Abstract
Introduction
This study aimed to estimate the contamination of water sources along the Tahoe Rim Trail (TRT) through evaluation of the presence and concentration of Escherichia coli, Giardia, and Cryptosporidium spp.
Methods
Sample sites were selected from 6 of the 8 sections of the TRT. Each stream was sampled 3 or 4 times during the summer and early fall of 2023. Samples were collected and analyzed for E. coli using Standard Methods 9223B and Giardia and Cryptosporidium spp. using EPA Method 1623.
Results
E. coli counts ranged from <1 to 29.2 most probable number (MPN)/100 mL. There was no difference in contamination with E. coli between streams (P=0.287) or sampling rounds (P=0.413). Giardia counts ranged from 0 to 7 count/10 L. There was a difference in contamination with Giardia between the highest and lowest concentration streams (P=0.028) but no difference in contamination between other streams or between sampling rounds (P=0.924). Cryptosporidium counts ranged from 0 to 1 count/10 L. There was no difference in contamination with Cryptosporidium between streams (P=0.589) or between sampling rounds (P=0.888).
Conclusions
E. coli, Giardia, and Cryptosporidium are present in the streams along the TRT. Results from this study show that the risk of illness from occasional direct consumption of water from the streams sampled is low; however, it is always advisable to filter or treat water from any backcountry source prior to consumption. More extensive studies are recommended to evaluate backcountry water sources in the Tahoe Basin to further evaluate the risk to human health from direct consumption
Keywords
Introduction
The Lake Tahoe Basin in Northern California and Nevada is one geographic region of the Sierra that attracts many recreationalists. It is home to the Tahoe Rim Trail (TRT), a renowned 174-mi loop encircling Lake Tahoe, and stands as one of America's most iconic long-distance trails. This scenic route offers locals and visitors access to an extensive network of trails, perfect for day hikes or extended backpacking trips. Since its completion, more than 3000 people have completed the entire trail, commonly a 10 to 15-d commitment, whereas more than 500,000 people explore sections of it annually. 1 Day hikers often have the convenience of bringing their own water, but backpackers on longer treks frequently rely on surface water sources. This reliance underscores the importance of understanding the potential risks of microorganism contamination in backcountry lakes and streams, which can pose health hazards to recreational users.
Several studies conducted in the Sierra over the past 30 y show that cattle and other livestock grazing above water sources may result in high concentrations of E. coli and other coliform bacteria in lower-elevation lakes and streams.2,3 Similarly, increased human activity appears correlated with increased risk for bacterial contamination. 4 Other organisms classically viewed as a concern for backcountry water sources, such as Giardia and Cryptosporidium spp., also pose a risk of contamination but are less consistently observed in backcountry settings than coliform bacteria.5,6 However, when present, the hardy Giardia cysts and Cryptosporidium oocysts can survive for weeks to months in cold water, leading to an extended period of potential infectivity in lakes and clear-appearing mountain streams. 8
Although some studies have evaluated the presence of E. coli and coliform bacteria and others the presence of Giardia and Cryptosporidium in backcountry sources,2,3,5,6 it is valuable to evaluate water sources along the TRT for all these organisms to provide a more comprehensive assessment of water contamination and risk of backcountry water consumption. This study aimed to estimate the contamination of water sources along the TRT via evaluation of the presence and concentration of E. coli, Giardia, and Cryptosporidium spp.
Methods
Field Site Selection
Given limited available resources and the complexity of the Tahoe water network, this study focused on sites designated as water sources by the TRT Association for individuals using the TRT.. 8 Each section of the TRT ranges from 16.4 to 32.5 m. One designated water source from 6 of the 8 total sections of the TRT were selected (Figure 1). Sampling focused exclusively on streams to ensure data consistency, excluding lakes and campsite pumps and thus eliminating 1 section of the TRT from sampling. Another section of the TRT, located in Desolation Wilderness, had recommended water sources that fit the criteria but were inaccessible for sampling given the remoteness of the region. Most streams sampled were large volume and fast flowing, except for Edgewood Creek, which dried up before our fourth sampling period.
Tahoe Rim Trail with sampling sites mapped using GPS coordinates. Map courtesy of AllTrails. www.alltrails.com
Water collection sites were accessed via trails intersecting the TRT, navigated using AllTrails (www.alltrails.com). All sites experienced significant foot traffic. Activities at or near to the sampling sites included day use, backpacking, dispersed camping, and fishing. Horseback riding is only permitted along the trail associated with Big Meadow Creek. Watson Creek was the only sampling site located downstream of a designated campground.
Sample Collection
Stream water was collected from a total of 6 sites over 4 rounds of collection spanning from July 10, 2023 to September 5, 2023. All sites produced samples throughout the entire collection period except Edgewood Creek, which dried up before the fourth round of sampling. Therefore, a total of 23 samples were collected and analyzed.
To decrease the risk of sample contamination, 5-gal buckets and funnels were first rinsed with water from the sampling site. Water was then collected in this 5-gal bucket and funneled into the designated collection containers. Samples for Giardia and Cryptosporidium were collected in 10-L (2.6-gal) disposable, collapsible containers provided by the Truckee Meadows Water Authority Laboratory. Samples for E. coli were collected in 100-mL sterile vessels provided by the Nevada State Public Health Laboratory. To ensure viability of the samples, they were all dropped off to the lab directly after or within 24 h of each collection with additional refrigeration. Researchers also took note of the GPS coordinates and elevation of each site to ensure consistency with subsequent sampling rounds.
Laboratory Sample Analysis
Organisms were harvested from the samples and subjected to quantitative analysis using standard microbiology techniques for the analysis of water contamination. E. coli analysis was conducted using Standard Methods 9223B by the Nevada State Public Health Laboratory. 9 Giardia and Cryptosporidium analyses were conducted using EPA Method 1623: Cryptosporidium and Giardia in Water by Filtration/IMS/FA by Truckee Meadows Water Authority (TMWA) Laboratory. 10 E. coli was reported in most probable number (MPN)/100 mL, and Giardia and Cryptosporidium were reported as count/10 L. Associated contamination of the samples with each organism between streams and between rounds was analyzed using a Kruskal-Wallis test.
Results
E. coli
E. coli counts ranged from <1 to 29.2 MPN/100 mL (Table 1). There was no difference in associated contamination with E. coli between streams (P=0.287) or sampling rounds (P=0.413).
Giardia counts ranged from 0 to 7 count/10 L (Table 2). There was a difference in associated contamination with Giardia between Watson Creek and Edgewood Creek (P=0.028) but no difference in associated contamination between the other streams or between sampling rounds (P=0.924).
Escherichia coli counts (MPN/100 mL) in samples from designated water sources of the Tahoe Rim Trail.
No data.
Giardia count/10 L in samples from designated water sources of the Tahoe Rim Trail.
No data.
Cryptosporidium
Cryptosporidium counts ranged from 0 to 1 count/10L (Table 3). There was no difference in associated contamination with Cryptosporidium between streams (P=0.589) or between sampling rounds (P=0.888).
Cryptosporidium count/10 L in samples from designated water sources of the Tahoe Rim Trail.
No data.
Discussion
Estimating the contamination and potential health risk associated with ingesting water from heavily trafficked streams designated as water sources along the TRT is vital for the safety of hikers and backpackers. Sampling evaluated water from selected streams along the TRT for concentrations of E. coli, Giardia, and Cryptosporidium. Overall, the analysis found no difference in associated contamination between streams or between rounds for E. coli or Cryptosporidium. There was a difference in associated contamination with Giardia between Watson Creek and Edgewood Creek but no difference between rounds.
E. coli
E. coli was chosen over total coliforms because it is a more specific indicator of direct fecal contamination.11,12 In the backcountry, one of the highest associated risk factors of contracting E. coli comes from drinking untreated water from areas that support livestock grazing. A 2006 survey of the Sierra Nevada range in California tested 60 lakes divided into 4 categories of 15 sites each: (1) high use by hikers, (2) high use by pack animals, (3) livestock grazing tracks, and (4) areas rarely visited by hikers or domestic animals. The study found that E. coli was present in 1/15 of the sites with heavy backpacking, 12/15 of areas with livestock (horses and pack animals), and 15/15 of the sites with cattle grazing, with counts ranging from 100 to 500 colony-forming units (CFU)/100 mL in the cattle-grazing regions, highlighting significant differences in fecal contamination depending on type of land use.. 13 Additionally, the apparent impact of cattle grazing on various watersheds in the Sierra occurs despite varying annual climatic conditions. 14
The laboratory analysis in this study measured E. coli in MPN/100 mL. This unit is different from what was used in the 2006 study (CFU/100 mL); however, studies have shown that either the two units is statistically indistinguishable or that MPN slightly overestimates counts of bacteria. 15 With this in mind, our data demonstrate counts of organisms orders of magnitude lower than those found in cattle-grazing regions of the Sierra. In the Lake Tahoe Basin, efforts have been made by entities such as the Advisory Planning Committee of the Tahoe Regional Planning Agency to introduce increased restrictions on cattle grazing and create new committees such as the Grazing Advisory Committee to preserve the ecosystem and protect water quality.. 16
When evaluating the risk of illness associated with ingesting E. coli from an untreated water source, the diversity in E. coli strains and serotypes becomes important. Data from this study is limited regarding the degree of risk of illness because specific strains or serotypes were not identified in our samples. However, using E. coli as a fecal indicator, our data do support the presence of fecal contamination in these water sources, which can be extrapolated to the potential risk of pathologic strains being present.11,12
Most E. coli strains are harmless to humans, but ingesting certain strains can cause gastrointestinal illness in healthy individuals.17,18 There are seven strains of diarrheagenic E. coli, including enterotoxigenic E. coli, enteropathogenic E. coli, enterohemorrhagic E. coli (EHEC), enteroaggregative E. coli, Shiga toxin-producing enteroaggregative E. coli, enteroinvasive E. coli, and diffuse adhering E. coli. 19 When comparing these strains, EHEC, especially serotype O157:H7, given its significance in public health, has a much lower infectious dose reported to be 10 to 100 CFU. 20 Our data show counts ranging from <1 to 29.2 MPN, which, depending on source concentration and volume consumed, have the potential to reach the infectious dose of EHEC serotype O157:H7. In a 2005 study looking at 350 E coli O157:H7 outbreaks in the United States from 1982 to 2002, only 3% of cases were linked to untreated water. These included contaminated well-water systems, spring water, ice, and nonchlorinated municipal water supplies. Most cases were foodborne from contaminated produce, beef, and dairy (52%), followed by person-to-person transmission (14%). 21 Contamination of produce, particularly lettuce, is likely secondary to contaminated agricultural irrigation water from livestock. Data from these studies support the idea that in the United States, E. coli-associated illness is less common from water sources in the backcountry compared with foodborne and person-to-person transmission.
Giardia and Cryptosporidium Spp.
Giardia duodenalis (synonymous with Giardia lamblia and Giardia intestinalis) is one of the most common intestinal parasites infecting humans, with 1 million estimated cases of giardiasis in the United States every year, with 50% of those being asymptomatic. 22 In 2021, researchers at the Centers for Disease Control and Prevention analyzed reported giardiasis outbreaks in the United States from 2012 to 2017. They found that among 703 cases, 370 cases (49%) were waterborne, followed by person-to-person transmission, foodborne, and unknown exposures. Of all waterborne exposures, the most common route of transmission was traced to recreational water sources (ie, treated pools or untreated lakes; 42%), followed by contaminated untreated freshwater drinking sources (28%). 23 One notable outbreak that led to 324 confirmed cases in Reno, NV in 1982 was traced to an infected beaver upstream of the municipal water supply. The beaver was relocated, and chlorination was increased to prevent future outbreaks. 24
Cryptosporidium parvum is one of the most common causes of waterborne diarrheal outbreaks, resulting in an estimated 748,000 cases annually in the United States, although only a fraction of cases is reported. 25 The most common modes of transmission are through recreational water sources, such as swimming pools and water parks (35%), followed by handling animals, especially cattle (15%), and poor hygiene at daycare centers and schools (13%). 26 In addition to grazing animals, multiple species of rodents, including the yellow-bellied marmot in the Sierra, have been found to be a potential zoonotic reservoir of Cryptosporidium. 27
Limited studies exist evaluating the infectious dose of G. duodenalis and C. parvum. Past publications estimate the infectious dose to be a count of 10 for G. duodenalis and 30 for C. parvum.28,29 Our data is limited when it comes to detailing risk of illness because the analysis conducted identified the Giardia and Cryptosporidium genera without identification of the species of origin. However, even if we assume that all the identified counts were from pathogenic species of Giardia (ie, G. duodenalis) or Cryptosporidium (ie, C. parvum), none of our samples reached the thresholds supported by the literature as the infectious dose for these organisms, suggesting that there is a low risk of illness. This conclusion is evident especially when taking into consideration the large volume of water (10 L) analyzed for each sample, a volume much greater than what an individual typically would drink from a single water source.
Several directions for future research have been identified. Due to the dynamic nature of contamination of water sources from humans or wildlife, the risk of contamination or illness is rarely constant. However, sampling a larger number of streams across the Tahoe Basin and extending the sampling period to include spring, summer, and fall over several years would yield valuable insights into potential fluctuations in contamination levels. This approach would allow the exploration of how factors such as temperature variations, wildlife migration patterns, and changes in water levels influence contamination dynamics. By sampling over a period of several years, changes in snowpack levels and their effects on pathogen loads may be observed. This also will be beneficial in the application of this data because hiking activity varies throughout the year. Additionally, further analysis to detect specific pathogenic strains of E. coli and species of Giardia and Cryptosporidium would be beneficial in increasing our ability to detail illness risk.
Limitations
Although the samples collected for this survey encompassed 6 of the 8 major sections of the TRT, only 1 stream per section was sampled, in 1 site, and thus may not provide full understanding of the pathogenic loads encountered by those using the trail. Sampling dates also coincided with the snowmelt of a historic year, with snowpack levels exceeding 200% of the normal values. 30 This increase in snowmelt may have decreased concentrations of pathogens measured in 2023 via a dilutional effect not typical of the average year.
Conclusions
Results from this study show that the risk of illness associated with occasional direct consumption of water from the streams sampled appears to be low. However, due to the dynamic nature of contamination of water sources from humans or wildlife, the risk of contamination or illness is rarely constant. Therefore, it is advisable to filter or treat water from any backcountry source prior to consumption. More extensive studies are recommended to evaluate backcountry water sources in the Tahoe Basin to further evaluate the risk to human health from direct consumption.
Footnotes
Acknowledgments
We thank the following individuals and groups: Allyn Jones, a medical student at the University of Nevada, Reno School of Medicine; Truckee Meadows Water Authority; The Nevada State Public Health Laboratory; the University of Nevada, Reno School of Medicine; and Lingchen Wang, a biostatistician with the University of Nevada, Reno School of Medicine Clinical Research Center.
Author Contribution(s)
Declaration of Conflicting Interests
The authors declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.
Funding
The authors disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This work was supported by the National Institute of General Medical Sciences [GM103440] from the National Institutes of Health Nevada Idea Network of Biomedical Research Excellence (NIH NV INBRE).
