Abstract

Objective

The objective of this study was to determine the risk of acquiring disease from popular Sierra Nevada wilderness area lakes and streams. This study examines the relative risk factors for harmful waterborne microorganisms using coliforms as an indicator.

Methods

Water was collected in the backcountry Yosemite and Kings Canyon National Parks and neighboring wilderness areas. A total of 72 sites from lakes or streams were selected to statistically differentiate the risk categories: (1) natural areas rarely visited by humans or domestic animals; (2) human day use–only areas; (3) high use by backpackers; (4) high use by pack animals; and (5) cattle‐ and sheep‐grazing tracts. Water was collected in sterile test tubes and Millipore coliform samplers during the summer of 2006. Water was analyzed at the university microbiology lab, where bacteria were harvested and then subjected to analysis using standardized techniques. Statistical analysis to compare site categories was performed using Fisher's exact test.

Results

Coliforms were found in none of the 13 wild sites, none of the 12 day hike sites, and only 3 of 18 backpacker sites (17%). In contrast, 14 of 20 sites (70%) with pack animal traffic yielded coliforms, and all 9 sites (100%) below the cattle‐grazing areas grew coliforms. Differences between backpacker versus cattle or pack areas were significant, p ≤ 0.05. All samples grew normal aquatic bacteria. Sites below cattle grazing and pack animal use tended to have more total heterotrophic bacteria.

Conclusions

Alpine wilderness water below cattle areas used by pack animals is at risk for containing coliform organisms. Water from wild, day hike, or backpack areas showed far less risk for coliforms.

Yosemite, Kings Canyon National Parks (NP), and adjacent wilderness are popular destinations for backpackers and day hikers from all over the world. The John Muir Trail traverses these areas, and thousands of hikers each summer travel from Yosemite through adjacent wilderness areas and Kings Canyon NP to the top of Mt Whitney in Sequoia NP. Although many hikers treat their drinking water either by filtration or by application of halogen compounds, treatments are not 100% effective. In addition, filters may become clogged and ineffective.

Coliform bacteria have been established as indicators of fecal pollution or contamination of waterways in the United States, including Giardia .1 Coliforms in wilderness areas may originate from one or a combination of sources: (1) wild animals; (2) humans from day use only; (3) backpackers; (4) horses and pack animals (stock); and (5) cows or commercially grazed animals. Coliform pollution of wilderness areas by humans may occur through inadequate burial and disposal of fecal material. In addition, bathing or swimming in lakes may also result in microbial pollution.2 Pack animals may pollute by deposition of manure either directly into lakes and streams or indirectly onto trails or meadows and have been documented to import Giardia into the Sierra wilderness.3 This manure may be washed into waterways by either summer storms or annual snowmelt.4 The US Department of Agriculture (USDA) Forest Service leases tracts in wilderness areas for cattle grazing.5 Cattle manure is known to contain many microbes pathogenic to humans, including Giardia , Cryptosporidium , and Escherichia coli O157:H7.6 Finally, coliform or other bacteria potentially may originate from natural, wild animal zoonotic reservoirs. Within the Sierras, over 3,000,000 acres of land have been designated as official wilderness by the National Park Service or USDA Forest Service and protected from development, logging roads, and motor vehicles.7 Most of these protected recreational areas range from 1,800 to 4,200 m in elevation. In addition to providing water to hikers and backpackers, these high alpine lakes and streams in NP and wilderness areas are also an important watershed because nearly 50% of California's household domestic water originates in the Sierra Nevada mountains.8 Therefore, the issue of microbial pollution by day hikers, backpackers, horses and pack animals, and also commercial cattle and sheep grazing is important to multiple users. Pollution may occur from one or more of several potential harmful substances including both microbial organisms and toxic substances.9 Microorganisms that may cause diarrhea include coliforms, pathogenic bacteria, viruses, and protozoa such as Giardia or Cryptosporidium .10 Although concerns have been raised regarding Giardia in the Sierra, many authors have presented data that other fecal pathogens such as enterotoxic E coli may play a greater role in mountain‐acquired illness.11–14 Debate has ensued on the impact of backpackers, cattle grazing, or livestock such as mules and horses polluting the watersheds in wilderness areas if any. We completed two studies in prior years that surveyed remote Sierra Nevada mountain lakes and streams.15,16 However, these studies did not provide the statistical power to show significant differences for all the risk factors. This current study has been designed to provide a direct comparison of risk factors.

We hypothesized that wilderness freshwater from watersheds that have different human or animal use patterns would have differing risk for the presence of coliform bacteria. Therefore, the purposes of the study were to analyze wilderness freshwater for coliforms and compare results from watersheds that have different exposures to native animals, humans, and domesticated animals.

Methods

Field site collection

Seventy‐two sites were prospectively selected to differentiate among environmental risk for different types of bacterial contamination in wilderness areas of Kings Canyon, Sequoia, and Yosemite NP, as well as the following USDA Forest Service wilderness areas: Carson‐Iceberg, Emigrant, Hoover, and John Muir. The Hall Natural Research Area adjacent to the eastern boundary of Yosemite and southern boundary of Hoover wilderness was also included. No overnight camping or motor vehicles are allowed in the Hall area and the remote areas used by day hikers. Risk classifications included: (1) wild areas not contaminated by humans or domesticated animals; (2) day hike areas used only by humans; (3) watershed areas used by backpackers without stock; (4) areas used by horses and pack animals (stock); and (5) cattle‐grazing tracts. The sites were risk stratified with the assistance of the National Park Service and USDA Forest Service based on user nights by backpackers, pack animals, and cattle allotments in grazing tracts. Cattle grazing is not permitted in NP, so all samples in cattle‐grazing tracts were taken from within Forest Service wilderness areas.

Field water collection

Water samples were collected from June through September in 2006. Water was collected in (1) sterile test tubes and (2) Millipore total coliform count samplers (Millipore Corporation, Bedford, MA, USA). Samples were collected 5 cm below the water surface, and care was taken not to disturb bottom sediment. All samples were collected in duplicate, and samples were cooled following standardized procedures and transported to the University of California Davis. Sample devices measured bacteria for 1 mL of sample. This was multiplied by 100 as per standardized procedure of reporting colony‐forming unit (CFU)/100 mL in the water literature. Water temperature was measured at each site using a stream thermometer (Cortland Line Company Inc., Cortland, NY, USA). Location and elevation were determined using United States Geological Survey topographical maps, guidebooks, backcountry rangers, and 30 years of author backcountry experience.

Bacterial analysis of water samples

Details of analysis for bacteria have been described elsewhere.15,16 The analysis for coliform counts and total bacterial counts required incubating Millipore counting plate paddles at 35°C for 24 hours. Bacterial colonies were counted and then harvested for further analysis. Colonies were initially plated onto sheep blood and MacConkey agars (Reel Inc., Lenexa, KS, USA). Lactose‐fermenting colonies from MacConkey plates were presumed to be coliform bacteria and subjected to further testing. Further screening and initial identification were done by subplating onto Cefsulodin Irgasan Novobiocin (Yersinia ) agar, sorbitol MacConkey agar, and Lysine Iron Agar and Tripple Sugar Iron tubes.

Statistical significance between groups was calculated with Fisher's exact test using STATA software (STATA, College Station, TX, USA).

Results

The results are summarized in Tables 1–5. Significant differences were found among sample groups. All nine samples (100%) taken from below areas in which cattle grazed or had recently grazed were positive for coliform growth. In areas frequented by pack animals, 14 of 20 samples (70%) had coliforms. In contrast, coliforms were found in only 3 of 18 areas where backpackers camped without the presence of horses or pack animals. None of the 12 sites used by human day hikers (and not used by stock) had coliforms, and similarly, none of the 13 wild sites rarely visited by humans or pack animals contained coliforms. The differences between wild, day hike, or backpacker versus either pack animal sites or cattle sites were statistically significant (p ≥ 0.05). No statistical differences were found when comparing wild, day hike, and backpacker sites to each other. No statistical differences were found in numbers of coliform bacteria based on water temperature or elevation. Among the 14 pack animal sites with coliforms, 7 were from lakes and 7 from creeks or rivers.

Table 1

Wild sites: rare human/stock watershed impact

Wilderness area Place Elevation (m) Temperature (°C) Coliforms (CFU/100 mL) Other bacteria (CFU/100 mL) 
Yosemite Johnston Pass Creek 2,780 13 None 2,100 
 Raymond Pass Creek 2,943 11 None 900 
 Florence Creek 2,819 7.5 None 300 
 Upper Yosemite Creek—Side Creek 2,501 11.5 None 6,400 
 Hoffmann Creek 2,560 12 None 2,600 
 Upper Middle Dana‐Gibbs Creek 3,016 10 None 300 
 Springs, Tioga‐Gaylor Lake trail 3,109 8.0 None None 
Kings Canyon Bago Springs Creek 2,840 8.0 None 3,900 
 Spring, north of Glen Pass John Muir Trail 3,353 7.0 None 100 
 Creek above Rae Lake Ranger Station 3,231 5.0 None 2,500 
 Creek Draining Lake 10,315 2,768 12.5 None 4,100 
 Lake 11,540 above Darwin Bench 3,517 9.5 None 1,100 
 Creek at McClure Ranger Station 2,926 9.5 None 1,500 
Wilderness area Place Elevation (m) Temperature (°C) Coliforms (CFU/100 mL) Other bacteria (CFU/100 mL) 
Yosemite Johnston Pass Creek 2,780 13 None 2,100 
 Raymond Pass Creek 2,943 11 None 900 
 Florence Creek 2,819 7.5 None 300 
 Upper Yosemite Creek—Side Creek 2,501 11.5 None 6,400 
 Hoffmann Creek 2,560 12 None 2,600 
 Upper Middle Dana‐Gibbs Creek 3,016 10 None 300 
 Springs, Tioga‐Gaylor Lake trail 3,109 8.0 None None 
Kings Canyon Bago Springs Creek 2,840 8.0 None 3,900 
 Spring, north of Glen Pass John Muir Trail 3,353 7.0 None 100 
 Creek above Rae Lake Ranger Station 3,231 5.0 None 2,500 
 Creek Draining Lake 10,315 2,768 12.5 None 4,100 
 Lake 11,540 above Darwin Bench 3,517 9.5 None 1,100 
 Creek at McClure Ranger Station 2,926 9.5 None 1,500 

CFU = colony‐forming unit.

Table 2

Day hike–only sites

Wilderness area Place Elevation (m) Temperature (°C) Coliforms (CFU/100 mL) Other bacteria (CFU/100 mL) 
Yosemite Budd Creek 2,622 10 None 4,600 
 Gaylor Lake 3,150 13 None 450 
 Upper Gaylor Creek 3,155 12 None 600 
 Lower Gaylor Creek 2,835 17 None 650 
 Granite Lake 3,176 16 None 200 
 Middle Fork Tuolumne River, Headwaters 2,438 10.5 None 3,400 
 Dana Fork of Tuolumne River 2,941 10 None 2,200 
Kings Canyon Bull Frog Lake 3,231 11.5 None 800 
 Creek in Meadow below Taboose Pass 3,353 11 None 2,800 
Emigrant Blue Lake Creek 3,048 12 None 1,100 
Carson Murray Creek 2,149 9.0 None 2,000 
Hall area Green Treble Lake—lower 3,010 9.0 None 400 
Wilderness area Place Elevation (m) Temperature (°C) Coliforms (CFU/100 mL) Other bacteria (CFU/100 mL) 
Yosemite Budd Creek 2,622 10 None 4,600 
 Gaylor Lake 3,150 13 None 450 
 Upper Gaylor Creek 3,155 12 None 600 
 Lower Gaylor Creek 2,835 17 None 650 
 Granite Lake 3,176 16 None 200 
 Middle Fork Tuolumne River, Headwaters 2,438 10.5 None 3,400 
 Dana Fork of Tuolumne River 2,941 10 None 2,200 
Kings Canyon Bull Frog Lake 3,231 11.5 None 800 
 Creek in Meadow below Taboose Pass 3,353 11 None 2,800 
Emigrant Blue Lake Creek 3,048 12 None 1,100 
Carson Murray Creek 2,149 9.0 None 2,000 
Hall area Green Treble Lake—lower 3,010 9.0 None 400 

CFU = colony‐forming unit.

Table 3

Backpacking sites

Wilderness area Place Elevation (m) Temperature (°C) Coliforms (CFU/100 mL) Other bacteria (CFU/100 mL) 
Yosemite Yosemite Creek 2,278 19 None 10,300 
 Booth Lake 3,001 15 None 1,600 
 Townsley Lake 3,154 4.5 None 400 
 Vogelsang Lake 3,147 100 2,200 
 Grant Lake 2,819 20 None 3,300 
 Ten Lakes #2 2,813 14 None 4,500 
 Ten Lakes #3 2,750 15 None 6,400 
 Ten Lakes #4 2,727 16 400 4,300 
 East Ten Lakes 2,865 19 None 3,600 
Kings Canyon East Creek at confluence of Bubbs Creek 2,494 15 None 3,500 
 Charlotte Creek 2,219 11 None 5,200 
 Charlotte Lake near Ranger Station 3,165 15 None 1,800 
 Upper Rae Lake 3,213 12.5 None 3,500 
 60 Lakes Drainage Creek 2,926 13 None 2,500 
 South Fork Kings River at Upper Paradise 2,134 12.5 None 600 
 Bench Lake 3,219 11.5 None 800 
 Darwin Canyon Creek 3,475 11 200 7,400 
 Goddard Canyon 2,576 11.5 None 4,500 
Wilderness area Place Elevation (m) Temperature (°C) Coliforms (CFU/100 mL) Other bacteria (CFU/100 mL) 
Yosemite Yosemite Creek 2,278 19 None 10,300 
 Booth Lake 3,001 15 None 1,600 
 Townsley Lake 3,154 4.5 None 400 
 Vogelsang Lake 3,147 100 2,200 
 Grant Lake 2,819 20 None 3,300 
 Ten Lakes #2 2,813 14 None 4,500 
 Ten Lakes #3 2,750 15 None 6,400 
 Ten Lakes #4 2,727 16 400 4,300 
 East Ten Lakes 2,865 19 None 3,600 
Kings Canyon East Creek at confluence of Bubbs Creek 2,494 15 None 3,500 
 Charlotte Creek 2,219 11 None 5,200 
 Charlotte Lake near Ranger Station 3,165 15 None 1,800 
 Upper Rae Lake 3,213 12.5 None 3,500 
 60 Lakes Drainage Creek 2,926 13 None 2,500 
 South Fork Kings River at Upper Paradise 2,134 12.5 None 600 
 Bench Lake 3,219 11.5 None 800 
 Darwin Canyon Creek 3,475 11 200 7,400 
 Goddard Canyon 2,576 11.5 None 4,500 

CFU = colony‐forming unit.

Table 4

Stock areas (horses and pack animals)

Wilderness area Place Elevation (m) Temperature (°C) Coliforms (CFU/100 mL) Other bacteria (CFU/100 mL) 
Yosemite Tuolumne River (Lyell Canyon) 2,804 16 200 3,000 
 Rafferty Creek 2,673 8.0 100 3,200 
 Fletcher Lake 3,095 10 None 400 
 Fletcher Creek 3,060 14 None 1,450 
 Merced Lake 2,195 12.5 200 1,200 
 Sunrise High Sierra Camp Drainage Creek 2,608 17 200 2,100 
 Cathedral Lake 2,837 14 100 1,800 
 Dog Lake 2,804 13 100 850 
Kings Canyon Bubbs Creek at Confluence Kings River 1,560 12.2 None 2,200 
 Bubbs Creek at Junction Meadow 2,469 14 200 4,700 
 Bubbs Creek at Vidette Meadow 2,896 10.5 None 6,000 
 Arrow Lake 3,154 12 None 1,400 
 Arrow‐Dollar Creek Trail crossing 3,145 12 100 2,700 
 Dollar Lake 3,115 12.5 300 7,400 
 Rae Lake (middle) 3,211 16 200 2,900 
 South Fork Kings River at Lower Paradise  13 300 2,800 
 Woods Lake 3,265 13.5 100 1,400 
 Copper Creek 1,555 13 None 3,000 
Carson Silver King Creek 2,316 10 100 1,100 
Muir South Fork San Joaquin River 2,390 15 200 20,000 
Wilderness area Place Elevation (m) Temperature (°C) Coliforms (CFU/100 mL) Other bacteria (CFU/100 mL) 
Yosemite Tuolumne River (Lyell Canyon) 2,804 16 200 3,000 
 Rafferty Creek 2,673 8.0 100 3,200 
 Fletcher Lake 3,095 10 None 400 
 Fletcher Creek 3,060 14 None 1,450 
 Merced Lake 2,195 12.5 200 1,200 
 Sunrise High Sierra Camp Drainage Creek 2,608 17 200 2,100 
 Cathedral Lake 2,837 14 100 1,800 
 Dog Lake 2,804 13 100 850 
Kings Canyon Bubbs Creek at Confluence Kings River 1,560 12.2 None 2,200 
 Bubbs Creek at Junction Meadow 2,469 14 200 4,700 
 Bubbs Creek at Vidette Meadow 2,896 10.5 None 6,000 
 Arrow Lake 3,154 12 None 1,400 
 Arrow‐Dollar Creek Trail crossing 3,145 12 100 2,700 
 Dollar Lake 3,115 12.5 300 7,400 
 Rae Lake (middle) 3,211 16 200 2,900 
 South Fork Kings River at Lower Paradise  13 300 2,800 
 Woods Lake 3,265 13.5 100 1,400 
 Copper Creek 1,555 13 None 3,000 
Carson Silver King Creek 2,316 10 100 1,100 
Muir South Fork San Joaquin River 2,390 15 200 20,000 

CFU = colony‐forming unit.

Table 5

Cattle risk watersheds

Wilderness area Place Elevation (m) Temperature (°C) Coliforms (CFU/100 mL) Other bacteria (CFU/100 mL) 
Carson Upper Clark Fork River 2,072 13 400 10,200 
 Lower Clark Fork River 2,316 12 600 12,600 
 Disaster Creek 2,366 12 550 11,000 
 Arnot Creek 2,000 13 100 4,600 
 Woods Creek 1,976 14 100 5,200 
Emigrant Kennedy Creek 2,244 14 200 8,600 
Hoover Buckeye Creek 2,377 450 4,700 
 Molydunite Creek 2,773 14 200 12,000 
 South Fork Walker River (Burt Canyon) 2,719 14.5 200 11,000 
Wilderness area Place Elevation (m) Temperature (°C) Coliforms (CFU/100 mL) Other bacteria (CFU/100 mL) 
Carson Upper Clark Fork River 2,072 13 400 10,200 
 Lower Clark Fork River 2,316 12 600 12,600 
 Disaster Creek 2,366 12 550 11,000 
 Arnot Creek 2,000 13 100 4,600 
 Woods Creek 1,976 14 100 5,200 
Emigrant Kennedy Creek 2,244 14 200 8,600 
Hoover Buckeye Creek 2,377 450 4,700 
 Molydunite Creek 2,773 14 200 12,000 
 South Fork Walker River (Burt Canyon) 2,719 14.5 200 11,000 

CFU = colony‐forming unit.

Noncoliform aquatic bacteria were also identified from the samples. The environmental heterotrophic bacterial counts are included in the tables. Although not statistically significant, total bacterial counts tended to be lower at the wild and day hike sites. No correlation could be made from temperature or elevation for either coliform or total bacterial counts.

Discussion

In this study, areas frequented by cattle or pack animals had the greatest degree of fecal contamination into the wilderness watershed. We are not surprised at the finding of coliforms below cattle‐grazing areas. In most of these areas, moderate amounts of cattle manure were observed during field collections. In some respects, finding coliforms below grazing areas serves as a positive control for the study. One might expect coliforms in watershed with high densities of cattle.4 However, finding coliforms in areas frequented by pack animals is significant. NP and USDA Forest Service wilderness have strict requirements on management of livestock in wilderness areas. It is not possible to exclude a human contribution to this finding, as high‐volume pack animals areas are also used by humans. However, the fact that very few backpacker‐only sites had coliforms would support the conclusion that microbial contamination in pack animal areas is a result of pack animal manure. In a prior study, we examined Sierra water for coliform bacteria.15 However, in that study, samples were taken from water primarily from sites used by both pack animals and humans and we were unable to fully determine the source. An additional study did not differentiate among areas used only by day hikers that did not see stock traffic but did find that samples from cattle and pack animal sites had significantly higher levels of coliforms compared to wild or backpack sites.16 This current study added sites that were used by human day hikers only and confirms the data from a prior year on the other risk categories. Pack animal sites are also used by backpackers; however, the low prevalence of coliforms in most of those areas used exclusively by humans in the absence of pack animals would suggest that pack animals are most likely the source in the combined use pack animal areas.

Pack animals produce high volumes of manure, which is deposited directly onto the surface of trails, soil, or meadows.17 Manure deposited on the ground can be swept into streams during summer rains or spring snow runoff.4 During the field operations of the study, pack animals were observed on several occasions to be defecating directly into bodies of freshwater. Fecal contamination as indicated by the finding of coliforms would place the watershed at risk for harboring microbes capable of causing human disease. Some of these infections are a potential threat to humans. This includes certain pathogenic strains of E coli , Salmonella , Campylobacter , Aeromonas , and protozoa such as Giardia , all of which have animal reservoirs. The organism Yersenia enterocolitica has been previously cultured in high alpine areas of the Sierra and may have a natural reservoir in small mammals and birds.18 Pack animals entering the High Sierra have been subject to analysis and Giardia found in their manure.19

Escherichia coli and other coliform bacteria can survive in aquatic environments for long periods of time depending on the nutriment availability, pH, and water temperature. The number of years that coliforms can survive in aquatic environments has been debated.20 A study of Lake Michigan shore water showed that E coli may sustain itself indefinitely in appropriate environmental situations.21 In addition to Giardia , cattle are noted to carry E coli strain O157:H7 at a rate of 1% to 30%, placing persons who drink untreated water below established cow pastures at risk for a very serious disease.6 Studies on this strain have also shown it to survive in cold water.6,22 In addition, many non‐O157 E coli are capable of inducing serious disease in humans.23

Finally, we wish to comment on the noncoliform bacteria found in the study. Aquatic bacteria are part of a normal ecosystem of lakes and streams.24 Indeed, if bacteria were absent, the normal food chain from frogs to fish, as well as the ecological balance, would be in jeopardy. A prior study identified many species, including Achromobacter species, Pasteurella haemolytica , Rahnella aquatilis , Ralstonia paucula , Serratia odorifera , Serratia plymuthica , Yersinia intermedia , Yersinia kristensenii , Yersinia frederiksenii , Pseudomonas species, etc.16 That study of wilderness water suggested a correlation between total bacterial counts and usage by backpackers, which was not found in the current study.

Recommendations

Hikers and backpackers should be extremely cautious with water in areas used by cattle. If water needs to be taken from lakes or streams to drink, it should be subject to maximal disinfection techniques. In areas where pack animals have been present, water should also be treated. In Sierra Nevada wilderness areas, water from alpine sidestreams that is free from domesticated animal activity has a very low risk of harboring pathogenic organisms and a minimal risk of illness if drunk untreated.

Limitations

Coliforms were used as an indicator of fecal pollution, and specific pathogenic microorganisms were not analyzed. Although samples were taken during times of the greatest use by humans and domesticated animals, these represent single point in time samples: additional samples at different times may have increased positive findings. Data in this report are applicable only to Sierra Nevada wilderness areas and not to areas with human habitation. The data may not be extrapolated to European or other international mountain areas. Finally, backpacker use was not quantified in terms of persons/night.

Conclusion

Alpine wilderness water below cattle‐grazing tracts or areas used by pack animals is at risk for containing coliform organisms. Water from wild, day hike, or back areas showed far less risk for coliforms.

Declaration of interests

The author states that he has no conflicts of interest.

References

1
American Public Health Association
.
Microbiologic examination
. In:
Clesceri
LS
, ed.
Standard methods for the examination of water and wastewater
.
20th Ed
. Baltimore, MD:
United Book Press Inc
.,
1998
.
2
McCarthy
TA
Barrett
NL
Hadler
JL
, et al .
Hemolytic uremic syndrome and Escherichia coli O121 at a lake in Connecticut, 1999
.
Pediatrics
 
2001
;
108
:E59.
3
Xio
L
Herd
RP
.
Epidemiology of equine cryptosporidium and Giardia infections
.
Equine Vet J
 
1994
;
26
:
14
17
.
4
Ramos
MC
Quinton
JN
Tyrrel
SF
.
Effects of cattle manure on erosion rates and runoff water pollution by faecal coliforms
.
J Environ Manage
 
2006
;
78
:
97
101
.
5
US Department of Agriculture Forest Service
.
Environmental assessment: rangeland allotments phase 1
. Sonora, CA:
Stanislaus National Forest
,
2006
.
6
Renter
DG
Sargeant
JM
Oberst
RD
Samadpour
M
.
Diversity, frequency, and persistence of Escherichia coli O157 strains from range cattle environments
.
Appl Environ Microbiol
 
2003
;
69
:
542
547
.
7
California Wilderness Coalition
. Available at: http://www.calwild.org. (Accessed 2007 Feb 1)
8
Carle
D
.
Introduction to water in California
. Berkeley, CA:
University of California Press
,
2004
. ISBN 0520–23580–0.
9
Goldman
CR
.
Four decades of change in two subalpine lakes
.
Verh Int Verein Limnol
 
2000
;
27
:
7
26
.
10
Rockwell
R
.
Wilderness water purity, especially in the High Sierra
.
Am Alpine News
 
2000
;
11
:
238
240
.
11
Rockwell
RL
.
Giardia update
.
California Mountaineering Club Newsletter
. Vol.
8
1997
.
12
Silverman
G
Erman
DC
.
Alpine lakes in Kings Canyon NP, California: baseline conditions and possible effects of visitor use
.
J Environ Manage
 
1979
;
8
:
73
87
.
13
Suk
TJ
Sorenson
SK
Dileanis
PD
.
The relationship between human presence and occurrence of Giardia cysts in streams in the Sierra Nevada, California
.
J Freshwater Ecol
 
1987
;
4
:
71
75
.
14
Zell
SC
Sorenson
MS
.
Cyst acquisition rate for Giardia lamblia in backcountry travelers to desolation wilderness, Lake Tahoe
.
J Wilderness Med
 
1993
;
4
:
147
154
.
15
Derlet
RW
Carlson
JR
.
An analysis of wilderness water in Kings Canyon, Sequoia and Yosemite National Parks for coliform and pathologic bacteria
.
Wilderness Environ Med
 
2004
;
15
:
238
244
.
16
Derlet
RW
Carlson
JR
.
Coliform bacteria in Sierra Nevada wilderness lakes and streams: what is the impact of backpackers, pack animals, and cattle?
Wilderness Environ Med
 
2006
;
17
:
15
20
.
17
Derlet
RW
Carlson
JR
.
An analysis of human pathogens found in horse/mule manure along the John Muir Trail in Kings Canyon and Sequoia and Yosemite National Parks
.
Wilderness Environ Med
 
2002
;
13
:
113
118
.
18
Harvey
S
Greenwood
JR
Pickett
MJ
Mah
RA
.
Recovery of Yersinia enterocolitica from streams and lakes of California
.
Appl Environ Microbiol
 
1976
;
32
:
352
354
.
19
Atwill
ER
McDougald
NK
Perea
L
.
Cross‐sectional study of faecal shedding of Giardia duodenalis and Cryptosporidium parvum among packstock in the Sierra Nevada Range
.
Equine Vet J
 
2000
;
32
:
247
252
.
20
Winfield
MD
Groisman
EA
.
Role of nonhost environments in the lifestyles of Salmonella and Escherichia coli
.
Appl Environ Microbiol
 
2003
;
69
:
3687
3694
.
21
Whitman
RL
Nevers
MB
.
Foreshore sand as a source of Escherichia coli in nearshore water of a Lake Michigan Beach
.
Appl Environ Microbiol
 
2003
;
69
:
5555
5562
.
22
Want
GD
Doyle
MP
.
Survival of enterohemorrhagic Escherichia coli O157:H7 in water
.
J Food Prot
 
1998
;
61
:
662
667
.
23
Khan
A
Yamasaki
S
Sato
T
, et al .
Prevalence and genetic profiling of virulence determinants of non‐O157 Shiga toxin‐producing Escherichia coli isolated from cattle, beef, and humans, Calcutta, India
.
Emerg Infect Dis
 
2002
;
8
:
54
62
.
24
Page
KA
Connon
SA
Giovannoni
SJ
.
Representative freshwater bacterioplankton isolated from Crater Lake, Oregon
.
Appl Environ Microbiol
 
2004
;
70
:
6542
6550
.