Mpox Cases Detectable Through Wastewater Surveillance, CDC Says

An absence of Monkeypox virus (mpox) detections in counties with wastewater surveillance signified a high probability that a large number of cases were not present, according to a study published in the CDC’s Morbidity and Mortality Weekly Report.

The researchers explained that the global mpox outbreak began in May 2022 when it started spreading widely outside countries through endemic transmission. They noted that patients with mpox can shed mpox DNA in urine, skin lesions, and stool, meaning that the number of infections can be tracked through wastewater surveillance. Consequently, the CDC’s National Wastewater Surveillance System began testing US wastewater for mpox in October 2022; by May 2023, testing was being conducted at more than 500 sampling sites in 49 states.

In this analysis, the researchers aimed to evaluate the sensitivity, positive predictive value (PPV), and negative predictive value (NPV) of using wastewater surveillance for detecting mpox cases. They defined sensibility as “the probability of a wastewater detection assuming that 1 or more persons was shedding virus,” PPV as “the probability that at least 1 person was shedding virus when a wastewater detection occurred,” and NPV as “the probability that no persons were shedding virus in the absence of wastewater detections.”

To evaluate these 3 areas, the researchers compared wastewater detections (mpox detected vs not detected) to the numbers of patients with mpox in a county who were shedding the virus; they noted that case ascertainment was assumed to be complete, and persons with mpox were assumed to shed the virus for 25 days after symptom onset. For the analysis, the researchers used samples collected between August 2022 and May 2023. Overall, the analysis included 3492 wastewater samples from 89 sites in 26 counties across 16 states; the researchers detected 281 mpox cases among the included samples.

Monkeypox virus (mpox) | Image Credit: AGPhotography - stock.adobe.com

From this analysis, the researchers discovered that wastewater surveillance sensitivity increased as the number of persons shedding mpox increased. More specifically, the sensitivity estimate during a given week was 31.7% (95% CI, 23.6%-40.7%) when at least 1 person was shedding mpox, 48.9% (95% CI, 33.7%-64.2%) when 5 or more persons were shedding mpox, and 76.5% (95% CI, 50.1%-93.2%) when 15 or more persons were shedding mpox.

Additionally, the PPV for predicting the presence of at least 1 person shedding mpox in a county on a given day or week was 72.6% (95% CI, 61.8%-81.8%) and 61.9% (95% CI, 48.8%-73.9%), respectively. Also, when mpox was detected in wastewater during a given week, the probability that 5 or more persons were shedding the virus was 34.9% (95% CI, 23.3%-48.0%), and the probability that 15 or more patients were shedding the virus was 20.6% (95% CI, 11.5%-32.7%).

Lastly, the NPV for predicting the absence of any patients shedding mpox in a county on a given day or week was 72.9% (95% CI, 70.5%-75.2%) and 80.3% (95% CI, 76.2%-84.0%), respectively. When the researchers did not detect mpox in wastewater during a given week, the probability that fewer than 5 persons were shedding the virus was 94.6% (95% CI, 92.0%-96.6%), and the probability that fewer than 15 persons were shedding the virus was 99.1% (95% CI, 97.6%-99.7%).

Based on their findings, the researchers explain that wastewater surveillance was "sufficiently sensitive" to detect even a single mpox case in these large samples. Conversely, they acknowledged their study’s limitations, one being that there may have been persons shedding mpox outside of areas covered by wastewater surveillance. Consequently, this would bias sensitivity and PPV estimates downward and NPV estimates upward, as wastewater surveillance cannot detect these cases. Despite its limitations, the researchers explained the study’s implications for public health practice.

"A single, isolated detection likely warrants a limited public health response," the authors wrote. "An absence of detections, in combination with no reported cases, can give public health officials greater confidence that no cases are present. Wastewater surveillance can serve as a useful complement to case surveillance for guiding the public health response to an mpox outbreak."

Reference

Adams C, Kirby AE, Bias M, et al. Detecting mpox cases through wastewater surveillance — United States, August 2022–May 2023. MMWR Morb Mortal Wkly Rep. 2024;73(2):37-43. doi:10.15585/mmwr.mm7302a3.