A recent study published in The Lancet Microbe summarized the evidence for respiratory transmission of mpox.

Study: Mpox respiratory transmission: the state of the evidence. Image Credit: QINQIE99/Shutterstock

Accelerated transmission of clade II mpox virus (MPXV) occurred in humans in 2022, leading to 1000’s of cases globally. MPXV acquisition routes include percutaneous exposure (direct skin exposure), mucous membrane exposure (oral, vaginal, and rectal mucous membranes), and infectious virus particle inhalation.

Infection sources include humans, animals, and contaminated fomites. In the present mpox outbreak, the prevalence of anogenital lesions in cases suggests that sexual contact is the first route of infection. In the current study, the authors described the evidence on MPXV respiratory transmission from studies conducted between 1961 and 2022.

Respiratory transmission of MPXV in animal models

Studies in non-human primates and prairie dogs have shaped the understanding of mpox transmission. These dogs are useful in studies as they’re at risk of infection, exhibit longer incubation periods, and might transmit the virus. Furthermore, prairie dogs are the one small animals that develop the characteristic skin rash observed in human mpox cases. Non-human primates are also helpful, given the genetic proximity to and similar disease presentation as humans.

A study showed that experimentally inoculating through the upper respiratory tract of prairie dogs can establish the infection model. In 2013, respiratory transmission with clade I MPXV was demonstrated in prairie dogs. Transmission to naïve dogs was not detected with clade II MPXV. Within the Seventies, studies on non-human primates also suggested respiratory transmission of MPXV.

Respiratory manifestations in humans

Severe respiratory distress has been observed within the late disease course with clade I MPXV infection. Oral lesions and oropharyngeal signs/symptoms have been reported historically and in the course of the 2022 outbreak. A sore throat was commonly observed in as much as 37% of cases in 2022. Cough was reported in nearly 50% of patients before 2022, but in 2022, it was less common, together with dyspnea and nasal congestion.

Historical and up to date studies have described viral isolation from oral/respiratory anatomical sites. In a case series in the UK (UK), clade II MPXV DNA was isolated from the upper respiratory tract when respiratory symptoms were absent and even after the resolution of skin lesions. In some patients, viral shedding continued from the upper respiratory tract for over three weeks.

Mpox outbreaks in humans

Most research on mpox in humans before 2022 has been conducted in Africans. Close contact inside households is implicated in long transmission chains spanning several generations. Within the 2003 outbreak in america (US), all subjects with symptomatic mpox were involved with prairie dogs or contaminated fomites.

Being near the infected animal was not related to MPXV infection. Furthermore, healthcare professionals weren’t infected, even when most reported inconsistent use of an N95 respirator or a surgical mask. The human-to-human transmission was documented in the course of the mpox reemergence in Nigeria in 2017 in five individuals, including one healthcare employee and 4 incarcerated individuals.

During 2018-21, eight travelers from Nigeria to the US, UK, Israel, and Singapore were diagnosed with mpox. 4 were symptomatic in the course of the flight, and make contact with tracing identified no additional cases. Notably, using face masks during flights in 2021 as a part of the general public health measures against coronavirus disease 2019 (COVID-19) could have prevented respiratory transmission.  

In endemic regions, healthcare personnel are at a better risk of infection than most people. Nevertheless, one study conducted in Colorado, US, in 2022 observed that healthcare employees exposed to mpox cases weren’t infected. Outbreak and clinical data showed that mpox transmission occurs during close contact for prolonged periods, and respiratory transmission between humans has not been reported in residential/congregate settings.

Concluding remarks

Together, the authors summarized the various sorts of evidence to know the contribution of respiratory transmission to the spread of mpox. Respiratory symptoms and the isolation of viral DNA from the respiratory tract suggest transmission by the respiratory route but don’t definitively reflect respiratory transmission.

Moreover, environmental surveillance studies observed replication-competent MPXV on surfaces and within the air, however the presence of the virus doesn’t imply infectivity or transmission. Further, outbreak analyses with improved documentation of exposure histories could help determine the definitive transmission mode. Thus far, the percentages of respiratory transmission appear low based on the available evidence, but studies should proceed to evaluate this possibility.