Poxviridae is a family of enveloped, brick-shaped or ovoid viruses. The genome is a linear molecule of dsDNA (128–375 kbp) with covalently closed ends. The family includes the sub-families Entomopoxvirinae, whose members have been found in four orders of insects, and Chordopoxvirinae, whose members are found in mammals, birds, reptiles and fish. Poxviruses are important pathogens in various animals, including humans, and typically result in the formation of lesions, skin nodules, or disseminated rash. Infections can be fatal. This is a summary of the International Committee on Taxonomy of Viruses (ICTV) Report on the family Poxviridae, which is available at ictv.global/report/poxviridae.

On November 4, 2005, the Poxvirus Program at the Centers for Disease Control and Prevention (CDC), a World Health Organization (WHO) Collaborating Center for Smallpox and other Poxvirus Infections, was contacted by a former volunteer with Médecins sans Frontières (MSF) with a request for information regarding a child with a generalized pustular rash illness in southern Sudan. On the basis of a review of a photograph of the child ( Figure 1 ), dated October 28, 2005, and limited clinical information describing a febrile period of disease, it was agreed that the findings were suggestive of a systemic orthopoxvirus infection. Orthopoxviruses include vaccinia, variola (smallpox), and monkeypox. Since monkeypox has been described in the Congo Basin and western Africa but has never been reported in southern Sudan, the potential emergence of orthopoxvirus infection in this region aroused concern. A rapid diagnosis was required to facilitate the appropriate public health response. An investigation by members of MSF found that small clusters of self-limited disease compatible with monkeypox had occurred that were not widely spread within the community. No deaths were reported among the patients with suspected cases.

Monkeypox virus (MPXV) is endemic within Africa where it sporadically is reported to cause outbreaks of human disease. In 2003, an outbreak of human MPXV occurred in the US after the importation of infected African rodents. Since the eradication of smallpox (caused by an orthopoxvirus (OPXV) related to MPXV) and cessation of routine smallpox vaccination (with the live OPXV vaccinia), there is an increasing population of people susceptible to OPXV diseases. Previous studies have shown that the prairie dog MPXV model is a functional animal model for the study of systemic human OPXV illness. Studies with this model have demonstrated that infected animals are able to transmit the virus to naive animals through multiple routes of exposure causing subsequent infection, but were not able to prove that infected animals could transmit the virus exclusively via the respiratory route. Herein we used the model system to evaluate the hypothesis that the Congo Basin clade of MPXV is more easily transmitted, via respiratory route, than the West African clade. Using a small number of test animals, we show that transmission of viruses from each of the MPXV clade was minimal via respiratory transmission. However, transmissibility of the Congo Basin clade was slightly greater than West African MXPV clade (16.7% and 0% respectively). Based on these findings, respiratory transmission appears to be less efficient than those of previous studies assessing contact as a mechanism of transmission within the prairie dog MPXV animal model.

Since the eradication of smallpox and the cessation of routine childhood vaccination for smallpox, the proportion of the world's population susceptible to infection with orthopoxviruses, such as variola virus (the causative agent of smallpox) and monkeypox virus, has grown substantially. In the United States, the only vaccines for smallpox licensed by the Food and Drug Administration (FDA) have been live virus vaccines. Unfortunately, a substantial number of people cannot receive live virus vaccines due to contraindications. Furthermore, no antiviral drugs have been fully approved by the FDA for the prevention or treatment of orthopoxvirus infection. Here, we show the inhibitory effect of one new antiviral compound, ST-246, on the in vitro growth properties of six variola virus strains and seven monkeypox virus strains. We performed multiple assays to monitor the cytopathic effect and to evaluate the reduction of viral progeny production and release in the presence of the compound. ST-246 had 50% effective concentrations of ≤0.067 μM against variola virus and <0.04 μM against monkeypox virus. In a dose-dependent manner, plaque size and comet tail formation were markedly reduced in the presence of the drug at low, noncytotoxic concentrations between 0.015 and 0.05 μM. Our in vitro phenotype data suggest that ST-246 inhibits variola and monkeypox viruses similarly by reducing the production and release of enveloped orthopoxvirus and support the development of ST-246 as an antiviral therapeutic compound for the treatment of severe systemic orthopoxvirus infections.

An ideal animal model for the study of a human disease is one which utilizes a route of infection that mimics the natural transmission of the pathogen; the ability to obtain disease with an infectious dose equivalent to that causing disease in humans; as well having a disease course, morbidity and mortality similar to that seen with human disease. Additionally, the animal model should have a mode(s) of transmission that mimics human cases. The development of small animal models for the study of monkeypox virus (MPXV) has been quite extensive for the relatively short period of time this pathogen has been known, although only a few of these models have been used to study anti-poxvirus agents. We will review those MPXV small animal models that have been developed thus far for the study of therapeutic agents.

Brincidofovir (CMX001), a lipid conjugate of the acyclic nucleotide phosphonate cidofovir, is under development for smallpox treatment using “the Animal Rule,” established by the FDA in 2002. Brincidofovir reduces mortality caused by orthopoxvirus infection in animal models. Compared to cidofovir, brincidofovir has increased potency, is administered orally, and shows no evidence of nephrotoxicity. Here we report that the brincidofovir half-maximal effective concentration (EC ₅₀ ) against five variola virus strains in vitro averaged 0.11 μM and that brincidofovir was therefore nearly 100-fold more potent than cidofovir.