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Using hydrogeological flow modelling to identify sources of hepatitis outbreaks
By Florian T. H. Kleinhoven Kincaid
Researchers have recorded multiple outbreaks of hepatitis A in Michigan since the 1950s. Between 2016 and 2020, outbreaks of hepatitis A most frequently occurred in southern mainland Michigan. Despite these events primarily occurring around metropolitan areas, such as Detroit and Grand Rapids, environmental monitoring revealed a possible connection to agricultural runoff from coastal farmland.
Reviewing prior literature showed that the hepatitis A outbreak in 2016 may have been related to a previously unexplored connection with biological groundwater contamination. We, therefore, employed a hydrogeological flow modelling approach to identify areas where residents would be at comparatively high risk of contracting hepatitis A. In the future, we propose that legislators use a similar method of assessment to identify regions to launch cost-effective, targeted, and impactful public health campaigns.
DISEASE CONTROL AND PREVENTION
The Michigan Department of Health and Human Services (MDHHS) has historically implemented disease monitoring and prevention initiatives. These campaigns primarily achieved prompt identification of hepatitis outbreaks through patient interviews. The intrinsically biased nature of the MDHHS’s interview-based approach to epidemiology resulted in the homeless, incarcerated, and drug-addicted being identified as the highest-risk societal groups.
The greatest difficulty in disease prevention lies in the tricky nature of diagnosing viral diseases. Hepatitis A is characterized by symptoms that may occur due to a variety of illnesses. It is, therefore, challenging to discern in the early stages. The imprecise nature of current analytical methods further impacts the reliability of diagnostic data. Moreover, underdiagnosing mild cases of hepatitis A likely resulted in a radically underestimated representation of the factual incidence.
We thus broadened the diagnostic scope to include general symptoms and characteristics of hepatitis A. The disease profile associated with hepatitis A is more commonly referred to as acute gastrointestinal inflammation (AGI) or inflammatory bowel syndrome (IBS). AGI or IBS are also known as gastroenteritis, and encompass symptoms including anorexia, nausea and malaise.
RISK FACTORS
Hepatitis A is caused by a group of pathogens collectively referred to as enteroviruses. These are small viruses made up of rudimentary genetic material called ribonucleic acid (RNA). Quantifying the presence of these viruses in the environment is challenging due to the highly mutagenic properties of RNA.
Hepatitis A may be transmitted from one patient to the next by direct person-to-person contact or through an intermediate vector such as food or drinking water. Drinking water is often prepared from surface water sources or freshwater from groundwater wells. Preventing environmental spills of fecal matter is, therefore, paramount to avoiding widespread outbreaks of hepatitis.
Untreated sewage spills have frequently occurred near Detroit. However, these events have only consistently been reported since 2014. The latest spill occurred in early 2020, when a Flint sewage treatment facility released over 87,000 cubic metres of wastewater into the environment. Most aquifer systems worldwide are recharged by surface water from lakes, rivers, and streams. It
U.S. cases of gastroenteritis recorded between the Great Depression and the early 2000s per 100,000 population equivalent.
is, therefore, possible that consuming untreated groundwater is a primary culprit of gastroenteritis.
In 2020, the Michigan Environmental Assistance Centre’s Drinking Water and Municipal Assistance Division reported that groundwater is commonly utilized for irrigation of food crops as well as direct freshwater supply. The risk of groundwater recharge contaminated with fecal matter entering the water table, therefore, warranted further investigation.
ENVIRONMENTAL MONITORING
Groundwater quality monitoring was successfully applied by researchers in Georgia, Kentucky and Wisconsin in the 1980s, 1990s and early 2000s. These projects recognized untreated well water consumption as a culprit of hepatitis A transmission. In many instances, boiling water before consumption dramatically lowered the number of infections, further supporting this thesis. Borchardt, et al. concluded in a 2004 study that sewage disposal into surface water streams might constitute the root cause of fecal groundwater contamination.
The Department of Environment, Great Lakes and Energy (EGLE) pursues their mission to protect Michigan's environment and public health through effective management of air, water, land, and energy resources. EGLE's Water Resources Division (WRD) aims to protect and monitor Michigan's swimmable, fishable, and consumable water resources.
Between 2014 and 2018, LimnoTech and White Water Associates collected microbial samples from surface sediment under the oversight of the WRD from 23 of Michigan's major watershed systems in 16 of Michigan’s counties (Figure 1). These efforts focused on assessing to what extent fecal pollution was likely present in Michigan’s waters.
We investigated whether animal and human waste from sewage spills and surface water runoff could contribute to hepatitis A transmission by consuming untreated groundwater. To that end, we applied a hydrogeological flow model to calculate the downstream viral particle concentration from a point source of entry to identify high-risk areas in the state.
Finally, we interpreted the modelled
Figure 1: Sediment E. coli assessment sites organized by county in Michigan.
groundwater quality data using a quantitative risk assessment framework to determine the probability of infection from contaminated groundwater source consumption.
ENVIRONMENTAL INDICATORS
The performance of the hydrogeological flow modelling approach relied on various key indicators. Firstly, identifying a suitable environmental indicator to ascertain the presence of fecal pollution was paramount to facilitating efficient, large-scale, field-based assessment. Secondly, the environmental indicator needed to be able to provide adequate estimates of the pathogen concentration in groundwater.
Indicator organisms are organisms whose abundance is characterized by a strong correlation with enteric pathogens and utilized as an environmental measure of fecal pollution. Microbial surface quality measurements often include high E. coli, a bacterium closely related to mammalian gut flora, including human intestinal colonies. We established the viability of long-term indicator organism survival in the natural environment through the combination of bio-environmental monitoring and chemical analysis.
The highest indicator organism concentrations tended to correlate with proximity to sewer outlets. Moreover, we noted considerable effects of tidal waves on pathogen concentration consistency in coastal areas.
Significant dilution due to meteorological events may also have affected the reliability of microorganism concentration measurements. Furthermore, chemical runoff from anthropogenic infrastructure in direct proximity to the sampling sites may have contributed to underestimated pathogen concentration in the underlying aquifer due to decreased indicator organism survival.
HYDROGEOLOGY
Michigan’s lithology consists primarily of loose sediment underlain by highly porous sand- and limestone bedrock. As a result, high transmissivities, a quantitative measure for the degree to which the lithology would allow groundwater to pass, are found along the south and northeastern regions of mainland Michigan. Groundwater recharge, therefore, likely originates from surface water in such geologically porous areas. Natu-
Figure 2: Enterovirus breakout curves for highest-risk regions in mainland Michigan and Keweenaw Peninsula. rally, groundwater recharge could also contribute to the subsurface transport of biological contaminants.
Retention of biological contaminants due to various physicochemical processes is highly likely. The filtration effect of groundwater transit through aquifer porous media causes a portion of the fecal pollution to be retained in the mineral’s matrix. As a result, the risk of exposure to contracting hepatitis from groundwater withdrawals would peak along with its travel time and distance due to its flow velocity. Figure 2 shows the most significant retention peaks in the catchments characterized by the highest risk of exposure to hepatitis.
RISK ASSESSMENT
We interpreted the hydrogeological quality data collected using the mathematical model using the Rose-Gerba risk-based approach, better known as a quantitative microbial risk assessment (QMRA), as a comprehensive method to study the distribution, and possible control measures of communicable diseases.
QMRAs are resource-efficient and effective in disease monitoring and prevention due to the lack of attrition found in extensive epidemiological studies, such as those employed by the MDHHS after 2016. Furthermore, the QMRA offers an ethical approach to prompt disease control and prevention in response to the 2016 outbreak.
Our QMRA focused on identifying in which Michigan counties the probability of contracting hepatitis A was the highest proportional to its population density. Data published by the U.S. Census Bureau in 2019 suggested that resident demographics at distances further away from major metropolitan areas were disproportionately disadvantaged by intermittent outbreaks of communicable diseases. However, the U.S. Census Bureau failed to identify the residents of exactly which counties were exposed to the highest risk of contracting communicable diseases.
In general, our results indicated that the state could be divided into five separate zones with different levels of risk on the mainland and the peninsula. The highest risk of exposure to hepatitis A per population equivalent occurred
in the northern counties of Michigan, where aquaculture is ubiquitous. Contrary to our expectations, the risk exposures were much higher in areas where agriculture was prevalent compared to the densely populated urban areas.
One potential explanation for this disparity may be that contact with untreated groundwater may be more prevalent in rural regions compared to cities such as Detroit or Lansing. Additionally, public water supply would likely pose a significantly reduced risk due to the federal water quality standards with which utility companies are required to comply. Evidence of higher concentrations of bovine E. coli in sediment samples from coastal areas further supports this hypothesis.
Since the early 2000s, Michigan's economy has been reliant on heavy industry and crop farming. The latter is situated mainly in and around the Kalamazoo watershed in central southern mainland Michigan. Large quantities of agricultural waste are discharged into the environment every day. On the other hand, fish farming is found along the northern coast of mainland Michigan, in and around Lake Michigan and Lake Huron, as well as in some dispersed locations in Lake Superior to the North.
What remains of Michigan’s heavy industry after the end of the economic boom of the 1950s is still most prevalent in and around the metropolitan areas of Detroit and Lansing. The presence of human gut flora in surface sediments, combined with the relatively low risk of exposure per population equivalent, leads us to ascertain that the introduction of chemical pollution from industrial runoff may harm the viability of fecal pathogens in the groundwater environment.
CONTROL AND PREVENTION STRATEGIES
Disease control and prevention are essential to safeguard public health. However, the effectiveness of prevention strategies such as vaccination campaigns is marred by challenges, including vaccine non-compliance and unregistered migration. Monitoring is nevertheless paramount to discern the relevant routes of transmission and predict
Figure 3: Risk exposure to contracting gastroenteritis and hepatitis A in Michigan between 2014 and 2018, expressed per 10,000 population equivalent.
prospective modes of contamination.
The highest risks of exposure were in northern mainland Michigan and on the Keweenaw Peninsula (Figure 3). We concluded that with the successful inoculation of around three-quarters of the most vulnerable demographics, the bulk of short-term hepatitis outbreaks could feasibly be averted. We proposed launching a vaccination campaign in these areas to inoculate 41,000 residents over 10-years to prevent future hepatitis outbreaks. We estimate the cost to be between two to five million dollars (US). This expense would represent a negligible portion of Michigan’s estimated gross state product of nearly US$500 billion in 2020, reported by the Department of the Treasury.
FUTURE INITIATIVES
No concrete plans exist to continue the expansion of our monitoring efforts in Michigan. The proposed method nevertheless offers a uniquely well-suited opportunity to identify more possible sources of pathogen transmission. Hydrogeological flow modelling approaches provide a viable and cost-effective means to identify sources of groundwater contamination contributing to hepatitis outbreaks. However, rather than continuing data collection of current monitoring sites over extended periods, data should be collected in dispersed locations to augment data density and continually reduce uncertainty.
Alternatively, future efforts might focus on identifying more exact methods to quantify the presence of pathogens in the environment than sequencing E. coli strains. While these bacteria are specifically present in the human gut and correlate strongly with fecal pathogens, their abundance is also affected by myriad environmental factors.
Nevertheless, the hydrogeological flow modelling approach applied in Michigan can be replicated successfully in comparable lithologies around the globe.
