Japan Confirms First Domestic Case of Severe Mpox Clade Ib

First Clade Ib Mpox Case Identified in Japan

Japan's Ministry of Health, Labour and Welfare, in conjunction with Kobe City officials, announced on September 16, 2025, the confirmation of the nation's first domestic case of the Mpox Clade Ib strain. The patient is a woman in her 20s who sought medical attention in Kobe after experiencing symptoms including a rash and fever. Genetic testing verified the presence of the Clade Ib variant, marking a significant development in Japan's monitoring of infectious diseases. Officials believe the woman contracted the virus during recent travel to Africa, and her condition is reported as stable.

Understanding Mpox Clade Ib and its Global Impact

Mpox, formerly known as monkeypox, is caused by the monkeypox virus (MPXV), which has two distinct clades: Clade I and Clade II. Clade I, which includes the Ib subclade, is primarily found in Central Africa and is generally considered more severe and potentially more transmissible than Clade II. Historically, Clade I has been associated with a higher risk of severe illness or death. However, Japan's National Institute for Health Crisis Management indicates that with appropriate treatment, the fatality rate for Clade Ib is estimated at less than 1% outside Africa, and no deaths from this specific subclade have been reported globally outside the continent.

The Clade I outbreak has been rapidly spreading across Central and East Africa, particularly in the Democratic Republic of Congo (DRC) and neighboring countries such as Burundi, Rwanda, and Uganda. Since January 1, 2024, the DRC alone has reported over 40,000 mpox cases and more than 100 deaths. The World Health Organization (WHO) had declared a public health emergency of international concern regarding the Clade I outbreak in Africa in August 2024, though this emergency status was lifted on September 5, 2025.

Symptoms, Transmission, and Japan's Preparedness

Mpox typically presents with symptoms such as a rash or mucosal lesions, accompanied by fever, headache, muscle aches, back pain, low energy, and swollen lymph nodes. Transmission occurs through close contact with infected individuals or animals, or with contaminated materials. Human-to-human spread is primarily via direct skin-to-skin contact, including intimate or sexual contact, and through respiratory droplets during prolonged face-to-face interaction. The Clade Ib variant also poses risks through prolonged close face-to-face contact with respiratory droplets or by touching contaminated items like bedding.

Prior to this new confirmation, Japan had recorded 254 mpox cases and one death, primarily from the Clade II variant, as of September 12, 2025. The first mpox case in Japan was identified on July 25, 2022. In response to the global situation, Japan's Foreign Ministry issued a Level 1 infectious disease alert for seven African countries in August 2024. The Ministry of Health has assured the public that Japan possesses established nationwide systems for testing, treatment, and vaccination, utilizing the LC16m8 vaccine for prevention. Authorities have stated there is currently no evidence of further community spread linked to this specific Clade Ib case.