Wastewater Monitoring: Norovirus Levels in Wastewater

This interactive dashboard includes the following sections (see definitions and methods below for more information about each item):

Description	Section
A legend with norovirus concentration levels categorized by color.
A graph showing the statewide average norovirus levels over time (use the slider to see additional dates).
Current statewide average concentration level for norovirus.
Current statewide average trajectory of norovirus levels (increasing, decreasing, or stable).
A map of Wisconsin displaying the most recent weekly level of norovirus for each sewershed across the state and the average level for each public health region (select each region to view the current regional level).
A heat map (colored squares) showing the weekly average level of norovirus for each location over time.
A scatter plot showing the normalized concentrations for all wastewater samples at the selected location over time. Colors of the points indicate the norovirus concentration category for each sample, and the line represents the trend over time.

Categorizing wastewater levels of norovirus

Genogroups

Genogroups are categories of norovirus that group together similar strains of the virus. All norovirus data reflects norovirus genogroup I and norovirus genogroup II added together (NovGI + NovGII).

Wastewater sample collection and testing

Participating wastewater treatment facilities in Wisconsin collect and submit samples of untreated wastewater for testing at the Wisconsin State Laboratory of Hygiene (WSLH) . Facilities typically send samples once or twice a week, which are received, processed and tested for different infectious diseases. For norovirus, viral RNA is concentrated and extracted from the sample and then tested using digital PCR for norovirus genogroups 1 and 2.

Normalization of norovirus concentrations

Raw concentrations for norovirus genogroups 1 and 2 are normalized by the population size and daily flow rate of the wastewater treatment facility. Outliers are removed, and the final concentrations genogroups 1 and 2 are added together to produce a final normalized concentration of norovirus.

Categorizing wastewater levels of norovirus for each location

The colored squares on the dashboard show norovirus levels in each community, ranging from very low to very high.

Weekly levels

The colored squares on the heat map (colored squares over time) and the map of Wisconsin show norovirus levels by week across all participating communities, ranging from very low to very high.

For each community, norovirus levels are summarized using data from the most recent week of samples. These values reflect the average across samples collected during that week (typically one or two samples), to represent overall weekly levels.

The weekly average concentration for each location is compared to the historical levels of norovirus observed at that location since wastewater monitoring for norovirus began. The amount of historical data available for comparison varies by location and is only used during periods of disease activity (for example, in-season), and when laboratory methods are consistent. Weekly levels are assigned to one of five categories based on comparison with the historical levels:

• Highest 20% (very high)
• 60th - 80th percentile (high)
• 40th - 60th percentile (moderate)
• 20th - 40th percentile (low)
• Lowest 20% (very low)

Individual samples and trends

Individual sample results (shown as colored dots on the trend graph) reflect norovirus concentration and associated levels measured on specific collection dates.

The samples are categorized using the same approach as weekly levels, by comparing each result to the historical range for that community. Individual samples are not averaged together before categorization. Due to high sample-to-sample variability in wastewater measurement, a trend-line is provided that shows the trajectory and the level over a rolling five-sample window, which uses a statistical smoothing technique called a spline.

For the region

The regional activity level summarizes norovirus wastewater levels across participating wastewater treatment plants within each public health region. At each site, norovirus wastewater measurements are combined and summarized as a weekly average, as described above. The weekly value for each location is then compared with that site’s historical wastewater data and assigned a category of very low, low, moderate, high, or very high.

To calculate the regional category, each site’s weekly category is converted to a numeric score:

• Very low = 1
• Low = 2
• Moderate = 3
• High = 4
• Very high = 5

The site scores are combined within each public health region using a population-weighted average. This means that wastewater treatment plants serving larger populations contribute more to the regional result than plants serving smaller populations.

The population-weighted score is then converted back into a regional activity category:

• Less than 1.5: Very low
• 1.5 to less than 2.5: Low
• 2.5 to less than 3.5: Moderate
• 3.5 to less than 4.5: High
• 4.5 or higher: Very high

A regional result is only reported when at least half of the expected sites in the region report, with a minimum of two reporting sites. If this requirement is not met, the regional activity level is not displayed.

Statewide average norovirus concentration (line graph)

The statewide norovirus concentration is displayed as a line graph over time on the dashboard. This average concentration (in billions of gene copies per person per day) represents the average level of norovirus across all sites in Wisconsin over time. Individual sites may have levels and trends that are different from the statewide average, select your local community for a detailed view.

To calculate a statewide average over time, we use the following methods:

1. For each site, norovirus levels are normalized by population and flow rate. Genogroups 1 and 2 are combined, and outliers are removed, as described above.
2. Using the normalized norovirus levels, a seven day average concentration is calculated for each site.
3. A daily average among all sites is created. This average is then calculated across seven-day periods (seven-day rolling average) to make changes over time easier to see.
4. Note that the statewide average will not be displayed on the graph until there are at least 50% of sites represented in the calculation to ensure that enough sites are represented.

Statewide average norovirus level (category)

The most recent statewide average concentration is compared with historical statewide average of norovirus calculated since norovirus monitoring began to determine the statewide average norovirus level. The current level is assigned one of five categories:

• Highest 20% (very high)
• 60th - 80th percentile (high)
• 40th - 60th percentile (moderate)
• 20th - 40th percentile (low)
• Lowest 20% (very low)

The most recent category is displayed on the dashboard as a colored box titled “current state level”.

Statewide norovirus trajectory

The statewide norovirus trajectory shows whether the current statewide norovirus level is increasing, decreasing, or stable compared to the average level calculated over the prior three days. To quality as a significant increase or decrease, measured differences must meet established thresholds based on statistical evaluation.

Norovirus can occur at any time of the year, but levels typically increase in the fall and peak during the winter months (November through March).

A community’s category reflects how current norovirus levels compare to its own past levels during that norovirus season (not other locations and not during the off-season). These levels are an indication of the level of norovirus activity in a community at a given time.

• A “very high” level means norovirus levels are among the highest levels that have been observed (top 20%) since we began testing wastewater in that community.
• A “high” level means norovirus is higher than what has been observed during a typical norovirus season for that location in the past.
• A “moderate” level means norovirus levels fall in the middle range of past observations during a typical norovirus season for that location.
• A “low” level means that levels are lower than what has been observed during a typical norovirus season for that location. If they follow a period of very low activity, “low” levels could be an indication that norovirus activity is beginning to increase for the season.
• A “very low” level means that norovirus activity is minimal or not measurable and no virus was detected in a sample. This is typical during the norovirus off-season.

We are still learning about wastewater levels and what they mean for communities. It is not possible to determine the exact number of people infected with a given virus based on wastewater levels alone. Therefore, it is more important to look at the category (not detected, very low, low, medium, high, very high) and the direction (increasing, decreasing, or stable) of wastewater levels in your location, region, or state, not the total number of gene copies in the results.

Can I compare results from one community with another?

We do not recommend comparing the exact concentrations (gene copies of norovirus per person per day) between communities. Each community's wastewater is different. For example, some have discharge from industrial sources and varying amounts of rainwater which affects viral concentrations differently. It is better to use data from a single community to see trends over time, and rely on the categorical levels (very low to very high) that have been adjusted for differences across sites, to understand the level of norovirus activity in a community, region, or state.