Review of characteristics of human monkeypox

By Tarun Sai LomteReviewed by Aimee MolineuxOct 20 2022

In a recent study published in Tropical Medicine and International Health, researchers conducted a scoping review of available literature on monkeypox (MPX).

Background

MPX is a zoonosis caused by the MPX virus (MPXV) of the Orthopoxvirus genus. Initially, the disease manifests with non-specific flu-like symptoms, gradually resulting in fevers, lymphadenopathy, and characteristic blistering lesions. Aging, pregnancy, and immunosuppression are the known risk factors for severe MPX.

A human MPX case was detected in the United Kingdom (UK) in May 2022 and subsequently in multiple countries. The evolution of the disease has been partly attributed to the cessation of smallpox vaccination, as vaccinated people show around 85% protection from MPX. Sexual contact, especially among men who have sex with men, has been attributed as an additional contributory factor for the ongoing transmission.

Moreover, several disease characteristics from the ongoing MPX outbreak remain less explicit. During the coronavirus disease 2019 (COVID-19) pandemic, scientific research has shown how to advance the understanding of viral pathology and the strategies to prevent and respond to resurgent zoonotic diseases, which could be leveraged for the current MPX outbreak.

About the study

In the current study, researchers performed a scoping review by accumulating evidence from the literature on human MPX. PubMed, SCOPUS, Hinari, and ScienceDirect were searched for eligible publications using relevant search terms. Studies were eligible if they described MPX etiology, epidemiology, management, presentation, and outcomes. Non-English articles, non-human MPX studies, and those with irrelevant or incomplete information were excluded.

Title/abstract screening was performed after removing duplicates. Full texts of publications that met the inclusion criteria were obtained. Two reviewers independently extracted data from included studies. The extracted data included author details, publication year, geographical location, study type, pox vaccination status, clinical presentation, transmission mode, treatment, and morbidity and mortality outcomes.

Findings

Literature searches identified more than 3000 articles, of which 509 duplicates were filtered out. Title screening further removed 2237 articles. Abstract/full-text screening was performed on 358 studies. After several rounds of screening, 77 articles were considered for the final analysis. Most publications were from the Democratic Republic of Congo (DRC), followed by the Central African Republic, the United States (US), the Republic of Congo, and Nigeria, among others.

Data on confirmed, possible, and probable MPX cases were available in 52 peer-reviewed publications. There were 1347 cases with a confirmed diagnosis, and 28,815 were suspected cases. Most patients were from the DRC (29,707) and Nigeria (184). The number of MPX cases increased every decade from the 1970s.

MPX cases were not reported outside Africa until 2003, when the US documented 47 cases due to exposure to exotic prairie dogs. Only a few studies said data on MPXV clades; most cases (29,905) belonged to the Central African clade. Sixteen studies reported on the pox vaccination status.

Of these, six studies observed that all MPX cases were non-vaccinated, whereas the remaining articles indicated that most cases (80% – 90%) were non-vaccinated. Vaccinated individuals also contracted MPXV, with the US having the highest proportion (21%) of infections among those vaccinated. Non-vaccinated individuals (3.6 per 1000) had a higher overall attack rate than vaccinated people (0.95 per 1000).

Ten studies reported data on the secondary attack rate (SAR). There was substantial heterogeneity in SAR across studies, with some reporting 0% SAR and others reporting up to 50%. The cumulative case fatality rate was 8.7%. The case fatality rate significantly differed between the Central (10.6%) and Western (3.6%) African clades. All cases of deaths were among children younger than ten years before the 1990s.

Conclusions

The scoping review suggested that most MPX cases are still reported in Africa, particularly in Nigeria, DRC, and the Central African Republic. Moreover, the case fatality rate was lower for the Western than for the Central African clade. Notably, all deaths caused by the Western African clade of MPXV were reported from an outbreak in Nigeria in 2017, suggesting that the Western African clade causes less severe disease than the other clade. The limited number of MPX epidemiologic studies restricts the ability to compute the global mortality rate.

Besides, the current epidemiologic data are limited by multiple confounding factors, such as poor quality of and access to healthcare in Africa. Thus, population-based studies are needed to precisely estimate the prevalence and case fatality rate of human MPX. Future studies must explore the morbidity outcomes of MPXV infection. The study’s limitations include the non-inclusion of grey literature and non-English publications.