O13.6 - Rectal and Pharyngeal Mpox PCR Testing of Men Who Have Sex with Men (MSM) Suggests that Rectal Testing is More Sensitive than Lesion Testing and that Most Infections Are Asymptomatic

Background: The clinical role of non-lesion mpox testing is ill-defined, and how often mpox causes asymptomatic infections is uncertain.

Methods: We performed mpox PCR testing on rectal and pharyngeal specimens collected from two cross-sectional samples of MSM sexual health clinic (SHC) patients in Seattle, WA, May-September 2022. One sample included participants in a prospective study. The second included randomly selected MSM. We evaluated the sensitivity of lesion, pharyngeal and rectal specimens in detecting mpox in symptomatic MSM and contacts to mpox. We also estimated the incidence of asymptomatic mpox in MSM based on the prevalence of asymptomatic mpox in our clinic, published data on duration of infection (20 days), and an estimate of the number of local MSM with a risk profile comparable to SHC patients (20% MSM). We used our incidence estimate and mpox surveillance data to estimate the proportion of all mpox cases that were asymptomatic.

Results: The study population included 704 MSM. Forty-six men had symptoms prompting mpox testing, 20 (44%) of whom had PCR+ lesions. Six additional symptomatic men had PCR+ tests from only rectal and/or pharyngeal specimens. Among 23 symptomatic men with PCR+ mpox and specimens from all 3 anatomic sites, lesion, rectal, and pharyngeal specimens were 83%, 100%, and 56% sensitive, respectively. Among 6 contacts to mpox, 3 had PCR+ lesions, 4 had PCR+ pharyngeal specimens, and 6 had PCR+ rectal specimens. Eight (1.2%) of 658 asymptomatic men had a positive rectal and/or pharyngeal mpox PCR. We estimate that 71% of all mpox cases in the population were asymptomatic. This estimate decreases if the mean duration of infection is longer, the size of the at-risk population is smaller, or the number of symptomatic cases is greater (i.e., symptomatic cases went undiagnosed or unreported)(figure). At least one-third of cases were asymptomatic under all modelled scenarios.

Conclusions: Although asymptomatic mpox was rare in our SHC, our findings suggest that asymptomatic infections likely account for a large proportion of all infections at the population-level. Rectal testing can identify infections in symptomatic patients missed by testing lesions alone and may be useful in evaluating contacts to mpox.