Monkeypox Outbreak in the United States-2003

The first outbreak of monkeypox in the United States occurred in 2003 and is also the first outbreak outside of Africa. This outbreak is associated with exposure to infected pets. Although there was no large outbreak, it was another demonstration of the ability of this zoonotic disease to adapt to new ecological environments.

Causes of the 2003 U.S. Monkeypox Outbreak

From May to July 2003, the U.S. monkeypox outbreak involved six states, and more than 70 cases were detected. The cause of the outbreak could be traced to African rodents imported into the United States from Ghana in April. An animal dealer purchased a shipment of pet groundhogs that were housed near these African rodents and thus contracted monkeypox. Then the groundhogs, which were household pets, caused monkeypox infections in humans.

Dissemination Paths

All of the monkeypox infections in 2003 were animal-to-human, with no human transmission and no deaths reported. Studies have shown that certain activities associated with animals are important causes of monkeypox infections. These routes of infection include:

• Touching the sick animal
• Touching the bedding of the sick animal
• Getting bitten or scratched on the skin by the sick animal
• Cleaning cages
• Other sick animal-related contact activities

Comparison of African and 2003 U.S. Monkeypox Characteristics

Characteristics	Comparison of African and U.S.
Epidemiological characteristics	Patients with DRC were more likely than US patients to have a visible rash, were more likely to spread, and had significantly more severe disease than in the US. Both being young and having an unvaccinated status was associated with more severe disease and mortality.
Single-gene phylogenies	The isolates collected from the US and Central and West African outbreaks were subjected to PCR amplification and RFLP analysis. The Gabon, Cameroon, RCG, and DRC isolates belong to the Congo Basin clade, while the Nigerian, Liberian, and US isolates belong to the West African clade.
Genome-wide phylogenetic analysis	Several geographically and temporally distinct monkeypox virus (MPXV) genomes originating from West Africa, Central Africa, and the US depict two distinct MPXV evolutionary branches: the West Africa/US and the Congo Basin. Fig. 1 Phylogeny of MPXV. (Likos A M, et al., 2005)

Tools for Pathogen Confirmation

Later virological studies proved that the West African clade MPXV was the cause of the outbreak. The following tools were used at that time to confirm that the pathogen was human monkeypox.

• Electron microscopy
• Polymerase chain reaction
• Immunohistochemical assay

The 2003 disease in the US appears to be milder than in the African region, which is related to the different virus strains. Therefore, in the face of human monkeypox outbreaks, we need to understand the molecular pathogenesis and epidemiological properties of MPXV so that we can improve the treatment and prevention of monkeypox.

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