquiring Lyme disease in these sites is low, but varies among trails, seasons, and years.