Monkeypox virus (MPXV), which is closely related to Variola virus, was first identified as a member of the Orthopoxvirus genus in 1959, causing eight outbreaks in primate colonies between 1958 and 1968 [1]. The first human infection caused by MPXV occurred in 1970 in the territory of Basankusu, Democratic Republic of the Congo (DRC), in a 9-month-old male infant [2]. Since then, sporadic outbreaks have been reported in some countries in western and central Africa, mainly among children in rural, rainforest areas [3], [4], [5], [6]. The MPXV outbreak gained worldwide attention in 2003 when it spread outside the African continent, including the United States [7], [8], [9], [10], [11]. A few cases were reported in the following 20 years [12], [13], [14], [15]. However, since the World Health Organization (WHO) declared the MPXV outbreak a "Public Health Emergency of International Concern" on 23 July 2022, as of 30 September 2023, a total of 91,123 confirmed MPXV cases and 663 probable cases, including 157 deaths, have been reported to the WHO from 115 countries and territories. Recently, the number of patients suffering from MPXV has increased dramatically, causing great concern. For this epidemic to be controlled, MPXV must be better understood and re-evaluated. In this context, we will review recent research on MPXV, including genomic and pathogenic characteristics, transmission, mutations and mechanisms, clinical characteristics, epidemiology, laboratory diagnosis, and treatment measures, as well as prevention of MPXV infection in light of the 2022 and 2023 global outbreaks.

Monkeypox virus (MPXV) is a pathogenic virus belonging to the genus Orthopoxvirus of the family Poxvirus. It has a double-stranded DNA (dsDNA) genome of approximately 197 kb, housing approximately 223 open reading frames (ORFs), along with variola virus (VARV, also known as smallpox, with humans as its only sensitive host), vaccinia (VACV), camelpox (CMPV), and cowpox (CPXV), all of which are pathogenic for humans and animals [16], [17], [18]. These orthopoxviruses are immunologically cross-reactive and cross-protective [19]. The 197 kb genome can be divided into three regions: a core region, a left arm, and a right arm containing inverted terminal repeats (ITRs). The core region is highly conserved; is responsible for transcription, replication and virion assembly machinery; and encodes approximately 181 proteins, while the arms, the variable region, are associated with pathogenicity and host range [20,21]. In addition to single-nucleotide polymorphisms, recent research has revealed that specific genes, multicopy genes, repeat sequences, and recombination fragments are primarily concentrated in the variable region through comprehensive analysis of the currently available whole-genome MPXV sequences [22]. Sequence comparisons confirmed that MPXV is a distinct species of Orthopoxvirus, not a direct ancestor or a direct descendant of VARV, the causative agent of smallpox [23]. The nucleotide sequences encoding essential enzymes and structural proteins in the central region of the MPXV genome were 96.3% identical to those of VARV but differed in the terminal regions, the places where most of the virulence and host-range genes are located [24]. The terminal regions were found to be 83.5-93.6% identical when the amino acid sequence similarity between the monkeypox virus strain Zaire-96 and two variola virus strains [strain India-1967 and strain BSH-75] was compared [24].

The genes with known functions present in MPXV but absent/fragmented in VARV include COP-A44L, a hydroxysteroid dehydrogenase that can influence virulence by increasing steroid production, leading to suppression of the immune system and therefore affecting the immune response; COP-B7R, an endoplasmic reticulum-resident protein that can either affect apoptosis or interact with and retain a normally secreted or cell surface-expressed protein in the endoplasmic reticulum that is important in the immune response; and BR-203, COP-E7R, COP-K4L, COP-B12R, and COP-K1L [25], [26], [27]. The genes with known functions present in VARV but absent/fragmented in MPXV include COP-C10L, an IL-1β that can influence virulence by blocking IL-1 receptors by binding to them and therefore allowing the virus to evade the effects of IL-1 activities; COP-E3L, an IFN-resistant protein that encodes a protein that has a C-terminal domain that binds double-stranded (DS) RNA and an N-terminal domain that binds Z-DNA, affecting IFN, a cytokine with antiviral activity; and COP-K3L, an IFN-resistant protein that affects IFN resistance and therefore influences virulence [28], [29], [30]. The proteins that are present in the MPXV but are fragmented in the VACV include COP-B19R, an IFN-α/β-binding protein that binds type I IFN-α/β and prevents it from binding to its cellular receptors, thereby inhibiting certain signaling pathways. These proteins are critical for the virulence of ectromelia virus (ECTV); BR-05/BR-226, a TNF-binding protein that secretes and binds TNF-α and TNF-β; and BR-207, a serpine-2/apoptosis protein [31,32]. Proteins missing in VACV expressed in MPXV may be a specific target for diagnosis, as well as for a subunit vaccine against mpox that may be developed in the future [33]. Each end of the MPXV genome contains an identical but oppositely oriented 6,379-bp terminal inverted repetition, which, similar to that of other orthopoxviruses, includes a putative telomere resolution sequence and short tandem repeats. Computer-assisted analysis was used to identify 190 open-reading frames containing >/=60 amino acid residues. Of these, four were present within the inverted terminal repetition. MPXV contains essential orthopoxvirus genes but is only a subset of putative immunomodulatory and host range genes. A recent frameshift mutation based on a 2-base insertion was fixed in a coding region that was identified at the 3′ terminus of the OPG191 gene, which encodes the MPXV gp168 protein. With this insertion, the protein was prematurely truncated among the virus population that was prevalent in 2022 [34].

MPXV is divided into two evolutionary clades: (1) Clade I, previously known as the Central African clade or Congo Basin clade, with a high case fatality rate of 1-12%; and (2) Clade II, once called the West African Clade, which is less virulent, with a lower case fatality rate of < 0.1%. Clade II is further divided into subclades: clade IIa and clade IIb [35]. The genetic differences between Clade I and Clade II are significant and are almost twice as large as the differences between subclades IIa and IIb [35]. Neither subclade descends from the other [36]. The order of evolution for the clades is clade I > clade IIa > clade IIb, with clade IIb evolving to become less virulent or adapting to other species, resulting in increased human transmission [37]. The two distinct clades presented a difference of approximately 0.5% in genomic sequence, primarily in the region that encodes important virulence genes, which accounts for the differences in clinical severity [38]. Among them, the strain that caused the global MPXV outbreak in nonendemic areas in 2022 was clade IIb [39,40]. At present, there are many subbranches of the IIb branch, including the A.1, A.1.1, A.2, A.3, and B.1 subbranches [41], [42], [43]. Clade I is largely limited to the DRC and is estimated to cause more severe disease and higher mortality than clades IIa and IIb [44]. Since the first imported MPXV case was reported in China in September 2022, the strain sequences reported to the Chinese Centers for Disease Control and Prevention all belong to the IIb branch [45,46]. Despite this, a number of sequences have been associated with the related A.2 lineage [47].

MPXV is resistant to dryness and low temperatures, allowing it to survive for months on surfaces such as clothing and bedding, soil, and crusts. However, VARV and VACV are sensitive to heat and can be eliminated in suspension tests within 30 min at temperatures between 55°C and 65°C. MPXV was inactivated in less than 5 min at 70°C and less than 15 min at 60°C, with no difference between viruses from the West African and Central African clades [48,49]. Common disinfectants such as 70% ethanol (≤1 min), chlorine-containing disinfectants, 0.2% peracetic acid (≤10 min), 1-10% probiotic cleaner (1 h), and ultraviolet light can effectively inactivate vaccinia viruses by at least 4 log10 in suspension tests and on artificially contaminated surfaces, as shown with different types of organic loads [50], [51], [52]. Additionally, the addition of hydrogen peroxide, sodium hypochlorite (0.25-2.5%; 1 min), 2% glutaraldehyde (10 min), and 0.55% orthophthalaldehyde (5 min) can effectively inactivate the virus on artificially contaminated surfaces, and hydrogen peroxide (14.4%) and iodine (0.04-1%) were effective in suspension tests [53], [54], [55]. Copper (99.9%) was equally effective against vaccinia virus and MPXV after 3 min [56]. An alkaline cleaner (0.9%) can inactivate the vaccinia virus on stainless steel carriers in 10 min, and UVC light (254 nm) has been shown to inactivate the aerosolized vaccinia virus within 7.6 s and inactive the vaccinia virus within 10 min [53,57]. Under practical conditions with different types of organic loads (compounds in the blood, respiratory tract and skin lesions), disinfectants with efficacy data obtained via suspension tests are preferred for the inactivation of the MPXV [58].