Ecological Archives E091-021-A1

Daniel J. Salkeld and Robert S. Lane. 2010. Community ecology and disease risk: lizards, squirrels, and the Lyme disease spirochete in California, USA. Ecology 91:293–298.

Appendix A. Information on methodology and parameters used for a "host-community" model.

Tick loads

We averaged larval tick load data from two studies that investigated host–tick relationships in oak–woodland habitats surrounded by chaparral and/or grassland, Mendocino County, in northwestern California and were carried out in 2002 and 2003 (Eisen et al. 2004a, b, Table 1). Host species included in the model were alligator lizards (Elgaria multicarinata), western fence lizards (Sceloporus occidentalis), brush mice, deer mice, piñon mice, California meadow voles, and dusky-footed wood rats. Tick loads were obtained for western gray squirrels in 2003 (Lane et al. 2005). Larvae can be difficult to observe on live small mammals in the field, and field inspections typically account for only 73% of immature ticks (Eisen et al. 2004b). Therefore, reported data on larval tick loads for Peromyscus spp. and wood rats were multiplied by 1.37 (Eisen et al. 2004b). Ticks on lizards are easier to count accurately and therefore were not adjusted. Tick loads for western gray squirrels were obtained immediately post-shooting, and remain unadjusted because the entire body was examined thoroughly for at least 20 minutes (Lane et al. 2005). Animals that do not regularly host I. pacificus larvae, such as brush rabbits (Sylvilagus bachmani) or black-tailed jackrabbits (Lepus californicus) (Eisen et al. 2004a), were not included in the model. These lagomorphs also are not abundant in the dense woodlands sampled. We include data only on I. pacificus because it is the principal vector of B. burgdorferi in northern California; other tick species are never or rarely found on lizards, and although squirrels do harbor Dermacentor occidentalis, this tick is an inefficient experimental vector of B. burgdorferi (Eisen et al. 2004a, Lane et al. 1994, 2005).

Host abundance

We used minimum number known alive (MNKA) estimates to calculate mammalian population sizes during the periods of peak immature tick activity. The MNKA is a retrospective estimate of population size that includes the number of animals captured at the current trapping session, plus any animals that had been captured and marked on previous trapping sessions but were known to be alive at the time of the current trapping session because they were captured during a later session (Hofmeister et al. 1999). Small mammal density estimates were calculated using the MNKA for 0.54-ha trapping grids in woodland bordered by chaparral, performed in 2003 (L. Eisen, R.J. Eisen, and R.S. Lane, unpublished data; Eisen et al. 2004a, 2009). Squirrel abundance was calculated by data on MNKA obtained from trapping sessions carried out in April–May 2006 and May 2007, the peak immature tick periods. Western gray squirrels were trappedwith Tomahawk live traps (7" × 7" or 9" × 9"; Tomahawk Live Trap Co., Tomahawk, WI) laid in 5 transects of 5 traps each, spaced 30 meters apart. Traps were baited with whole pecan nuts and peanut butter, wired open for a minimum of 2 weeks prior to trapping, and re-baited on 3–4 occasions during this period. During trapping sessions, traps were opened prior to sunset and left open for the following 2.5 days, and checked every 4–5 hours during daylight starting an hour after dawn. Traps were closed at night during cold weather and in the afternoons during hot weather. Upon capture, squirrels were restrained using a canvas handling bag (Koprowski 2002). At first capture during each trapping session, animals were briefly anaesthetized with isoflurane (Isothesia; Abbott Laboratories, North Chicago, IL), to allow individual marking of squirrels. We marked squirrels with ear tags in 2006 and used passive integrated transponder tags (Biomark, Boise, ID) inserted subcutaneously in 2007 because ear tags were frequently removed by squirrels. Squirrels were released at their sites of capture, and any individuals recaptured during the same trapping session were identified and released. Squirrel captures from trapping sessions that occurred in July 2006, January and July 2007, and February 2008, provided additional data for the MNKA for May 2007. We were unable to use other approaches, such as MARK, to calculate population numbers because of variation in trapping efforts during these additional trapping sessions, and because squirrels lost their individual identification numbers during the 2006 field season. Lizard density estimates for oak woodland surrounded by chaparral were taken from Eisen et al. (2004a). Animal handling and capture procedures were reviewed and approved by the Animal Care and Use Committee at the University of California at Berkeley, and complied with guidelines from the California Department of Fish and Game and the National Institutes of Health.

The mean distance between traps where squirrels were captured consecutively was 70 m. We used this distance to estimate the trapping area by buffering the locations of traps using a merged 70 m diameter buffer. Trap locations were obtained using GPS, and imported to ArcGIS (Environmental System Research Institute 2006, ArcGIS computer software version 9.2, Redlands, California, to perform spatial operations. The total area estimate was 6.96 hectares and the resultant squirrel densities in oak woodlands at Hopland were 4.9ha–1 in April–May 2006 and 3.6ha–1 in May 2007. Thus, at the peak larval abundance period, mean (± SD) squirrel abundance was 4.2 (0.9) ha–1. We consider our buffer size to be reliable, as squirrels were most frequently captured at identical trap-sites within trapping sessions, implying limited movement. Most between-trap movements were also between-session, e.g., movement between capture in May and July 2007. Furthermore, a newly placed trap, approximately 50 m from the nearest trap of an established transect failed to capture marked individuals but did capture new, unrecognized animals. Previous estimates of western gray squirrel home range sizes in California range from 0.12–0.34 ha for males, and 0.47–0.62 ha for females (Carraway and Verts 1994), and our buffered trap area is approximately 0.38 ha.

Reservoir competence

Reservoir host competence is defined as the proportion of molted nymphs infected with B. burgdorferi that result from larvae that had fed upon a particular host. Data on reservoir competence of Peromyscus spp. were derived from experimental investigations by Peavey and Lane (1995). For squirrels, we used reservoir competence observed in the laboratory during the two months following infection (80.5%), as we assumed that squirrels in the wild would have been recently infected with B. burgdorferi during the peak tick season (Salkeld et al. 2008). The reservoir competence of wood rats (31%) was an average based on laboratory data for transmission of B. burgdorferi sensu lato by wood rats to I. pacificus (24%; Brown and Lane 1992; 38%; Brown and Lane 1996). Although more recent work has suggested that B. burgdorferi sensu lato isolates from wood rats in woodlands are predominantly B. bissettii, wood rats are also infected by B. burgdorferi sensu stricto (Brown et al. 2006) and these estimates are the best among those that have been published. Ultimately, no wood rats were actually infected with B. burgdorferi at the study sites (Eisen et al. 2009), and therefore any contribution of wood rats to local disease risk in our model system is restricted to hypothetical investigations of the model’s sensitivities.

In California, the western fence lizard, Sceloporus occidentalis, and alligator lizards, Elgaria spp., host large numbers of larval ticks (e.g., Eisen et al. 2001, 2004a,b), but the immune systems of these reptiles contain proteins in their blood that destroy spirochetes (i.e., belonging to the alternative complement pathway), and therefore they do not act as reservoir hosts of B. burgdorferi (Lane and Quistad 1998, Wright et al. 1998, Kuo et al. 2000). Therefore, all larvae that had fed on lizards were considered to be uninfected. Most birds host small numbers of I. pacificus larvae, but their reservoir competence is unknown, so they are not included in the model.

Borrelia burgdorferi infection prevalence in mammalian hosts

Information on the prevalence of B. burgdorferi infection in small mammals captured in 2003 in oak woodland surrounded by chaparral was taken from Eisen et al. (2009). We used an infection prevalence of 80% for western gray squirrels in the host community model (Lane et al. 2005), because this prevalence datum was obtained for squirrels that were examined during the same period in 2003 as the prevalence data for the other small mammals.

TABLE 1. Host abundance, larval Ixodes pacificus infestation loads, host Borrelia burgdorferi­–infection prevalence, and vertebrate host reservoir competence (% nymphs infected from larval exposure) parameters used in the model.


Host abundance/ha1

Number of Ixodes pacificus larvae per host2

Borrelia burgdorferi infection prevalence3

Reservoir competence

Alligator lizard (Elgaria multicarinata)



0see text

0see text

Western fence lizard (Sceloporus occidentalis)



0see text

0see text

California meadow vole (Microtus californicus)





Dusky-footed wood rat (Neotoma fuscipes)





Brush mice (Peromyscus boylii)





Piñon mice(Peromyscus truei)





Deer mouse(Peromyscus maniculatus)





Western gray squirrel (Sciurus griseus)

4.2 (range 0 – 10)c




1All abundance data were obtained from investigations carried out at Hopland, in oak woodland surrounded by chaparral, during peak immature tick periods in 2003, apart from data on western gray squirrels, which were obtained in April–May of 2006–2007.

2 We used an average of I. pacificus larval loads, which were obtained from oak woodland habitats from Mendocino County woodlands during the peak tick-activity periods of 2002 and 2003. Reported tick loads of small mammals (Peromyscus spp., wood rats, and California meadow voles) were corrected (× 1.37) to account for detection difficulties (see text).

3 Infection prevalence was obtained from mammals sampled in oak woodland surrounded by chaparral, at Hopland, in 2003. Sources of data include: aEisen et al. 2004a; bEisen, Eisen, and Lane, unpublished data; cOriginal data; dEisen et al. 2004b; eLane et al. 2005; fEisen et al. 2009; gAverage derived from Microtus and I. scapularis in NE USA from Mather et al. 1989 and Markowski et al. 1998; hAverage derived from Brown and Lane 1992, 1996; iPeavey and Lane 1995; jSalkeld et al. 2008.


Brown, R. N., and R. S. Lane. 1992. Lyme disease in California: a novel enzootic transmission cycle of Borrelia burgdorferi.Science 256:1439–1442.

Brown, R. N., and R. S. Lane. 1996. Reservoir competence of four chaparral–dwelling rodents for Borrelia burgdorferi in California. American Journal of Tropical Medicine and Hygiene 54:84–91.

Brown, R. N., M. A. Peot, and R. S. Lane. 2006. Sylvatic maintenance of Borrelia burgdorferi (Spirochaetales) in northern California: Untangling the web of transmission. Journal of Medical Entomology 43:743–51.

Carraway, L. N., and B. J. Verts. 1994. Sciurus griseus. Mammalian Species 474:1–7.

Eisen, R. J., L. Eisen, and R. S. Lane. 2001. Prevalence and abundance of Ixodes pacificus immatures (Acari: Ixodidae) infesting western fence lizards (Sceloporus occidentalis) in northern California: temporal trends and environmental correlates. Journal of Parasitology 87:1301–1307.

Eisen, L., R. J. Eisen, and R. S. Lane. 2004a. The roles of birds, lizards, and rodents as hosts for the western black–legged tick Ixodes pacificus. Journal of Vector Ecology 29:295–308.

Eisen, R. J., L. Eisen, and R. S. Lane. 2004b. Habitat–related variation in infestation of lizards and rodents with Ixodes ticks in dense woodlands in Mendocino County, California. Experimental and Applied Acarology 33:215–233.

Eisen, L., R. J. Eisen, J. Mun, D. J. Salkeld, and R. S. Lane. 2009 Transmission cycles of Borrelia burgdorferi and B. bissettii in relation to habitat type in northwestern California. Journal of Vector Ecology, in press.

Hofmeister, F. K., B. A. Ellis, G. E. Glass, and J. E. Childs. 1999. Longitudinal study of infection with Borrelia burgdorferi in a population of Peromyscus leucopus at a Lyme disease – enzootic site in Maryland. American Journal of Tropical Medicine and Hygiene 60:598–609.

Koprowski, J. L. 2002. Handling tree squirrels with a safe and efficient restraint. Wildlife Society Bulletin 30:101–103.

Kuo, M. M., R. S. Lane, and P. C. Giclas. 2000. A comparative study of mammalian and reptilian alternative pathway of complement-mediated killing of the Lyme disease spirochete (Borrelia burgdorferi). Journal of Parasitology 86:1223–1228.

Lane, R. S., R. N. Brown, J. Piesman, and C. A. Peavey. 1994. Vector competence of Ixodes pacificus and Dermacentor occidentalis (Acari: Ixodidae) for various isolates of Lyme disease spirochetes. Journal of Medical Entomology 31:417–424.

Lane, R. S., and G. B. Quistad. 1998. Borreliacidal factor in the blood of the western fence lizard (Sceloporus occidentalis). Journal of Parasitology 84:29–34.

Lane, R. S., J. Mun, R. J. Eisen, and L. Eisen. 2005. Western gray squirrel (Rodentia: Sciuridae): A primary reservoir host of Borrelia burgdorferi in Californian oak woodlands? Journal of Medical Entomology 42:388–396.

Markowski, D., H. S. Ginsberg, K. E. Hyland, and R. Hu. 1998. Reservoir competence of the meadow vole (Rodentia: Cricetidae) for the Lyme disease spirochete Borrelia burgdorferi. Journal of Medical Entomology 35:804–808.

Mather, T. N., M. L. Wilson, S. I. Moore, J. M. C. Ribeiro, and A. Spielman. 1989. Comparing the relative potential of rodents as reservoirs of the Lyme disease spirochete (Borrelia burgdorferi). American Journal of Epidemiology 130:143–150.

Peavey, C. A., and R. S. Lane. 1995. Transmission of Borrelia burgdorferi by Ixodes pacificus nymphs and reservoir competence of deer mice (Peromyscus maniculatus) infected by tick-bite. Journal of Parasitology 81:175–178.

Salkeld, D. J., S. Leonhard, Y. A. Girard, N. Hahn, J. Mun, K. A. Padgett, and R. S. Lane. 2008. Identifying the reservoir hosts of the Lyme disease spirochete Borrelia burgdorferi in California: the role of the western gray squirrel (Sciurus griseus). American Journal of Tropical Medicine and Hygiene 79:535–540.

Wright, S. A., R. S. Lane, and J. R. Clover. 1998. Infestation of the southern alligator lizard (Squamata: Anguidae) by Ixodes pacificus (Acari: Ixodidae) and its susceptibility to Borrelia burgdorferi. Journal of Medical Entomology 35:1044–1049.

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