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In May 2022, human mpox (previously known as monkeypox) cases were reported in several countries which previously did not report locally transmitted infections, e.g. Europe, North America and Australia. This heralded the start of the 2022 mpox outbreak, which was primarily driven by human-to-human close (including sexual) contact. Globally, the number of mpox cases peaked in August 2022 and has been on a downward trend. As of 11 January 2023, a cumulative total of 84,400 laboratory-confirmed mpox cases including 76 deaths (case fatality rate of 0.09%) have been reported. As of 17 January 2023, there were a total of 21 confirmed mpox cases reported in Singapore (excluding an isolated case of mpox diagnosed in 2019). As of 21 February 2023, 110 countries worldwide have reported mpox cases.

The monkeypox virus is a member of the Orthopoxvirus genus that also includes the smallpox (variola), cowpox and vaccinia viruses. Although the monkeypox virus is not directly related to the variola virus, smallpox vaccination may prevent mpox infection.

Transmission of mpox

Spread of mpox may occur when a person comes into close contact with an infected animal (rodents are believed to be the primary animal reservoir for transmission to humans) in areas where there are animal reservoirs (i.e. West and Central Africa), or more commonly, another infected human (which is the most common mode in the 2022 outbreak), or materials contaminated with the virus. The virus enters the body through broken skin (even if breaks are not visible), the respiratory tract, or the mucous membranes (eyes, nose, or mouth or anogenital mucosa). The incubation period in humans ranges between 5 to 21 days (typically 6 to 16 days). Outside West and Central Africa, the ongoing outbreak of mpox has primarily affected men who have sex with men (MSM).

Clinical features of mpox infection

In 'classic' mpox, the majority (>70%) of infected persons are symptomatic, and early symptoms of mpox include fever, headache, muscle ache, backache, lymphadenopathy, and a general feeling of exhaustion (asthenia). Within 1 to 3 days after the onset of fever, infected persons will develop a maculopapular rash, often starting from the face before becoming generalised (centrifugal distribution), including involvement of palms and soles in up to 75% of cases. The lesions progress to become vesicles and then pustules, before crusting occurs in approximately 10 days, which then spontaneously fall off. The disease is typically self-limiting, with symptoms usually resolving spontaneously within 14 to 21 days. However, the infection can be fatal, particularly in young children, with a reported mortality rate of 1% to 10% during outbreaks.

In the 2022 outbreak, presentation has often been more atypical and variable in terms of order of onset of fever versus rash, and extent of dissemination of rash. The disease caused by mpox in the global 2022 outbreak has been milder than in ‘classic’ mpox as the virus subtype (known as ‘clade’) is different.  About 72% of cases report prodromal systemic symptoms (e.g. fever, fatigue, muscle aches). Anal and genital lesions are reported commonly, and lesions may initially appear quite non-specific.

Atypical features include:

1) Only a few or even just a single lesion, lesions which begin in the genital or perineal/perianal area and do not spread further.

2) Lesions appearing at different (asynchronous) stages of development.

3) Appearance of lesions before the onset of fever, malaise and other constitutional symptoms.

4) Symptomatology may masquerade as other sexually transmitted infections (STIs), and mpox may occur concurrently with other STIs. Patients remain infectious from the onset of fever until the vesicles/lesions have scabbed over and have separated, with normal skin underneath (re-epithelialisation).

Persons with an unexplained rash AND at least one of the following accompanying symptoms (fever, headache, backache, lymphadenopathy, myalgia, asthenia (profound weakness) AND within the last 21 days:

  • Travelled to mpox affected countries OR

  • Had close contact with confirmed case OR

  • Had a history of sexual or intimate in-person contact with persons in a social and sexual network experiencing mpox activity (including men who have sex with men (MSM) and commercial sex workers) and presents with lesions on or near the possible route(s) of exposure during the sexual activity e.g. genitals, anus, oral or hand regions.

should be evaluated for mpox infection.

The diagnosis of mpox can be confirmed by testing by PCR (polymerase chain reaction) which is a type of molecular test, which is performed at the National Public Health Laboratory (NPHL). When a case is suspected, clinicians should contact the NPHL via their hospital's diagnostic laboratories, and send swabs from vesicles (throat swabs and blood are less preferred alternative due to lower sensitivity) in a dry and sterile tube (without universal transport medium) for further diagnostic testing.

Suspect and confirmed mpox cases should be isolated, preferably in a negative pressure isolation (NEP) room, if admitted. Healthcare workers caring for these cases should practice strict hand hygiene and don personal protective equipment including disposable gowns, gloves, N95 masks or equivalent, as well as eye protection. Usually no specific antiviral treatment is required for majority of cases, and clinical management is supportive. Suspect or confirmed cases may also be referred to the National Centre for Infectious Diseases or the Department on STD Control (DSC) for further evaluation. Healthcare providers should refer to the latest guidance from the Ministry of Health for specifics.

As mpox is an Emerging Infectious Disease, all suspected cases of mpox should be reported to MOH via the submission of the MD131 Notification of Infectious Diseases Form through the Communicable Diseases Live & Enhanced Surveillance (CDLENS) system or by fax to 6221-5528/38.

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