Detecting Mpox Cases Through Wastewater Surveillance — United States, August 2022–May 2023

Discussion

This study is the first to examine the performance of wastewater surveillance for detecting mpox cases using empirical data. Wastewater surveillance had a sensitivity of 14% on a given day for detecting the presence of at least one mpox case. However, most sites were collecting more than one sample per week. Weekly sensitivity for detecting the presence of at least one mpox case was substantially higher (32%). As the number of cases shedding virus increased, weekly sensitivity increased to 49% for detecting five or more persons and 77% for detecting 15 or more persons shedding virus. Weekly PPV and NPV were both high (62% and 80%, respectively).

Although sensitivity might seem low compared with clinical testing, each wastewater sample represents thousands to millions of persons. Results show that wastewater surveillance was sufficiently sensitive to detect even a single mpox case in these large, pooled samples. These findings contrast those for SARS-CoV-2 (the virus that causes COVID-19), for which the minimum number of cases required for a wastewater detection is thought to be much higher (8–38 cases per 100,000 persons for detection rates at 50% and 99% probability, respectively) (7). Unlike SARS-CoV-2, high levels of Monkeypox virus are present in skin lesions as well as in urine and stool (1,8–10), and poxviruses are highly stable in the environment (9).

When Monkeypox virus was detected in wastewater on a single day or week, there was most likely (but not always) at least one case present in the county. Wastewater detections in the absence of known cases might have been the result of travelers, commuters, patients experiencing prolonged shedding, or subclinical or unreported infections (i.e., wastewater detections might have reflected true infections that had not been detected by case surveillance). Moreover, when Monkeypox virus was detected in wastewater on a single day or week, it was rare that 15 or more persons were shedding virus in the county. Because most samples were collected in fall 2022 or later, after case counts in the United States began to decline, large numbers of cases were infrequent. In addition, when Monkeypox virus was not detected in wastewater, there were most likely zero cases (and almost certainly no large numbers of cases) present in the county. High NPV can likely be partially attributed to low disease prevalence during the study period.

Limitations

The findings in this report are subject to at least five limitations. First, because data on viral shedding patterns and clinical case information were lacking,***** variations in case shedding patterns were not included. Second, persons shedding Monkeypox virus might have resided within counties included in the analysis but outside areas covered by wastewater surveillance. This limitation would bias sensitivity and PPV estimates downward and NPV estimates upward, because wastewater surveillance cannot detect persons shedding virus if they reside outside covered areas. Third, because data from all sites serving a county were combined, results could not be stratified by sampling or testing methods or site population size and thus represent average estimates of wastewater surveillance performance across sites and time. Fourth, because case counts during the study period were low, estimates for detecting large numbers of cases are highly uncertain. Finally, although mpox is a nationally notifiable disease in the United States and all cases should be reported, and studies suggest that most cases are diagnosed, some cases might remain unreported (6).

Implications for Public Health Practice

The findings in this report can help guide the public health response to Monkeypox virus wastewater detections. Because wastewater surveillance is sufficiently sensitive to detect very few mpox cases, a single, isolated wastewater detection might not warrant a large public health response. Moreover, because most wastewater detections during the study period resulted from five or fewer cases, the public health response to a single wastewater detection might be scaled to one recommended for small case numbers, as long as mpox case counts remain low. Finally, nondetection of Monkeypox virus in wastewater, in combination with no reported cases, can provide reassurance to public health officials that large numbers of cases are not present in communities where wastewater surveillance is occurring. Wastewater surveillance for Monkeypox virus has a sensitivity of 32% for detecting a single case, with sensitivity increasing to 49% and 77% for detecting five or more and 15 or more cases, respectively. PPV and NPV for Monkeypox virus wastewater surveillance are high (62% and 80%, respectively). Wastewater surveillance can be a useful complement to case surveillance for guiding the mpox outbreak response.